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TEC HE EIT ratorium v Scheepshydromechanica Mekelweg 2 - 2628 CD DELFTSixth symposium on developments of interest to yacht architecture, under the auspices of the HIS WA. This symposium was held on 6-7 th. September 1979,
Symposium
Yachtarchitecture
'79
4
6-7 september 1979, RAI Congrescentrum, Amsterdam, The Netherlands
6TH SYMPOSIUM ON DEVELOPMENTS OF
INTEREST TO YACHT ARCHITECTURE
1979
LIST OF PAPERS
Joseph Cougeon, Cougeon Brothers Inc., U.S.A.
THE USE OF A WOOD-RESIN COMPSITE FOR MARINE CONSTRUCTION
David Beach, Boating Industry Association, U.S.A.
SAFETY FOR SMALL CRAFT
Dick Newick, Richard C. Newick High Performance Boats, U.S.A.
CONSIDERATIONS RELATIVE TO A TRIMARAN DESIGN
K. Nomoto, Osaka University - Faculty of Engineering, Department of Naval Architecture,
Japan page 64
BALANCE OF HELM OF A SAILING YACHT - A SHIP HYDRONAMIC APPROACH ON
THE PROBLEM
Olin J. Stephens II, Sparkman & Stephens Inc., U.S.A. page 90
A PHILOSOPHY OF YACHT DESIGN
Lars Larson, Statens Skeppsprovningsanstalt Goteborg, Sweden page 101
THEORETICAL PERFORMANCE PREDICTIONS FOR THE 5.5 MTR YACHT ANTIOPE
M. C. Meijer, Technical University Delft - Ship-Hydromechanics Laboratory, The Netherlands page 135
SHIPPROPELLERS FOR SMALL CRAFT
Brian Grant, Gear Test, England page 176
A JOURNALISTIC APPROACH TO THE TESTING OF SMALL CRAFT EQUIPMENT
B. E. Perry, Ian Proctor Metal Masts Ltd., England page 203
SPARDESIGN, CONSTRUCTION AND STRENGTH CALCULATIONS
Published by: Uitgeverij Interdijk by., Amstelveen, Holland, tel.: 020-45 37 51. Telex 10179 Indyk NL
page 6
page 29
Edited bij a committee under the chairmanship of Prof. ir. J. Gerritsma.
Members of the committee: W. de Vries Lentsch Jr. (vice-chairman); G. W. W. C. Baron van Hoevell; Dr. J. van Vollenhoven
INTRODUCTION:
Under the auspices of HISWA (The Netherlands Federation for Trade and Industry in the field of Shipbuilding and Aquatic Sports) biennially (since 1%9) an international Symposium on de-velopments of interest to yacht architecture and yacht and boatbuilding is organised.
The Symposium 1979 is- like its predecessors - intended to assist in stimulating research and spreading data that can be of benefit to yacht architecture and the yacht and boatbuilding indus-try.
The demand for the printed papers has been so great, that only a very limited number of copies of the papers of the 1st ( in Dutch) and the 2nd. 3rd, 4th and 5th ( in English) are still available.
Dr. J. van Vollenhoven, Director General HIS WA
6
THE USE OF A WOOD-RESIN COMPOSITE FOR MARINE CONSTRUCTION by
Meade A. Gougeon
President - Gougeon Brothers, Inc.
Abstract
Wood is an excellent engineering material with unique physical properties that ideally suit it for use in constructing lightweight marine craft.
Being a hygroscopic material, wood is plagued by a number of
serious problems that are moisture related. Because of these problems
and the introduction of more modern materials, the use of wood has
declined greatly.
With the help of modern technology, most of the problems with wood can be solved by incorporating wood into a composite with a
proper resin material.
Three different construction methods are presently being used to construct hulls with the wood-resin composite.
THE USE OF A WOOD-RESIN COMPOSITE FOR MARINE CONSTRUCTION Before we can present a meaningful discussion of the wood-resin composite, the main ingredient, wood, must be fully understood from
the engineer's viewpoint. Unfortunately, the physical properties of
wood are not widely known or understood in the marine field. This is
not too surprising when one considers that even the best naval archi-tecture schools give wood only token mention concentrating their
efforts on the metals and fiber reinforced composits. Wood is an
immensly complex subject, and we can only cover those pertinent points in this paper that are necessary to develop a clear understanding of
the wood-resin composite.
Wood as an Engineering Material
In considering wood as an engineeringmaterial, it is pertinent to note that "wood" is not a single material with one fixed set of mechan-ical properties, but rather includes many species which possess a wide
range of properties, which depend upon both the species and the density
selected. The range of properties is considerably wider than what is
generally available with most other types of materials, where some
variation of properties can be attained by means such as alloying or
tempering, but where little variation of material density is possible.
Wood, on the other hand, can be selected over somewhat more than a full
order of magnitude in density, from 6 lbs/cuft or even less for
select-ed grades of balsa, to over 60 lbs/cuft for certain species of
hard-wood. The flexibility this can provide the wood structure designer is
obvious; since low-density species can be selected for efficient use
as core materials, or for panels or beams where stiffness or buckling
resistance is of primary importance. High-density species can be
selec-ted where there is a need for high strength and modulus per unit volume, such as in panel skins or in structural members which must occupy
con-strained geometric volumes. The full range of intermediate densities
provide a match for requirements anywhere between these extremes. In
this regard, it is worth noting that the physical properties of wood
are roughly proportional to its density, regardless of species, since
the basic organic material is the same in all species. Thus, changing
density is rather like compressing or expanding the net strength and
elastic stiffness into different cross-sectional areas with little net
variation to total properties per unit mass.
Given that the strength and modulus of wood vary approximately in proportion to its density, it is easily shown that the length of a
8
solid wooden panel which is stable against buckling will vary inversely with its density, while the length of a solid wooden column stable against buckling will vary inversely with the square root of its
den-sity. Therefore, approximately a factor of ten in unsupported panel
length, or a factor of three in unsupported column length, is readily
available to the designer of wooden structures. Designers of
struc-tures using other materials can perhaps best appreciate what this
means by imagining that a factor of ten of density variation were
somehow readily available for the steel, or aluminum, or composite,
with which they regularly work.
Granted that the density variation of wood can be of advantage to
the wooden structure designer, one must also inquire how good are its
net properties per unit mass relative to other structural materials.
There are, after all, other light variable density materials available,
such as the expanded foams. For modern structures where weight is an
important issue, the strength-to-density ratio (specific strength) and
modulus-to-density ratio (specific modulus) are two very important
numbers to consider, since they determine how much strength and stiff-ness you can get for a given mass of material.
A typical grade of Douglas fir, a moderate density species, will possess approximately the following properties:
Fir Density
Compressive Strength
Tensile Strength
Modulus
To easily compare this to other materials, the table below
indi-cates the strength and modulus required of the other materials to
achieve exactly the same strength-to-weight, and modulus-to-weight,
possessed by Douglas fir.
