Propulsion and Shafting

7/27/2019 Propulsion and Shafting

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Review of Engine Shafting, Propulsionand Transmission Systems

Key Considerations for IndustryBy

Dag FriisBob McGrath

Christian KnappOcean Engineering Research Centre

MUN Engineering


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Scope:

Components of the Propulsion System

Where engine power goes

Propeller types

Propulsive efficiency

Cavitation

Selection Guidelines

What can I do with my as-installed system?

Testimonials and Simulations

Conclusions


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Propulsion and Shafting System:

Is a massive family that includes:

Main Engine

Gearbox

Shafting

Shaft couplings Journal and stern tube bearings

Propeller

Must be designed to work in harmony

Changes or problems with one component effect the entiresystem


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System Power Evaluation:

Indicated Power Delivered Power Shaft PowerBrake Power


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Where Engine Power Goes:Gear Losses

4%Shaft

Losses3%

Prop Losses(thrustdeduction)

25%

PTO (ifapplicable)

20%

Remaining48%


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Propeller Types:

Fixed Pitch

Least Expensive in initial cost

Efficient for wide range of operations


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Controllable Pitch Propeller: Controllable Pitch

Great for multi-modeoperations.

Engine RPM remainsconstant while pitch isvaried for different loadingconditions, or bothsimultaneously


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Nozzles:KORT RICE

Depends on application and how much clearance you have if

using a nozzle makes sense

Built Low Speed Efficiency Loses operational efficiency

when majority of time spent

steaming

Built for Steaming Efficiency Multiple options by going

with either speed or towing

nozzle


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Mewis Duct:

Designed for vessels with poor inflow due to hull form

Stabilizes water inflow to propeller. Uniform load distribution.

Rotated fins pre-swirl the flow, generates higher load on propeller and

more thrust

Guaranteed efficiency gains (when designed and optimized for vesseland coupled with rudder technology)


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Achieving Good Propulsive

Efficiency: The Power characteristics of the engine

have to be matched to the best possiblepropeller characteristics for thisapplication.

The main propeller characteristics are:

Diameter Pitch

RPM Number of Blades Blade Area Ratio


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Efficiency: The greater the propeller

diameter the more efficientthe propeller, i.e. choose thelargest propeller that can be

reasonably accommodatedin the available propelleraperture.

Propeller clearances (inches)

Prop diameter (inches)

60 72 100

minimum maximum minimum maximum minimum maximum

a 4.8 12.0 5.8 14.4 8.0 20.0

b 4.8 15.0 5.8 18.0 8.0 25.0

c 9.0 18.0 10.8 21.6 15.0 30.0

d 1.8 3.6 2.2 4.3 3.0 6.0


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Cavitation:

Cavitation occurs when the pressurein an area of the propeller fallsbelow the vapour pressure.

This results in bubbles or Cavities ofsteam forming

The problem is that when the steamcavities collapse on the surface ofthe propeller it leads to erosion ofthe blade material

Collapse also generates noise


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Cavitation:

TIP CAVITATION

SHEET CAVITATION


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Choice of Blade Area Ratio:

The smaller the blade area ratio the higher the open waterpropeller efficiency

The choice is made on the basis of choosing the smallestratio that will give satisfactory propeller performance froma Cavitation point of view.


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Hull and Propulsion System

Interaction: Interaction Between Hull and Propeller

Flow speed through the propeller (Wake fraction )

Effectiveness of the thrust developed by propeller (ThrustDeduction Fraction)

Hull Geometry and Characteristics

The higher the L/B ratio the better the flow of water to the propeller

Results in a more gradual change in direction of water flow

Lowers likelihood of flow separation and eddy making

Increases flow velocity through propeller

Results in more uniform flow velocity through propeller


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Selection of Propeller

Characteristics: In order to be efficient, the propeller characteristics

have to be selected based on:

Maximum Allowable Propeller Diameter

Flow conditions at the propeller (hull form)

Cavitation

Operational Scenario

Operating RPM (gear ratio)


