building fully cored rescue boats

8/2/2019 Building Fully Cored Rescue Boats

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2006 InternationalSandwich Symposium

APRIL 20TH - SEATTLE - WASHINGTON - USA

Building Fully-Cored Rescue Boats

by Rolf Eliasson - B. Sc., M.A.

sponsored by:

ZE


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Building Fully Cored Rescue Boats 2

2006 Sandwich Conference

The Author - Rolf Eliasson - B. Sc., M.A.

After winning the Yachting World International Design

Contest in 1976 Rolf founded R.E.Yacht Design. Since this

time he has produced well over 100 designs ranging from

an 8 ft. dinghy to a 73 ft. cruising yacht. His design work

for Swedish-based, Nimbus Boats covers some 22 power

boat models. Rolf estimates that the total number of boats

built from his designs now amounts to over 6,000. In 1991

he became a member of the ISO working group respon-

sible for writing standards for the EU Pleasure Boat Direc-

tive. Together with Prof. Lars Larsson, head of the naval

architecture department of Chalmers Technical University in

Gothenburg, he wrote the standard reference work Prin-

ciples of Yacht Design.

Introduction

Colin Archers rescue boat, the epitome of seaworthiness

and performance, in her time. A good tradition to build

on...

The Main Reasons for Fast Light Rescue Boats

Studies made by the SSRS (Swedish Sea Rescue Society

in the early 1990s showed that the majority of all accidents

involving pleasure boats in Swedish waters occur within 10

nautical miles from shore. When this was taken into consid-

eration, together with the fact that people die mostly from

hypothermia and not by drowning (the water temperature in

the Gulf of Bothnia and the north of the Baltic Sea is only

39-41F [4-5C] well into June) it was clear that the speed

capability and as short as possible readiness was the tar-

gets to aim for.

As a result of this the SSRS decided to concentrate its new

builds and replacement programme on light, fast lifeboats.

Clearly, the ASTRA-type was too slow for the majority of

assignments. Something new was needed. Discussionswithin the SSRS led to the following;

Requirements:

1. Maximum speed in excess of 30 knots.

2. All weather capacity.

3. Self-righting.

4. Limited ice-going capabilities.

5. Shallow draught.

6. Stretcher places for emergency transports.

7. Easy recovery of PIW (Person in Water).8. Easy on-station repairability.

9. Redundancy.

Consequences:

1. Suitable hull design and light weight.

2. High speed maneuverability in rough seas.

3. Low centre of gravity and large deckhouse volumes.

4. Rugged bottom construction.

5. No room for propellers and rudders means waterjet.

6. Interior arrangement to be laid out consequently.

7. Low freeboard aft.

8. No exotic materials or methods.

9. Built in redundancy of key parts/equipment.

Resulting Design

1. The shape of the still waterline gives a hint of the fore

body design. Not too sharp but neither too full. A deli-

cate balance.Fig. 1 - Pre-1995, ASTRA, heavy, slow and reliable.


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appear. Not too sharp and deep forefoot to avoid broaching

in following seas, moderate deep-vee deadrise (a balance

between an easily driven and a softly riding hull), simple

prismatic shape of the hull that is good for the waterjets

the performance and ease of construction. The boats were

to be built without using any female molds. This led to a

design with only developed surfaces in order to use whole

sheets of foam thereby minimizing the number of joints.

Fig. 2 - Main features of the new SSRS rescue vessel.

2. Negative deadrise of chine strake to increase lift.

3. Negative deadrise of planing strakes for the same rea-

son as 2. Strakes taper towards the bow, and dead-rise gradually becomes positive, not to slam too hard in

heavy seas.

4. Boundary of wetted surface at 40 knots. Rails go into

this area, but no further.

5. Spray deflection area of the rails.

6. Wetted surface at 40 knots, about half of static wetted

area.

7. Waterjet intake. No rails, keels or other devices in front

of them, to give a clean flow of the water to the jets.

8. Raised part of bottom beside the jets to enable proper

operation in reverse.

9. Modified RIB-type collar. It is not filled with air, but with

an elastic polyurethane foam, covered with a skin of

tough polyurethane and Kevlar. Tapering of the collar

forward is important, since otherwise too much buoy-

ancy might develop when running into a head wave,

capsizing the boat backwards.

