On the comparison between the ISO Norm and the independent private companies: the expression of the demand and the capacity Tommaso Coppola University of Naples « Federico II », DIN (Department of Naval Engeneering), Naples, Italy

Francesca De Santis University of Naples « Federico II », DIN (Department of Naval Engeneering), Naples, Italy

ABSTRACT: The aim of this paper is to compare the differences between the structural scantling of the yachts given by the European ISO and the ones given by the National private companies. The European structural dimensions rule is the ISO 12215 section n° 5, which since the 23th October 2008 it has been acknowledged by the Italian UNI. The private companies rules compared with the ISO Norm are the RINa, the LR, the DNV and the ABS ones. First of all a comparison has been made between the fields of application of the considered Regulations. Local loads acting on all the main points of the hull have been analyzed and finally the formulas given by the European ISO and by the National private companies to determine the minimum thickness requirement and the mechanical properties of the laminate itself have been analyzed. The results obtained, underline that the private companies regulations are more restrictive than ISO Norm, in determining the loads acting on the hull. But to build a vessel according to the ISO requirements it is not sufficient dimension it following the national private companies regulations, because in the determination of the mechanical properties the National companies are usually less restrictive.

1 INTRODUCTION

There are many regulations to follow to dimension a craft and they are very different. The present work was born to compare the most used of these regulations analyzing the differences and the analogies among them, comparing these to the new ISO norm, which is a standard to follow if the product is to be commercialized in the European Area. The purpose of this work is to verify that dimensioning the crafts following the National regulations (the private companies rules) is sufficient to obtain the EC trademark, or better, that the National rules are more restrictive than the Standard European norms. 2 LIMITS OF APPLICATION AND FURTHER GENERALITIES The RINa, the LR, the DNV and the ABS are independent private company, whose certifications

have a double function: on the one hand they give to the consumers a chance to choose between all the different elements according to objective criteria; on the other hand, they also help the shipyards to follow the same shared standards, in order to simplify and rationalize the activities, beside increasing the valuation of the organization itself. It is to underline that the first four Regulations, as well as the ISO Norm, are applied in Europe, while the ABS is mostly applied in the United States and in North America in general. Firstly it should be distinguish between the Certification and the Classification: the Certification checks the compliance with a particular legislation while the Classification is a voluntary certificate, a free choice of the producer, that wants to optimize his product or a category of products, with the intent to increase the markets interest and the value of the product itself. In particular the certification considered in this paper is the CE Marking: this is an indicator of a product’s compliance with EU legislation and

IX HSMV Naples 25 - 27 May 2011 1

enables the free movement of products within the European market. By affixing the CE marking on a product, a manufacturer is declaring, on his sole responsibility, conformity with all of the legal requirements to achieve CE marking and therefore ensuring validity for that product to be sold throughout the European Economic Area (EEA). This also applies to products made in third countries which are sold in the EEA, therefore the CE marking does not indicate that a product was made in the EEA, but merely states that the product is assessed before being placed on the market and thus satisfies the legislative requirements to be sold there. The only purpose of the European Norm is to give to the Producer all the different guidelines to follow to obtain the CE trademark. It is not for commercial scope; the ISO Norm establishes the regulations to respect and the aim of the CE trademark. It can be attributed by any recognized board, which can verify that the product complies to the European standards. On the contrary it is necessary to follow the national independent private companies rule, if that product needs to have the certification of that particular board (Classification), in order to have more prestige. At the beginning, the private companies (such as RINa, Lloyd’s Register and Det Norske Veritas) were involved mainly in this second purpose and only successively, when the European directive 94/25/EC came into force, the private companies brought into the European Community Standards. An important difference among the analyzed regulations is their fields of application that are sintetized in the following table. Table 1. Fields of Application of considered regulations

Length (m) Speed (kt) Displacement (t)

RINa ‘95 L ≤ 60 v ≤ 60 -

RINa ‘09 L ≥ 16 v ≤ 10L1/2 -

LR L ≥ 24 - -

DNV - v ≥ 25 Δ < (0,13LB)1,5

ABS 24 ≤ L ≤ 61 - -

ISO 2.5 ≤ L ≤ 24 v ≤ 50 - All the considered rules make a difference between planing and displacement units: the former category includes those crafts in which the total weight is supported by hydrodynamic lift, the latter category includes those yachts whose weight is fully or predominantly supported by the hydrostatic forces. The RINa considers in addiction a third category: semi-planing yachts. These are yachts that are partially supported by the buoyancy of the water that they displace and partially by the dynamic pressure

generated by the bottom surface running over the water. This last category is not considered by all Private Boards, because some of these think that the Semi-planing mode is just a transitory condition, to pass from low to high speed. Table 2. Speed limits for displacement and planing units

Displacement Units Planing Units

RINa v/L0,5 ≤ 0,4 v/L0,5 > 0,4 LR Γ (*) < Γ > 3

DNV v/L0,5 ≤ 0,4 ABS v ≤ 2,36 L0,5 v > 2,36 L0,5 ISO v/L0,5 < 0,5 v/L0,5 ≥ 0,5

(*)Γ = Taylor Quotient = v/L0,5

The ISO Norm gives a more accurate craft classification, because it doesn’t make a difference just between craft, but it distinguishes two type of sailing, giving only one set of formulas and suggesting to evaluate the pressure in both condition considering the heaviest condition. The lawmaker of the ISO 12215-5 marks four design categories that identify particular conditions of the sea and wind. The craft subject to CE trademark should be built in accordance with their design category. In fact, as pointed out below, there is a coefficient (kDC) which takes into account the craft design category in any pressure formula. The RINa doesn’t mark the “design category”: the formulas given by it take into account the sealing condition by appropriate multiplicative corrective factors. The LR and the DNV marks six services area: these restrictions (ranging from G1 to G6 for LR, and from R0 to R5 for DNV) define the maximum distances from the nearest Harbour or safe anchorage. Obviously the restrictions are related to the zones, areas and seasonal periods as defined in the International Convention on Load Lines. The ABS doesn’t considers expressly the design categories, but, analogously to the RINa, they are taken into account in the pressure formulas by a service dynamic factor, which considers displacement, speed and sea conditions. All regulations split the craft in different stresses areas: bottom, side, deck, superstructure (Fig. 1). It can be observed that the Regulations define these areas at roughly the same way with some differences; the RINa, in particular, defines the element of separation between bottom and side as the chine. Therefore, the ISO considers Side only the dry part of the hull, while for both RINa Rules the

IX HSMV Naples 25 - 27 May 2011 2

Side area can be both dry and wet (underwater). According to that, the ISO Norm considers as bottom an area greater than the one considered by RINa. So, being the bottom area of the hull subject to higher stress (due to forces like gravity, pressure, inertia which act on the bottom), it appears that the ISO Norm is more conservative than RINa Rules in the determination of the load areas of the craft.

