This
series of articles is written exclusively for marine surveyors to
help identify the wide range of structural defects that can be found
in boats and yachts. Because there is such a diversity in types
of hulls, design styles and an ever-expanding array of new construction
materials, it is difficult for surveyors to keep up to date on cause-and-effect
evaluations.
Whether the surveyor deals exclusively with prepurchase
surveys, insurance claims or marine expert related matters, learning
how to locate, detect and evaluate is a critical factor in the surveyor's
work. This essay deals with basic principles of hull design, along
with cause and effect analysis of hull failures. It will set the
necessary foundation for this continuing series of essays.
Improper design and the improper selection and
use of materials is the primary cause of most non-damage related
structural failures. Contrary to common belief, actual manufacturing
defects only rarely figure into structural failures. It should come
as no surprise to any surveyor that the boat building industry,
much like the automotive industry which, after more than 70 years
of mass production, backed up with their enormous financial resources,
is still fraught with frequent design defects. But unlike the automotive
industry, boats are not manufactured in units numbering millions,
rather 10's and 100's at best.
Because of this, design faults are spread over
a very wide array of different builders and tens of thousands different
models over the years so that rarely do major design errors ever
become widely documented. To make matters worse, there are very
few avenues for dissemination of information, and virtually no one
who maintains any kind of database on hull failures. This essay
will attempt to illustrate the most common defects, the cause and
the visible effects that the surveyor can use as a basis for conducting
a thorough structural survey.
Structural Principles Before we go directly
into reviewing problems, its important that we first review the
major principles of hull design. From and engineering standpoint,
fiberglass boats have similarities to both bridges and aircraft
airframes. A discussion of these similarities will help us to better
understand the forces that act on a boat hull, and the structural
principles required to build one.
Boats are similar to bridges in that the hull must
have a framing system to support it because the hull itself, like
a bridge, spans a fluid substance. Whereas a bridge spans air, a
hull spans water, and while water is more dense, it is still a fluid
and offers lesser means of support that solid ground. Further, when
a boat is hauled out and set on blocks, often only one at each end
of the hull, that hull then literally becomes a bridge spanning
open air. Unless the hull has an adequate system of framing and
girders to span the unsupported sections, like a bridge it will
buckle and collapse.
We can add to this the fact that boats are dynamic
objects; they often travel at high speeds over rough water and even
occasionally, if not frequently, become airborne. Thus, the stresses
on a boat hull are far more than a matter of just gravity and mass,
but are multiplied by velocity and compounded by slamming. And as
anyone who has ever done a belly-flopper off a diving board knows,
water becomes hard as a rock when a wide, flat object falls upon
it squarely.
Most bridges do not consist of a flat deck supported
by girders underneath. Rather, most bridges are either in the form
of a truss, or they are suspended from above by a combination of
rigid and flexible supports. A boat is also similar to this principle
since the hull bottom and sides do not alone constitute the entire
structural framework. Boats that lack weather decks and superstructures,
for example, are far weaker than their cousins who do have these
additional structures. Thus, decks and superstructures also constitute
major structural elements of most boats and ships.
And here it is that fiberglass boats develop similarities
to modern jet aircraft. Aircraft utilize the principle of monocoque
construction. That is, the body of the aircraft does not have a
frame but essentially is the frame. The skin of the aircraft and
the framing system are so closely integrated that they essentially
become one structure and its hard to tell where one ends and the
other begins. Modern jet aircraft are essentially flying pipes with
wings, and it is from this engineering principle that they gain
their strength, despite the extremely light construction.
Modern fiberglass boats make use of this principle
of monocoque construction and in this way are more closely related
to aircraft than they are to their wooden-boat ancestors from which
they evolved. A wood boat is the sum of its many parts while a fiberglass
boat hull is essentially one component. The combination of molded
hull and deck joined together creates a unified whole that is much
stronger than the sum of its parts. But boats are proportionately
far heavier than aircraft and are subjected to different stresses.
Aircraft don't fly off the tops of waves; boats do. While the bottoms
of hulls take the major brunt of stresses, and must be designed
to withstand them, the monocoque construction still plays a major
role in providing strength to the overall structure.
