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-THE ELEVENTH CHESAPEAKE SAILING YACHT SYMPOSIUM
Stars and Stripes Design Program for the 1992 America's Cup Chris Todter, Team Dennis Conner, San Diego, California, USA David Pedrick, Team Dennis Conner, San Diego, California, USA Alberto Calderon, Team Dennis Conner, San Diego, California, USA Bruce Nelson, Team Dennis Conner, San Diego, California, USA Frank Debord, Team Dennis Conner, San Diego, California, USA Dave Dillon, Team Dennis Conner, San Diego, California, USA
ABSTRACT
The Team Dennis Conner (TDC) design program for the 1992 America's Cup is presented in an overview form. The team members are listed. The spectrum of design tools available are discussed, highlighting the usefulness and emphasis of each. The design tradeoffs will be presented in general form, including a discussion of the monoplane/multiplane appendage tradeoffs. The importance of the structural design aspects and methods will be presented. An appreciation of the full size performance feedback to the design will be covered.
NOMENCLATURE
b= q= e=
induced drag force for monoplane induced drag force for biplane aircraft weight (which, for the yacht becomes the force perpendicular to the keel's hydrodynamic axis and to the remote apparent water velocity geometric span remote dynamic water pressure efficiency factor Dib=induced drag force for biplane Lift on wing of biplane with span 1 Lift on wing of biplane with span 2 geometric span of wing 1 geometric span of wing 2 remote dynamic water pressure a complex function of span ratio and gap to span ratio, independent of stagger
INTRODUCTION
Following is a discussion of the overall design program of the Team Dennis Conner America's Cup effort for 1992. The team members are listed, scheduling strategy is discussed, the design tools used are described, and the methodology of the design process is shown to the extent possible within the limitations of confidentiality restrictions, and within the scope of this paper.
207
TEAM MEMBERS
The Design Team consisted of the following members:
PRINCIPAL DESIGNERS David Pedrick Pedrick Yacht Designs
Alberto Calderon Advanced Aeromechanisms Corp
Bruce Nelson Nelson-Marek Yacht Design
TECHNICAL COORDINATOR Chris Todter
TEAM DENNIS CONNER OPERATIONS MANAGER Bill Trenkle
TECHNICAL ANALYST Frank Debord Scientific Marine Services
ASSOCIATE DESIGNERS Matt Brown Advanced Aeromechanisms Corp
T.J. Perrotti Pedrick Yacht Designs
Scott Vogel Nelson-Marek Yacht Design
RESEARCH ASSOCIATES Bill Burns Advance Aeromechanisms corp
Scott Ferguson Pedrick Yacht Designs
CONSULTANTS (in alphabetical order)
Boeing Corporation Ed Tinoco, Paul Bogataj, Winfreid Feifel, Bill Herling, Arvel Gentry
General Motors Technical staffs Group
Newport News Shipbuilding Steve Slaughter, Paul Miller, Dave Dillon
Ove Arup Peter Heppel, Patrick Dallard
Rasmussen and Associates Willem Kernkamp
Science Applications International Corporation Eric Schlagater, Carl Scragg, Don Wyatt, John Kuhn
South Bay Simulations Bruce Rosen, Joe Laiosa
CONTEXT FOR THE DESIGN PROGRAM
A new class of yacht was used for the 1992 America's Cup. This new International America's Cup Class was the result of the political and TV pressure to produce a modern exciting match racing boat for San Diego conditions. Several of the Design Team members were involved in the class formulation which produced a deceptively simple Rule formula allowing significant latitude of design tradeoffs between sail area, displacement, and characteristic length.
As with any new rule and the intense concentration of talented designers trying to beat the rule, there were numerous interpretations required to keep the event fair and yet not stifle creative progress. This process was skillfully handled by the Technical Director and measurers of the International America's Cup Class (IACC). Generally, the Rule turned out to be very good, with a wide variety of hull forms and parameters present in the 1992 America's Cup and some of the closest racing the America's Cup has ever had.
The reason for this introduction to the new class is to make the point that all the syndicates started with virtually a clean sheet of i;:.aper in terms of design. We were faced with finding the optimum hull/appendage/sail plan combination quickly, with no prior database of design knowledge for this class. This prospect led us to believe that the team who developed the best tools, and learned the fastest would have the best chances of success. In simple terms, given other factors such as financial resources being equal(which of course they are not), the team with the best Velocity Prediction Program (VPP) could win the race. This is a gross simplification, but its basis is true.
STRATEGY
The overall design program was developed around what was initially to be a 3 boat program. A lot of effort went into understanding the strategy related to when to build the boats including, of course, what levels of R & D and anticipated gain increments would be achievable at various points along the way.
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This may seem like a waste of time and effort but our group felt that to compete with the richer syndicates, even with our modest 3 boat program, we had to make every test and every move count. The strategy became more critical and required even more planning as it became apparent that financial constraints would limit us to fewer boats.
The trade offs considered included the desire to have a two boat program for testing certain design aspects such as tandem appendages, or rig proportions at full scale. This thinking was opposed by the desire to put as much R & D into what might be the final design (the second boat), thus waiting to build, which would leave us without a full scale testing program. It was decided to wait as long as possible with the second boat so we could put as much design effort into it as possible. This meant that we had to test multiple appendages in other ways (described later) and study the rig proportion trade-offs by observing our competitors.
Our design program was faced with another setback later in the year after putting an intense effort into what was to be the final hull design, when the "drop-dead" day passed without our syndicate being able to commence building. We had determined the exact timing of the build and outfit cycle for the second construction and had targeted the latest possible day for having the design complete enough for the construction to start. As the date passed, we had to resign ourselves to the fact that modification, possibly including multiple appendages, and refinement were our only avenues of improvement left.
We had, of course, an appendage program underway, both looking at biplane appendages as well as more conventional bulb/keel/rudder improvements, but the emphasis on the appendage aspect was heightened at this point.
DESIGN TOOLS
The tools available to the Design Team, and to some extent any yacht designer who can afford to use them, cover a wide range of technologies, costs, level of accuracy, and confidence. some tools are best suited to hull parametric evaluation, others to detailed appendage design. The selection of the right tool for the particular job and more importantly the interpretation of the results is the key to successful modern yacht design.
Each of the design tools available and used by the Design Team is listed below, with a discussion of their advantages and disadvantages and their applicability.
The towing tank is the classical and ,still the most reliable and relied on design tool, although one of the most expensive.
Methodology. Test procedures and data
analysis techniques were, for the most part,
based on those developed by the Partnership
for America's Cup Technology (PACT) in the
Spring and Summer of 1990. A great deal of
effort was spent on quality control during
testing. Relevant code predictions and
previous tank results were always used during
testing to provide real-time assessment of
results. Data was expanded using the PACT
appendage stripping technique and expanded data
was faired by fitting splines through forces or
moments versus speed, yaw and heel. An example
of raw and faired data is shown in Figure 1.
During data expansion, variable wetted length
and wetted area were used to more accurately
determine wavemaking drag. Although this does
not significantly affect total drag
predictions, it does provide more accurate
wavemaking drag values for comparison with
Computational Fluid Dynamics (CFO) results.