Figure 1. Stresses areas

Another important difference between the considered regulations is the estimation of the application point of acting loads: the Load Points. According to RINa the reference point (pdr) is always the panel outline which is lower than other. The LR individuates the load point as the position of centre of gravity of the item being considered. The following definitions are usually to be applied: the position of the load point is to be taken at one third of the panel or strake height or at the stiffener mid position. The load point for which the design pressure is to be calculated according to the DNV corresponds to the midpoint of horizontally stiffened plate. For vertically stiffened plate it has to be considered the lower support or the lower edge of plate when the thickness is changed within the plate. The ABS doesn’t mark explicitly the load points, but they are implicitly defined assigning the formula to determine the design pressure: in these formula, in fact, there are terms which are related to the element that is considered and indicate the calculation point. The ISO 12215-5 divides two cases: the first is one in which the considered element is fully in a specified design area, the second is one in which the considered element extents over two or more design areas. In the first case the pressure is considered uniformly distributed on whole element, with the value measured in the middle of the panel or in the mid length of the stiffener; in the second case the design pressure is a constant pressure over the entire area, which is calculated as a weighted average between the two (or more) pressures measured in the middle of every part of the panel.

Figure 2. Example of Load Points

3 STRESSES ON THE HULL

3.1 Load Criteria

The stresses on the hull are considered in accordance with the analyzed regulations. In all considered regulations the scantling requirements are based principally on providing adequate local strength. In particular the ISO 12215-5, which is the rules that considers the less long craft, specifies that issues such as deflection under normal operating loads, global strength and its connected shell and deck stability are not addressed, but the adopted criteria may need to be supplemented by additional considerations deemed necessary by the designer of the structure. The formulas given by all regulation are based on different kind of load which are considered acting on the craft. The RINa, since 1995 edition, subdivides the local forces in Internal loads: usually considered in case of concentrated loads (such as tanks or big machinery); external hydrostatic pressure, due to hydrostatic heads functions of draught of the considered point; external wave pressure, due to wave loads; external impact pressure, due to slamming. The scantling formulas of RINa are obtained considering four loads conditions, each one given by the combination of forces caused by weights present on yacht, outer hydrostatic load in still water, forces of inertia (due to the vertical and transversal acceleration of the yacht) and impact pressures. The LR provides that to correctly dimension a craft, design pressures for the shell envelope including exposed decks are to include the effects of combined static and dynamic load components. In addition, the effects of impact or slamming loads are to be considered. It gives different formulas to evaluate each one component, and finally it gives global formulas which, in function of the sailing and the considered element, take into account the different influencing factors. The loading conditions considered by the DNV include the static and dynamic pressures; these last

IX HSMV Naples 25 - 27 May 2011 3

ones are considered due to the wave and to local impact, such as slamming. The DNV explicitly says that the local vibrations due to impulse from engine and propeller blades are not considered in the given formulas. Finally, the ABS regulation doesn’t explicit the loads considered in its formulas. 3.2 Stresses on bottom

The RINa considers three formulas to evaluate the pressure acting on the bottom

p1= 0,24L0,5 (1-ho/2T) + 10 (ho+aL) p2= 15 (1+av)Δ/(LCS) gFLF1Fa)

p3≥ 10 D The formula for the evaluation of the pressure are basically equal in 1995 and 2009 RINa, but in the new Rule there are some specifications. The first formula (p1) seems to have hydrostatic nature, just depending on factors which represent hydrostatic head (T e ho) as well as the function of longitudinal position of pdr (coefficient a). It can be noted that the first term of p1 formula [0,24L0,5(1-ho/2T)] is always negligible in comparison to the second one [10(ho+aL)]. In fact the factor into brackets (1-ho/2T) is always lower than 1, being the ratio ho/2T lower than 1; moreover in the first addendum (1,24L0,5) the square root of length L compares. The second term, instead, is always relevant (the Length is of grade 1 and has 10 as multiplicative coefficient). Moreover it is observed that the first addendum is as much negligible as greater is ho, as shown in fig. 3.

Figure 3. Values of p1 calculated for 24 m length yacht

More specification there are in p2 formula. In this formula there are factors which depend on vertical acceleration, besides hull shape and position of pdr. This last one formula has a dynamic nature, in fact it is not considered for displacement craft. In the 1995 RINa edition, the maximum design value of vertical acceleration in LCG (ao) is given by the designer, who defines it by taking into account

the operating condition of the yacht; the ao value is recommended to be not lower than 1,5g (g =9,81m/s2). In the p2 formula the value av is considered equal to ao . On the contrary, the 2009 RINa Rule considers the vertical acceleration in LCG (aCG) which corresponds to the average of the 1% highest accelerations in the most severe sea conditions expected. Generally, it is to be not less than:

aCG = S v/L0,5 This formulas has a testing nature, but it is observed that, making S explicit (S=0,65CF , CF = 0,2+0,6/(v/L0,5)), aCG fluctuates in line with relative speed v/L0,5. The considered value av in 2009 RINa p2 formula, in addition, considers the longitudinal distribution of pressures also along the hull; made by the factor kv, (longitudinal distribution factor) which is calculated by a graphic given in function of longitudinal position of pdr (Fig. 4).