Steel Density 7.8(487 lbs/cuft) Compressive 112,500 psi Tensile 225,000 psi Modulus 30 x 106 psi .52 (32.5 lbs/cuft) 7500 psi 15,000 psi 6 2 x 10 psi Equivalents of Fir Aluminum 2.7(169 lbs/cuft) 38,942 psi 77,885 psi 10.38 x 106 psi Fiberglass Composite 1.9 (119 lbs/cuft) 27,403 psi 54,807 psi 7.3 x 106 psi
Those familiar with the typical properties of steel, aluminum, or
fiberglass composite, will recognize that these numbers indicate
Douglas fir to be a competitive structural material on a per unit
weight basis. It might also be noted that the number cited for the
Typical shop laminates we have produced, in fact, exceed the strength
and modulus numbers cited.
It should be pointed out at this time that the preceding
consid-ered the properties of wood along its grain direction. The same piece
of fir which displays 15,000 psi tensile strength along its grain
direction will have something like 300 psi tensile strength across
its grain. That is a 50 to 1 variation in tensile strength depending
on the load direction. The other physical properties of wood are also
distinctly anisotropic, although not to as great a degree. What this
means is that the wooden structure designer may have to take explicit measures to deal with crossgrain or shearing forces within the wooden structure which could safely be regarded as negligible by the designer who uses a conventional material with isotropic properties, such as
steel or aluminum. It also means that in cases where large loads
flow in more than one direction, that wood fiber will have to be
arranged to align with all of these loads. For cases where the large
loads are confined to a single plane, a structure such as laminated
veneer or plywood can meet the requirements. Where loads in all three
axis exist, the designer must use more sophisticated approaches
tai-lored to the loads and geometry. One of the key elements in our
success with lightweight wood composite boat structures has been an
ability to identify directional loads so that we could align our wood
fiber accordingly.
Wood has an exceptionally high work of fracture (around 104J/M2) with an almost total resistance to crack propagation that is so
famil-iar with metals. By its very nature as a
fiberous material, wood
has excellent resistance to repetitive fatigue loading. When one
con-siders that nature has spent millions of years in the serious business
of competitive survival to develop strong trees which must stand
re-/
peated and highly variable loads from wind for long periods of time
(some redwoods are several thousand years old), it is not surprising
to find that wood is an efficient structural material with very
re-spectable fatigue properties. Unfortunately, the fatigue testing done
with wood has been minimal in comparison to other materials; but results to date indicate that infinite fatigue life
occurs somewhere
between 307 to 40% of ultimate. When building high performance
rac-ing craft where safety margins are not particularly important,
we
often use as high as 100% of ultimate with surprisingly good long
term
performance. This is probably due to the unusual
failure mechanism of wood.
10
In boats, wood is usually used in bending so that tension and
compression forces occur in the same part. Failure will always be in
compression which is the weakest part of wood. As compression loads
are applied to the individual wood cells, they can deform considerably
before initial failure begins to occur. This deformation allows the
wood cell to share some of the burden with those cells closer to the natural axis which, in effect, transfers some of the load to the
ten-sion side. In this way, the actual load on a wooden beam before total
collapse occurs may be up to twice the theoretical compressive stress.
No doubt this ingenious mechanism was designed by nature to allow
trees to withstand occasional violent gusts of wind. For boats, this
failure mechanism together with good fatigue life and resistance to
stress concentrations make wood a potentially very safe material; and
to quote Mr. J. E. Gordon, "a material with which one can vary nearly
get away with murder."
It appears that nature happened on a very good thing when it
invented a cellular structural material in which a significant portion
of its volume is composed of air. Man has tried to duplicate this
phenomina with the development of expanded foams, but even the best of man-made foams is a poor match for wood on a weight-to-strength or
stiffness basis. Efforts to develop superior materials have generally
centered around the exotic high-modulus fiber composites such as boron
or carbon. While these materials exhibit an extremely high Young's
modulus per unit weight, they do not have the desireable low-density
resistance to buckling of wood. Rather than continue his pursuit of
high-modulus fibers, man might better spend his time developing a
material with holes in it. The efficiency of low-density materials
is clearly illustrated in Figure I where wood (spruce) is compared
with a number of materials in various roles.
While the Young's modulus of wood is low in comparison to other materials, its efficiency as a panel is superior to even that of a
carbon fiber composite. Panel stiffness is the single most important
physical property of a material for boats. A boat hull and deck has
significant amounts of surface area that must all be adequately
sup-ported against high-point loading at the least cost in weight.
Lightweight panels can be built with higher-density materials by
bonding two separate skins on either side of a lightweight core of
foam or end grain balsa. Unfortunately, this procedure is quite
will probably not provide a panel that is as efficient as wood.
Cer-tainly the complexity and difficulties of engineering structures with
cored laminates are well known.
FIGURE I.
THE EFFICIENCY OF VARIOUS MATERIALS IN DIFFERENT ROLES
Where:
E/p is specific Young's modulus
-/T.
-7-
is weight-cost of carrying a compression load in columnis weight-cost of a panel in buckling
Taken from the book "Structures" by J.E. Gordon. Conclusion
Wood is an engineering material that possesses some unique poten-tial for boat construction that is unavailable with any other material. Specifically, wood has high-weight-to-strength values, is almost
immune to crack propagation, is resistant to stress concentration,
and has excellent resistance to fatigue loads. The density range of
wood seems to be ideally suited for boats to provide the highest
stiff-ness at the least cost in weight. These are all the advantages; but
like all engineering materials, wood has its share of problems which
we shall discuss in detail.
Historical Problems with Wood and a Modern Solution
With the development of modern engineering materials, such as steel, aluminum and fiber reinforced plastic composites, the use of wood as a serious engineering material for sophisticated structures
has greatly diminished. The reasons for this are generally well known,
wood can deteriorate from rot, and be dimensionally unstable. The
11
Material
Young's modulus, Specific gravity
EMN/m2 p grams/c.c. E/p
VT --ZIT
P PSteel 210,000 7.8 25,000 190 7.5 Titanium 120,000 4.5 25,000 240 11.0 Aluminum 73,000 2.8 25,000 310 15.0 Magnesium 42,000 1.7 24,000 380 20.5 Glass 73,000 2.4 25,000 360 17.5 Brick 21,000 3.0 7,000 150 9.0 Concrete 15,000 2.5 6,000 160 10.0 Carbon-fiber composite 200,000 2.0 100,000 700 29.0 Wood(spruce) 14,000 0.5 25,000 750 48.0
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The fact that the consistency of wood can vary within a single tree together with fluctuation in physical properties because of moisture
level change can cause difficult quality control problems.