10. For stability reasons the deckhouse is relatively large.

Due to its larger size, the 65 footer (20 meter) boat is more

slender than the 40 footer (12 meter), but the same features

Fig. 3 - The hull lines of the SSRS-2000 RESCUE.

For a planing boat, the limitation of just using developed

surfaces is not necessarily a bad thing, and with good com-

puter software it is not even difficult. As can be seen in Fig-

ure 3, the sides are almost slab sided with no flare, but on

the finished craft the topside-covering collar is designed to

provide the flare. More of that later.

Some Personal Sandwich History - Why Not a Schooner?

Fig. 4 - The dory schooner Saharah, 1974.

This is an early (1974) fully-cored sandwich construction

with PVC-foam and glass/polyester laminates. All parts are

built this way - hull, deck, superstructure, bulkheads and


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stiffening system. In those days it was considered unwise

to build an offshore going sailboat in full sandwich. The ex-

perts sentenced it to an early grave, broken up by the sea.

They were wrong.

Why Not a High Speed Powerboat?

Fig. 6 - The SSRS-1200 RESCUE, 1995. (Photo: Dan Ljungsvik)

shifted one inch (2.45 cm) to the side. This resulted

in the only high-tech items onboard, the jack-shafts

between the engines and jets, made of epoxy/carbon fi-

bre (to save weight) exploding in a cloud of black dust. Itwas interesting to inspect the boat afterwards. No cracks in

the laminates anywhere, the engine beds/bottom stringers

were intact as were the attachment of the elastic vibration

dampers of the engines. But the feet of the engines them-

selves were not. They were bent, and that was the rea-

son for the engines moving one inch across the boat. Later

discussions with the engine manufacturer, who did some

reverse engineering, showed that the impact had resulted

in forces in the engine room of 10 g. The boat withstood

the test though, but the engine people gave us some goodadvise, dont run boats at that speed (30 knots) in those

sea conditions.

Basic Construction

Fig. 5 - 70 knot sportsboat Thundercan, 1985.

Ten years later (1985) this high speed design was made,also using fully-cored construction in line with the schooner.

One big difference was that the laminates were vinylester/

kevlar/carbon/glass. Also, of course, much higher density

cores were used. The second big difference was that the

fuel tank (gasoline) was integral with the hull, for weight sav-

ing and capacity reasons. The tank was stuffed with a kind

of metal mesh that is used in F-1 racing cars, to make them

less likely to explode on impact. With all this; a fully cored

70 knot boat designed for offshore use and an integral gas

tank, the verdict from the experts was even stronger- If itdid not blown as a result of a huge gas explosion, the bot-

tom core would be pulverized at the first high speed slam-

ming impact. Well thats what the experts said. Again they

were wrong.

Incidentally, this boat was designed on the forepeak of the

schooner Saharah during a cruise along the south coat of

Norway. Together they became a rescue boat!

Ten years later in 1995 this (Figure 6) rescue boat was de-signed. The picture shows the vessel landing after a jump

from a 13 ft. (4 meter) wave. As can be seen the spray

rails and negative deadrise of the chine strakes are do-

ing their job as the topsides virtually run dry. One mis-

hap happened during this photo session though (yes,

this shot was taken from a helicopter). One landing from

a wave ended with the craft on its side. Result: a big

bang and loss of propulsion. Reason: the engines had Fig. 7 - Basic construction of SSRS-1200 RESCUE.


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Fig. 8 - SSRS-1200 - combination engine beds/bottom stringers.

All surfaces are of developed type to simplify building over

male molds with full sheets - easy! This minimizes the num-

ber of joints in the core and the risk of voids. By always

laminating onto the core, it is possible to obtain a good fiber

ratio (+50%) without resorting to vacuum bagging. Not us-

ing vacuum bagging was a request from the SSRS who

wanted the capability to carry out repairs on location. The

stiffening system consists of a few heavy stringers, rein-

forced keel and chine supported by structural sandwich

bulkheads - simple!

The fuel tanks are integral with the hull to save weight andobtain enough volume for the required range - 350 NM at

full speed 10 hours. The cross sections in Figure 7 show the

shape of the polyurethane collar giving flare to the topsides,

and also adding to the stability at moderate heeling angles.

room stiffeners/engine beds to give an unbroken continuity

of the longitudinal stiffening system. Also seen in the pic-

ture are the tank baffles of GRP with limber holes in them

adding to the stiffening of the bottom panel. The trench be

tween the tanks is used for piping. There is a diverter valve-

block so you can run either or both engines from both tanks

or select just one tank.

