Anyone
who has ever seen airframe construction, particularly jet aircraft,
understands why aircraft can be built with skins that are extremely
thin. And while an aircraft isn't subjected to the same type of
forces as a boat hull, the fuselage is the hull and must be strong
in different ways. Rather than being framed, one could correctly
say that an airframe is corrugated, for that's exactly what it is.
The skin can be extremely thin because the frames are so close together.
Boat hulls, of course, are not built that way,
although they could be. Wooden canoes or clinker construction is
similar. Instead, modern fiberglass boat hulls relay on a limited
number of major girders and frames. Girders, or stringers as they're
called in yacht construction, serve a dual purpose of both supporting
the bottom and providing longitudinal rigidity to the hull. Frames
provide lateral support but very limited transverse stability so
that they have only one purpose and that is to support the bottom.
It is very helpful to think of a boat bottom as
an upside down bridge. The main difference is that bridges are not
subjected to any force from the under side. But boat hulls are subjected
to forces from both sides. It is also helpful to think of a boat
hull not as a continuous, single skin, but as being made of panels
that span the stringers and frames. In hull design terms, the span
between supports are referred to as panels. These are the unsupported
distances between supports.
When designing a hull, it is the thickness and
strength of the unsupported panel to resist bending forces that
is of critical importance, precisely because the panel is not supported.
Our previous discussion talked about the differences between flexible
and rigid hulls. The amount of flexibility of the hull panel is
dependent on frame spacing and strength. For the purposes of this
discussion, we'll assume that the framing system is completely rigid.
There are very few pleasure yachts built in which
the framing is so close and the panels so thick that some bending
does not take place under heavy load conditions. In fact, flat fiberglass
panels have a high modulus of elasticity, meaning that they can
bend a lot without damage to the panel. This is one of the features
of reinforced plastic that makes it so forgiving. But that forgivingness
induces the tendency for designers to stretch things a little too
far in terms of what they can get away with. If that weren't true,
we wouldn't have so many boat hulls with structural failures.
Fiberglass laminates, because they're not a rigidly
controlled, machine made substance, are subject to human error and
variance in their uniformity. Neither the thickness nor the quality
of the lamination are subject to much control. This means that while
the same laminating schedule may be maintained throughout a model
line, the resultant strength of fiberglass hulls can vary widely
from boat to boat. Tests have shown that laminate strength on nominally
"good" laminates can easily vary by +/-33%. By "good"
it is meant that the laminate has no major defects, but rather simply
variance in resin/glass ratios. This also explains why one of an
apparently same series of hulls fails while others don't. All surveyors
who are serious students of hull failures have encountered this
anomaly that often seems to defy explanation.
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| Example of hard spot caused by improper
stringer design and installation. Bottom hinges around hard
edge of stringer wood core. At right, wood core is elevated
by a soft material so that it does not touch the hull skin
and the load is bourn by the more flexible tabbing. |
When investigating bottom panel failure, it is
economically unfeasible to attempt to evaluate the strength of a
large panel, particularly one without a uniform shape. To do so,
every square inch of the panel would have to be analyzed. However,
if we could, there can be no doubt but that we'd find all sorts
of imperfections and defects. To illustrate an extreme example,
in one case I found two candy bar wrappers laminated into a hull.
Accumulated saw dust caused by the lay-up shop being located near
the carpenter shop is yet another, not to mention the fact that
lamination is a sloppy, dangerous job that often entails a very
high turnover rate in workers that makes training very difficult
and costly.
The point here is that final product values usually
end up considerably below design strength unless the designer leaves
a healthy margin of error. Assuming of course, that a degreed engineer
is involved which, in many cases there isn't. So what we end up
with is many builders who utilize trial and error and experience
as a means of determining the lay-up schedule. The conscientious
builder will usually slightly over build, while the profit-minded
builder will skimp where ever possible. And here is where the problems
begin.
Panel or Laminate Failure There are three
primary causes of panel failure: inadequate design strength or thickness,
design shape error, and lay-up faults. These can be stand alone
problems, or may appear in any combination of the three, including
all together.
Assuming that a hull is properly framed out, and
that the laminate does not have serious imperfections, panel damage
and failure can occur when the panel is too thin. While fiberglass
is flexible, there are limits on how much it can bend before structural
deformation causes the plastic to start disbonding or shattering.