Figure 4. RINa Coefficient kV

The 2009 RINa calculates av by the formula

aV = aCG kV . Differently from the 1995 RINa edition, the 2009 one gives a formula to evaluate transversal accelerations; this formula should be used in direct calculation for units which have more decks or with upper structures with not minor effects. The transversal accelerations in the RINa’s rules edited in 1995, were completely neglected. Some inconsistencies encountered in the formulas coefficients remained unresolved in the regulation passage from 1995 to 2009 edition. The most evident inconsistence is found in the p2 dynamic pressure: the F1 factor, which is related to hull bottom deadrise and is given by a ratio between the angle of the bottom in the section x (βx) and the one in the section containing LCG (βLCG). If βx is equal or bigger than 50°, the coefficient F1 should be equal to zero or minus quantity. F1 is a multiplicative coefficient in p2; on the consequences, the p2 value should be zero or a minus quantity in that section. This means that, in the considered section, the pressure forces are directed towards the hull outside just because of the hull shape, but this is

IX HSMV Naples 25 - 27 May 2011 4

impossible without concentrated internal local loads, such as tanks or machine system. For the last formula (p3) given by the two RINa Rules, it can be observed that the minimum value of the RINa requested pressure is an hydrostatic head which depends on D (Depth of bulkhead) and not on T (Draft). This is explained by the choice of a parameter that is an implicit security factor. The LR gives two sets of formula to evaluate the pressure acting on the bottom: it distinguishes crafts operating in non-displacement mode and crafts operating in displacement mode (Table 3 – 4). Table 3. Design pressures for non-displacement craft Category / location Craft Type Symbol Plating pressure

Mono-hull craft

Basic craft PBP

Greater of HfSfPs Hf Sf Cf Pdl Hf Sf Gf Cf Pf

Bottom shell Craft with foils or other lifting

devices PBP

Greater of HfSfPs Hf Sf Gf Pfb

Side shell PSP PBP Table 4. Design pressures for displacement craft Category / location Craft Type Symbol Plating pressure

Mono-hull craft Bottom shell

Basic craft PBP

Greater of HfSfPs Hf Sf Gf Pdh Hf Sf Gf Pf

Side shell PSP PBP

The formulas considered by the LR to evaluate the forces acting on the shell take into account different factors and seem to be the most accurate among the analyzed formulas. The bottom formulas consider both hydrostatic and wave pressures

Ph = 10 (Tx – (z – zk )) Pm = 10 fz Hm Pp = 10 Hpm

Obviously the hydrostatic pressure Ph depends only on the position of the considered element and in particular on its hydrostatic head (Tx – (z-zk)). For the hydrodynamic wave pressures two formulas are considered: the former take into account the vertical distribution factor, evaluated by a formula in function of the immersion and of hull length. The other coefficient that compares in the formula (Hrm) represents the vertical relative motion; it is evaluated by a formula depending on the wave head, on the hull shape and on the longitudinal position of the considered point; in addiction the vertical

relative motion depends on the speed of the craft and on the Froude number The latter formula (Pp), instead, depends on the longitudinal position of the load point. The coefficient Hpm is function only of the length of the craft and of the longitudinal position of the considered element. To better identify the weight of each component of the pressure we can use the figure proposed by the LR (Fig. 5):

Figure 5. LR Combined pressure distribution

More than the combined pressure Ps, the LR takes into account the impact pressures and gives different formulas to evaluate these last ones. The first distinction is between the pressure due to the bottom slamming and the pressure due to the bow flare slamming. For each one of these two cases, the displacement and non-displacement mode are considered. The bottom slamming pressure evaluated for displacement mode depends on the longitudinal position of the considered point, on its draft, on the length of the hull and on the speed too; it is evaluated by a practical formula and it should be taken not less than Pm. For displacement craft, instead, the formula given by the LR considers the displacement, the vertical acceleration and the shape of the hull. For the bow slamming pressure, the formulas given by the LR consider only the craft type, the length and the relative speed. This is obvious if it is considered that the bow flare slamming regards the fore part of the hull and it is connected to the bow shape and to the speed. The bottom slamming, instead, is connected to the draft and to the speed over than the craft shape. Differently by the RINa formulas, in the LR ones there were not encountered inconsistencies. The DNV considers influencing the scantlings of stiffened panels different pressures and forces (static and dynamic sea pressures, internal loads from dry cargos…).

IX HSMV Naples 25 - 27 May 2011 5

The DNV gives only one formula to evaluate the sea pressure

pb= 10 ho +(ks - 1,5 ho/T) CW which considers together the static and the dynamic forces. The formula, in fact, takes into account the hydrostatic head (ho) and a wave coefficient (Cw) evaluated in function of the hull length. Moreover in this formula a sea load distribution factor (ks) is considered; this factor is graphicated in function of the longitudinal position of the load point and in function of the block coefficient of the hull. The DNV gives also formulas related to the bottom slamming effects

psl b= 21/tan(βx) ka kb Cw (1-20TL/L) psl= 1,3 kl (Δ/A)0,3 TO

0,7 (50-βx)/(50-βCG) aCG The base slamming pressure on bottom (psl b), which should be applied to all craft, depends on the deadrise angle (βx) and therefore on the hull shape, over than the hull length and the draft. In this formula it is observed that the inconsistencies detected in the RINa formulas are overcome. In fact the deadrise angle (βx) is imposed to be taken between 10° and 30° (the ISO Norm will operate in the same direction). This is important if the second formula is considered (psl); this formula should be applied to craft with relative speed not less than 3. This formula can be compared to the p2 formula of the RINa. These both formulas directly depends on the vertical acceleration, on the hull shape (in particular related to the shape in the section which contains the center of gravity), on the fully load displacement, on a longitudinal distribution factor (kl) and on the dimension of the panel considered. The coefficients kl given by the RINa and by the DNV are both evaluated by a graphic, even if the values corresponding to the same longitudinal position are different (Fig. 4, Fig. 6). Different are also the multiplicative coefficient and the formula to evaluate the vertical acceleration, but it can be concluded that the two formulas have the same structures.