DRAG
FIG 1. TANK DATA
Thanks to sponsorship by Cadillac, Team Dennis
Conner was fortunate to have access to the GM
Technical Center Aerodynamics Laboratory. The
size of this wind tunnel permitted tests with
1:3.5 scale models. This resulted in some
model fabrication cost savings since models
built for the tank could be used in the tunnel
as well, and it permitted direct comparison of
tank and tunnel data at model scale.
Methodology. Tests were completed using
an imaged canoe body mounted with the
waterplane vertical in the tunnel. This
arrangement is illustrated in Figure 2. The
model includes actuators to control yaw angle,
rudder angle and tab angle from the control
room. In addition, it permitted attachment of
image appendages and provided for variable keel
location. Tunnel speeds permitted testing to
keel Reynolds Numbers up to 1.3 x 106.
Typically configurations were tested at the
towing tank Reynold s Number and this maximum
Reynolds Number. Certain configurations were
tested at additional Reynold s Numbers to
provide data for extrapolation.
209
FIG. 2 WIND TUNNEL
SPLASH is a linear free surface panel method
potential flow program developed by South Bay
Simulations. It has been under a continual
development process for several years,
producing accurate wave drag, lift and induced
drag estimates for various heel, and yaw
settings.
While the absolute accuracy of the code is very
good, the best use of this type of code is in
doing comparative or derivative studies. This
means that the code is "anchored" to some known
data such as a tank test, then relatively small
geometric variations of the hull or appendage
are run producing differences from the
baseline.
This is a large complex code requiring
experience in the use of similar codes or
preferably the use of SPLASH. The duplication
of a set of tank test results takes several
hours of CRAY computing time and several days
of job submission and interpretation, but when
used properly is a very powerful tool.
Non-Free Surf ace CFO Codes
Into this class of computer flow simulations
fall codes such as the proprietary potential
flow code used by Boeing for wing and body
design, and Boeing's boundary layer codes which
were applied in the design of TDC keels and
bulbs. The same comments made for SPLASH apply
here, the practitioner must be skilled and the
codes require significant computer resources to
run. The results, when properly interpreted,
are excellent.
VPP/RMP
The VPP is the glue that ties all the analysis
and test data together, combining the hydrodynamics of the hull and appendages with the aerodynamics of the sails and rigging.
In the simple case, the VPP takes a set of tank data extrapolated to full size lift, drag, and roll moment, and finds the equilibrium boat speed and wind angle for optimum upwind, downwind, and reaching performance. The weight distribution and rig geometries are additional inputs to the VPP, which produces a table of speeds and times for the various legs of the AC course for selected windspeeds.
The VPP can take hull and appendage data from tank tests, wind tunnel tests, from SPLASH runs, other CFD {any rational or arbitrary alteration of the lift and drag multidimensional surfaces is possible) and produce the estimates for performance.
The VPP can also produce its own internal estimates of hull and appendage performance to use in the equilibrium prediction process. This is called the LPP (Lines Processing Program) and is based on some simplified formulas for wave drag and appendage effective span (originally from the IMS formulation but modified to better predict IACC yacht performance). This process is understandably less accurate than inputting tank data but is still useful for analyzing small variations around a known baseline.
It should be noted that these predictions are only as good as the input data including the assumptions made for the sail forces. This is the reason that full size sailing performance feedback to the VPP is important.
The RMP {Race Model Program) is the tool that takes the performance predictions for various design candidates and tests them against each other (in the computer) using the wind speed data collected over the past ten years from the race venue. The boats are run against each other using the wind as recorded at the appropriate time of day, with realistic match racing ahead and behind penalties. A win/loss percentage is developed for each boat in this manner and thereby the best candidate can be determined.
sea Keeping Codes
It is fundamental that moving a yacht through the water takes energy, and the best performance comes through maximum extraction of energy from the sails and the minimum dissipation of energy by the rest of the yacht in attaining its speed. Punching into waves absorbs energy, which, for a given sail force, reduces the amount of energy available to sustain speed. Pitch and heave motions send additional radiating wave formations away from the hull. Other transient effects of hull, keel, rudder, and sail fluid dynamics accumulate losses compared to more ideal steady-state flow.
210
TDC's oceanographic research showed that the sea state off Point Loma is frequently more severe than what is normally associated with a given wind velocity. Therefore, good rough water performance was a factor in the design teams' hurried analysis of Stars & Stripes and then became a priority area of study for the development of the final design.
Seakeeping codes are relatively large computer programs used to predict the motions of and forces acting on vessels and structures caused by ocean waves. These codes have been under development for more than 30 years. The available codes have to date been of limited use to yacht design, however, for several reasons.
The first major reason is that the priorities of ship design are much different than yacht design. A racing yacht design has speed as its main goal, while ship design is more concerned with motions, particularly rolling and slamming. The ship emphasis on motions rather than added resistance, which is required for yacht design, means that relatively little research on added resistance has be undertaken.
The second explanation for the lack of yacht seakeeping codes is that yacht shapes are substantially different than motor yachts or ships, which are essentially "wall-sided" well above and below the waterline throughout most of their length. Yachts have considerable flare in their transverse sections, changing shape dramatically over a relatively small vertical range near their waterline. They nearly always have shallow-sloped after overhangs, and bow shapes vary greatly from the more plumb-profiled, finer waterlined "destroyer" type to the shallow-sloped, blunter-waterlined "spoon" or "meter" type. Furthermore, the heel angle of a sailing yacht is held nearly constant by the sail forces, while rolling ships average upright with large excursions in either direction. Consequently, the assumption of "wall -sidedness", which has simplified some of the codes to the extent of making them solvable (in some cases, they are not solvable without this assumption), has worked well for ships but renders most of the codes unsuitable for yachts.
Given the general difficulty in predicting added resistance, the sailing yacht's flared shape, and asymmetry introduced by heel angle, the previously existing seakeeping codes had very limited application in Stars & Stripes design. PACT's research evaluated several ship codes, and helped identify two promising candidates for yacht design, the Lin-Reed strip theory code, and SWAN, a JD linear panel code attributed to Paul Sclavounos of MIT.
TDC went on to explore the Lin-Reed code.
While it was not particularly good at
quantitative evaluations of different hull
shapes, it proved very useful in predicting
differences in added resistance due to changes
in radius of gyration (distribution of mass
away from the center of gravity), position of
the vertical center of gravity, and in
correcting model test data for the towing
position.
The SWAN code, a more sophisticated and
fundamentally grounded seakeeping code, and
Paul Sclavounos' ray theory bow shaping code
(which consideru the forces caused by the
encountering wave's impact on the 2D bow shape)
were both considered promising and the TDC team
was anxious to try both but they were
considered too developmental at the critical
design time. The SWAN code has since proven to
be an accurate and useful code in subsequent
PACT research.
The limitations of seakeeping codes as of
March, 1991, led to a conunitment by TDC to an
ambitions series of rough-water model tests
(described later). Sufficient testing was
performed to develop generalized, parametric
relationships for added resistance of IACC
yachts from experimental data, with guidance
and refinements from the Lin-Reed seakeeping
code and theory. This ability to predict added
resistance of arbitrary IACC candidate yachts
was added to the VPP/Race Model evaluations in
the last few months before the freeze date of
our final design. Since the final design was
not built, the rough-water capability developed
was used to assist in on-going refinements to
Stars & Stripes.