We feel that the demise of wood as a serious engineering material
is both unfortunate and premature. With the help of modern technology,
most of the problems with wood can be solved in a practical manner. For the past ten years, we have used wood in composite with plastic resins to build high-performance sailing craft; specifically, iceboats and multihull craft that must be built at high strength and
stiffness-to-weight ratios in order to be successful. In part, our success has
been due to the fact that wood itself is an excellent engineering material; but our ability to solve the moisture problem with wood, however, has been the key to the development of wood as a practical engineering material especially for use in a hostile marine
environ-ment.
To better understand what we have done to achieve our solution, a discussion of the interrelationships between moisture and wood is
need-ed.
Moisture is a major ingredient of all wood in the living tree. Even wood that is properly dried or cured will have a significant
per-centage of its content by weight being moisture. This will typically
range from 8% to 15% of the oven dry weight of the wood, depending
upon the atmospheric conditions in which the wood exists. Figure II
shows the ultimate moisture content of wood when subjected to various
relative humidities at a temperature of 70°F. Unfortunately, the
sub-ject is a little more complicated than the chart portrays because 50%
relative humidity is much different at 40°F. than at 70°F. (warm air
holds more moisture than cold air); but every area will have an
aver-age year around moisture and temperature level that will determine the
average wood moisture content level. In our Great Lakes area, wood
seems to equalize at about a 10% to 12% moisture content when dried in
a sheltered but unheated area. The real problem with wood is that its
moisture level is rather quickly influenced by short term changes in
atmospheric conditions. In the Great Lakes area, we continually have
extremes of dry and humid climate conditions that are compounded by
temperature extremes of 100oF. between winter and summer.
Wood cells are quite resistant to the invasion of moisture in a
liquid form, but moisture vapor as a gas has a sudden and dramatic
effect on wood by being able to easily and quickly pass through the
condi-tions is thought to be the leading cause of wood to age prematurely.1 Conversely, wood in its natural state as a living organism will remain
at a relatively constant moisture level during its entire lifetime
until it is harvested.
This sponge-like capacity to take on and give off moisture at the whim of the surrounding environment in which it exists, is the root
cause of all of the problems with wood. Specifically, varying moisture
levels in wood are responsible for: (1) dimensional instability,
(2) internal stressing that can lead to checking and cracking of the
wood, (3) potential loss of strength and stiffness of the wood, and
(4) wood decay due to dry rot activity.
Dimensional instability has always been a limiting factor for the use of wood in many engineering applications where reasonable tolerances
must be maintained. To complicate matters, the dimensional instability
of wood has never been constant and varies widely between species of wood, with radial grain wood (cut perpendicular to grain) in most
species being more stable than is tangential wood (cut parallel to
grain). The dimensional change of wood due to mositure change always
occurs first on the outer surface causing differing moisture levels to
occur within the same piece of wood. This can lead to internal
stress-ing that often is the cause of checkstress-ing and crackstress-ing on the wood
sur-face.
Moisture has a significant effect on the strength of wood. Dry wood is much stronger and stiffer than is wet wood. The reason for this
is the actual strengthening and stiffening of the wood cell walls as
they dry out. If wood is taken at its fiber saturation point of 20%
and allowed to dry to 5% moisture, its end crushing strength and
bend-ing strength may easily be doubled and in some woods tripled. The
result is that wood has the potential to be an excellent engineering
material when dry but only a mediocre material when wet. This causes a vexing problem for the engineer who may not be able to determine the
level of moisture content that can be maintained in the structure he is
designing and must assume a worst case situation.
Of all of the problems with wood, dry rot decay is the most known
and feared. Dry rot is a misleading term since dry
wood does not rot;
1Wood has been taken out of the tombs
of Egypt that has been over 3,000
years old. Because of the constant
temperature and humidity in which
it was stored, the wood has lost none of its original physical
prop-erties.
14
there, in fact, must be rather exact conditions in order for dry rot
spore activity to occur. (1) The moisture content of wood must be at
or near the fiber saturation point of 257 (rot is unknown in wood with
a moisture content less than 207). (2) There must be an adequate supply
of oxygen available to the rot spore fungi, i.e. the wood must not get
too wet. (3) The temperature must be warm, 76° to 86°F. is ideal
al-though fungi have been known to be active at temperatures as low as
50oF. (4) There must be the proper kind of food. Some woods such as
western red cedar are resistant to rot because of the tannic acid in
their cellular makeup.
Although there are many types of rot fungi that can destroy wood,
in North America there are two species of the brown rot family that
predominate. They are very hardy creatures that seem to survive large
temperature extremes in a dormant state waiting only for the right
con-ditions to occur to become active. Efforts to control the brown rot
family have had only limited success and generally center around
poi-soning the food supply with various commercial wood preservatives.
Our approach to solve this problem is quite different as will be
ex-plained.
Wood's benefit of being a low-density material becomes a
disadvan-tage when it must be joined. The use of fasteners as a joining medium
is a limiting factor on the load transfer capacity between wood parts.
Only a properly designed adhesive bonded joint can utilize the full
potential of wood. Historically, this has created a problem because
wood bonding has only been totally successful when performed under
ideal conditions. Complicated structures such as boats present
diffi-cult bonding conditions with potential for failure; thus, most
design-ers or builddesign-ers have been reluctant to use this joining method and
revert to the less efficient but more predictable fastener concept of
joining wood parts.
The Wood-Resin Composite
As we have discussed, most of the problems that we have with wood
are moisture related. Therefore, a primary goal of incorporating wood
into a composite with a resin is to provide maximum protection against
moisture to the wood fiber. Our basic approach is to seal all wood
sur-faces with our proprietary resin system.1 This same resin system is
1
We began the development of our resin system in 1969 specifically to
solve the problems that we had with wood. The makeup of our resins are
trade secrets of our company. For the past eight years we have marketed
used as the bonding adhesive to make all joints and laminates with the
goal that they too will be secured from moisture. The lamination
it-self can usually be counted upon to offer secondary moisture
protect-ion. To build our structures, we usually laminate thin veneers
to-gether and count on the glue line between each veneer to serve as a
secondary moisture barrier. For instance, when using 1/8 inch thick
veneers in a 1 inch thick laminate, we would provide 7 glue lines that
must each be penetrated to increase or decrease the moisture content
of the entire laminate.
The success of the wood-resin composite-nethod depends on the
ability of the resin employed to resist moisture passage. Our resin
system is the most effective moisture barrier that we know of and has proven itself through actual usage over many years in marine
construc-tion. We can not claim that this resin system forms a perfect moisture
barrier, but it does slow the passage of moisture into the wood to such a great extent that any moisture change within the wood itself is
minimal. If dry wood encapsulated in our resin were put in a steam
bath and left there for several months, the moisture in the wood would
eventually rise. However, the rate of moisture change in a wood-resin
composite is so slow under normal changing atmospheric conditions that
the wood inside remains at a virtually constant moisture level that is
in exact equilibrium with the average annual humidity and is able to
easily resist violent seasonal moisture fluctuations. With the
mois-ture content of the wood stabilized at constant levels, we are able to
maintain a set standard of physical properties together with excellent
dimensional stability. Dry rot is eliminated as a problem by keeping
the moisture content below that required for dry rot activity and also
by completely sealing the wood from an oxygen source that is a necessary
ingredient for the root spores to survive. Our testing has shown that
even if wood should reach a moisture level high enough to support rot
spore activity, the rot spores still can not exist without adequate
oxygen.