Figure 4. DNV Coefficient kV

The ABS distinguishes the pressures acting on the bottom of planing and semi-planing yachts and displacement ones. For the first two categories it considers the three formulas

pb= N1 Δ/(LwB) (1+n) FoFv1 pt= 7,5 k1 N d FD Fv2 po= 9,8 (D + 1,22)

while for the last category it gives tabled values. The pb and the pt have a dynamic nature: they both depends on design area factor (FD) and on a longitudinal distribution factor. About this it can be observed that the factors which compare in the two formula (FV1, FV2) are different (Fig. 7, Fig. 8) and contrary to what was seen for RINa and DNV, the have not only different value, but different trend too. In the first formula pb the pressure value depends on the displacement and on the length hull too. Moreover by the coefficient n, it depends on the deadrise angle and the speed, and differently by the other regulation, in this factor there is a dependence on the trim angle too.

Figure 7. ABS Coefficient FV1 and FV2

The last formula po can be considered as a minimum value of the pressure. Similarly to the RINa, the value is function of the depth instead of the draft. Also the ISO Norm considers three formulas

PBMD = PBMD BASE x kAR x k DC x kL PBMP = PBMP BASE x kAR x kL

PBM min = 0,45 mLDC0,33+(0,9 LWL x k DC )

to evaluate the pressure acting on the craft bottom, which represent respectively: the bottom pressure in the displacement condition (PBMD), the bottom pressure in the planing condition (PBMP) and a minimum value (PBM MIN) which should be considered in any case. The biggest value obtained by the formulas is to be taken into account. The first difference between the ISO Norm and the other considered regulations is clear: in the all private companies regulations a preventive division is made between displacement and planing craft, even if both formulas (static and dynamic) are considered for planing craft; the ISO doesn’t make a previous distinction, but it calculates the pressures acting in both cases valuating the most critical condition. The displacement condition pressure PBMD depends on design category (kDC), on craft maximum load (load displacement condition mLDC), and on the basis displacement mode bottom pressure (PBMD BASE

IX HSMV Naples 25 - 27 May 2011 6

=2,4mLDC0,33 +20), which has just a static nature,

depending on the cube root of displacement (it is a length unless density). Besides, the PBMD also depends on longitudinal position of considered element (kL Longitudinal pressure distribution factor) and on the dimensions of the panel (kAR). The planing condition pressure PBMP depends on the same coefficient of the PBMD and similar observations can be made: it can be observed that even if at first it seems do not depend on design category through kDC, this factor is included in the minimum planing pressure formula (PBMP BASE =(0,1mLDC/LWLBC)(1+kDC

0,5 nCG). Furthermore it is clear that the minimum planing pressure must be related to a dynamic load factor: nCG. This factor has been introduced to take in consideration the negative acceleration supported by the craft either while slamming into an encountered wave at speed or falling from the crest of a wave onto its trough. It should be the equivalent factor of vertical acceleration av considered by the RINa, but it is clear that the RINa Rule considers just vertical acceleration, while the ISO Norm consider in general the effects of the impact with waves. A maximum value of nCG is fixed equal to 7. The limitation on nCG is due to the limitation of speed by the crew to keep the slamming accelerations within acceptable comfort and safety limits (the crew of "super sports" or racing boats accepts a harder riding than a family cruiser, but need special body support, shock damping seats or equipment to prevent injury from high g's. 3.3 Stresses on side

The same approach can be applied to evaluate the formulas to be used to calculate the side pressures. Both RINa Rules use the same formula for planing and displacement yachts. They consider a minimum value equal to 10h1, where h1 is the distance, in m, from the pdr to the straight line of the beam of the highest continuous deck. The RINa formula is derived from a practical test: p1 (p1= 66,25 (a+0,024) (0,15L-ho) The side pressure depends on pdr position from fully load waterline, on pdr longitudinal position (through coefficient a) and varies proportionally to length (L). It is interesting to observe that the minimum value of side pressure is given in function of h1, while the value p1 is function of ho (Fig. 8).

Figure 8. RINa side vertical distance

For planing yachts for the zones located forward of 0,3 L from the PPAV, the RINa considers another formula to evaluate the minimum side pressure in function of the hull shape in the pdr zone, instead of the draft or pdr position. The LR requires using the same value of the bottom pressure to dimension the side of the shell (Table 3–4). The formula given by the DNV to evaluate the side pressure is a linear function of the vertical distance from the waterline (ho) and the wave coefficient:

ps= a ks (CW – 0,53 ho). Analogously to the RINa, the DNV considers a second formula to evaluate the pressure in the fore part of the hull. This is a complex formula where it can be highlighted the dependence on the hull shape analogously in the RINa formula. The ABS distinguishes displacement and non-displacement craft, giving tabled value for the former category and only one formula for the latter (ps= k1h + 0.20 pb). The side pressure value given by the ABS is a linear combination of the bottom pressure evaluated with the first formula (pb) and of the distance (h) of the lower edge of the panel to the freeboard or main weather deck at side (in any case h should not be taken less than 0,5D). The ISO Norm formula for side pressure calculation have the same structure of bottom pressure formulas

PSMD =[PDM BASE+kZ (PBMD BASE - PDM BASE ]kARkDC kL PSMD =[PDM BASE+kZ(0,25PBMD BASE - PDM BASE ]kAR kDC kL

PSM min = 0,9 LWL x k DC Analogously to bottom pressure there is the distinction between displacement mode acting pressure (PSMD) and planing mode acting pressure (PSMP), considering the different factors which influences the loads. Compared to bottom factors, a new coefficient is introduced: kZ (Fig. 9). This is the side pressure reduction factor. It is given by: kz=(Z-h)/Z, where Z is the height of top of hull or hull /deck limit above the fully loaded waterline and h is the height of centre of panel or middle of stiffener above the fully loaded waterline.

IX HSMV Naples 25 - 27 May 2011 7

The kZ factor makes a weighed average between the pressure acting in fully load waterline (where the value of pressure should be calculated with bottom pressure formula) and the pressure evaluated on hull/deck limit.