Structural Codes
Four major groups worked with the principal
designers during the structural design and
analysis phase. Ove Arup performed a
preliminary finite element analysis (FEA) of
the initial structural configuration. Newport
News Shipbuilding (NNS) was involved in the
hull structure and keel detailed design and
analysis, and General Motors did research for
the second boat design. In addition, Rasmussen
and Associates performed hydroelastic analyses
on several of the appendage iterations and
configurations.
In the detailed structural design and analysis
stages, two principal types of structural codes
were used- a laminate analysis program and
finite element analysis programs. The laminate
analysis program was used to compare initial
laminate designs based on stiffness and
strength in the major directions. once this
process was complete, the laminates were input
into the finite element models and evaluated
against a set of applied loads.
The laminate analysis program used by NNS was
GENLAM, a public domain program available
through Think Composites of Stanford
University. GENLAM is based on the laminated
plate theory for analyzing composite plate
structures and will run on almost any personal
computer (PC). GENLAM can calculate the
211
stiffnesses and strengths of asymmetric hybrid
laminates and will predict factors of safety
for a set of applied loads based on the TSAI-WU
failure criterion. Since it was possible to
analyze many laminates quickly and dismiss
specific combinations of materials and ply
formats early, much time was saved in the
finite element portion of the analysis.
A finite element program works with a user
generated mathematical model of the geometry
and material properties of a structure to
perform an analysis that can determine the
deflections and internal stresses in the
structure for a set of applied loads. The four
major commercial finite element codes used in
the analysis of Stars & Stripes were MSC/PAL2,
COSMOS/M, I-DEAS, and MSC/NASTRAN. The
preliminary analysis done by ove Arup was
performed using I-DEAS, Sparcr:aft used an in
house code, Newport News Shipbuilding worked
with MSC/PAL2 and COSMOS/M and General Motors'
work was with NASTRAN. MSC/PAL2 and COSMOS/M
are both PC-based programs, while I-DEAS and
NASTRAN are most widely used in mainframe or
workstation environments.
The major portion of the finite element work
was accomplished using COSMOSIM. This program
can analyze models with up to 15,000 nodes or
60,000 degrees of freedom while running on a
PC. COSMOS/M allows for input: of geometric
data via a generic graphic format file (such as
.DXF), output by programs such as AUTOCAD.
This made for easier model building by allowing
the direct input of lines from the designers.
COSMOS/M can also output model files for
NASTRAN analysis, which aided in transferring
model changes between NNS and General Motors.
scale Sailing
Scaled testing relies on the principle that if
all linear dimensions are accurately scaled,
other than second order effects caused by the
vertical wind speed gradient and foil and sail
Reynolds number degradation, the performance of
the scaled yacht can be extrapolated to full
scale. This method has some benefits over full
scale, primarily lower cost, but still suffers
from the inherent difficulties of boat-on-boat
testing.
TDC used this method for some intermediate
level testing of the tandem keel configuration
as discussed below.
Full Size Data Analysis
The analysis of full size sailing data is
extremely important to the design process since
it provides the feedback on how well the
performance predictions are borne out in real
life. A proprietary data analysis package was
developed to take the logged data such as boat
speed, wind speed, wind angle, and heel and
apply dynamic adaptive filtering algorithms to
the data to reduce the noise. This smoothed
data was then assembled statistically to
produce sailing targets for real time
optimization. These targets also served to
quantify the performance envelope for
comparison with the VPP predictions.
FIG 3. TYPICAL POLAR DATA PLOT
DESIGN OF STARS & STRIPES (USA 11)
Introduction
The first boat was commenced as early in the program as possible, to give us a boat for the May, 1991 World Championships. The basic parameter set for the first boat was defined in just two days, wi~hin three weeks of first assembling the design team. The concept for the first boat was for it to be a test platform and it was to be our best effort at an optimized design for San Diego conditions while consciously avoiding parametric extremes which would have limited our flotation and tuning flexibility.
The keel/mast step/chainplate structure was designed to allow radical movements or alternate configurations of appendages and as such was an overkill for a standard fixed position rig and keel, a penalty she still carries.
The deck layout and in fact deck construction had been done earlier (ahead of the boat) to allow us as much hull design time as possible.
The detailed internal structural design and skin layup was developed over the course of the ensuing construction with drawing being delivered just before construction.
Parameters
The design team reviewed all the data available at that time which included the limited TDC tank testing described below, the PACT series of tank tests, some pictures of the French and Italian boats (the first two IACC boats built), some wave drag CFO runs by South Bay Simulations (on PACT hulls), and numerous VPP runs including a rough water model based on the analytical approach developed during the Stars & Stripes 1987 design program. A basic parameter set of length, displacement, sail area, and beam were determined.
The principal designers then developed several iterations of hull lines plans until all offices were satisfied and the plug for the hull was started.
212
There were numerous other basic decisions which had to accompany, or closely follow the hull lines plan development, such as:
a. main sail area versus foretriangle area b. estimates of the structural requirements
and weight c. size of fin and rudder d. bulb shape e. mast and rigging stiffness
Fortunately, the design group had been engaged independently in this research for the previous year and needed only refine the calculations based on the final range of length, weight,
sail area.
TDC Towing Tank Program
In addition to relying heavily on early parametric tank test results provided by PACT, Team Dennis Conner completed model tests at four different towing tanks. Late in the summer of 1990 several models were tested at 1:8 scale at the Davidson Laboratory, exploring choices in hull size, proportions and bow shape. These tests were used to extend the PACT parametric series and results were tied to
the 1:3.5 PACT scale tests by duplicating one PACT scale model.
In November, 1990, after commencing construction, a series of 1:3.5 scale tests were initiated at Arctec Offshore Corporation with a model of USA 11. For the following 14 months the team used this facility as the primary source of calm water tank tests. Early
in the development of the tandem appendage configuration, exploratory tests were completed at small scale at a university tank. In addition, the team completed proprietary seakeeping tests at the University of Michigan.
Table 1 summarizes the 1:3.5 scale tests completed for development of a second hull and optimization of USA 11. These tests were closely related to the wind tunnel test program and the SPLASH CFO work described in the following sections.
The first series of these tests was used to characterize USA 11, establish sensitivity to small changes in displacement and complete some appendage stripping exercises. Next a set of appendage tests was completed to provide guidance in planning wind tunnel work and a data set that could be used to assess free surface effects.
Most of the remaining 1:3.5 scale tests focused on testing designs for the second canoe body and developing the tandem keel configuration. In May, a canoe was tested which was designed using a wavemaking resistance optimization scheme developed by SAIC. In August, the final two canoe bodies were tested. These designs were developed with input from the Michigan seakeeping tests and SPLASH runs. The final testing completed by the team was an appendage test series completed in February, 1992.
Tank testing was the primary experimental tool
used to develop the tandem keel configuration
since the free surf ace effects were believed to
be critical, and foil position was a major
issue. Three test series were completed,
beginning with a proof of concept series in
February 1991. Based on these results, a more
extensive series of tests was completed in May.
A final test series was completed in August
which was an attempt to better understand the
viscous effects associated with this
configuration.