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Selection of Propeller

Characteristics: Propeller type has to be selected based on operating regime:

Fixed Pitch is best suited to a single speed operation Fixed Gear Fishery, i.e. Propeller Designed for Cruising Conditions

Controllable Pitch when towing fishing gear Nozzle can be detrimental for boats that spend a major portion of

their time steaming to and from the grounds due to the increaseddrag at cruising speed

Nozzle Propeller when towing fishing gear, and affordable This is only likely to be the best alternative if the vessel spends most

of its time towing gear Usually fitted with controllable pitch to optimize performance at

both operating conditions


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What can I do with my as-installed

system?

Have Clearance? Increase your diameter /decrease rpm(mind tip cavitation)

No Clearance? Alter pitch and gear ratio (mind cavitation)

Clearance AND Pitch restricted? Alter gearing ratio (mind cavitation andprop loading)

Reduce unnecessaryhotel loads (extra deep freezes, clothes dryers, T.V.s, cabinlights, etc)


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Testimonials:

86 diameter propeller achieving best fuel econ. and highest speed at lowest rpm

0

5

10

15

20

25

30

35

0 50 100 150 200 250 300 350 400

GPH

RPM

64'11" RPM VS Fuel Economy

60" Diameter

66" Diameter

86" Diameter


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Testimonials:

28 diameter propeller achieving highest speed at identical RPM

0

1

2

3

4

5

6

7

8

9

10

0 5 10 15 20 25 30

Speed (kts)

Prop Diameter (in)

Identical 34'11" Vessels, Speed vs Prop Diameter at 660 RPM

22" Diameter

26" Diameter

28" Diameter


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Simulations:

35 fixed gear vessel:

Altered as-built prop from 25 to 30 diameter

Achieved 12% fuel savings per hour

65 mobile gear vessel:

Constrained in diameter due to as-built specs

Achieved 2% fuel savings per hour by altering pitch

Greater savings achievable by altering of gear ratio


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Conclusions: Have your propeller checked by a qualified professional for suitability of Diameter,Pitch, RPM, and Blade Area Ratio and resulting efficiency for your operation

If you are towing fishing gear a significant part of the time, a controllable pitch andpossibly a nozzle propeller may be the best choice

If you are not towing gear a well designed fixed pitch propeller is your best option

Check that changing propeller and/or gear ratio makes economic sense for theremaining vessel life.

Time and money spent in R&D can save and even make you money in the long term,but the analysis has to be done.

Remember your decisions should make business sense.


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QUESTIONS?

VS


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Propeller Cavitation Design Chart:


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Considerations for Outboards:


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Propeller Design Chart:


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Symptoms and Causes:


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Nozzle Propeller: If flow separation occurs around the

nozzle one will get a significantincrease in drag, i.e. reducing the

efficiency of the nozzle-propeller

Nozzle-Propeller diameter will be lessthan for regular propeller, thereforeresulting a reduction in efficiency


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Propeller Types:


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Propeller Design Parameters:

The Optimal Open Water Efficiency:

Rises with increase of Propeller Diameter

Rises with increase of Propeller Speed of Advance This is governed by hull characteristics and its effect on

slowing of the water flow through the propeller disk (wakefraction)

Decreases as the Blade Area Ratio Increases Governed by cavitation avoidance considerations


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Efficiency: The greater the propeller

diameter the more efficientthe propeller, i.e. choose thelargest propeller that can be

reasonably accommodatedin the available propelleraperture.

This is done by allowing forreasonable propellerclearances in order to reducelikelihood of pressure pulse

vibrations being induced inthe local hull structure.

Propeller clearances (inches)

Prop diameter (inches)

60 72 100

minimum maximum minimum maximum minimum maximum

a 4.8 12.0 5.8 14.4 8.0 20.0

b 4.8 15.0 5.8 18.0 8.0 25.0

c 9.0 18.0 10.8 21.6 15.0 30.0

d 1.8 3.6 2.2 4.3 3.0 6.0

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Propulsion and Shafting