Figure 9. Side pressure reduction factor

The value PDM is the one evaluated for deck pressure, so the displacement pressure PSDM is calculated as the average between the minimum pressure on the bottom and the pressure on the deck. Analogously the PSMP is evaluated as an average between the hull/deck limit and the bottom pressure, but only the 25% of the latter is considered; this means that in planing mode the static bottom pressure has a lower weight of the one in displacement mode. It is to be noted that, the side planing pressure calculated in the same condition with the planing craft formula is lower than the one calculated with the displacement craft formula. About the minimum value PSM min it can be noted that it is just a part of the PBM min; in fact it is given by the second addendum of the minimum bottom pressure, not considering the contribution due to the mass. 3.4 Stresses on deck and superstructure

In last analysis the load on the deck and superstructure considered by all regulations is reported. RINa considers, in both analyzed editions, only hydrostatic heads acting on decks, giving tabled value in function of the considered zone type (if sheltered or not) and in function of the vertical position of the deck (over or under the pdc, where pdc is intended as the first deck above the full load waterline, extending for at least 0,6 L and constituting an effective support for side structures). Analogously to the bottom and side pressure, to evaluate the deck pressure the LR gives two sets of formulas: the former for the non-displacement crafts and the latter for the displacement ones (Table 5).

Table 5. Design Deck pressures Craft Type Symbol Plating pressure

Non-Displacement PWDP Greater of

Hf Sf Gf Cf Pwl PCD

Displacement PWDP Greater of

Hf Sf Gf PWH PCD

In the formula of Pwh and Pwl can be observed that, differently from the RINa, the LR considers depending on speed (for displacement craft) and on vertical acceleration (for non-displacemet crafts) the forces acting on the deck. This is rational if it is considered that the forces acting on the deck are commonly attributed to the water boarding, which is strictly connected to the vertical acceleration and to the speed of the craft. Therefore it can be concluded that the LR is more accurate even in the evaluation of the deck pressure. The DNV imposes that the design deck pressure is proportional to the vertical distance from the load point to the deck itself. Therefore it doesn’t consider the longitudinal position of the load point or if the considered point is sheltered or not The ABS too gives tabled value to evaluate the pressure acting on the deck. Such as RINa Rules, the pressure values are tabled in function of the position of the considered point on the deck, but the vertical position of the deck itself is not considered. Differently by the other regulations, the pressure value is expressed as linear function of the length of the hull. The ISO Norm has a different approach, considering more than a minimum hydrostatic head (equal to 5 kN/mm2): according to the ISO the deck pressure value depends on waterline length (LWL), besides the shape (kAR) and position of considered element (kL), and more over on design category (kDC) of the craft

PDM = PDM BASE kAR k DC kL For loads acting on superstructure the RINa Rule doesn’t consider explicit forces: in both editions it gives only a minimum thickness to build decks and bulkheads of superstructures. In no way does RINa consider the dimensions and the position of the element considered. Coherently to the other formulas, the LR distinguishes the two sailing to evaluate the pressure acting on the superstructures: displacement and non-displacement crafts. Both formula (table 6) are based on the value of pressure evaluated considering the deck pressure (which is function of the craft type) and the position of the considered element (front, side,…).

IX HSMV Naples 25 - 27 May 2011 8

Table 6. Design Superstructure pressures Craft Type Symbol Plating pressure

Non-Displacement PDHP Greater of Hf Sf Gf Cf Pdhp

Displacement PDHP Greater of Hf Sf Gf Pdhp

The DNV gives three formulas to evaluate the pressure acting on the superstructure. It gives a value which is evaluated by a formula that have the same structure of the side one and two values which are function of the vessel length and of the angle formed by the considered surface and the deck. The ABS gives tabled values to evaluate the pressure acting on the superstructures. It distinguishes the different side of the superstructure or deckhouse (front, side, top) and gives two limit values in function of the length of the hull: a minimum value for length equal or less than 12 m and a maximum value for length from 30.5 m to 61 m (for vessel between 12 and 30,5 m the design pressure is to be obtained by interpolation). . The ISO Norm, instead, takes into account (considering a formula similar to the one proposed for the deck) the previewed sea condition (kDC), the shape of the considered element (kAR) and moreover it takes into account the location of the considered element (front, side, above deck,…) by the introduction of another factor (ksup = superstructure and deckhouse pressure reduction factor) evaluated by a table in function of the position, the orientation and in function of the type area:

PSUP,M = PDM BASE kAR k DC kSUP In this assumption the maximum load on superstructure is depending on the previewed sea condition. 4 EXAMPLE

The Fig. 10 reports the diagrams of the pressures values calculated with all considered formulas at the keel of a boat whose characteristics are shown in Table 7. Table 7. Principal characteristics of reference vessel REFERENCE VESSEL LOA = 27,60 m CB = 0,48 LH = 23,98 m v = 40 kt LWL = 21,82 m Cs = 5,18 m B = 6,47 m v/L0,5= 8,17 BWL = 5,64 m CF = 0,27 D = 2,87 m S = 0,18 T = 1,10 m aCG = 1,50 g ∆ = 66,60 t βLCG = 12 °

The graphic shows that, using one or the other regulation, the value of the pressure can be very different. It can be observed that the values given by the ISO are the same for the whole craft length: this because the only factor which can diversify the pressure value along craft is kl and for the considered craft it is constantly equal to 1.

Figure 10. Bottom pressures evaluated for the reference unit

The only values that exceed the maximum ISO pressure are the ones obtained by the pb of the ABS in the forward part of the hull and the pb of the DNV. These both have a dynamic nature; in fact the ABS formula contains the vertical acceleration and the DNV formula is related to the slamming. Moreover by the figure it can be observed that the minimum and the static pressures calculated by the ISO and by the other regulations assume comparable values. In general it can be concluded that the Private Societies general approach is different by the ISO Norm one mainly because they have different origins and they have different scope: the RINa, the LR, the DNV and the ABS are private owner Societies having, apart from the two mentioned exceptions, an implicit tendency to be more conservative for safety reasons and self-protective objective. The same RINa underlines its scope in the part relative to the generalities of the board; in fact it says explicitly that the class assigned to a yacht reflects the opinion of the Society that the yacht, given the intended use and within the relevant time frame, complies with the Rules applicable at the time the service is rendered. Probably this is the reason why the ISO 12215-5 is less restrictive than RINa and than other national regulations: the first gives a guideline to have the CE Mark, the second ones give a Mark, named “Class” in the private societies exception, which gives to the