We, of course, did not invent the principle of laminating wood; this process had been commonly used for a number of years.
There are,
however, some significant differences with our method.
First, a
wood-resin composite laminate as we construct it is composed of
a very high
resin content by weight, considerably higher than what is considered
normal. This high percentage of
resin-to-wood ratio is desireable for
several reasons. Enough high-density plastic is
available in the
16
composite to provide sufficient moisture protection to all of the wood
fiber. Secondly, our resin has excellent physical properties with the
potential to improve the composite structurally. Wood is considerably
stronger in tension than it is in compression. The resin that we have
developed is just the opposite, being much stronger in compression
than it is in tension. By properly mating the two materials, one
compliments the weakness of the other with the potential for more
strength than either would be capable of on its own. How much extra
strength that is developed is dependent on several variables which
include wood density, wood resin ratios, and geometric configuration
of the laminate.
Most wood laminating adhesives require high pressures (up to 75
psi) and heat to assure effective bonds. Achieving high laminating
pressure can be very expensive with a high capital expense for tooling.
High pressures also severely limit the size of a laminated part that
can be made. With our adhesive system, we are able to make perfect
bonds1 at very low laminating pressures. In many cases, only contact
pressure is needed between wood pieces because our adhesive has suf-ficent physical properties to easily span small gaps. Low laminating
pressures have the positive effect of lowering the cost of wood
bond-ing and also makes bondbond-ing practical in situations that once would have
been considered too risky. In our custom boat construction, we rely
primarily on staples that are supplied by air powered staple guns for
laminating pressure together with various simple mechanical clamping
mechanisms. When warranted, we use vacuum bag pressure against a
mold that can provide up to 13 psi laminating pressure at nominal costs
for equipment and tooling. Vacuum bag pressure techniques are
espe-cially effective with low volume production which is so typical of the
marine industry.
Economics and Manufacturing Methods
Labor continues to be the largest single cost element in boat
con-struction no matter what material is used. Wood has always been a
labor intensive material, and our efforts to reduce labor hours with
our wood-resin composite approach have only met with limited success.
We are presently cost competitive with any other construction material
1The term "perfect bond" indicates that a bonded joint tested to
des-truction will have a failure mode within the wood fiber itself rather
than the resin bond, i.e. the bond is stronger than the grain strength
in the custom or semicustom range; and in some sizes and types of
boats, we seem to have a definite cost advantage. In production
quan-tities, G.R.P. construction with its use of a female mold has an
obvious advantage over any competitive method that we can presently
offer. With small boats, however, this advantage is mainly in reduced
finishing time (a molded gel coat finish in comparison to a hand
applied finish). We and others have laid up small boat
hulls (up to
20 feet in length) using male molds with vacuum bag pressure in labor hours that approach being competitive with a G.R.P. layup of the same
size. We are presently experimenting
with female molds in a program to develop large wood-composite windmill
blades with the goal of
devel-oping a molded gel coat type finish. If this program
proves successful,
we may be able to use the same technique with
wood-composite boat
hulls to lower expensive finishing costs. Methods of Hull Lamination
In our own boatbuilding practice, we have used three basic methods
of laminating hulls with the goal of providing the best product at the
least amount of cost to the customer. We have titled these three
methods the "mold method," the
"strip plank method," and the "stringer
frame method." Each method can be varied to provide a great deal of
flexibility. Each of the methods also has advantages and disadvantages
for a given boatbuilding situation which we will discuss
briefly.
A male mold or a plug is a form over which wood veneer can be
laminated into the shape of the hull desired.
The mold merely serves
as a base upon which pressure
can be exerted to facilitate a bond
between wood laminates until the adhesive element
cures. Usually,
either staples or a vacuum bag is used to apply this pressure.
When
staples are used for pressure, a very simple riband-type mold can be
used where perhaps only 50% of the mold surface is
solid. With the
vacuum bag process, the mold must be both solid and air tight to
create a vacuum; and, of course, this more thorough type of
mold is more expensive to produce.
The biggest advantage of
the mold method is reproducibility; you
can make any number of identical hulls from a single mold.
An adequate
mold can be built with minimal cost in man hours and materials
allow-ing economical low unit cost for toolallow-ing. Even a sophisticated mold
for a vacuum bag pressure system would only
cost at most a quarter of
that tooling needed for similar size G.R.P. hull
production.
18
The disadvantage of the mold method is that even a simple mold
presents an expense that may be difficult to justify if only one hull
is to be built. The larger the boat, the bigger this problem becomes.
Thus, the strip plank method of construction was a natural evolution of the mold method where the mold, in effect, becomes a permanent part
of the hull. Most molds are stripped planked anyway; and with a little
alteration, we found that it was easy to allow the strip planking to
become a permanent part of the structure rather than remain on the
mold frames. Besides the obvious efficiency in savings of labor and
materials, a further benefit is derived with this method with the
capability to install all bulkheads, frames, and other interior items
directly in the setup stage rather than having to laboriously install
these items later into an already completed hull which is necessary
with the mold method. We also found that this method is structurally
efficient with the strip planking providing the longitudinal fiber in
a hull scantling system. Subsequent layers of veneer are then bonded
on the exterior strip plank surface to provide both diagonal and
athwartship strength and stiffness. Well placed bulkheads and
inte-rior appointments are used to further strengthen and stiffen the hull
laminate. Thus, we are able to efficiently produce an exceptionally
rigid and strong monocoque hull structure and, at the same time,
pro-duce a smooth interior that is almost devoid of the normal interior
framework associated with wooden boats.
We usually choose low-density wood for our strip planking so that
we can achieve the thickest hull skins at the least amount of weight
to provide the maximum panel stiffness to further reduce the need for
any supporting framework on the interior. A side benefit of the
extraordinarily thick hull skins has been the very good insulating
and sound deadening qualities which are desireable for boats cruising
in northern climates. Unfortunately, there is a minimum hull skin
thickness of around 7/8 of an inch in which a hull can practically be
built with this method. This is due to the fact that it is
impracti-cal to strip plank material less than 1/2 inch thick. In addition,
to eliminate the normal interior frame structure, we feel that a
mini-mum of three laminations of 1/8 inch veneer are needed over
the
exterior surface of the strip planking. This results in the minimum
7/8 inch thick hull skin panel which will weigh approximately 2 pounds
per square foot. This panel weight limits the practicality of the
some heavier displacement cruising boats in the mid-20 foot range
built with this method.