Table 1 - Summary of 1:3.5 Scale Tank Tests
Date Description
Nov, 1990 Baseline, Displacement,
Stripping
Jan, 1991 Appendage Series
Feb, 1991 Tandem Keel Proof of
Concept
May, 1991 canoe No. 2
May, 1991 Tandem Keel
Aug, 1991 canoes No. 3, 4
Aug, 1991 Large Model Tandem Keel Conventional
TDC Wind Tunnel ~ Program
Table 2 summarizes the tests completed. The
first series of tests was designed to provide
the design team with a set of baseline data and
an improved understanding of how the tunnel
should be used in conjunction with tank tests
and CFO. The model was tested with and without
imaged appendages, and appendages were added
one at a time. In addition, this series
included rudder and flap sweeps and a series of
keel and bulb position variations.
In April of 1991 the first series of bulb and
winglet variations was completed. This was
extended in July and October. Also in October,
a foil test was completed in support of the
tandem keel configuration development. Final
bulb and winglet tests were completed in
January, 1992.
Table 2 - summary of Wind Tunnel Teat Program
Date Description
Feb, 1991 Baseline tests, Keel and bulb position
Rudder and flap sweeps
Apr, 1991 Bulb and bulb/winglet
Jul, 1991 Bulb and bulb/winglet
Oct, 1991 Bulb and bulb/winglet
Oct, 1991 Foil tests
Jan, 1992 Final bulb/winglet
213
Structure
Significant effort was devoted to structural
analysis of the first boat by the design team
and consultants from Newport News Shipbuilding
(NNS) and Ove Arup. Structural weight and
stiffness were recognized as major performance
parameters in these new IACC yachts, and the
design team needed to rapidly develop a sound
and efficient structural design for the first
boat.
The structural design of Stars & Stripes, was
complicated by the fact that it was also
intended to be a testing platform capable of
accommodating a wide range of rig and keel
positions. An initial baseline structural
design was developed for FEA modeling and
analysis, which indicated the areas of highest
stress and deflection under the sailing loads
provided by the design team. This FEA model
was then set up for detailing of the model in
the areas of interest and analysis of the
effects of varying the rig and keel positions.
Hull laminates and internal structures were
subsequently modified and optimized to minimize
longitudinal hull deflection and torsional
rotation of the sections between the mast, keel
and rudder. Detailed local models of the mast
step, chainplates and keel attachment structure
were also created to optimize the laminates for
minimum weight with the specified allowable
stress and deflection limits.
Newport News Shipbuilding and the design team
also developed a keel structural analysis
optimization program based on righting moment
maximization at specified heel angles. Design
trade-offs between keel fin size, weight and
bending stiffness were aided by these analyses.
Final keel fin structural optimization was
guided by a detailed FEA model analysis by NNS
of the specified fin geometry developed by the
design team from CFO analyses provided by
Boeing.
FIG. 4 FEA MODEL SHOWING EXAGGERATED DEFLECTION
Design Objectives. In all of the structural design and analysis performed for Stars & Stripes, there were a number of major design objectives that drove the work:
Stiffness The hull and keel were designed to minimize deflections(maximize stiffness) and righting moment.
Adaptability Since the first boat was meant to be a trial horse and test platform, a large amount of adjustability was built into the design.
IACC Rule Laminates were designed to meet the rule for minimum weights and thicknesses, while maintaining maximum stiffness.
Strength A minimum factor of safety was kept within the major structure to avoid costly, time consuming or catastrophic breakdowns.
Analytical Approach. The first step in the structural design was to create an initial, conceptual configuration and use this as a starting point to analyze the impact of reducing material (and weight) or using different structural configurations versus the changes in stiffness. once these initial trade-offs were completed, the preliminary and detailed design stages were initiated. At thie point, the major bulkhead locations were generally fixed, and the design and analysis centered more on the laminates that would meet the minimum rule requirements while providing the maximum stiffness. This led to many timeconsuming analysis iterations. The initial keel design studies were also started, with a large effort centered on identifying the best keel fin material based on the trade-off between fin weight, bulb deflection, and cost.
One of the most complicated areas of the detailed design phase was devising a method of attaching the keel fin to the hull to minimize bulb deflection, while allowing a large amount of fore and aft position adjustment. other areas completed during the detailed design
phase include the keel fin structure, bonding requirements for all structural panels and final attachment methods for the keel mast and rigging.
All of these stages used finite element analysis (FEA) to evaluate the designs as they were drawn, leading to a large measure of interaction between the designers and the analysts. The basic approach of the FEA was to use a series of global and local analyses, where design changes were assessed for their impact on both the overall hull stiffness and on local stiffness, such as keel or shroud deflection. The global model consisted of the entire hull and major internal structure, with loads applied to model the keel and rigging.
214
This model had a coarser mesh than the local model, but could predict overall hull deflections. The local model was more detailed, with a finer mesh representation of the center section of the boat, spanning the mast step/chainplate area to the rudder. This model predicted the local deflections and gave more accurate prediction of the laminate strengths in these areas.
Once the boat had been constructed and sailed and it became apparent that a second boat would not be built, there were many modifications proposed to reduce the weight penalties that had been built in for keel and mast adjustment and to remove structure in other areas to reduce the pitching gyradius. All of these proposed changes were first modeled and evaluated using the FEA tools before being implemented.
Testing. A material testing program was .undertaken to provide the team members with refined values for the material properties that were used in the structural design and analysis. This project was a joint and cooperative effort between the design team, Goetz Marine Technology, General Motors and the Florida Institute of Technology. In addition to the standard material properties testing,
this test program also covered such areas as beam and bulkhead attachment schemes, where a number of candidates were constructed and tested to evaluate strength and stiffness.
Testing was also performed on the boat to confirm the results of the analytical models. During dockside testing, the rigging and mast were loaded up and hull deflections measured. This and similar tests helped to validate the global FEA model which had previously been used to predict the various deflections. The predicted results gave good agreement with the measurements, allowing somewhat more aggressive design margins to be used.
Strain gauge testing was also performed on the internal hull structure and keel fin. These results validated the local hull and keel FEA models and also provided information about the expected dynamic effects of sailing on keel stresses, so that keel design could be refined for the next iteration.
Conclusions. Based on the initial objectives, the design was very successful. The boat was quite stiff, keel and mast movement were easily accomplished overnight, and there were few structural failures. As could be expected however, Stars & Stripes proved to be somewhat over built for her final use as a competitive IACC yacht. This helped to prevent structural failures, but the weight penalties involved in making her a test platform had to be carried through the racing program.
RESEARCH AND DEVELOPMENT FOR THE FINAL DESIGN
Yf.f Feedback l.!;:Q!!l Stars ~ stripes
The first hull was not tank tested until after
the design was frozen and under construction.
This may, on the surface, sound backwards but
the truth is, the first boat was to be the
baseline for any subsequent development. It
was necessary to know the performance of the
first hull and rig very accurately, and know
how it performed relative to the competition.
As discussed above, there are still significant
aerodynamic and unsteady effects present in
actual sailing conditions which are not modeled
well in the VPP. By determining the
performance of the full size boat accurately,
and knowing the flat water, steady state tank
characteristics of the hull, it is
theoretically possible to determine some of the
aerodynamic and unsteady characteristics.
This is an arduous task, since the performance
characteristics are typically masked by very
noisy data, with a tremendous number of
unmeasured variables, but significant progress
was made in the improved determination of these
variables.
This aerodynamic knowledge can, for example,
now be better modeled in the VPP, which affects
the optimization of the subsequent hull
characteristics and cause optimization to a
different point in the design space.