IX HSMV Naples 25 - 27 May 2011 9

yacht something more than the only symbol CE which represents the suitability to European market. 5 SCANTLING: MINIMUM THICKNESS In all regulations the scantlings of Fiber Reinforced Plastics, at least for the yachts, are made identifying a minimum thickness which has to be realized: this means that the resistance verification criteria used to verify the considered elements are the ones used for isotropic and homogeneous material, such as steel. Actually, the peculiar non-isotropic feature of a composite material should be taken into account to determine the stress which it is able to resist; it is needed to evaluate more factors such as used woven type, the sequence of layers, the base material (glass, kevlar, hybrid,…), as well as the interconnections among layers. Therefore the thickness t on its own is not the binding factor to design: it is usually added to another more relevant and binding factor the Glass Content of Reinforcement in Laminate ψ (or in a single layer); this parameter is clearly defined as the ratio between fiber mass contents (p) and total mass of the laminate (q):

ψ = gc = p/q More than the thickness, ψ is the fundamental parameter to determinate mechanical and physic properties of composite final material because they are so near to single fibres properties as greater is their relative content in the laminate. The ratio ψ is expressed in terms of mass, instead of volumes; this reveals the intrinsic empirical nature of the proposed formulas, because it is clear that the mass is easily measured in practical tests for both fibres and resins. The analyzed regulations define the minimum requirements for FRP laminates in different ways. Both RINa Rules, LR and ISO Norm give formula to calculate the characteristics of the materials in function of ψ. RINa regulations and ISO Norm, in addiction, evaluate the thickness of a single layer (on the consequence the thickness of the laminate itself) in function of ψ, while the LR gives a formula expressed in function of the Elasticity Moduli of each component (matrix and fibers). The formulas given by these regulations will be analyzed in the next paragraph. The RINa minimum thickness formulas express the thickness in function of the panel dimensions, over than the ultimate strength of the laminate. One evaluates the thickness considering the acting pressure, while the latter considers the depth of the craft. This means that if in the former formula the real pressure acting on the vessel shell is considered,

the latter formula is based only on the hydrostatic head, being the thickness value proportional to the depth of the vessel itself. The evaluation that the LR makes of the minimum thickness is based on the acting pressure and, differently by the RINa, it considers the Elasticity Modulus instead of the ultimate strength. The DNV requires that the reinforcement of a single skin laminates have to contain at least 40% continuous fibers. Such as the other regulations, the minimum thickness value is expressed as function of the ultimate stress, but in the DNV formula the ultimate tensile stress is considered instead of the flexural one. The DNV relate the thickness of a laminate to the deflection too. For a given design pressure the required thickness is related to a laminate deflection factor (δ = w/t) and an allowable combined bending and membrane stress (σc). To determine stresses and deflections in FRP laminates, the DNV accepts either direct calculations or simplified method. The simplified method is applicable only under fixed conditions: the principal directions of reinforcement are parallel to the edges of the panel and the difference in the modulus of elasticity in the two principal directions of reinforcement should be not more than 20%. The simplified method gives a formula to evaluate the thickness and the membrane stress (σc). Maximum values of δ and σc are tabled for the various structural members. The direct calculations previewed by the DNV are based on the determination of the failure strength ratio: the criterion on that they are based is the Tsai-Wu one. Moreover, the DNV provides for the possibility to use recognized computer programs to make these calculations. In these specifications, the DNV seems to be more accurate than the others regulations, that don’t specify the failure criterion on which are based. The ABS fixes minimum properties for the warp direction of a Basic Laminate: this is a laminate that consist of general-purpose polyester resin and alternate plies of fiberglass mat and fiberglass woven roving which has a minimum glass content of 35% of weight. The American Regulation gives a formula to evaluate the thickness of a basic laminate which is based on the glass content, but it underlines that the appropriate verified minimum physical properties are to be used in the scantling equations. The properties of laminates have to be determinate by destructive tests of panels simulating the laminates used in actual production. The ABS establishes all the tests that are needed to determine the mechanical properties of F.R.P. laminates.

IX HSMV Naples 25 - 27 May 2011 10

Such as the other regulations, the ABS gives formulas to evaluate the minimum thickness that has to be realized in function of the acting pressure, of the dimension of the panel and on the design stress. The ISO Norm too gives a formula to evaluate the minimum value of thickness and similarly to the others it is based on the acting pressure, on the stress design and on the dimension of the panel. Moreover the ISO formula takes into account the curvature factor according to the ABS and the DNV formulas. To consider that all these formulas are based on local strength requirements. 5.1 Materials Properties

To facilitate the design of a vessel, the regulations give some formulas to evaluate the mechanical properties of the laminate. Among the analyzed regulations only the RINa, the LR and the ISO Norm give formulas to that end; the DNV and the ABS base the determination of the mechanical properties of the laminates only on the tests that have to be executed on them. The RINa, the LR and the ISO fix minimum values of ψ: if they are not reached, the composite is not suitable to be used (in naval fields at least) because the physic and mechanical required characteristics are not realized. It is important to note that even if there are no more differences between 1995 RINa and 2009 one, the 2009 edition is the only one qualified version to be compared with the LR edited in 2010 and the ISO 12215-5 of 2008. The ISO Norm is more accurate than RINa Rule and LR, because the first gives different formulas in function of used fiber (Glass, Carbon or Kevlar), considering new type of material in nautical market; the RINa considers only the glass fibers and gives some generic formulas for carbon fibers, while the LR gives formulas related only to the glass fibers. The formulas used by classification Societies are obtained by practical tests and they don’t give suitable values for all type of laminate; in fact they only consider the material composition percentage and their aggregation mode respect to total. It should be noted that both RINa Rules and ISO Norm give indications to evaluate ψ of one layer in function of woven’s kind and in function of production technologies (Table 8, Table 9) and by the table we can appreciate the differences that there are among different fabrics.