Bending, as we know, causes tension on one side of the laminate
and compression on the other. Compression causes the plastic to
crumble around the glass fibers. Tension causes interlaminar sheer
that works to separate the plastic from the fibers, or ply from
ply.
Now, in a typical laminate, particularly one using
weaves, we have fibers running in all directions. On the tension
side, the fibers prevent the plastic from deforming up to the limit
of the strength of the fibers. When the stress exceeds the strength
of the bond of the plastic to fibers, these fibers then pull loose.
When the bending is repeated hundreds or thousands of cycles, this
process then results in significant weakening of the panel. You
can't see this damage, but it is there. This weakening becomes progressive
and so the panel starts bending more and more. Eventually, stress
cracks begin to appear, usually first on the exterior, but also
on the interior particularly, if the inside of the hull is gelcoated
or painted so as to show up the cracks.
Hard Spots or Hinge Effect If this condition
continues long enough unchecked, it can eventually result in fatigue
failure of the panel. In the real world, this description of factors
is rarely this simple. All sorts of design defects and other faults
may exist to compound the situation.
Simple panel failure caused by inadequate thickness
is both common and easy to detect. Panel failure unrelated to any
other factors always occur near the center of the panel, or the
periphery of the dimple caused by deflection. That is because the
center is the area least supported by frames.
Panel failures that occur close to, or exactly
at the intersect of a frame (here a frame is meant to be any structural
member), then there is a contributory cause. This is known as the
"hard spot" or "hinge effect." Obviously, were
the panel thick enough, no hinging would take place, so hinging
is always compounded by other faults.
When a panel bends, at some point near a frame
that bending is going to be resisted by the frame. If the bending
occurs exactly at the intersect of the panel and frame, there exists
at this point an abrupt resistance to bending. This sudden resistance
causes a bend to occur with a very short radius, and it is the radius
of the bend that has everything to do with how much bending can
occur without damage or failure.
The shorter the radius of the bend, the greater
the compression and tension load, the sooner structural deformation
begins. This is why frames and bulkheads should never make sharp
intersects with either bottom or side panels. Short radius bends
are prevented by adding bosses or fillets in way of the panel/frame
intersect. This spreads out the load over a wider area, increases
the radius and reduces compression/tension loading. As previously
mentioned, panel defection in itself is not a bad thing.
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Stress cracking is one of the visible effects of
panel deflection. In this case, the panel was dimpled or "oil
canning" as the eliptical array of the stress cracks
indicate. The number of cracks tends to indicate the severity
of the deflection or bending. |
This short radius or sudden change in direction
is what is referred to as hard spots or hinge effect. It means the
panel is bending sharply around the frame or anything else inside
the hull that is rigid such as a deck support post or a fuel tank
bed in contact with the hull.
Stress Cracks are the warning signal that
a panel is bending beyond the limits of its strength. Stress cracks
can appear either as a result of a one-time event such as slamming
hard off of a wave, or it can be the result of repetitive stress
cycles. This is one of the things that makes the evaluation of stress
cracking so difficult. My 30 years of experience suggests that fairly
large numbers of boats sustain single incident stress cracking with
no evidence that the damage becomes progressive. On the other hand,
this can be extremely hard to know with any certainty because no
one has the ability to follow the life history of a boat. Yet, because
the surveyor encounters so many boats with stress cracks on the
bottom, it is the surveyor's task to make that evaluation.
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| Natural hinge points such as chine flats
and other angular surfaces require the build up of extra laminations
called fillets which add extra strength. Lacking these, bending
and stress cracking is likely to occur. |
Most stress cracking occurs as a result of repetitive
panel bending or hard spots caused by improper design of internal
components, combined with inadequate panel thickness. This type
of stress cracking is usually progressive because the bending is
not intermittent, but occurs nearly every time the vessel is used.
The great difficulty, and therefore the great danger to surveyors,
is that most boats get used more often in calm water conditions
so that a potentially dangerous condition can exist for years without
ever resulting in a failure. It can happen that a boat with a weak
bottom is used only in protected waters where pounding almost never
occurs. Then, suddenly, the boat is moved to another location where
the conditions are different, and now the hull is subject to frequent
slamming and stressing.