Seakeeping Experimental Research
Sailing yacht performance prediction, whether
by computer modeling or physical model testing
in a towing tank, is normally examined in a
smooth-water environment. This is because both
means of analysis involve a steady-state
condition of the yacht that is far simpler than
in rough water, where the dynamic interaction
between.the moving sea surface and the moving
yacht introduces additional dimensions of
complexity.
The dynamic response of the yacht to the
encountered waves involves variations due to:
a. hull size, proportions, and details of
shape b. displacement and corresponding
distribution of displacement
c. boat speed d. wave length and height e. yacht's angle of incidence to the waves
f. heel angle g. change in aerodynamics of the pitching
sail plan
All but the last were researched and
satisfactorily quantified by the TDC design
team, with decisive input into the final
design. In fact, earlier knowledge of these
influential factors led to the design team's
choice of parameters in the first-generation
design, Stars & stripes, committed in
September, 1990.
215
As noted previously, PACT had initiated
research in seakeeping codes, which the TDC
design team endeavored to utilize as
appropriate. Meanwhile PACT also sponsored
initial tank test procedures and testing of its
baseline design at the University of Michigan
during February-May, 1991. A limited but well
chosen model test series was undertaken by TDC
and completed in June 1991. The test program
and investigation of other yacht seakeeping
research focused on the effects of:
a. beam, length, displacement and gyradius
b. wave length, steepness, and incidence
angle c. boat speed
The object.ive was to determine an empirical
prediction scheme for added re~istance of
candidate IACC designs using generalized
relationships of test data to hull
characteristics.
Rough water tank testing is far more time
consuming and costly than smooth water tests.
smooth water is easily defined and created
repeatedly, while waves are not as well
behaved. Target wave profiles are chosen, but
imperfect wave generation and the fact that the
waves change shape as they progress down the
tank add larger error bars to the measurements
than desirable. Measurement of the wave
profile in front of or very near the moving
model is difficult and introduces other
experimental errors.
It should be noted that the TDC testing, and in
fact most seakeeping testing, was done in
"regular" waves. This means waves of constant,
repeating wave length and height as opposed to
the ocean where the sea state is
characteristically called "random". The energy
content of a sea state can be measured and
represented statistically as a function of
mixed component lengths (with a unique
relationship between wave length and
frequency), called a "sea spectrum". Different
wind velocities and other factors cause
different spectra. The energy content of waves
within a limited bandwidth of wavelength can be
integrated from the area under the spectral
curve. Similarly, the entire spectrum can be
defined by values for a finite number of
regular wave lengths (frequencies). See Fig 5.
600
500
N' ( 400
~ = N 300 (
~ "' a:
200
100
~
\
x J J
\
_/
(r-.....
I I\ \
~
"' r--.....
3.0
2.5 u
2.0 r N (
5.
1.5 ~ c: Gl Cl
1.0 ca
i 0.5 CJ)
r--0 0.0
FIG 5.
0 25 50 75 100 125 150 175 200 Wave Length (fl)
SEA SPECTRUM AND ADDED RESISTANCE OPERATOR
Because the random sea state itself can be described by the superposition of regular waves, it follows that models can be tested in regular waves and the results then extrapolated to any arbitrary sea state by spectral analysis. In fact, testing in approximately 10 regular wave lengths adequately describes the response of the yacht's motions and added resistance for making random sea predictions. (See added resistance operator on Fig. 5). Another convenient mathematical relationship that appears to hold true enough for yachts is the linearity of responses to wave height. In the case of motions, doubling the wave height produces twice the motion amplitude and added resistance increases approximately in proportion to wave height squared. By multiplying the resistance "response amplitude operator" for added resistance by the "sea spectral density", one obtains the added resistance curve (Fig. 6), whose area (when consistent units are used) is the added resistance of the yacht in the prescribed conditions.
Because only the total resistance of the model can be measured in the towing tank, added resistance is the total resistance minus the calm water resistance. There are enough physical variables in rough water testing to require several runs to be made at each boat speed and wave length and height to determine a statistically significant value of resistance. It can take half an hour or more for the water to be calm enough for the next run to be made, and an accurate set of calm water data for the model must be taken as well. Given TDC's priorities, limited time and money, analysis concentrated on added resistance, rather than motions, although they also were measured and logged.
By testing a systematic series of models, the quantitative effects on added resistance due to a change of each of several key parameters could be measured. After considerable effort in data analysis, relationships were found between the principal characteristics of the models and added resistance so that nearly all of the test measurements could be predicted within experimental accuracy. With the confidence of this generalized, rough water predictive ability, the design team could make accurate allowances for added resistance in waves in drag calculations of candidate designs for analysis by the VPP.
YS
450
400
~ 350 g. 300 E 2 250
i en 200 ., ~ 150 0
~ 100 er:
50
0 0
I\ I I~ \ I \
' \.....,
I \
I ""' u -....... ---25 50 75 100 125 150 175 200
Wave Length (ft)
FIG 6. ADDED RESISTANCE CURVE
216
Seakeeping turned out to be a very important factor in the outcome of the 1992 America's Cup, as anticipated by the design team. Considering the size of the matrix of test conditions and test cycle times, one can appreciate that seakeeping tests are rarely undertaken in yachts--even for the America's cup. More progress in both testing and analytical solutions is recommended for the next America's Cup.
Volume Distribution
The design team worked together with the Ship Hydrodynamics division of SAIC LaJolla to investigate the effects of hull volume distribution on wave and total resistance. Initially, optimized upright sectional area curves were developed with a thin-ship based wave drag code, which included estimated viscous drag effects, for IACC hull forms at lengths, volumes, buoyancy centers and target speeds as specified by the design team. A comparison of these results with the TDC baseline hull form provided direction for the development of an alternative hull form for testing.
In an effort to further investigate and optimize the details of the hull form, a slender-ship based hull geometry optimization code developed by SAIC LaJolla was employed to generate optimum hull forms within the naval architectural constraints specified by the design team. This panel-method code, which combined the slender-ship theory wave resistance calculations and ITTC (with form correction) viscous drag estimates to predict total resistance, was modified to optimize heeled, as well as upright, hull forms at the speeds specified by the design team.
Initial studies with the code generated some irregular hull forms which included bumps and hollows precluded by the IACC rule. The code, which iteratively moves panels on the hull surface as it converges towards the minimum resistance configuration, possessed the useful capability of producing gradient plots which indicated which panels could be moved with the least impact on the total resistance. With the gradient plots providing design guidance, a smooth hull form was created which the code predicted to have significantly less total resistance at speeds above 8.2 knots and similar or slightly greater resistance at lower speeds.
While the above results appeared promising, questions remained regarding the reliability of the method and the predicted form drag associated with the resultant hull shape. It was determined that a towing tank model test was needed to establish the true potential of the optimized hull form.
The results of the tank test at Arctec were
less encouraging, with the total resistance
being similar to or greater than the TDC
baseline in all conditions, although the hull
form factor was determined from Prohaska plots
to be similar to the baseline. An analysis of
the longitudinal wave cut data using Sharma's
method for calculating wave drag failed to
conclusively reveal any increase in wave drag
for the model relative to the baseline,
However this now appears to have been due to a
mis-calibration of the wave drag probes. It
was noted that the measured sinkage and trim of
the optimized model were both greater than the
baseline, which was attributed to changes in
the afterbody shape. Based on these results,
the design team elected to return to the use of
thin-ship based optimal sectio~al area curves
for guidance on hull volume distribution.
