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Here's
yet another good reason why, when you spend a lot of your hard-earned
money for a boat, you shouldn't automatically assume that the builder
knew what he was doing when he built the boat. How the components
of a boat are fastened together, particularly the hull and deck,
can make a world of difference. Unfortunately, there are a lot of
builders, as well as boat owners, who don't understand that different
applications require specific methods of fastening. Here are some
examples of what can happen when the wrong fasteners are used.
The photographs below illustrate design and construction
errors that violate some very fundamental rules. One would think
that after 40 years of fiberglass boat building, virtually all builders
would know that you do not use screws to fasten parts of the hull
together, or even attach hardware to the vessel. Obviously, the
Australian builder of this $400,000 boat either did not know that,
or did not care, for this builder used screws to attach just about
everything together on this boat.
Why not use screws? In a word, "glass."
Even a mechanical idiot wouldn't attempt to drill a hole in a plate
of glass and put a screw in it, yet the primary constituent of "fiberglass"
is just that: glass. And when you run a threaded screw into glass
reinforced plastic, the glass shatters right along with the plastic.
Thus, screw attachments into FRP are extremely weak, as the problems
with this boat clearly demonstrates.
In this case, the builder used screws to attach
the deck to the hull of this forty footer. It is true that he did
glass over the hull/deck joint on the inside, but that didn't help
much because he then ran the screws for the rub rail right through
the hull. That resulted in water leaks at every screw hole as shown
in photo at bottom of page. But that was only part of the problem.
Since the deck did not fit the hull very well, there was a nearly
one inch overlap at the stern as shown in photos below. The builder
simply filled the gap with putty which later fell out, leaving the
large gap.
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Subject vessel with stern rub rail
removed. |
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Illustration showing
how a typical hull/deck lap joint is put together with screws.
In this case, the improper fit of the deck leaves a large
gap between the two parts, making the joint even weaker
than it normally would be. Notice how easily this joint
can break apart. See photos below for closer detail. |
Since this occurred at the stern only one foot
above the water line, wave action caused water to enter the gap
which eventually worked its way into the hull core. But this is
only the beginning of the owner's problems. A large, wrap-around
windshield was attached to a balsa cored deck with screws, most
of which had pulled loose. Further evaluation showed that a lot
of other hardware was also attached by screws. The owner began to
suspect a problem when leaks in the overhead developed down below.
This lead to the discovery that it was the windshield screws that
were leaking into the balsa cored deck. Then the owner hired a surveyor
to check out the rest of the boat. This led to the discovery of
the water saturated hull core and cracks in stringers, frames and
bulkheads.
Several months of trying to get the builder to
take the boat back have resulted in consistent denials by the builder
that most of these problems are real. In the meantime, the owner
is faced with the prospect of an international lawsuit.
Decks That Don't Fit You can see the kind of problems
that develop when a deck shell doesn't exactly. Somehow the mismatched
parts have to be pulled together. But to simply force them together
puts a stressing load on the fasteners and the parts being forced.
This can later result in either the fasteners letting go or stress
cracks developing. And surely when the boat bumps against a dock,
the impact will likely cause the whole thing to break apart.
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Close up view of the mismatched
deck joint. |
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The gap is seen to
be nearly one inch. The builder's solution was to pack the
gap full of putty that later fill out. |
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Here's the result on the interior: water leaking
into the hull around every screw. To make matters worse, the
hull is cored and water got into the core. Notice how badly
these stainless fasteners are corroding. This is the result
of closed-cell, errosion corrosion. |
What the builder should have done is to at least
mitre the oversize part to adjust the fit and then relaminate the
area. Apparently that was too much work; better to just pass the
problem onto the buyer. Bearing in mind that the molds will turn
out identical parts, to solve the problem it is necessary to correct
the error in the mold. That makes for a lot of costly work, so a
lot of builders solve the problem by just ignoring it, turning out
one defective boat after another.
Unfortunately, mismatched hull and deck shells
is a fairly common occurrence. The problems that it causes typically
only begin to manifest themselves after several years or more. The
first signs are stress cracking along the toe rail, or whatever
area the excessive gap exists that was forced together. Other signs
are internal leakage and guard rails that loosen for no apparent
reason. Often we will find the guard rail molding screws backing
out. If the deck is screwed on, over the years these fasteners will
progressively loosen all around the hull until most of the entire
deck becomes loose, at which point we find guard rails loose all
over. I vividly recall one Wellcraft 30' Scarab Sport that I surveyed
on which the entire deck had come loose. Every screw holding the
hull to the deck had pulled loose.
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Here's a more graphic illustration
of what happens to stainless screws (and even small bolts)
used below the water line. There were a half-dozen here that
completely erroded away. Unless you know what type of fastener
and how to intall it, this is the likely result. |
When looking at new boats, this condition is fairly
easy to spot if you know what to look for. Simple sight down the
hull sides looking for any signs of buckling or unevenness. On power
boats, this will usually be evident at the stern; a dimple will
appear in the hull side for decks that are too small, thus it pinches
the hull when forced in place. For oversize decks, locating an excessive
overlap is a little more difficult, especially if its covered up
with an angled rub molding. Walk all the way around the boat, sighting
upward and looking for a gap. It may be one that is filled in with
some sort of caulking and even painted over.
Another thing we can do is to test all the rub
rail fasteners with a screwdriver. If the screws spin, then we know
for sure that the fasteners aren't holding.
For sail boats this is somewhat less of a problem,
occurring mostly with smaller boats. It is not a problem at all
with decks that sit on an inward flange, e.g. a hull flange that
angles inward, on which the deck sits flat and makes a horizontal
lap. The problem occurs on those with an outboard, vertical lap
joint where the screws or bolts are run in horizontally. In this
case, the excessive overlap is likely to be at the stern, although
it can occur anywhere. Stress cracks along the joint and interior
leaking are likely to be the first signs of a problem.
See Related Article Hull Design Defects Part II
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