There is no better illustration of this than the
offshore racer type boat, a long skinny hull equipped with tremendous
horsepower. In the so-called "cigarette" type boat, the
deck provides a major part of the hull strength that, lacking a
strong deck, the hull would buckle. These decks are not "hull
covers" but designed as structural elements. These race boats
are true monocoque structures because the hull and deck structures
are not screwed or bolted together, but literally bonded together
to become one piece.
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| Here's a good example of poor design and construction
detail. Utilizing a glass over plywood framing system, there
are no fillets under the frames or stringers which are butted
hard against the hull. This creates hardspots with the propensity
for stress cracking. In addition, the length-to-height ratio
of the tall stringers creates instability where the stringers
are likely to buckle under inpact loading. Additional framing
between the stringers is needed to stabilize them. Also note
that there are only two hull side stiffeners so that flexing
of the sides is likely to cause hull/deck joint breakage.
In the forward section, a dog leg in the stringer profile
can be seen |
Dynamics of Hull Stress Power boat hulls
are essentially modified rectangles with a shallow vee on the bottom.
When a boat falls off, or slams down off a wave, the bottom impacts
the water and suddenly stops its downward movement. This sudden
stop sends shock waves up the hull sides that are then transmitted
to the deck and any upper structures that may exist. In the meantime,
while the hull suddenly stops its downward movement, everything
inside the hull wants to continue on downward, creating even more
stress.
When the hull impacts the water, the resultant
stresses work to cause the hull to want to buckle transversely and
longitudinally. The impact with the water is never uniform along
the length of the hull so that one end, or one side, of the hull
is more stressed than the other. One effect is to try to break the
boat in half like snapping a stick in half. The other effect is
to bow the hull sides inward or outward, the effect of bending along
the horizontal plane. Yet another is twisting or torsional stress
along the entire length of the hull.
In actual operation under heavy conditions, the
hull sides of most boats will deflect to greater or lesser degrees
depending on how well it is designed. This is the result of impact
loading, bending and torsional loading on the hull caused by high
velocity over waves, porpoising and so on. If you've ever wondered
why so many boats have rub rails falling off and weak and damaged
hull/deck joints, you probably thought that this was primarily due
to hitting up against dock pilings. But the real reason is that
many boats have poorly designed hull/deck joints that are simply
lap joints screwed together. It is the stress transferred from the
hull bottom to the hull sides and thence to hull/deck join that
causes the screws that join these parts together to break loose.
Putting screws into fiberglass is a terrible means of making connections.
Screw joins are simply too weak to work effectively.
So it is that the deck - and the superstructure
that is often integral with the deck, i.e., are molded as one piece
- are not only part of a unified structure, but also absorb much
of the load initially induced on the hull. This also accounts for
much of the damage and cracking found in and around deck structures,
and why on many boats windows, doors and hatches and portholes just
never seem to stop leaking. The whole structure is working so that
no amount of caulking, bedding and gasketing can ever stop the leaks
because they just open up again
These are the effects of stress on the exterior
boat hull and structure. But the stress doesn't end there for we've
not yet considered the hull framing system. The framing system consists
of stringers, bulkheads and frames in more conventional construction.
Yet increasingly builders are seeking to reduce costs and streamline
production by eliminating much of the detail work involved in the
framing system. They are doing this by again utilizing the principle
of monocoque construction which takes the form of premolded "liners"
or so-called 'grid liners," a premolded combination internal
framing system and accommodation components. And rather than bonding
these parts together with conventional tabbing or taping, instead
they are being glued together with some sort of adhesive putty.
Although the use of liners has been around for
a long time, the combining of a framing system with a liner is new.
And as any experienced surveyor can see, it poses some obvious problems,
but that's a subject I'll deal with in Part II. In the meantime,
the conventional stringer, bulkhead and frame system is the method
used by about 98% of all boats over 30 feet.