Table 8. ISO ψ value for standard woven

Type of ply reinforcement

Open mould simple surface

Open mould complex surface Vacuum bag

Chopped strand mat sprayed up 0,30 0,25 0,36

Chopped strand mat hand lay up 0,30 0,25 0,36

Woven roving 0,48 0,36 0,58 Roving-mat combination 0,46-0,18 R 0,35-0,11 R 0,56-0,22 R

Multidirectional fabric 0,50 0,38 0,6

Unidirectional fabric 0,55 0,41 0,66

Table 9. RINa ψ value for standard woven

Type of ply reinforcement

Open mould simple surface

Open mould complex surface Vacuum bag

Chopped strand mat sprayed up 0,22 0,17 0,28

Chopped strand mat hand lay up 0,22 0,17 0,28

Woven roving 0,40 0,28 0,50 Roving-mat combination

(0,46-0,18R)-0,08

(0,35-0,11R)-0,08

(0,56-0,22R)-0,08

Multidirectional fabric 0,41 0,30 0,50

Unidirectional fabric 0,46 0,32 0,57

The RINa gives lower ψ values than the ones given by ISO for all woven’s kinds; so it safeguards itself evaluating ψ; on the consequence imaginable it safeguards itself in the determination of mechanical laminate’s properties too. In particular it is observed that the differences in industrial technique of vacuum bag are bigger than the ones in manual laminations. This is justified considering that RINa data are based on a reduced number of experimental tests in compare the data used by ISO Norm, but the most important thing is that the RINa’s data are based on tests made when the new technologies were not so developed as today in naval fields. A comparison can be made between RINa, LR and ISO formulas used to evaluate laminates’ physic and mechanical properties (Table 10). Some formulas, except the explicitation, are equal; so the values given by the three regulations coincide for the same ψ. For other formulas, instead, there are some differences probably due to different sample examined. Differences are observed in the Ultimate Tensile Strength σut (Fig. 10) and in Elasticity Moduli E (Fig. 11) formulas.

IX HSMV Naples 25 - 27 May 2011 11

Table 10. E-glass mechanical properties for Woven Rowing and Crossplied (in N/mm2)

Table 11. E-Glass mechanical properties for Unidirectional (in N/mm2)

LR formulas Chopped Strand Mat ISO formulas ISO formulas

Parallel to the fibres Perpendicular to the fibres σut 200 ψ + 25 880 ψ2 + 140 ψ + 140 42 σuc 150 ψ + 72 250 ψ + 190 105 E (15 ψ + 2) 103 46600 ψ2 + 7200 ψ + 7250 48600 ψ2 – 39000 ψ + 12500 G (1.7 ψ + 2.24) 103 14380 ψ2 - 10560 ψ + 3840 14380 ψ2 - 10560 ψ + 3840 Regarding σut it is clear that the formulas given by RINa and by ISO have the same structure (square dependence by ψ), even if they have different multiplicative coefficient, while the LR formula express σut linearly depending by ψ .

Figure 9. Ultimate tensile strength values for different E-glass

woven type (in N/mm2)

Moreover, it can be observed that for ψ included among 0,30 and 0,60 the ISO Norm gives σut values greater than the ones given by the RINa (they are always comparable), while for ψ values lower than 0,30 and higher than 0,60 the values given by RINa are greater than the ISO ones. The ψ values usually realized on great surfaces such as hulls are among

0,35 e 0,50, so as far as σut is concerned, the ISO Norm is less conservative than the RINa Rule. The values obtained by the LR formula, instead, seems to overestimate the tensile strength for low values of ψ, while seems to underestimate them for high ψ. It is observed that the RINa joins all woven’s kinds under the same formula, while the LR and the ISO Norm give specific values for particular reinforcement kind (Table 11) the ISO Norm considers unidirectional woven distinguishing ultimate loads along the parallel and perpendicular directions to the fibers; it considers composite material obtained by sprayed MAT, which are cutted in small pieces and sprayed by particular machines. The LR considers the Mat too, giving different formulas to evaluate the Ultimate tensile strength and the Elasticity modulus too. The different mechanical characteristics of these type of reinforcement are plotted in Fig. 1 To observe that the value given by ISO and LR for Mat are comparable, even if the LR overestimate always the ultimate tensile strength. Other considerations can be made about Elasticity Modulus (Fig. 10): the RINa and the LR give different coefficients (instead of the one given by ISO Norm), considering three formulas for Tensile, Compressive and Flexural Modulus of Elasticity. The formulas given by the RINa and the LR for Tensile and Compressive Modulus are based on the same schema (linear dependence by ψ); also the formula of the only one modulus of elasticity given

RINa formulas LR formulas ISO formulas

σut Ultimate tensile strength 12780 ψ2- 510 ψ + 123 400 ψ - 10 800 ψ2- 80 ψ + 37

Et Tensil modulus of Elasticity (38 ψ - 4.75) 103   (30 ψ - 0.5) 103   /  

σuc Ultimate compressive strength 150 ψ + 72 150 ψ + 72 150 ψ + 72 Ec Compressive modulus of Elasticity (40 ψ - 6) 103   (40 ψ - 6) 103   /  

σuf Ultimate flexural strength 502 ψ2 + 107 502 ψ2 + 106,8 502 ψ2 + 107 Ef Flexural modulus of Elasticity (33.4 ψ2 + 2.2) 103   (33.4 ψ2 + 2.2) 103   /  

τu Ultimate inplane shear strength 80 ψ + 38 80 ψ + 38 80 ψ + 38 E Inplane modulus / / 38000 ψ - 5000 G Shear modulus of Elasticity (1.7 ψ + 2.24) 103 (1.7 ψ + 2.24) 103 1700 ψ + 2240

τu inter Interlaminar (out of plane) shear strength 22,5 - 17,5 ψ / 22,5 - 17,5 ψ

IX HSMV Naples 25 - 27 May 2011 12

by the ISO Norm is based on the linear dependence by ψ.

