building fully cored rescue boats
TRANSCRIPT
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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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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.