Stringers In power boats, stringers provide
the majority of the longitudinal hull resistance to bending in the
vertical plane. The apex of the vee at the bottom or keel adds additional
strenght. This is qualified by whether the deck is also designed
to give the hull longitudinal rigidity. Depending on design, some
decks, particularly on motor yachts with very short decks and lots
of windows, are so small as to add very little additional strength.
On the other hand, the typical flybridge sport fisherman with its
long foredeck, relatively small windows and strong house sides,
adds a great deal of rigidity to a hull. So it is that we can now
understand why there is a lot more to the strength of hull than
just the framing system. In monocoque, or semi-monocoque construction,
the whole structure must be considered. And it is precisely here
that so many untrained "designers" who lack a solid background
in engineering, make their mistakes.
Mistakes involving stringer design and installation
are legion, about which a whole book could be written. And yet the
principles for creating an effective stringer system are very simple
and easy to achieve. Surely there are not many designers or builders
who do not understand this. Or are there? Problems usually arise
as a result of other design and marketing considerations. Typical
examples are when a designer wants to create a small boat with 6'6"
headroom or wants to install unusually large engines. The machinery
spaces, which are not subject to appearance and marketing considerations,
are usually sacrificed.
In order to get the 6'6" head room or make
high profile engines or other equipment fit, the principles of proper
stringer design are often sacrificed. In other words, the principles
of sound hull design get sacrificed for marketing considerations
and the surveyor needs to be constantly aware of this fact. Its
the primary reason why, in this day when all is known how to build
a good boat, bad boats are still being built. Give the customer
what he wants, even if the product is going to fall apart.
The principles of good stringer design are simple.
They must run uninterrupted from one end of the hull to the other.
They must be of adequate height to width ratio, i.e., structural
modulus, to resist impact loading on the hull skin, be of sufficient
strength to carry the engine load, be stabilized against lateral
movement if high profile, and be securely attached to the hull so
that they don't break loose. The profile, or top of the stringer,
should run in a straight line. If there are any changes in the profile,
then special design reinforcements must be added.
 |
Dog leg in stringer which was cut
down to make the engine fit. The stringer proved to be so
weak that the engine bounced every time the hull hit a wave,
ultimately bending the shaft and wrecking the transmission.
Also notice the hard spots created by the fuel tank mounting
pads at top of photo that caused stress cracks in the hull.
|
These principles are often compromised by designs
that utilized dog-legs, step downs, step ups (meaning an inconsistent
profile along their length), perforations with large and ill-placed
holes, inadequate section modulus and numerous other faults. In
nearly all the cases that I have seen, there is no compelling reason
why these faults should have occurred. What these design faults
unfortunately suggests is that the designers really don't understand
the basic engineering principles. Yet in most cases of failure that
I have seen, the builder could have had his cake and eat it too
by giving a little more thought to the problem. What is compromised
in one way can always be built up in another. There's always an
alternative solution. The builder just didn't take the time to consider
it.
Bulkheads serve two very distinct functions.
First, bulkheads act as transverse frames. More importantly, the
bulkhead is the structural element that prevents torsional stress
or twisting of the hull. Unified with a stringer system, they form
a structural web and a truss. Remove the bulkheads and its rather
like removing the trusses from a bridge or a roof. The overall strength
can be reduced to the point of structural failure. And because of
the efforts of interior designers to produce small boats with the
appearance of wide open interior spaces by the elimination of full,
and even partial bulkheads, that hull structures begin to fall apart.
 |
Here's what often happens when a large cut out is
made in a structural bulkhead. In this case, the 3/4"
plywood was fractured in three places. |
One builder that produced a 34 footer which had
only one partial internal bulkhead - an engine room bulkhead that
was only slightly more than half the height of the freeboard of
the hull - resulted in severe structural failures in much of the
model line. You probably know the boat, the 34 Wellcraft Grand Sport.
In this model line, not only did major hull skin and stringer failures
occur, but in many cases the single plywood bulkheads fractured
from side to side.
Even companies with reputations for building very
rugged hulls occasionally make silly mistakes. In a nearby photo
you will see the result when Bertram decided to make very large
cut outs in the centers of plywood bulkheads to save weight. They
unthinkingly removed all the strength from the plywood bulkhead
with predictable results; the bulkheads fractured.