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The hinge effect, or stress cracking initiated by
hinging off of an internal structural such as a stringer,
produces parallel cracks such as these. The deposits made
by weepage of styrene based fluids indicates that the cracks
penetrate well beyond the gelcoat. |
Evaluating stress cracks is very difficult and
there are no clear-cut rules. Every case has to be evaluated independently.
Yet hard spots are easy to identify because the area of cracking
is usually small. It may show as a star burst pattern, as concentric
circles of cracks, or a short series of parallel, arcing cracks.
These are easily evaluated simply by going to that exact point on
the interior to see what is causing it, and then to recommend a
method of eliminating the hard spot. Moreover, reinforcing an apparently
weakened panel is usually not difficult to accomplish.
Cracking due to overall panel deflection
is also easy to detect, but much more difficult to evaluate. On
bottoms painted with anti-fouling paint, remember that the paint
is very brittle and does an excellent job in magnifying cracks.
Cracks will usually appear in the paint long before they will show
up in gelcoat which is usually a bit more pliable. By removing the
bottom paint, we can usually find out if the cracks also show up
in the gelcoat. Be careful not to obliterate cracks in gelcoat by
scraping as this is easy to do. If cracks don't show up in the gelcoat,
I'll use my finger and try to rub dirt over the surface to try to
get invisible cracks to show up, then wipe clean with a rag. If
the cracks are serious or old, they'll stand out. If nothing shows
up under the paint, it would be a fair assessment to assume that
the weakening is not serious.
The age of the boat plays a very important
role in evaluating the significance of stress cracking. This is
because older boats have been subjected vastly greater number of
stress cycles than newer boats. If cracks show up in the paint on
an older boat, but don't appear visible in the gelcoat, or are only
faintly visible, I usually dismiss them. If the condition has existed
for a long time and there's no evidence that it is highly progressive,
I feel safe with that judgment.
The prominence of cracks is another indicator
of their significance. When cracks initiate, they usually start
out as a very fine fissure. As cracks age, the very sharp edges
of the crack will erode over time. That means that the appearance
of the crack will be wider or more prominent. Cracks that stand
out prominently should be regarded as a red flag. Cracks that are
old and progressive will stand out clearly, even after you've scraped
bottom paint away. At this point, the cracks will appear as a clear
black lines. If bad enough, they will clearly reveal a fissure.
Remember that stress cracks that appear on the exterior bottom are
the result of the tension side of the bend since compression loading
tends not to produce cracks.
Examining the same area on the interior is likely
to tell us much more about the significance because the tension
loading may appear on this side also, depending on how the panel
is bending. However, this is only true if the interior is coated
with gelcoat or paint. If it is a raw laminate surface, stress cracking
may or may not show up, especially if the surface is very dirty
and permanently stained. Dirt and oil may work its way into the
stress cracks and completely obscure them, even if you wipe the
surface clean.
The number of parallel cracks is another
indicator of how serious the condition is. When there are 4 or more
parallel cracks, there is good reason to believe that panel bending
is going beyond load limits. But, again, we have to evaluate in
terms of age. If its a fairly new boat with a three or more parallel
cracks, odds are that this is a progressive condition that could
ultimately lead to panel failure.
Sail and power boats tend to exhibit different
cracking patterns. This is because sail boat bottoms are usually
curved while power boat panels tend to be flat. Flexing convex curves
result in the condition known as oil-canning which produces large
dimples that can reveal circular patterns of cracking, or cracking
that appears in a parallel series of arcs. This condition should
be considered as dangerous with a high potential for ultimate failure.
Power boat panel defection tends to parallel either
hull stringers, bottom strakes or bulkheads. It will only show up
as curving arcs if the panel defection is severe in conjunction
with oil-canning. This happens rarely, so when it does, beware that
the problem is very serious indeed. The most common cracking is
found inside the concave curve of a strake. This is because the
strake forms a natural hinge point, or a hard spot. Strakes that
are not filled and filleted almost invariably end up causing stress
cracking. This is easy to determine by looking to see if there is
a strake depression on the interior of the hull. If there is, the
strake has not been filled and filleted and is the cause of the
cracking.