The 65 footer (20 meter) is built in basically the same way

With one big difference. The fuel tanks are not integral with

the hull. They are, however, integral with the structure.

Fig. 9 SSRS-1200, integral fuel tank.

The longitudinal inside tank wall and longitudinal baffle are

designed as tall top-hat stiffeners connected to the engine

Fig. 10 - The basic structure of the SSRS-2000.

Figure 10 shows the transverse stiffening system, togeth-

er with the structural soles. The longitudinal stiffeners and

skins are omitted. The fuel tanks, shown as cylinders, are

tied into bottom floors with their internal baffles (the tanks)

continuing the floors. Being situated closer to the neutra

axis of the hull compared to being integrated with the hullthe tanks are not as heavily stressed. The reason for not

going the 1200 route is simply that I didnt dare. On a larg-

er, leaner boat the hull girder is more susceptible to bend-

ing and twisting in a seaway compared to a shorter, more

beamy vessel. Anyhow, both ways work for these two de-

signs. The 1200 boat has been in service now for ten years

and to date no structural deficiencies have been reported.

Fig. 11 - The hull reinforcements for the SSRS-2000.


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Figure 11 shows the hull reinforcements of the 2000 boat.

The greenish bands are where additional bi-directional (dou-

ble bias) reinforcements are laminated to the inner skin. This

is where the stiffeners (stringers, bulkheads and frames) are

laminated to the hull. The 1200 boat is built in a similar way.

The reason is clearly seen in Figure 12.

The problem here are the little red dots shown. This is where

it should have been radiused putty for the tabbing, but it is

often missing creating a line-void along the stiffeners edge

This is not good and even more so if a regular polyester is

used, with its inferior gluing properties. There is a real risk

that the tabbing will start to peel off under load.

The second method (B) also uses a pre-formed stiffener

but here the flanges are glued to the hull. Two important re-

quirements must be satisfied. The glue/putty must cover the

entire flange area and it must possess enough elongation

before breaking, otherwise there is a risk that it will crack as

a result of a slamming impact. One interesting technique

used by a Finnish yard, is to create the gluing pressure to

the stiffening system by sucking air out from the inside of the

stiffeners. It is not vacuum bagging rather vacuum gluing

Tests made as early as 1980s by the Swedish State TesLaboratories (Statens Provningsanstalt) showed that on the

average, the (B) method was six times stronger than the (A

method - mainly because of (As) weaker peel strength.

Method (C) is built in-situ over a core. The form core is

puttied down to the hull with good radiuses between the

stiffener sides and hull before being laminated over. Provid

ing the stiffener flanges are well tapered and the secondary

bonding surfaces are clean and roughened, this makes an

excellent stiffener to hull joint - especially when vinylesteor epoxy is used. This is how the rescue boats are built

The (C) method also lends itself to vacuum bagging tech-

niques.

Stringer Details of the SSRS-2000

Fig. 12 - The moment of distribution of a strip of a hull panel.

The bending moment in a hull panel over a support (frame,

stringer or bulkhead) is double of that in the mid-panel po-

sition, when the panel is fixed at the ends. This is exactly

what classification societies assume. Also the coming ISO

standard 12215 assumes this when calculating the scant-

lings. So, consequently the critical skin will be the inner one,

in compression. Although the scantling rules recognize this,

(the 1200 is dimensioned according to DNV High Speed

Craft and the 2000 is dimensioned to the ABS High SpeedCraft rule) the boats in question are rescue boats and it is

unthinkable that they should break up due to heavy weath-

er. That is the reason for the reinforced bands, shown in

Figure 11. In addition there is a difference in how the stiff-

eners are attached to the hull. Three typical examples are

shown below.

Pre-formed

Stiffener

Pre-formed

Stiffener

In-Situ

StiffenerFlangeFlangeTabbing Tabbing

Sandwich

Panel

Sandwich

Panel

Sandwich

Panel

A B C

Fig. 13 - Stiffener attachments.

Starting from the left (A), the most common method in pro-

duction boat building (at least in Scandinavia) to fix a stiffen-

ing system to the hull, is to pre-form it in a separate female

mold, trim the edges to the hull contour (more or less) and

then laminate it with tabbings (marked in blue in Figure 13).