Some of the more successful boats with this method are the Ron Holland designed Golden Dazy, a Canada's cup winner, and the Britain Chance designed centerboarder Bay Bea, a U.S. Admiral's cup contender.
The stringer frame method of laminating is probably the most widely used method of building a custom one-off laminated hull. The
basic procedure is to erect frames at a given interval (some may be only temporary) which are then covered with longitudinal stringers in
sufficient numbers to serve as a beginning molding surface. Like the
strip plank method, the stringer frame method eliminates the need for
a mold. It also has the advantage that you- can install interior
members such as bulkheads and permanent frames during the setup. The
biggest advantage of the stringer frame method is that you can use it
successfully with just about any size and type of boat. The stringer
frame method has become very popular because it has the advantage of requiring the least amount of hours to produce a hull for a custom
one-off boat. This method also has the potential to produce the best
strength and stiffness-to-weight ratio hulls, particularly in situa-tions where there is little compound curvature such as exists in
cata-maran or tricata-maran type hulls.
The stringer frame method also has its share of disadvantages. The main problem is that the builder must begin laminating the hull
skins over what in reality is an inadequate mold. A good deal of
skill on the part of the builder is necessary to overcome this initial
obstacle when applying the first two
laminations
of veneer. With thelongitudinal stringers set at 5 to 8 inch intervals, the first
lamina-tions must be installed with great care to insure a fair molded
sur-face. It is only after the first two laminations are bonded in place
that a good solid mold surface exists over which normal molding can
take place. A further disadvantage of the stringer frame method is
that it results in a cluttered interior. Unlike the mold or strip
plank methods which result in smooth interior walls, the stringers and frames inherent with this method not only take u valuable inter-ior room but are also more difficult to keep clean.
The concept of the load-bearing skin, well supported by a string-er frame system, was first developed by the aircraft industry. By substituting load-bearing skin panels in place of fabric, aircraft designers found that they could build substantially better strength-to-weight structures which became a significant factor in the
20
ment of modern aircraft. With the development of plywood, the marine
industry borrowed the load-bearing skin concept and, with a few modi-fications, were able to construct light-weight hull and deck
struc-tures. Because boats use water as their medium rather than air and
because hulls sink when punctured, thicker skins are usually necessary as a practical measure in relationship to the supporting framework that
are common in normal aircraft construction scantling systems. Even so,
skin thicknesses on boats built with the stringer frame system are usually much less when compared to the skin thicknesses necessary with
either the mold or the strip plank method. The basic concept behind
both the mold and strip plank method is to produce a total monocoque
self-supporting load-bearing skin; the stringer frame supported skin,
however, is only a partial monocoque in that it can only be load
bear-ing when properly supported by framework.
With hulls that have significant amounts of compound curvature, we usually prefer a thick skinned total monocoque-type hull; but with hulls that have significant amounts of flat or straight runs, the
framework supported partial monocoque is prefered. Our decision
be-tween the two, other than for purely technical reasons, is usually
influenced by economics and the personal tast of the client.
Our basic laminating material has been 1/8 inch thick sliced
veneers. We have also used thinner plywood from 1/8 to 1/4 inch thick;
but structurally, plywood is not as efficient as veneer on a
weight-to-strength or stiffness basis. (Because of several extra glue lines
with-in a plywood lamwith-inate, it is usually considerably more dense than
veneer.) Plywood also costs at least twice as much as does veneer on a
per square basis. Sliced veneer should not be confused with the more
normal rotary cut veneer that is used in most plywood panels. Rotary
cut veneer is made by rotating a log in a lathe with a sharp knife,
then peeling off a thin layer of veneer from the rotating log. Sliced
veneers are made by first cutting the log into quarters, then slicing
off a given thickness of veneer by moving the quartered piece down upon a fixed blade, slicing off a veneer that usually measures from 8
to 15 inches in width and anywhere up to 17 feet in length. The
slic-ing process provides edge grain orientation within the veneer providslic-ing
a very stable and flat material which makes the laminating process much
easier, i.e. less pressure is needed to secure the veneer laminate in
proper position. One-eighth inch thickness is about the maximum that
Fortunately, the 1/8 inch thickness seems to be an ideal choice for
most laminating situations. Larger hulls can be laminated with thicker
stock, but the cost of sawing and the associated waste and material is usually not worth any benefit that can be gained (there is no waste in
the slicing process).
Material Cost for the Wood-Resin Composite
Contrary to popular belief, the cost of high-quality wood is quite low especially when compared to other efficient competitiors in the marine field such as aluminum or a cored GRP laminate. The cost of
wood, of course, varies by species, quality and dimensional size. One
of the more common species that we use is Douglas fir which can be purchased in sawn dimensional stock at approximately 40¢ per pound or
slightly less than one dollar per board foot. Sliced and trimmed 1/8
inch Douglas fir veneers ready to use would cost approximately 80¢ per
pound or 300 per square foot. The price of our resin is at present
$1.80 per pound; and assuming that it is used in a ratio of 207 resin to 807 wood fiber to form a composite, the resultant per pound cost of one dollar is lower than the cost of aluminum or a foam cored CRP
lami-nate on a weight basis. To be added to this is always the possibility
that on a per pound basis wood may be the more efficient material for a given application, saving some weight and cost of material in the
process.
The most interesting aspect of wood is its future cost potential. Over the past five year period, the price increases on top quality
(clear) Douglas fir have been considerably less than the inflationary
rate. This in part is due to the fact that very low levels of energy
are needed to turn a tree into usable stock (veneer or dimensional
boards). In comparison, many materials such as aluminum require high
levels of energy to produce and have increased in price at a much
higher rate. Figure ZI:shows the relative amounts of energies required
to produce all of the common materials while Figure IV shows the struc-tural efficiency of various materials in terms of the energy needed to
manufacture them. In both cases, wood is by far the most efficient
material per unit of energy required to produce. We feel that as
energy costs continue to escalate in proportion to other costs, the price of wood will decrease in relationship to the other materials
because of its minimal dependence on energy. Rising energy costs should
further favor the use of wood by putting a premium on lightweight
con-struction.
2 2
It is difficult to predict the future of the wood-resin composite. We feel that its use in custom and semicustom boat construction will continue its present rapid increase in popularity, but its future as
a production material is less clear. As we have discussed, the rising
cost of energy will have a positive effect but will probably never be
enough to overcome present high labor costs. An improvement in
pre-sent technology will be needed to make the wood-resin composite an
economically competitive material in volume production. Over the past
30 years, GRP technology has developed to its present fine art. In
comparison, wood-resin composite manufacturing technology is quite new;
and there is a great deal yet to be learned. The next five to ten
year period will see a period of intense activity by ourselves and
others to develop better manufacturing techniques to solve this last
major problem with wood. For sure, we can safely predict that we will
all see a steady increase in the use of wood in the marine field in
the years to come.