Structural Optimization
Design Objectives. The design objectives
for the second boat were similar but not
identical to the first boat:
Stiffness
Weight
The hull and keel were designed to
minimize deflections(maximize stiffness)
and righting moment.
A major effort was made to reduce
unnecessary weight, especially by fixing
the keel and mast positions, integrating
major structural components, and removing
redundant structure.
IACC Rule Laminates were designed to meet the rule
for minimum weights and thicknesses,
while maintaining maximum stiffness.
Strength A minimum factor of safety was kept
within the major structure to avoid
costly, time consuming or catastrophic
breakdowns.
Analytical Approach. For this design,
the team already had a strong baseline to work
with from the design iterations performed for
the first boat. Therefore, the basic approach
was to take what was learned from testing and
sailing the first boat and from observing the
competition and improve upon that. There were
three major paths taken in the structural
optimization of the second boat:
a. Eliminate internal structure (bulkheads,
longitudinals, etc.) or relocate for
maximum stiffness and minimum weight.
b. Optimize ply orientations of all major
laminates in the hull structure for
stiffness.
217
c. Reconfigure the deck to lower the hull
center of gravity, reduce hull weight,
and increase stiffness.
General Motors performed a design of
experiments on the global FEA model in which
the variables were either stringer location or
removal. Many combinations of variables were
analyzed, and the results graphed to find the
greatest structural benefit dependent on these
variables. In this case, the objective was to
reduce overall hull deflection (increase hull
stiffness). These analyses were used to
streamline the internal structural
configuration and greatly reduce the weight,
while maintaining desired hull stiffness.
The laminate optimization was also performed by
General Motors. Here performing a design of
experiments would have been prohibitively
costly and time consuming due to the nearly
infinite number of combinations of ply
orientations in the hull and hull structure.
Instead the internal design optimization
capabilities of NASTRAN were used. In NASTRAN
Version 67, the program uses a directed
optimization routine to evaluate changes in
structural properties such as plate thickness
or laminate ply orientations, and finds the
best combination of these properties based on a
given objective function. The objective here
was to optimize hull stiffness while keeping
the laminate at the IACC Rule minimums.
The hull and internal structure were broken up
into about 20 separate laminates, and each of
the ply orientations within was allowed to
vary. The objective function (minimum bending
and torsional deflection) was input, and
NASTRAN calculated the resulting optimized
laminates. These laminates were then modified
slightly to make them practical for
construction.
The deck configuration studies were conducted
jointly by the designers and Newport News
Shipbuilding. Several proposed configurations
were chosen by examining the changes made by
other teams and looking at our own design
objectives and how they could be achieved.
These configurations were then input into the
global FEA model and evaluated based on overall
hull stiffness, torsional stiffness in the
cockpit area, hull weight, and hull center of
gravity.
Results. Since the second boat was never
built, it is difficult to tell for certain the
gains made over Stars & Stripes(USA 11), or how
well it would have fared in competition. The
analytical results, however indicated that
between the improvements found through each of
the three optimization paths, the second boat
design was approximately 15% stiffer than the
first, and had a considerable (but still
confidential) improvement in weight and center
of gravity.
VPP/Parametric Optimization
A significant effort was expended, particularly with the new class of yacht to be designed, to determine the tradeof fs within the IACC Rule for the basic parameters of length, sail area, and displacement. In addition, the waterline and deck beam must be traded off against the necessary stability. While the beam doesn't appear in the class rule, it is a very important parameter.
For example, since the basic weight of the hull and rig has a practical minimum due to the skin scantlings and mast weight requirements, there is a limited amount of stability available due to displacement or the position of the vertical center of gravity (VCG). Increasing the beam increases the boat's stiffness, but at the expense of added drag, particularly in ocean waves. Similarly, increasing the displacement increases the stiffness at the expense of higher drag, but within the rule, adding displacement buys more sail area.
As every designer knows, adding length will increase the boat's speed potential, at least above drifting conditions, but the IACC rule reduces sail area if the length is increased. There is also the issue of the bow shape and its effect on the measured length. The long, overhanging bow shape has a shorter static waterline length than the destroyer type bow, for the same rated length, but tends to pick up sailing waterline length with speed and heel, which the destroyer bow does not.
A method to find the optimum set of parameters for the expected conditions of seastate and wind was required. As often the case with real-life problems, the process requires optimization of several variables without enough data input. A combination of LPP derivatives and statistical regression was used to find the "sweet zone" in the design space. These methods were applied to the design of the "second boat" which was tank tested and predicted to be a dramatic step forward from the baseline Stars & Stripes, but which was never built.
APPENDAGE RESEARCH AND DEVELOPMENT
Basic Appendage Considerations
IACC racing yacht appendages need to perform two primary functions:
a.
b.
Rigidly support ballast as required for lateral roll stability
Generate lateral forces required for tracking and maneuvering
These requirements must be met while producing a minimum amount of drag over a wide range of speeds, heel angles and loads. The maximum draft of the yacht in measurement trim is effectively limited to 4.00 meters by the IACC rule which also limits the span of any winglets and the number of moveable appendages and axis position of any moveable appendages to within the centerplane of the yacht.
218
Ir. order to analyze and ration&lly develop an optimized appendage configuration for an IACC yacht, it was first necessary to define the operational conditions and quantify the performance deltas associated with changes in drag and stability over the range of operating conditions. VPP derivative studies were used to derive these deltas, including the effects of changes in stability due to lateral keel bending. Drag polars produced for each configuration were then integrated over operational values for lift to predict performance deltas for configurations with sim.ilar stability characteristics. The deltas could be adjusted for the effects of differing stability from the VPP derivative studies. Weighting factors based on statistical wind speed data were then applied to the performance deltas to develop a composite overall performance figure of merit for each configuration.
Monoplane Configuration
The conventional, or monoplane, keel configuration was approached with the above criteria in mind, starting with an analysis of the hydrodynamic loading conditions that must be sustained. The VPP and other proprietary keel/rudder trade-off codes were employed to determine a basic lateral area for the keel and rudder, knowing the tank test trim tab, leeway and load sharing optima. This determined a starting point from which the structural analysis could be undertaken.
The goal of the structural tradeoff is to maximize the righting moment with some tradeof f for drag. For example, if the keel were made thicker to minimize the bending, (bending results in a loss of righting moment) the increased thickness would add drag which must be tolerated all around the course.
The selection of the keel sectional shape had to consider the structural aspect as well as the hydrodynamic effects. Some sectional shapes provide more stiffness but not necessarily less drag or less weight. The trade-offs here relate to the position along the chord line of the maximum thickness, therefore the structural stiffness, but also the keel's drag characteristics in laminar or turbulent flow as well as its tolerance to transient higher load conditions, such as tacking.
The selection of materials for the keel is another righting moment trade-off, with cost as a non-trivial parameter. A composite keel for example, is lighter than its steel counterpart, but for the same sectional shape, will deflect more under load, and therefore the net righting moment must be optimized differently in terms of structure and section, for different materials.
Later generation keels tended to get smaller, as the range of loadings became better known and the upwind/downwind trade-offs became better understood on the race course. In addition, winglets sprouted in the later stages, which change the keel optimization.