Figure 10. E-glass Moduli of Elasticity (in N/mm2)

The only difference between RINa and LR Elasticity Moduli formulas are in the Tensile ones. The values obtained by the LR formula for ψ until 0,55 are bigger than the ones obtained by the RINa formula, therefore the LR overestimate the Elasticity Tensile Modulus for commonly used ψ. Unlike the other, the RINa and LR’s formula, which evaluates the Flexural Modulus of Elasticity, has a quadratic dependence by ψ . The linear dependence by ψ is addressed to the exclusive influence of impregnation percentage, while the quadratic dependence is addressed to an explicitation of the dependence by the woven’s kind, in particular by the combination of warp and weft. In last analysis it is interesting to observe that the E values given by ISO Norm are in the middle between the ones given by RINa for Compressive and Tensile Modulus, while the Flexural Modulus values are very different from the other, starting from comparable values corresponding to lowψ and arriving to be very different for high ψ values. Because of in naval field the Aramidic fibers have been “recently” introduced, just the ISO Norm considers them, giving suitable formulas in function of woven’s kind and in function of ψ (it is known that aramidic fibers keeps less resin than glass fibers, then higher ψ values are considered). Even if the RINa and the LR considered have been published after the ISO norm, they don’t give suitable formulas to evaluate the mechanical properties of other kind of fibers. Differently by the Aramidic Fibers, Carbonic ones are considered both by ISO Norm and by RINa, the latter probably because the carbon fiber composites have been widely used in pleasure craft field (most in the high performance sail boats). Differently by

glass fibers, the values of mechanical properties given by RINa and ISO are very different for the same kind of woven and for fixed value of ψ (Table 12). Table 12. Carbon fibers mechanical properties (in N/mm2) RINa formulas ISO formulas σut 740 ψ - 65 990 ψ - 90 E 75000 ψ - 6730 100000 ψ - 9000 σuc 460 ψ - 40 610 ψ - 55 Ec /   /  

σuf 2.5 σut /(1 + σut/σuc)   /  

τu / 40 ψ - 31 G / 5100 τu inter 35 * 103 /

Figure 11. Ultimate tensile strength values for Carbon (inN/mm2)

It is important to note that only RINa gives a formula to evaluate the Ultimate Flexural Strength and the Interlaminar Shear Strength (ISO Norm ignores those factors), while ISO Norm gives formulas to calculate Shear Strength and Shear Modulus (RINa ignores those factors). Most probably these differences depend on the kind of wovens: the ISO 12215-5 considers only High-Strength Carbon fibers, while RINa is more generic and includes all kind of carbon (Low Modulus, Intermediate Modulus, High Modulus, Very High Modulus). So in the case of RINa formulas higher safety margins are obtained: in fact the ultimate tensile and compressive strenght given by RINa are considerably lower than the ones given by ISO Norm (Fig. 11). It is needed to consider that it is not sufficient to justifie the differences between the moduli of elasticity. The differences between the moduli of elasticity calculated by RINa and ISO Norm can be justified by other reasons.

IX HSMV Naples 25 - 27 May 2011 13

6 CONCLUSION

The present analysis has shown that there is a substantial difference between the Private companies regulations and the ISO Norm in the evaluation of the pressure values obtained for the same vessel. Instead the considered regulations are substantially based on the same scheme, there are some differences in the determination of the values of the minimum thickness that should be reached to satisfy the requirement of each regulation. About the mechanical and physical characteristics, instead, it is observed that RINa, LR and ISO approach is practically the same (this prove how difficult it is to change and introduce new element to better evaluate the composite materials properties). It has been observed that the ISO can be compared to the RINa in the evaluation of the ultimate tensile strength. About the Modulus of Elasticity it has been observed that the RINa and the LR have the same approach in the determination of it, while the ISO is substantially different. It has to say that where the values given by the regulations for each property are always comparable. For commonly used values of ψ (0,35 ÷ 0,50), we can observe that the LR overestimates the properties of the laminate both for the tensile stress strength and for the elasticity modulus. Therefore it can be concluded that if we use the national LR to dimension the laminate, the ISO requirements are not satisfied. Acknowledgements The paper has been financially supported by University of Naples “Federico II” founds.

References R.I.Na. (1995) Regolamento per la Costruzione e la Classificazione delle unità da diporto, Registro Italiano Navale, Genova R.I.Na. (2009) Rules for the classification of the yachts (part B), Registro Italiano Navale, Genova L.R. (2010) Rules & Regulations for the Classification of Special Service Craft 2010 Vol 3, Lloyd’s Register, London L.R. (2010) Rules & Regulations for the Classification of Special Service Craft 2010 Vol 6, Lloyd’s Register, London D.N.V. (1993) Rules for Classification of High Speed and Light Craft, Det Norske Veritas Classification as, Høvik A.B.S. (2000) Guide for Building and Classifying Motor Pleasure Yachts, American Bureau of Shipping, Houston ISO 12215-5:2008 (e) Small Craft-hull construction and scantlings-part 5: design pressures for monohulls, design stresses, scantlings determination, ISO copyright office, 2008 UNI EN ISO 8666 - Small craft Principal data, ISO copyright office, 2002 L.R. (2010) Rules & Regulations for the Classification of Special Service Craft 2010 Vol 1, Lloyd’s Register, London L.R. (2010) Rules for the Manufacture, Testing and Certification of Materials, Lloyd’s Register, London Migliaresi C., Pegoretti A. (1998) “Materiali compositi”, ENCO Journal, Vol. 8 Giovanni Carabelli (1986) Il poliestere rinforzato e altri materiali compositi, ITIP Norman Nudelman (1990) Principles of Fiberglass Boat Design & Construction, Dipl. N.A., Westlawn Institute of Marine Tecnology, Stamford, Connecticut U.S.A. Steve Sleight (1985) Modern Boat Building Materials and Methods, Westlawn Institute of Marine Tecnology, Stamford, Connecticut U.S.A. Bertorello C. (1998) Tesi di Dottorato “Analisi Strutturale per costruzioni Navali in Plastici Rinforzati”, Università degli studi di Napoli “Federico II” R.I.Na. (2009) Rules for the classification of the yachts (part A), Registro Italiano Navale, Genova

IX HSMV Naples 25 - 27 May 2011 14