Figure II. HUMIDITY CHART Cr)
70° F
0 10 20 30 40 50 60 70 80 90 100Material
FIGURE
APPROXIMATE ENERGIES REQUIRED TO PRODUCE VARIOUS MATERIALS n = energy to manufacture Oil equivalent
Joules x 109 per ton tons
Steel(mild) 60 1.5
Titanium
800 20 Aluminium 250 6 Glass 24 0.6 Brick 6 0.15 Concrete 4.0 0.1 Carbon-fibre composite 4,0.00 100 Wood(spruce) 1.0 0.025 Polyethylene 45 1.1Note: Al! these values are very rough and no doubt controversial;
but I think that they are in the right region. The value
given for carbon-fibre composities is admittedly a guess;
but it is A
guess founded upon manyyears
of experience indeveloping similar fibres.
FIGURE
IV.
THE STRUCTURAL EFFICIENCY OF VARIOUS MATERIALS IN TERMS OF THE ENERGY NEEDED TO MAKE THEM
(These figures are based on mild
steel
as unity. Theyare only very
approximate.)Taken from the book "Structures"
by J. E. Gordon.
23
Energy needed to endure a Energy needed to produce given stiffness in the a panel of given
Material structure as a whole compressive strength
Steel 1 1 Titanium 13 9 Aluminium 4 2 Brick 0.4 0.1 Concrete 0.3 0.05 Wood 0.02 0.002 Carbon-fibre composite 17 17.0
'Results of tests on small, clear straight grained specimens. tvaiues In the first line for each species are from tests of green material; those in the second line are adjusted to 12% moisture content.) Specific gravity is based on weight when oven dry and volumewhen green, or at
12% moisture content.
'We and others use these species with WEST System products.
'Modulus of elasticity measured from a simply supported, center loaded beam, on a span depth ratio of 14 to 1. The modulus can be corrected for the effect of shear deflection by increasing it 10%.
Yiecnanicai
pro,)erties1 of
selected woods''Impact
Static bending Compression Compression Shear Tentjan Side bending
parallel to perpendicular parallel to perpendicular hardness height of Modulus Modulus Work to grain-maximum to grain-fiber grain-maximum to grain-max- load per- drop
caus-of of maximum crushing stress at pro- shearing lmum tensile pendicular Ins complete rupture elasticity' load. strength portional limit suength strenuth to grain failure
Specific Million Pounds per
-Species gravity Psi 1,51 cubic Inch Psi Psi Psi Psi Pounds Inches
Ash, white .55 9,600 1.44 16.6 3,990 670 1,380 590 960 38 .60 15,400 1.74 174 7,410 1,160 1,950 940 1,320 43 Balsa, medium .17 2,900 .58
-
1,805 100 300 118 100 Birch, yelow 55 8,300 1.50 16.1 3,380 430 1,110 430 780 48 .62 16,600 2.01 20.8 8,170 970 1,880 920 1,260 55 Cedar, Alaskan .42 6,400 1.14 9.2 3,050 350 840 330 440 27 .44 11,100 1.42 10.4 6,310 620 1,130 360 580 29Cedar, Northern white .29 4,200 .64 5.7 1,990 230 620 240 230 15
.31 6,500 .80 4.8 3,960 310 850 240 320 12
Cedar, Port Orford .39 6,600 1.30 7.4 3,140 300 840 180 380 21
.43 12,700 1.70 9.1 6,250 720 1,370 400 630 28
Cedar, western red .31 5,200 .94 5.0 2,770 240 770 230 260 17
.32 7,500 1.11 5.8 4,560 460 990 220 350 17
Douglas fir, coast .45 7,700 1.56 7.6 3,780 380 900 300 500 26
.4s 12,400 1.95 9.9 7,240 800 1,130 340 710 31
Hickory .64 11,000 1.57 23.7 4,580 840 1,520 NA NA 74
.72 20,000 2.16 25.8 9,210 1,760 2,430 NA NA 67
Lauan, light red .41 7,500 1.44 NA 3,750 NA 840. NA 500 NA
.44 11,300 1.67 NA 5,750 NA 1,090 NA 590 NA
Mahogany, Honduras .45 9,300 1.28 9.6 4,510 NA 1,310 NA 700 NA
11,600 1.51 7.9 6,630 NA 1290 NA 81-0 NA
Meranti, dark red .43 8,600 1.50 8.9 4,450 NA NA NA 560 NA
12,100 1.63 11.7 6,970 NA NA NA 630 NA Okoumet Gaboon .37 7,300 1.14 3,900 NA NA NA 380 NA Pine, loblolly .47 7,300 1.40 8.2 3,510 390 860 260 450 30 .51 12,800 1.79 10.4 7,130 790 1,390 470 690 30 Pine, longleaf .54 8,500 1.59 8.9 4,320 480 1,040 330 590 35 .59 14,500 2.98 11.8 8,470 960 1,510 470 870 34 Pine, white .34 4,900 .99 5.2 2,440 220 680 250 290 17 .35 8,600 1.24 6.8 4,800 440 900 310 380 18 i/amin .59 9,800 1.57 9.0 5,395 NA 994 640 NA NA 18,400 2.17 17.0 10,080 1,514 1,300 NA Spruce, black .38 5,400 1.06 7.4 2,570 140 660 100 370 24 .40 10,300 1.53 10.5 5,320 530 1,030 520 23 Spruce, Sitka .37 5,700 1.23 6.3 2,670 280 760 250 350 24 .40 10,200 1.57 9.4 5,610 580 1,150 370 510 25 Teak .57 11,000 1.51 10.8 5,470 1,290 1,070 .63 12,800 1.59 10.1 7,110 1,480 2,030
1. A simple riband mold is used to produce a 30 foot hull laminate which
consists of 6 layers of 1/8 inch thick veneer.
The 1/8 inch thick veneers were held in place with staples until a
resin cure could take place. After the first two layers were bonded,
the alloy staples were left in place on subsequent layers to save
time.
\ 20 1-0 civ\
3. A setup for the strip plank method incorporates
temporary frames that are later removed, but
per-manent bulkheads and frames can be installed in
the setup stage to save time.
4. Strip planking that is neither tapered nor beveled (to save time) is
temporarily fastened to the mold frames until the bonding adhesive
between the plank edges reaches a cure. The planks are also bonded
to any permanent members in the setup.
5. The first layer of 1/8 inch thick
veneer is applied at a diagonal angle over the strip planking.
6. The setup for the stringer frame method also
uses temporary mold
frames, but bulkheads, frames and other interior items can be
effic-iently installed at this point.
28
12o y\av\,,
7. The first layer of veneer is very carefully
in-stalled over the stringers. The pads serve to
help line up the veneer edges between stringers
where they are not supported.