Ballast Bulbs. The function of the ballast
bulb is conceptually very simple,. It provides
most of the righting moment required to sail
upwind. As such, it is generally desirable to
have the maximum weight of highest density
material, in the lowest drag arrangement
possible.
The IACC rule limits the bulb material such
that nothing denser than lead can be used, and
limits the maximum draft of the keel/bulb to
4.0 meters. A round torpedo-like bulb was the
starting point since it has reasonable drag for
the necessary volume. However it was soon
obvious that squashing the bulb somewhat lowers
the vertical center of gravity as well as
allowing a longer keel span (since the bulb on
the end of the keel robs the keel of aspect
ratio and thus increases its induced drag).
Squashing the bulb, however, isn't magic,
since the surf ace area of the bulb increases
rapidly and therefore the drag of the bulb
itself increases.
A significant effort went into the TDC bulb
design program in an effort to find the best
compromise of shape for the bulb. The optimum
shape considers all of the above, attempting to
pack the lead volume low (for stability
reasons), trying to minimize the adverse span
effects, and of course ensuring that it does
not fall off under the worst possible loading
case.
Winglets. Winglets have been around
aerodynamic circles for many years, and made a
well known appearance in the 1983 America's Cup
on Australia II. Their theoretical benefits
and trade-offs are very complex but can be
summarized as follows:
In the IACC class, the draft is limited to 4.0
meters which limits the keel span to something
less than optimum. By adding carefully
positioned winglets attached to the bulb,
sloping downward lightly, the induced drag can
be reduced, resulting in a slight improvement
upwind. Unfortunately, adding this surface
area below the water results in higher viscous
drag. For the winglets to be a net gain, the
induced drag benefit upwind and tacking (and to
a much lesser extent, reaching) must outweigh
the added viscous drag which must be carried
upwind and downwind. .
In summary, winglets can be made to improve the
IACC yacht performance upwind at some penalty
downwind. The design trade-off study revolves
around this upwind/downwind strategy and the
optimization of size, shape and position of the
winglets.
Biplane Configuration
The ~ ~ Stripes Tandem. During the
Challenger Series for the 1987 America's cup,
the "twin rudder" USA demonstrated that there
were high performance alternatives to the fin
keel monoplane. Therefore, it was necessary to
study multiplane designs in preparation for the
1992 America's Cup.
219
USA, however, was developed under the 12 Meter
Rule, in which a short draft keel is coupled to
a deep draft canoe body. The 12 Meter Rule led
to the structural optimization of USA with tri
plane appendages. This enabled the complete
separation between the ballast function
(assigned to a lead torpedo supported by a
short ventral strut) from the hydrodynamic
sideforce function carried out by the two
foils, (fore and aft of the ballast). Although
geometrically a triplane, the "twin rudder" USA
was, from a hydrodynamic standpoint, a high
performance biplane. Due to the significantly
different displacement and draft
characteristics of the new IACC rule, as
opposed to the 12 Meter Rule, it was necessary
to begin the 1992 multiplane design effort with
a clean sheet of paper.
Various multiplane appendage solutions were
considered within the constraints of the IACC
Rule, attempting to match the Stars & Stripes
canoe body, which had originally been designed
as a conventional monoplane. No attempt was
made to design an integrated canoe/multiplane
solution.
The TDC multiplane appendage work was carried
out over a 13 month period from the
commencement of R & D until the design freeze
for full size fabrication. A brief review of
the TDC tandem program is presented below.
Benefits of Multiplanes. The general
multiplane configurations which are attractive
under the IACC Rule considered by the TDC
design team were:
a. USA type triplane (Fig. 7)
b. twin foil "tandem" approach
The triplane solution, as examined by the
design team, encountered adverse wake
interference problems when constrained by the
IACC Rule limitation of two movable appendage
surfaces. It also required very critical
volume/wetted area/span tradeoffs, which had to
be evaluated with hydroelastic considerations.
Given limited resources, and time constraints,
this approach was not pursued beyond initial
feasibility studies.
On the other hand, the twin foil tandem
solution appeared less restricted by the IACC
Rule, and offered the promise of several
significant benefits compared to the classical
monoplane fin keel configuration. Two of the
tandem benefits are amenable to brief
discussion within the scope of this paper.
FIG 7. TRIPLANE
Tandem Induced Drag Benefits. According to wing theory, if the maximum span of a wing system is specified, minimum induced drag is obtained by an equal-spanned multiplane of maximum gap. The relation between induced drags of a monoplane, Dim, and of a biplane wing Dib having a wide biplane gap Gb between its wing members is discussed in Ref. 4, summarized for convenience in the following equation:
and for the biplane:
Dib={(L1/b1) 2+2*z*L1*L2/b1*b2+(L2/b2)2}* (l/Pi*q)
( 1)
(2)
Equation 2 was chosen for elliptic loading to clearly show the distribution of the induced drag forces of the wing members of the biplane, with the middle term quantifying the mutual induction effects.
If the spans and lifts of the wings are made equal, governed by a draft limit, for example, it can be shown that the induced drag efficiency ratio of the biplane relative to the monoplane is:
(3)
According to equation (3), for equal spans, the biplane's induced drag with values of z less than 1 is always less than the monoplane and this occurs when the gap between the wings of the biplane has finite values. For zero gap, z becomes 1, and the biplane's induced drag advantages are lost.
The classical aircraft configuration for equation (3) is the unstaggered biplane with large gap. It is noted, however, that the unstaggered feature serves only a structural purpose (i.e. efficient wire bracing), since, according to equation (2), induced drag is independent of stagger. (Stagger is the distance between front and rear members of a biplane wing in the chordwise direction.)
The classical gapped biplane airplane configuration cannot be applied to an IACC Rule yacht because all appendages must be on the hull centerline. This does ~ot rule out a centerplane biplane as an appendage configuration utilizing a staggered twin foil keel properly called a "tandem" (Fig. 8). This design approach, however, would appear to rule out the induced drag benefit of the biplane configuration shown in equation (2), since by definition, a centerplane tandem must have zero geometric gap between its foils.
Nevertheless, equation (2), under upwind sailing conditions, can be properly interpreted with certain care, as having a significant hydrodynamic gap, even if the foil's centerplane geometric gap is zero.
220
Equation (3) for a yacht must be handled with care since its formulation in wing theory assumes a limitless flow field. Therefore, its application to a yacht requires the mathematical substitution of half of the limitless flow field on one of the aerodynamic biplane's sides, (say at a constant geometry boundary of y=O) by the yacht's water-air interface applied adjacent to the roots of the yacht's tandem foils (a constant pressure boundary which is not, however, a gravitational equipotential in the vicinity of the hull). Thus, equation (3) is not rigorously applicable to a yacht. The theoretical treatment of the necessary water-air interface substitution is clearly beyond the scope of this paper.
It should also be remarked that equation (3) cannot compare a biplane against a monoplane having winglets, unless equation (1) is changed first, increasing the monoplane's effective span by the positive effect of the winglet.
FIG. 8 TANDEM
Tandem Structural Benefits. The rationale for a tandem within the IACC Rule may be derived from considerations other than induced drag benefits under multiplane wing theory. This is because the principle IACC monoplane solution is a high aspect ratio fin keel with a streamlined bulb trailing the fin's
tip, as in Fig. 9. This type of bulb design offers low interference drag with the fin keel, and also minimizes kelp entanglement since the bulb does not protrude forward of the fin at the intersection. However, a trailing bulb has certain adverse structural characteristics which develop as the boat heels:
1. The weight of the trailing bulb twists the slender fin keel, reducing the righting moment contributed by the bulb's center of gravity.