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Stress cracking appearing transversely across a
bottom strake. In this case, it was caused by a 27' boat having
only one structural bulkhead, located in the wrong place and
no transverse framing. Torsional twisting of the hull caused
the cracking which is on the verge catestrophic failure. |
Improperly installed stringers often cause hard
spots, particularly in smaller boats, that results in cracking and
possible ultimate long term failure. This is usually very easy to
detect by sounding the bottom and locating the stringers. Again,
it is the age of the boat and the severity of the cracking that
determines its significance.
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These stress crack appearing on the interior bottom
under an engine were nicely shown up by black diesel oil via
the capillary effect. The 11 parallel cracks indicate that
the degree of panel bending is severe and the possibility
of failure must be considered. In this case, there were an
insufficient number of transverse frames. |
Delamination, contrary to what one might
expect, is not a common occurrence in conjunction with panel deflection
and stress cracking on solid laminates. In fact, it is very rare.
Out of 3600 surveys, I can't ever recall having found any. Panel
bending does not produce enough interlaminar shear to cause ply
separation unless the panel contains defects and the bending is
very serve. Still, its a good idea to tap around a bit when cracks
are visible. However, delamination is often found after complete
panel failure occurs - i.e. the panel splits open - but this happens
as a result of the final fracturing and stress initiated during
the final failure mode, so it is not wise to use the absence of
delamination as a positive evaluation factor.
Cored Bottoms are an altogether different
story. But then a cored bottom is a problem just waiting to happen
anyway. Coring a hull bottom is just plain foolish, no matter what
any builder or the glowing reports in the magazines may tell you.
Remember that these people get their income from advertising revenue
derived from the people who advance these materials. In other words,
they are biased. Core materials are simply too weak and hull bottoms
take too much of a beating for cores to survive. When we find cored
bottoms, the presence of stress cracking should be regarded with
the same reaction as to skin cancer. Horror! Here, stress cracking
raises the potential for water ingress into the core, with all the
attendant problems that poses, including the potential for delamination.
When cores are involved, ply separation or delamination is highly
likely. Consider the hull guilty until proven innocent.
We should be especially wary of sailboats with
cored bottoms. If you get sued after a sailboat core fills with
water or its keel falls off, take your lumps because you deserve
to get sued for not finding the problem. There is only one safe
way to handle a cored sailboat bottom, and that is to declaim all
knowledge of what is going on inside that core. You can't see it,
test it or know what is happening inside unless a failure is already
well advanced. Failures involving cored bottoms are legion. Even
worse, it can happen that there are no visible, outward signs of
trouble before failure occur. Failures can occur suddenly, and
without warning. Disavow all responsibility, in writing, in
detail, and all ability to determine the condition of the hull.
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To determine whether a hull is cored or not, look
for those areas on the interior hull where the core terminates.
In this photo, the core can clearly be seen standing out around
the bow of this yacht. |
Hull Sides Stress cracking on hull sides
is something that generally did not happen until the mid 1980's
when builders began skimping on hull side laminate. Hull sides have
gotten so flimsy in the last decade that its almost laughable if
the construction of some of these boats wasn't so pathetic. Hull
side cracking is a problem that shows up almost exclusively in low
to mid price range boats. The cheaper the boat, the more likely
you'll find it.
This is rarely a problem to the hull side itself.
The cracking usually occurs as a result of severe panting because
the sides are thin and unsupported. The sides themselves don't fail
because the panels are so large and the flexing occurs over such
a large area that the radius of bend is too large to cause damage
to the laminate strictly as a result of panting. However, because
the stress is transmitted vertically up the hull side, the forces
of interlaminar shear are very high. Therefore, delamination becomes
a distinct possibility. Nowhere is this more true that in vessels
that have been repowered with more powerful engines. In this case,
total hull side failures have been known to occur even with high
quality boats.
Hull side panting can be a problem because there
are other things attached to the hull sides. Panting of the sides
can cause disturbance not only to internal components, but also
to the hull/deck joint. A boat with floppy hull sides is not very
likely to have the deck bonded to the hull, but rather just be screwed
on. Panting hull sides almost invariably results in shearing of
the hull/deck joint and complete loosening of the fasteners. This
is why we find so many boats with the rub rails falling off. The
screws holding the rub rails on are set into the hull/deck joint
and the shearing load applied to these screws breaks them loose.
See Also SCREW IT!
Hull sides should be generally sounded (cored or
solid) and closely examined for stress cracking. Stress cracking
is most easily telegraphed through painted on boot stripes because
the paint is more brittle, so look for it there. Because side failures
are rare, the cracking needs to be evaluated in terms of the whole
structure. The cracking is more likely to be a sign of other problems.