Fig. 14 - Bottom Stringer.

The lay-up is simple. Seven plies of 800 g/m2 double bias

glass, each of 27.5 in (700 mm) width staggered 1 in. (12


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mm). The lay-up is started and finished with a layer of 300

g/m2 CSM. The crown of the stringer is additionally rein-

forced by five layers of unidirectional glass of 900 g/m2 per

ply, sandwiched between the DB-layers. When using a resin

with good gluing properties (e.g. epoxy or vinylester) it really

doesnt matter in what sequence the plies goes in. Thats

the reason for the asymmetric appearance of the flanges to

the hull. It does simplifies the lay-up though as you can use

the same width glass for the entire lamination.

load assumptions made in the standard. If using a more

accurate computation method, FEA, beware of the big risk

DO NOT use load assumptions coming from scantling rules

based on classical structural theory with materials working

within their elastic limits. NEVER use a calculation mode

that does not correspond to the load assumptions. May

I remind you of the two AC-boats that broke and sank a

couple of years ago? It was surely not due to lack of com-

putational power...

Fig. 15 - Engine Bed.

The engine beds sit on top of the bottom stringers, formedover a core of 100 kg/m3 PVC, with eight layers of 800 g/m2

DB-glass. On top of that is a 16 mm (5/8) stainless steel

flat-bar for the engine-feet bolts. The entire thing is then

laminated over with exactly the same lay-up as the bottom

stringers. The method is the same for the 1200 that really

stood up to a full scale test during the photo session previ-

ously mentioned.

Figure 16 shows the relatively simple stiffening system that

is needed, when using a fully cored hull and stiffeners, with

mechanical properties similar to each other so they cooper-

ate fully. If you go into advanced composite lay-ups, with

fibers that have big differences in strain properties, then

things get a bit complicated. When the laminate schedule

is complex and cannot be regarded as quasi-isotropic, a

laminate stack analysis should be made, to determine the

first ply failure. A guide for this is presented in an annex (H)

of the coming ISO standard 12215-5, which works with the

8 x 800 double bias

16 mm stainless steel

4 x 30 mm - H100 core

Same laminate as hull bottom

Fig. 16 - The SSRS-2000 stiffening system.

An important consideration when designing a fully cored

fast powerboat is the cores ability to absorb shock loads

coming from slamming in a seaway. To do that, the core

should not be too rigid, but be able to flex a little to reduce

the blow and absorb the impact energy. The ISO 12215

standard recognizes this and allow cores with an elonga-

tion to break of more than 35% to use 65% of their ultimateshear strength, while more brittle foam materials can only

use 55%. Balsa and honeycomb cores are allowed jus

50%.

To hammer the above reasoning down a little bit deepe

when it comes to the use exotic fibres and strain, look at

the Figure 17. As we can see the Kevlar is the strongest one

while Boron is the stiffest closely followed by carbon fiber

Usually when the exotic fibers are used together with glass

reinforcement there are some specific consequences. If, fo

instance we have a laminate consisting of Kevlar 49, Car-

bon HT and ordinary E-glass, the carbon fiber is full loaded

when strained to 1.2% (the vertical line in Figure 17). Here

the carbon develops its highest strength value of almost

2 GPa, and if strained any more it will break. The other fi-

bers in the laminate have their maximum strength at much

higher strain values: Kevlar at 2.7% and E-glass at 3.8%

To make all the fibers in the composite co-operate, the tota


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strain must not exceed 1.2%, which means that the Kevlar

can only be used to 1.2 GPa and the E-glass to 0.5 GPa ,

roughly half their maximum values.

the other fibers are not allowed to develop their assumed

strength. Another thing to keep in mind is to use a resin

with a higher strain level than the fibers, to discourage the

start of micro-cracking. Due to the high strength of these

exotic fibers much higher demands on the resins adhesive

characteristics must be made. Polyester is not a particularly

good glue, whereas vinylester or an epoxy formulated fo

laminating are respectively good and excellent glues with

high strain values thereby making it possible to utilize the ful

properties of the high performance fibers.

To sum up. If you are using exotic fibers, skip the glass

when in a sandwich construction. In single skin it can be

useful with glass as a bulking material to build thickness

With cored structures, a proper sandwich core is so much

lighter and builds thickness so much faster, so why bother

with glass at all. Alternatively stick to glass entirely if you arenot too weight sensitive.