2. The ensuing torsional deflection of the slender fin keel opens up the potential for hydroelastic flutter, at least at the high speeds encountered when reaching (water dynamic pressures of approximately 400 lb/ft2)
Both of these problems can be greatly
alleviated if the conventional rudder of Fig. 9
is strengthened and moved forward on the canoe,
as shown, with the rudder's span extended
downward and forward to structurally engage and
support the rear end of the bulb. This type of
structural solution greatly decreases the
flutter potential and reduces the decay of
righting moments with heel which otherwise
characterize the trailing bulb/keel
configuration of Fig. 9. Nonetheless, the
tandem solution requires careful and rigorous
trade-off studies between foil stiffness and
wetted area.
!
ow~~-
u JJ FIG. 9 MODIFIED MONOPLANE
Tank Test Results. The TDC tandem
configuration was first developed
theoretically. It was then tank tested using
small scale models at Reynolds numbers known to
be very low. Results were sufficiently
favorable to justify a short but successful
program at 1/3.5 scale at the Arctec towing
tank. These tests confirmed a significant
upwind benefit, compared with the TDC baseline
configuration. The gains in lift/drag ratio
upwind were of sufficient magnitude to warrant
moving forward with a tandem program for manned
sailing on a half sized boat.
Hobie 33 Tandem Keel Tests. The TDC
design team realized early that sailing a
tandem appendage configuration new to the
America's Cup required a significant on the
water time to develop maneuvering techniques
and to optimize sails for the configuration.
Thus the original tandem program at TDC was to
include 1/2 scale IACC yachts, one tandem and
the other a baseline monoplane configuration to
investigate handling qualities and performance.
This was a cost effective and scientifically
valid procedure to optimize the design of the
full size boat through sailing tests of it
smaller counterpart, prior to fabrication of
full size appendages. It also would have
reduced the adverse impact of not having a full
size trial horse in the TDC program.
Unfortunately, due to funding limitations, the
exact one half scale boats were not built.
Instead, an attempt was made to evaluate the
sailing qualities of the tandem using
lengthened Hobie 33's. They reasonably closely
approximated 1/2 scale displacement, length,
and beam but in order to scale the righting
moment, the bulb densities had to be altered.
221
The other non-scale element was the sail area
and rig height, partially compensated for with
new fully battened, large roach mains.
Performance comparisons were difficult since
the non-scaled nature of the boats made
mathematical extrapolation to a full size IACC
yacht impossible but the handling qualities of
the tandem were found to be satisfactory, and
the decision was made to proceed with full size
appendage fabrication for retrofit to Stars &
stripes.
Sailing the Stars ~ Stripes Tandem. To
compensate for the lack of accurate one half
scale sailing tests, TDC endeavored to get as
much sailing time for the full size tandem as
possible before the start of racing. An
experimental full size optimization of
sail/foil yaw balance was practical due to the
large range of mast position possible in the
deck and structure of Stars & Stripes
(fortuitously designed with a large range of
movement since this was to be a test boat
anyway). Tests and sailing trials were planned
for the month preceding the first Defender
Round Robin in January, 1991. The timing and
strategy were planned to allow the use of the
opponent as the test boat in the first, low
scoring Defender round.
Delays in design finalization, and fabrication
of the tandem appendages, however, forced Stars
& Stripes to race the first round robin with
its proven and efficient monoplane
configuration. These races confirmed that
Stars & Stripes had good performance level
compared to Jayhawk, but was less successful
against Defiant.
TDC's tandem appendages were installed in stars
& Stripes after the first round robin, a
Herculean task including removal of the
monoplane appendages and controls, installation
of new structure in the canoe, installation and
alignment of the tandem appendages and totally
new control system.
Thus the Stars & Stripes tandem configuration
entered round robin 2 under less than ideal
conditions and preparation. There were only
four days in the water prior to the start of
round 2 to carry out the IACC required rule
flotation measurements, check and debug the
control system, and tune the boat (primarily
yaw balance) with no trial horse and whatever
limited wind conditions were present. In
addition, the surface finish of the appendages
and joint fairings were incomplete due to time
limitations.
It is a matter of record that the Stars &
Stripes tandem lost the only 3 races it sailed
by large time margins. A significant yaw
balance problem became evident in the strong
winds of races 2 and 3 forcing depowering of
the main when close-hauled, and was obviously
damaging to the upwind performance.
The large time losses of the tandem in the first 3 races of round robin 2 compared to the performance of the monoplane in round robin 1 were attributed by most observers as a configuration problem inherent in the Stars & Stripes tandem.
However, the assessment of the tandem's performance in round robin 2 was difficult to make for two reasons. One was heavy seas, which had not been present during the first round robin. As it turned out, the largest waves of the entire Defender trials were encountered by the Stars & stripes tandem in round robin 2. The second factor was the speed of the new America3 boat USA 23, the eventual winner of the America's Cup making its first appearance in round robin 2.
A closer look at some of the performance highlights of the tandem sheds a somewhat different light on the configuration, however. For example:
February 8 In relatively calm water and light winds (which make the yaw imbalance less critical), the Stars & Stripes tandem showed its most promising upwind performance against America3 , particularly in the lighter, flatter early stages.
February 11 In conditions unusually rough even for San Diego, while suffering upwind due to yaw balance problems, Stars & Stripes was able to gain a total of 23 seconds against America3 on the downwind legs.
Immediately after the February 11 race, the America3 syndicate offered TDC the option of withdrawing the tandem configuration and replacing it with the original monoplane, but declined to allow the changing of the mast position, which would have helped alleviate the yaw balance problem with the tandem. Thus, to avoid further large losses, while still trying to raise funds to carry the syndicate through to the America's Cup, TDC had no alternative but to replace the tandem with the monoplane configuration and continue with round robin 2. Winning the first race with the re-fitted monoplane provided a much needed morale boost to the sailing team.
Lack of time to sail and tune or refine the tandem configuration before entering the Trial races undoubtedly contributed to its disappointing debut, and perhaps masked the true potential of the configuration which was thus never realized.
222
CONCLUSIONS
The Stars & Stripes design effort was a cost effective, multi-disciplined program which both utilized available technology and advanced the state of the art in many areas. It boldly and briefly experimented with a retrofit tandem configuration, the potential of which was unreached in the TDC one boat campaign with limited resources. With no option but to concentrate its sailing and development efforts on its only boat, the team was successful in extracting the utmost potential from the original design.
REFERENCES
1. Beck, R., et al, (1989) "Motion in Waves," SNAME Principles in Naval Architecture, Vol. III, Chapter 8.
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4.
Gerritsma, J., "Performance of Light and Heavy Displacement Sailing Yachts in Waves," Second Tampa Bay Symposium SNAME, Sec. 1, February 1988.
Pedrick, D., "The Performance of Sailing Yachts in Oblique Seas," Chesapeake Symposium SNAME, No. 10, January 1974.
Calderon, A.A., "The Keelless Twin Wing 12-Meter Yacht USA", Volume 34, Ancient Interface XVII, 1987.