Suspect bad deck joins and look for broken bulkhead tabbing. Also
watch out for through hull fittings that may be loosened or damaged
as a result of panting.
Liners Builders are always searching for
ways to reduce labor costs, and one of these is the use of interior
liners. Experienced surveyors are all too familiar with the problems
that liners present. First is that a liner tends to obscure all
internal structural members so that the surveyor cannot make an
evaluation of the hull structure. Secondly, liners tend to preclude
the use of proper bulkheads and frames because a liner can't be
placed where a structural bulkhead exists. Thirdly, the design of
the liner needs to substitute for the structural members is has
displaced. Fourth, the liner usually affords no access to examine
its structure and how it is attached.
Liners are most commonly used in boats up to about
32' but have been found in boats up to 42'. The larger the boat,
the greater the potential for trouble because of this tendency of
liners to displace or eliminate traditional framing methods. Boats
with liners over 30' are known to have a disproportionately larger
number of structural problems, a situation that is entirely predictable.
When liners displace bulkheads and frames, several
things happen. First, the hull becomes highly prone to twisting
or wracking. When bulkheads are eliminated, unsupported panel size
naturally increases. And when that happens, panel deflection and
failures increase. Because the liner usually covers up so much of
the interior, this makes the surveyors job doubly difficult and
exposes him to more risk of failure to locate serious problems.
Grid/Liners are a new development in which
the designer attempts to include all of the vessel's framing system
into a full, complete hull liner. So far, the use of grid liners
is limited to only a few builders of small boats, but the idea is
likely to spread because it presents the possibility of eliminating
all the difficult laminating detail work of bulkheads and stringers
inside the mold. With a grid liner, the detail work can be transferred
to a low profile mold on the shop floor that is more accessible
and easier to work.
While this may streamline production, this method
has a number of problems. One is that the liner has to be bonded
to the hull, and obviously the builder cannot laminate it to the
hull once it is set into place. The only solution, of course, is
to glue the liner into the hull. The problem with adhesives is that
they only work perfectly under perfect conditions, something we
don't see much of in boat building.
The only things that glue together well are parts
with identically uniform surfaces. For example, gluing two pieces
of wood together that are perfectly flat makes for a very strong
joint. But allow the slightest surface irregularity and the joint
becomes very weak. That's not just true of wood but any material.
Unfortunately, the interiors of laminated hulls can hardly be called
uniform. Will the grid/liners remain bonded to the hull? Only time
will tell. Our experience with bonding putty in cored hulls tells
us that there's not likely to be any better level of success in
this application than for foam cores. See related article Hi
Tech Materials.
Essentially what they are doing is spreading the
glue on the interior of the hull, and then dropping the liner in
and hoping that a complete bond takes place. The builder
will never know because he can't see the results. The bonding surface
is just as likely, indeed, probably more so, to be full of voids
or gaps where the two parts are not bonded together. And a void
in a glue joint or laminate is a stress initiator that propagates
delamination.
Sail boats utilize grid/liners more frequently.
Fortunately, a sailboat hull is considerable more amenable to this
design, both by its shape and the fact that they are not subjected
to the forces of high speed. Even so, one of the largest boats built
with a full grid/liner was a Hunter 60 that experienced total liner
disbonding and failure. Yet even their smaller models were widely
known for liner failures.
If this method gains wider acceptance, its going
to pose a whole new range of problems for surveyors.
Interior Effects of Weak Hulls When liners
are used, they either have to sit on top of stringers, the bottom,
or be suspended from the hull sides. In either case, the liner is
not completely isolated from the hull, and if the hull is experiencing
problems with excessive panel deflection or panting, that deflection
is most likely going to be transmitted to the liner in one way or
another.
In sailboats, liners are either tabbed, glued to
the hull or both. In powerboats, liners usually rest on top of stringers
and are usually joined to the hull at the deck, whether by bonding
or mechanical fasteners. Flexing of the hull is usually transmitted
to the liner. Linered boats usually have a number of wood components
inside such as cabinets, trim seating and the like. These components
are usually fastened to the liner with screws. If both the hull
and liner are flexing, then it is common to find evidence of this.