8. The completed hull is being rolled over. Note the heavy resin
coat-ing of all the interior parts.
SAFETY FOR SMALL CRAFT
David D. Beach
Staff Naval Architect
National Association of Marine Mfg. Chicago, U.S.A.
The invitation to present this paper was made earlier this year when
I was in the midst of a number of projects that were then taking
travel and drawing board work. When I could accept the invitation,
it was, I presumed, because of my commentaries and discussion over
the years in a number of meetings of the International Council of
Marine Industry Associations (ICOMIA) where the subjects have been
safety oriented. Participation in the early development of the
ICOMIA Safety and Quality Standards, and speaking out on the general subject of product liability law suits made your committee aware, no
doubt, of the background I possessed in these matters. So, I
pre-sented a draft outline which said that I'd discuss those areas of
small craft design where "unsafe" conditions could be designed into,
or built into small boats. This was accepted, and the formal
prepa-ration of the paper started.
However, before I get into anything that has to do with this general
subject, there's a couple of points I'd like to make. One is that
I'll assume the professional designers and builders here are fully
cognizant of the fact that there are voluminous recommended practices,
voluntary standards and mandatory regulations in many countries. These relate to "safe" design and construction practices for yachts.
30
The other is that I do not intend to dwell on the details of those
standards, their good points or their shortcomings. What I hope to
do is to present a discussion of some of the boat accident cases in which I've been involved, and to present my thoughts and observations
thereon. My observations are likely to be somewhat philosophical and
will, I hope, serve to cause some reflective thinking by designers as
to the extent of their responsibility.
Before lecturing on a general topic of this nature one must first
look at several definitions which will prescribe the scope of the presentation.
First the definitions:
1. "Small Craft." We will arbitrarily limit this
discus-sion to craft which fall generally into that rather
vague classification of smaller "Yachts" and I will,
also arbitrarily, discuss boats in the general length
range of 9 to 18 Meters (30' - 60'), give or take a
little bit. Because I am primarily power boat oriented,
I assume from the beginning that this paper will be
similarly oriented. I shall bring into the examples
such sailing craft experience as I have, or am aware of.
"Safety." I like the dictionary definition that says it
is freedom of danger or risk. So we shall discuss design
and production practices that insure freedom from danger
or risk, and illustrate these by examples of practices
that do produce danger and risk. A short statement might
safety conscious and knowledgable can produce a yacht
design in which there is freedom from danger and risk, or is a safe boat.
3. "Seaworthy." I'll throw this word in, as it appears
frequently in the courts. A seaworthy boat is one that
is suitable for a specific voyage with respect to its
construction, its machinery and equipment, its outfit
and its crew. A craft may be seaworthy for one trip or
outing, but can well be unseaworthy for a more extended
trip, outing or passage on other waters. All safety
considerations must reflect intended or anticipated use!
Now, putting these three definitions together, an expanded title for this lecture would be approximately as follows: "The design and
con-struction of pleasure boats between 9 and 18 Meters in length which
are free of danger and risk when used in the manner intended."
Next, it seems to this designer that there are a number of specific parts, or areas, of yacht design where attention should be directed
to design and construction practices commonly seen.
I would classify
them as to whether the design, by itself, produced an unsafe or
un-seaworthy craft and as to whether the specifications and construction were wanting as to the production of a safe and seaworthy craft.
In the area of basic design I would place the fundamental Naval
Archi-tectural matters of stability, trim changes underway,
spray
suppres-sion and dryness of deck which result from form
matters, maneuvering,
seakindliness of easy motions in a seaway, and the like.
Together
with these, I would lump in such arrangement items as would limit
helm
32
visibility, make access and egress awkward, or put the passengers
and/or crew in precarious situations in the normal use of the craft.
Perhaps in this category I would include subdivision and
compartmen-tation and the calculations to include flocompartmen-tation volumes. Although,
as to this, I think that the number of floodable length curves I've
seen for boats and yachts under 18M. could be counted on the fingers
of my hands. That would include only two that I've done in thirty
odd years of practice.
In the area of specifications and construction we would place the
matters of strength and integrity of the hull and structure, the
strength and operational control of all mechanical systems, the
mini-mizing of potential fire and shock hazards in the electrical systems
and the integrity of, and provision for maintenance of, the fuel
sys-tem including the provisions for vapor and fume control. There are
others.
Now, in the matter of design factors which involve the fundamentals
of yacht naval architecture, I consider that these have long been
well explored and quantified in the classrooms and in the available
texts. The trained professional, with an appropriate design office
apprenticeship under the careful eye of an "old timer" is aware of
these matters, and they become as a second nature to him. He
automa-tically considers the options he has, and the effect of changes in
one parameter on another characteristic. Or at least that's the way
it's supposed to be. These basic considerations will, in this
dis-cussion of safety, not be explored further except in several examples
Before discussing problems resulting from failure to consider basic principles of Naval Architecture let me elaborate on what I observe
to be a facet of the U.S. scene. I have no idea as to whether or not
this observation is applicable to this side of the Atlantic or to the
Far East. However, from some of the things I've seen at European
boat shows I'm strongly inclined tc believe it's a universal situation.
We all know that production of boats and yachts, in America as well
as elsewhere, has expanded considerably in the last decade or so.
A
substantial portion of this growth has been in the new products of
small boat builders who have expanded their product line into bigger
boats. A number of these small builders
do not either retain outside
competent consultant designers or have in-house technical expertise.
A result of this is that an occassional unsafe design reaches the
market place. An example of this situation
follows.
Case One:
A manufacturer of one boat type produced a trailerable boat of that
same generic type, but of a new and unconventional
appearance
arrange-ment. In the U.S. a trailerable cruiser is mited to
an overall beam
of 8' - 0" (2.43 meters approximately), that limitation
being the
width of a vehicle which can be trailed on the highways
behind a motor
car. This 8' 0" wide boat was water tested, we assume,
with reason-able loads, but perhaps it was not anticipated
that high speed turns
would be attempted with a high concentration of
passengers sitting on
the cabin top at the topside steering station.
Such a turn was
attempted, with such a loading, and the results were as could have
been predicted. The high center of gravity
combined with low
34
verse stability and good speed rolled the boat outboard in the turn, resulting in substantial loss of life.
In this case, it was brought out, in the reports of the investigations
and in the newspaper stories on the subsequent law suit that no boat
designer or builder there employed was fully aware of the relations
between centrifugal force, dynamic or static stability and rudder
induced loads that could produce heel. Available text books explain
these relationships, but the builders had not read the books!
This case illustrates the point made a few paragraphs earlier. It is
not uncommon for the expanding production building firm to consider
their in-house employees can develop larger craft from their
exper-tise from smaller ones. It behooves the management to question the
technical competence of the staff when new product types are
contem-plated. This may not be done when the organization is small and the
general manager may be the owner-president who is doing the
"design-ing."