The mention of Kevlar leads us to impact strength. As we al

know Kevlar is used in bullet proof vests so the assumption

could be that it must be really good in this respect. This is

not always the case. To start with the material used in vests

is not the same kind that is used in boat building. Secondly

the fibers are locked into a resin matrix in boats which is

Fig. 17 - Strain vs Strength of different fibers.

If we are using all the materials at their maximum strength

and disregarding the strain, the stiffest fiber will break be-

fore the structure is loaded to its maximum, since this fi-

ber will then take on too big a load. To put it another way:

0

0.50.4

1.4

2.0

1.0

1.5

2.0

2.5

3.0

1 2 3 4 5 6

Tensile Strain (%)

Kevlar 49 Kevlar 29

E Glass

Polyester T-68

Boron

Carbon HT

TensileStress(%)

0 1 2 3 4 5 6 7 8

43

62

52

0

88

54

0

(J/kgm2)

FRP-1 glass mat 22

28

41

20

32

23

24

FRP-2 glass/aramid (93/7%)

FRP-3 glass

FRP-4 glass spray-up

FRP-5 glass

FRP-6 glass/aramid (90/10%)

FRP-7 glass

Amoun

tofcontinuousfibers(%)

F

ibercontent(vol.%)

Fig. 18 - Specific absorbed energy of FRP laminates.


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not the case for the vests, where the fibers can move so

that they basically catch the bullet. In 1997 the Technical

Research Centre of Finland (VTT) made a study of Local

impact strength of various boat-building materials (publica-

tion 317), by Martin Hildebrand.

Figure 18 shows the specific absorbed energy for seven

different FRP laminates.

All the laminates in Figure 18 are made with polyester res-

in. As can be seen, both a high fiber content and a high

amount of continuous fibers produce higher specific impact

strength values. A perhaps more surprising result is the 7%

and 10% values respectively for FRP.2 and FRP.6, both of

which contain aramid fibers (Kevlar). Against all popular

belief the laminate (FRP.6) with more aramid in it performs

worse than the laminate (FRP.2) with less aramid. The rea-son is that FRP.2 is thinner 0.14 in. (3.5 mm) than FRP.6 0.3

in. (7.7 mm) and it is the specific energy absorption that is

measured. In absolute terms the FRP.6 absorbs 56 Joules

compared to FRP.2s 33 Joules. Still, the specific values tell

a lot about a laminates effectiveness to absorb an impact.

The winner in this test series is FRP.3 because of its high

continuous fiber content (88%), relatively thin laminate 0.15

in. (3.8 mm) as well as a high fiber content (41%).

It is interesting to compare FRP.5 and FRP.7 that contain

the same reinforcements, with the exception that FRP.7 has

more CSM in the middle and thus a quasi-sandwich lay-

up is created. Due to this configuration, the specific impact

strength is increased by 16%. So to make thin laminates

with a thickness building core, really makes sense.

In the spring of 2001, I performed an accidental full scale

test of the impact strength of a sandwich laminate. This

happened two days before my wife and I were planning to

set sail. The result of this mishap was a six week delay.

It is interesting to note that the inner laminate was virtu-

ally undamaged. The planking structure that is visible is

not the foam planks but the imprint of them onto the inner

laminate. It really shows a very good adhesion between the

inner laminate and the foam core. Thanks to the excellent

energy absorbing properties of the core, the damage was

limited. Had this been a single skin hull, the damage would

have been much worse, and more difficult to repair.

Fig. 19 - The fall.

Fig. 20 - The damage

Fig. 21 - The repair.

As previously stated all boats shown here have structura

bulkheads of foam sandwich construction for strength and

weight reasons. This route is seldom taken by production


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Fig. 22 - The result, four years later in Curacau.

boat builders in the leisure sector. The reason is said to be

economic, and although this partly true I have a suspicion

that it goes deeper than that. It is called ignorance of howmuch better a sandwich bulkhead co-operates with the hull

compared to the ubiquitous plywood, how much weight

can be saved, how much lighter the non-structural interior

elements (even made of wood) can be built and how much

this will enhance the performance of the boat, be it power

or sail.

In the ISO 12215-5 standard there is simple method of ob-

taining scantlings for sandwich bulkheads by transforming

a known plywood thickness to sandwich.