And we know how engine room fore and aft bulkheads
constitute one of the foremost structural elements of medium size
yachts, and we've witnessed what happens the builder unthinkingly
decides to cut a big hole in the bulkhead and install a door. For
whatever reason, it did not occur to the builder or designer that
he was destroying the structural integrity of the bulkhead.
 |
This is another good example of the structural integrity
of a bulkhead being defeated by cutting it full of holes.
It is perforated like a postage stamp and is destined to fail.
|
To do their job, bulkheads must be adequately secured
to the hull bottom, sides and underside of the deck. Judging by
30 years of inspecting fiberglass boats, its a fair statement to
say that many builders don't think that this is very important considering
the large number of bulkheads that surveyors find to be broken loose.
Probably at least half of all boat builders don't tie the bulkhead
to the deck, and often for good reason. The bottoms of their boats
are so flexible that the bulkhead will telegraph the deflection
of the hull into the deck, causing damage to the deck. Therefore,
it they leave a gap at the top, at least it won't tear the deck
apart, just everything else that the bulkhead is attached to, or
is attached to it.
While we've been talking so far about structural
bulkheads, bulkheads come in several varieties, including full,
partial and nonstructural partitions. While I know of no published
rules on the subject, my own rule is that to be classified as a
full bulkhead (1) it must span the width of the hull, (2) span no
less than 75% of the depth of the hull and be attached to the bottom,
(3) have no openings larger than 50% of the height of the bulkhead,
and (4) such openings must be centered in the vertical plane and
be adequately strengthened to compensate for the cut out. An opening
that effectively cuts the bulkhead in half is not a full bulkhead
but a partial. For maximum effectiveness, the bulkhead must be attached
to all four sides of the hull.
 |
Floor frames under main mast of
large sail boat. Properly designed by the designer, the builder
apparently saw nothing wrong with drilling the frames full
of holes. Here you can follow the fracture along the perforated
effect of the holes at right and left sides. Frame was so
weakened that ply separation also occurred. A marine surveyor
got sued because he either did not find or report this condition,
which was far more extensive than this photo shows. |
Partial bulkheads are really nothing more than
frames and do not serve any greater function than frames. It is
a mistake to call a hull partition with two doors in it a bulkhead,
for it is really only a partition, or a partial bulkhead at best.
Surveyors often mistake partitions for bulkheads. Remember that
to be classified as such, a bulkhead must be serving the purpose
of tying the four sides of the hull together (bottom, deck and sides).
If its shot full of holes and openings, its not achieving that purpose.
Partitions simply serve the function of separating
one space from another while providing little, if any major structural
strength. Builders often make the mistake of thinking that partitions
are structural bulkheads and this is because they don't have any
trained engineers or designers on staff. And just because a partition
may be taped into the hull does not mean that its structural; the
taping is usually there just to hold the partition in place, not
the partition to hold the hull together. Sail boats and some smaller
power boats often have plywood partitions that are screwed to bosses
on an inner liner. Again, these should not be mistaken for bulkheads.
Frames serve the purpose of stiffening panels
between bulkheads and stringers. Fiberglass boats often lack frames
where they are needed. Obviously, if a panel is flexing too much,
additional framing would prevent that condition. Some builders scrimp
on frames because frames create additional detail work and adds
more to labor cost. Fortunately, where excessive panel weakness
is discovered, adding frames after the fact is usually fairly easy
to accomplish. So long as there is accessibility, correcting panel
weakness is usually not difficult or costly.
Rigid or Flexible Hulls Aluminum and steel
boats are examples of vessels built to be completely rigid. By the
nature of the material, these hulls will not tolerate flexing. Fiberglass
boats, however, are another story. Fiberglass boats can be designed
to be either flexible or rigid. For example, if you examine Bertram
hulls built over the years one can see a very abrupt change in hull
design philosophy. Somewhere in the mid 1980's, Bertram made a transition
from very rigid hulls to fairly flexible hulls. And as the Bertram
engineers have proved from years of extensive R&D (they were
one of the few boat builders that took R&D seriously) you can
build light, floppy hulls without danger of them falling apart.