Look for screws backing out, misaligned parts, cracked moldings
and little piles of wood dust that indicates friction against the
wood. Unusually large gaps between parts or things like built-in
refrigerators backing out of their holes are often indicators of
trouble.
Extensive stress cracking in liners is another
indicator. Theoretically, the liner should not be subjected to much
stress, so when cracks appear the condition requires careful evaluation.
Be particularly alert to stress cracks around companionway doors
and at the bottom corners of the sole or foot well. Serious cracks
in these areas are a strong indicator of serious working. Another
critical area in power boats is the coaming around the windshield
area where the fore deck terminates into the cockpit. Cracks in
all three locations indicate serious trouble. If that's the case,
also examine the hull/deck joint. If the screws are loose at the
mid section, the hull is probably bending excessively and may indicate
a serious structural design flaw.
The Effects of Speed The faster a boat goes,
the more stress it is subjected to. It follows, then, that high
speed boats are considerably more vulnerable to design defects.
This also means that that any evidence of stress cracking or other
problems needs to be evaluated relative to the vessel's speed, as
well as it's age.
Unless you've had experience with high speed vessels
in rough water conditions, its hard to appreciate the extreme forces
involved. Considering how hard it is on the human body, its a little
easier to imagine the stress on the hull. High performance boats,
those which are intended to give the impression that they're capable
of being operated fast and hard, are those that are most susceptible
to problems simply because they are used harder. And because of
that, they are substantially less tolerant of design flaws.
A good example of this is a 41' Cigarette race-style
boat which was really a tripple engine, 1400 HP, luxury go-fast
boat. This boat was designed with stringers that steps in them.
That is, that the stringers had different heights at different locations
in the hull. At the only full bulkhead in the vessel - the cockpit/cabin
bulkhead, the stringers stepped down from 24" high to 12"
high. This created a serious stress initiator point which caused
the stringers to fracture at this point. Not only that, but the
bulkhead had broken loose because the hull was bending longitudinally
so bad that the hull sides were bowing outward and the hull/deck
joint popped open.
This boat had a full interior liner and no part
of the hull other than the aft engine room was visible. The failures
were foretold by serious cracks in the cabin liner, around the companionway
door, as well as the very loose guard rails. These cracks were sufficiently
severe that it was clear that they were not the result of normal
stress or improperly designed curves or a generally weak liner.
The combination of all these indicators pointed to a hull that was
starting to break in half.
In another case, also a Cigarette, the builder
had tried a three, rather than four stringer arrangement, with one
stringer on the centerline. Apparently the designer did not know
that no stringer was needed at the vee of the bilge because this
was a natural strong point. Yet stringers were needed outboard on
the bottom panels where there was now only one on each side instead
of two. The stringers on both sides fractured and the bottom split
open.
Yet another was the case of a Wellcraft 40 footer
which had only one transverse bulkhead, and in which the transverse
frames were not bonded to the stringers. These stringers were very
tall, glass over plywood, and when pounding occurred, the stringers
buckled because, lacking any bonding to transverse members, they
had no lateral stability because they were too thin.. This hull
began to self-destruct during the delivery from builder to the owner
in less than 30 hours operating time.
Another builder designed and built perfectly good
stringers, but then proceeded to drill them full of three inch diameter
holes for reasons known only to the builder. The degree of ignorance
displayed by these builders was truly astonishing.
Examples like this should lay to rest forever any
assumption that boat builders always know what they're doing. All
too often they don't. Surveyors should be mindful of the fact that,
more often than not, boats are designed not by naval architects,
but by people with no formal training whatsoever. This is not to
say that unschooled designers are not qualified. Many are if only
by experience of trial and error. Unfortunately, too many people
from the marketing department are actively involved with structural
design when they shouldn't be.
These issues are raised, not to be gratuitously
critical, but to point out how easy it is for a surveyor to take
structural design for granted, and to fall into the trap of not
looking closely. Luxury and high speed are rather like trying to
mix water and oil. The mix is not easy to achieve. The effects of
speed multiply relative to the mass or weight of the vessel. To
create a high speed yacht that does not start to fall apart when
abused - as they are likely to be - requires some serious engineering,
engineering that as often as not is lacking. Paying close attention
to these warning signs will go a long way toward keeping the surveyor
out of trouble.
Related
Reading: Hull Design Defects Part
I
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