Taking a bulkhead in the SSRS-2000 as an example we get

a plywood thickness of 0.7 in. (18 mm) with a hull depth o

8.36 ft. (2.55 meters). A good quality plywood of that thick-

ness would weigh up to 2.8 lbs/ft2 (13 kg/m2). By switch

ing to sandwich with skins of stitched glass rovings (as-

suming a hand lay-up) we can easily reach a fiber content

(by weight) of 50%. This gives us an ultimate compression

strength of 148 N/mm2, of which we use 74 N/mm2 and an

in-plane modulus for the skins of 14,000 N/mm2. A prope

core thickness for this span is 1.2 in. (30 mm) with a density

of 5 lb/ft3 (80 kg/m3). The result is staggering; for strength

reasons the skins have to be 0.02 in. (0.6 mm) thick and fo

stiffness reasons only 0.02 in. (0.3 mm) thick! This gives a

skin requirement of [imperial to be input] (500 g/m2) of rein

forcement on each side.

Using a commonly available stitched roving of [imperial to

be input] 600 g/m2

we end up with a weight of [imperiato be input] 2.4 kg/m2. Added to this is the weight of the

core, also [imperial to be input] (2.4 kg/m2 ), which brings

the total panel weight to [imperial to be input] (4.8 kg/m2). I

is just a shade more than a third of the plywood panel! The

actual scantlings of the rescue boats bulkheads are heavie

though, since they are designed to be watertight collision

bulkheads. So, to use plywood just because it looks good

(at least some plywoods do) you have to pay a heavy weight

penalty. If you really need the wood-look, it is always pos-

sible to surface the bulkhead with veneer. Personally I reallydo not think that painted bulkheads need to look ugly. Two

examples are given is Figures 24 and 25.

Fig. 23 - The ISO 12215-5 scanting standard


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Fig. 24 - Interior of the SSRS 1200.

Fig. 25 - Interior of Saharah.

Fig. 26 - The SSRS-2000 (#2) under construction.

Fig. 27 - The launching of the first SSRS-2000.

To sum up, here some pictures of the SSRS-2000. Simple

developed surfaces everywhere, suitable for sheet materi-

als. Clean mono-hedron hull form and not too deep a fore-

foot. Just two planing strakes each side, but they are big

and at the correct position.

Computer simulations take the guesswork out, and are

good companions to experience.

Computer Stability Simulations

Perhaps the most important computer simulations are the

ones concerning a vessels stability characteristics. Not only

can you check that the stability is positive all around the

clock, but you can select different VCG positions, varying

displacements and alternative deck house designs.

Doing all this manually is impossible, in my world.

Fig. 28a - 60 degrees heeling.

Fig. 28b - 90 degrees heeling.


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Fig. 28c - 135 degrees heeling.

Fig. 28d - 180 degrees heeling.

Fig. 29 - The SSRS-2000 at full speed.

REFERENCES

The following is a list of the references used to compile this

paper.

ABS: 1997. Guide for Building and Classing High Speed

Craft. American Bureau of Shipping, New York.

Blount, D, Codega, L: 1991. Dynamic Stability of Plan-

ing Boats. Fourth Biennial Power Boat Symposium, Mi-

ami.

Caprino,G, Teti, R: 1989. Sandwich Structures Hand-

book. Il Prato Pelf SpA, Padua.

DIAB: 1991. Divinycell Technical Manual H-Grade. Divi-nycell International AB, Laholm.

DNV: 1985. Rules for Classification of High Speed Light

Craft. Det Norske Veritas, Oslo.

Hildebrand, M: 1997. Local Impact Strength of Various

Boatbuilding Materials. Technical Research Centre of

Finland, Espoo.

ISO/TC 188/WG18, ISO/FDIS 12215-5: 2006, Hull con-

struction-Scantlings - Part 5: Design pressures fo

monohulls, design stresses, scantling determination

International Standards Organization, Geneva.

Larsson, L, Eliasson, R: 1999. Principles of Yacht De-

sign. Adlard Coles, London.

Savitsky & Brown: 1976. Procedure for Hydrodnamic

Evaluation of Planing Hulls. Marine Technology.

VTT: 1997, VTT-NBS Extended Rule, Technical Research

Centre of Finland, Espoo.

building fully cored rescue boats

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