Moreover, there is a legitimate need to attempt to reduce costs
by reducing the weight of the most costly materials. All you have
to do to see how this is possible is to look at the aircraft industry
which has invested billions in R&D.
In recent years, boat builders have been observing
and borrowing some of the fruits of this technology. Unfortunately,
aircraft and marine design principles, while having similarities,
are not the same. Equally unfortunate is the fact that some boat
builders attempt to incorporate this new technology directly into
their products without any R&D of their own. And herein lies
the problem.
It is entirely possible to take just about any
hull and reduce its glass/resin content by 25-35%. In fact, back
in 1985 I undertook such a project by taking the plans for a 55'
Hatteras with a design weight of 72,000 lbs and redesigned to come
in at 42,000 pounds, including a huge 50% reduction in the weight
of the basic hull structure. This was done by applying basic airframe
design with modifications for marine. The end result had two serious
problems that were anticipated. First, the hull weight was reduced
by means of an intricate framing system. The problem with that was
that anything that was saved on materials cost was more than offset
by increased labor costs of achieving the detail work.
Even less did I anticipate the effect on how the
hull would handle with a 41% overall weight reduction. Scale model
testing revealed the boat to be so light that it would pitch and
roll so violently that it would be uninhabitable to human beings.
It developed a whip-snap roll in a 3' sea that would literally throw
people off the deck. Or when pitching, launch them like a trampoline!
So much as for ultra light boats. Weight is a factor that provides
stability.
But the project did prove the viability of ultralite,
flexible hull construction. Rather like the old Cleveland Browns
Rubber Band Defense, designed to bend but not break. The point here
is that builders can get away with a lot of shortcuts if they know
how to do it right, and if the increased labor costs don't make
it impractical. Its easy to design a flexible hull that flexes without
breaking. What do I mean by flexible? Well, if on a sea trial you
run a tape measure between the top of the engine stringer and the
underside of the deck, you'll probably be surprised to see the stringers
flexing by as much as 1/2" even on what you consider to be
a well made boat. If you were to string diagonal measures from one
corner of a large compartment to another, in the manner used to
measure squareness of square or rectangular structures, you will
find that when you put a boat into a hard turn, one of those measures
is going to go very slack. That's because the hull is being twisted
by the torsion of the turn.
The early models of the 60' Hatteras Convertible
were a prime example of a large hull that was inadequately bulkheaded.
These hulls would twist so badly that when you put it into a hard,
full speed turn, the propeller shafts would bind up in the bearings.
And you can just imagine the effects on shafts, engines and transmissions!
This was not so much a matter of a boat with not enough bulkheads,
but rather the bulkheads that it did have were poorly designed and
executed.
Design-wise, rigid hulls are easier to design and
build. With a flexible hull, very rigid attachments of internal
components becomes a problem because the flexing starts to tear
everything loose. The designer overcomes this by making the interior
sort of "free floating." For example, in designing a flexible
hull, you do not use the hull or framing system (stringers and structural
bulkheads) as a foundation for the interior components such as the
sole and cabinetry work. Instead, you build a shelf on the upper
hull sides and literally suspend the interior from the shelf. That
way when the bottom flexes and the hull sides pant, it doesn't work
so hard to tear the interior apart.
Conversely, if the designer is confidant that the
hull is rigid, he can go ahead and place the soles on top of stringers
(although this is never a really good idea) and attach components
to bulkheads or hull sides. For slow speed boats that don't skip
across the tops of waves, this is the way its usually done. The
hull isn't going to flex that much that its going to rip the interior
apart. Whereas the slow boat builder can get away with all sorts
of haphazard design, the fast boat builder cannot.