Rule One From Case One. "If you've never done it before, make sure
you're right, even if your ego must suffer by hiring a qualified and
proven expert to review the decisions and data that establish the new
boat design features."
What would the consultant have done in such an instance? It seems to
this designer, that if the preliminary drawings of the proposed
pro-duction product were presented to me for analysis and comments, I
would have done several things.
realize that always our hindsight is better than our foresight, so what we mention here is based on an analysis of what should have been
done initially. We have learned, in the courts, that it is very easy
for the plaintiffs attorneys to state that the builder was negligent
for not anticipating all that might be wrong. And, in the courts,
we have experienced the extreme difficulty of explaining why things
were not done. So, the consultant, who has been through this legal
routine, becomes very much worth his fee. He has learned, through
the hard method of experience, how to do the following:
Anticipate the possible use of the boat in all
likely conditions. At the same time certain
misuses are fairly obvious. The consultant
cannot consider the normal use of a boat. He
must consider all the possible things the
inex-perienced operator or crewman might do. Some
of these misuses of the boat might be said
to be foolhardy---and some damnfoolhardy. And
finally he must be aware of the fact that the damn fools get smarter every year and can think up new ways to misuse a boat.
Consider the basic questions of preliminary
design; weight, weight disposition, and the resultant trim attitude and static stability
curve. Review any arithmetic done by the builder,
if any was done.
Consider attainable speed, using at least sim-ple weight/horsepower ratios for planing boats,
36
and some more sophisticated methods for higher
speed displacement craft. Use the Barnaby/Birt
tables as a basic minimum, but do estimate speed
to be reasonably assured. With the steering
con-figuration, heeling movements can be estimated, and with the speed and estimated turning radius, a resultant roll angle can be predicted.
While nothing here is precisely calcuable, it is
sufficiently in the ball park to point to poten-tially unsafe conditions.
Now, what would be the options of the boat builder, had he been aware of the potentially hazardous condition an imprudent or uninformed
operator might permit to exist? The builder has at least two easy
options to prevent such an accident as has been outlined above, and
one more severe.
1. He can by design restrict the areas to which passengers
have easy access. Here he could set the rails in from
the deck edge, rake them inboard, or cut off the bow portion by a rails set athwartship some distance aft
of the stem.
Post placards and notices that would limit the occupacy of the topside areas to not more than a set number of
persons.
This second option is based on a principal of American
law relating to product design. That principal states
which may produce hazard or risk, and if that hazard
or risk is not obvious to the user of the product,
then the manufacturers and seller of the product has
a duty to warn the user as to it.
These placards, or notices and paragraphs in the owners instructions do not remove the hazard, or make the boat any more inherently safe, but they do reduce the liklihood of any unpleasant events which involve
the unsafe characteristic.
3 Take the whole project back to the drawing board.
Now you know the likely speed, and some basic work
can suggest a different beam perhaps, a hardened chine, a different vertical weight distribution, maybe a
dif-ferent keel/skeg configuration or a new rudder shape
to change the rudder induced forces. The combination
of all three will certainly minimize the liklihood
of a capsize.
This example was selected because of the increasing number of smaller
yachts which are incorporating topside steering stations and flying
bridge configurations that include lounge and/or seating areas. The
boat manufacturer should be aware of their potentially unsafe design
and that such craft may be unseaworthy in some conditions of wind and
wave.
Case Two:
This is a similar case; in which a failure to consider basic naval
architectural principals is involved. The basic facts are simple.
38
The boat produced was one having a long foredeck and a steering
sta-tion well aft of amidships. This boat was involved in a fatal
col-lision with an anchored boat.
The boat, other than at rest, very low speeds or at full throttle,
ran with a very high bow-up attitude. The helmsman could not see
the horizon over the bow. The occupants of the stricken craft saw
the approaching craft several hundred meters away, but could not make
themselves seen to the operator of that craft. They jumped
over-board at the last second, but two died regardless.
An analysis of the craft, which the estates of the deceased alleged was a faulty design showed a number of interesting facts. The center
of gravity of loaded boat was about 30% of the length of the bottom
from the transom, and the center of the chine planform was only a bit
aft of amidships (narrowing chine line aft), and there was a high
deadrise prismatic bottom from approximately amidships to the transom.
I daresay that there are at least ten recent texts, technical
trans-action papers and research reports from the model basins that
demon-strate and illudemon-strate this highly undersirable combination of design
features. Yet the pre-trial depositions brought out the fact that
the boat was developed from a boat 20% shorter in length and with a
different arrangement and load distribution. The plug, or mold
pat-tern, was produced from the plug of the earlier boat, and extended
aft almost five feet (1.52 M) with the lines running in the same way
the smaller boat was molded. Nothing was put on paper first! The
prototype was tested for top performance speed primarily and for
throttle were not even recorded. The catalog went to press with a photograph of the beached boat at a shore picnic and another showing
the boat running nicely at about 30 miles per hour (50 K.P.H.) with
a pretty water skier astern. Nothing showed the 7.5 degree bow-high
attitude in the hump speed range.
The boat in this case was a "safe" boat, insofar as it's occupants were concerned, but could hardly be called "safe" when it was
con-sidered in relationship with other boats on the same waters.
Using this example, what could have been said which would have been of value to the management of the company, that lost the 1.8 million
dollar lawsuit for wrongful death? I think that the Rule of Case One,
stated previously is entirely applicable here. Also, I think that
there's some additional comment about the need for critical analysis
of complete test results before releasing a prototype to production,
and immediately producing three or four boats a week.
Certainly the boat should be tested for struc-tural adequacy at the worst impact loadings
which would come from top speed operation. At
least twenty five hours in sea states greater
than 2 would be minimal. (I know of one
alumi-num small boat builder who tested runabouts for a full 100 operating hours at the top speed his
operators could sustain!)
In these days of rising fuel costs, with the
U.S. consumer costs approaching those in effect
in Continental Europe, it would seem necessary
40
to record (for the owners manual, at least)
the fuel use rates over the complete throttle
range. While making those measurements, ample
time exists to make others, and to observe the
operational characteristics of the boat.
Measure-ments of trim angle, noise level, turning radius
and the like are all important for the designers
future use. They are also important to show the
suitability of that prototype for the likely uses
to which it will be put. These measurements
would have been the basis for determining that
the whole project should have gone back to the drawing board, or substantial changes should have
been made before turning the project loose to
production.
It was no defense in this case that the owner
operator of the boat could have corrected the
bad trim angle of the boat by buying and
instal-ling a set of adjustable trim tabs. The plaintiff
used this in his allegations of negligent design
against the boat builder and also for his claims
of negligent operation against the owner operator.
Let us spend the remainder of the time here alloted and talk about
safe design that is not fundamental Naval Architecture. Let us look
at some cases where other steps in the design process produced