There are limits, of course, to just how far a
designer can go with flexibility. In terms of rigidity, we're talking
about the difference of the bottom flexing 1/4 to 1/2" or not
at all. With the increasing lust for speed and advent of high performance
diesels, flexibility causes serious problems. Flexibility is okay
for slow or moderate speed vessels, but becomes disastrous to high
speed yachts. The reason is not so much inherent in the hull structure
itself, but rather in the drive train. Delivering a thousand or
more horsepower through a long and large diameter shaft demands
higher tolerances of the drive system, and therefore mandates more
rigid hulls, not less. Along the length of a 30' drive train, the
hull must be absolutely rigid; it cannot deflect or twist lest the
whole drive system be thrown out of alignment.
To gain an appreciation for the significance of
this, just look at the massive structural system found in high performance
Hatteras or Vikings, shown below. When you're dealing with a quarter
million dollars or more worth of engines and transmissions, it doesn't
pay to fool around. Mistakes are just too costly. On recent survey
of a high performance 48 Hatteras and I was absolutely astounded
at the massive stringer system in this boat. Although I had seen
it before, I didn't really appreciated how large it was. The width
of the top hat bottom supports actually covered nearly 50% of the
bottom panel area.
 |
Stringer system of a 48' Hatteras
Hi Performance Convertible. Note that the width of the top
hats are about the same as the width of the bottom panel spans.
This is a good example of structural overkill, yet demonstates
the builder's concern with strength. Also note the webs between
stringers under the engine mounts that provide extra stability.
Despite the appearance, these top hats are actually quite
thin. When slamming occurs, the thin sections will absorb
much of the impact, hence the web sections to increase stability
and insure that the engine beds do not move. |
Now, did it cost the builder more to do it this
way than in the usual way? Not likely, they just had to spend some
extra time thinking about what to do. The actual execution and materials
cost was probably no higher than any other design. The point here
should be painfully obvious; ultimately it costs more to do it wrong
that to do it right.
The bottom line is that whether a hull is successfully
flexible or rigid is dependent on design and function. In a high
speed vessel, everything else about a hull can be flexible, but
the foundation of the drive system must be absolutely rigid. Another
point to remember is that the smaller the diameter of the shaft,
the more bending it can tolerate. Shafts from 1" to 1-1/2"
can tolerate a heck of a lot of bending caused by a flexing hull.
But when you get up to 2" diameter, these powerful systems
will not tolerate movement of the foundation and the systems will
begin to self-destruct.
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The importance of stringer stability
is revealed by this stabilizing strut, in addition to the
mounting frame above it. Yacht: 56' Magnum, 2600 HP. With
this kind of horsepower, the mounting system and shafts will
not tolerate movement. |
Material Trends If you read industry magazines
like Composites and Professional Boat Builder, its
hard not to be impressed by these advertising vehicles efforts to
influence the use of aerospace composites and techniques into boat
building. Every issue of these two magazines devotes a major part
of its space to promote the use of exotic materials and very complex
technology for building pleasure craft. In an industry known for
its trial and error, seat of the pants methods of development, one
could effectively argue that high technology is probably the last
thing this business needs to become involved with.
In my estimation, what they are attempting to do,
is to promote and transfer these high tech materials from the aerospace
industry, which was backed up by the bounteous source of federal
tax dollars, to an industry well known for its critical capitalization
problems. They are promoting the very same technology utilized in
the production of military war planes such as the F117 and B2 bombers
(the later of which has a $2 billion per copy price tag) to the
construction of pleasure craft. Viewed in this light, the economics
of this trend don't look very promising.
Currently the experimentation with these materials
is largely confined to custom boats with very wealthy patrons who
are willing to foot the bill in order to posses the latest and greatest.
However, there has been some extension into production building,
mainly so-called niche markets such as race boats, both power and
sail. And to the extent that it is clear that the production boat
building industry does not possess the necessary capital resources,
nor the profit margins to sustain them, their incorporation of this
technology into production building is very likely to continue along
the lines of trial and error. What this portends for the surveyor
are the risks of failing to locate design failures during surveys,
failures involving design, materials and construction techniques
that fall into the realm of the experimental. Make no mistake about
it, experimentation with new materials directly into a product is
the norm, not the exception.
With this basis understanding of the principles
of good hull design, we can now begin to study the effects of what
happens when these principles are violated.
Related
Reading: Hull Design Defects Part II
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