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One of the most
frequently asked questions that a marine surveyor gets is, "Should
I buy a boat that has blisters?" This is a question that I've
wrangled with for many years, and after a great deal of research
involving thousands of boats. This essay will answer your questions
a bit more directly than some of our more detailed blister essays,
which many of you found hard to understand. That's understandable
because this is a very complex subject. But be prepared that the
answer is populated with a lot of ifs, ands, and buts.
Let's face it, an awful lot of boats have blisters,
so that finding one that doesn't (or won't get them) can be a difficult
proposition. The short answer is that if at all possible, you should
try to avoid that, if for no other reason than the potential expense
you may face in the future. That expense may not result from the
absolute necessity to repair the blisters, but the position you
may find yourself in when it comes time to sell the boat. Particularly
with newer model boats, say 1 - 3 years old, it is not unusual for
buyers demand a reduction in price, or that the blisters be repaired.
For older boats, its usually much less of a problem,
for the fact is that moderate blistering on an older boat rarely
impedes the sale. Unfortunately, another fact of boating life is
that there is a great deal of misinformation on this much talked-about
subject. One common misconception is that blisters seriously weaken
and/or damage boat hulls. In 30 years of surveying and examining
around 4000 hulls, I have seen less than 10 cases where blisters
have resulted in serious structural degradation of a hull where
it was weakened to a point where some type of failure was immanent.
What is a blister? First, let's understand
that all fiberglass hulls absorb water to some degree because both
the gel coat finish on the exterior, and the fiberglass reinforced
plastic is porous. Since water is a solvent, it will react with
the plastic resulting in the water and solvents in the plastic mixing
to create a weak solvent solution, usually with styrene. This then
softens the gel coat somewhat and, combined with a bit of gas or
fluid pressure, results in the blister.
Moisture Meters Here's a subject I get a
lot of questions about, and one that I want to address it upfront.
Since I have already stated that all fiberglass boats absorb water
to some degree, and often without causing blistering, it follows
that the use of a moisture meter is useless. If you doubt this,
please see the essay "Illustration
of Water Absorption From Hull Interior"
on this site. It shows a hull that has been completely water saturated
for 10 years, but has not developed blisters. Moisture meters measure
only the surface moisture, and since gel coat and paint is very
porous, the moisture meter is only going to tell you what you already
know; its wet. It cannot tell you anything about the propensity
of a hull to blister. While these instruments have their uses, predicting
whether a hull is prone to blistering is not one of them. See
also Moisture
Meters on Boat Hulls.
Are blisters harmful? Yes, but. This is
a question of how much harm. Blisters form at the interface between
the gel coat and what is called the skinout mat, which is a layer
of chopped, short-strand fiberglass that is used to prevent the
coarser weave pattern of heavier fiberglass cloth from telegraphing
through to the finish surface. You've probably seen boats with a
checkerboard pattern showing on the surface, and this is the reason
why. Now, fiberglass fabric, being made of bundles of very fine
glass fibers, is very porous also, most especially the outer layer
of mat. Once the gel coat absorbs water, the fibers in the mat that
are unsaturated with resin then spread the water around via the
capillary effect.
Blistering involves only the gel coat and surface
mat in 99% of the cases. This is due to the fact that the structural
fabrics, such as roving, get saturated better. Its also because
the water is less likely to penetrate beyond the mat and, even if
it does, woven fabrics do not have the weak gel coat factor and
are much too strong to allow whatever pressure may develop within
a void to cause a separation. The incidence of blisters occurring
within structural laminates is extremely small.
 If
the resin used to make the hull is of a lower quality that will
react with water, a process known as hydrolysis, which means nothing
more than becoming saturated with water and dissolving, then the
hull is poised to develop blisters. Many other factors also come
into play here, such as how well the mat layer is bonded to the
gel coat.
Since the vast majority of blisters occur between
the mat and gel coat (depicted in illustration above), this bond
has to be fairly weak for the blistering process to occur. If the
bond is strong, then blisters will not occur, even though there
is a lot of water absorption. This is a very general, even generic,
description of the blistering process. There are frequently numerous
other factors involved which I will not address here.
Aside from the damage it causes to the surface,
most of the damage done by the blister is to the gel coat and the
skin out mat, which is not a structural part of the hull laminate.
Remember, the mat is only there to prevent the fabric pattern from
showing through to the surface. The obvious question is now, "But
isn't the water dissolving the rest of the plastic resin in the
laminate?" The answer to that is "No, its not." At
least not to any considerable degree.
You may have noticed that I have not used the word
"osmosis" that we hear so much these days. Technically,
water passing through the porous gel coat is not osmosis; its just
water passing through a porous material. However, the blistering
process may involve osmosis, a process which concentrates solvents
within the space formed by the blister void. This concentration
of solvents does indeed dissolve the plastic, but fortunately the
amount of fluid involved is so small that it does not seriously
threaten the laminate.
Of course, the larger the blister, the more concentrated
solvent is present, the more damage it will cause. Therefore the
amount of damage, and therefore structural weakening caused by blistering,
is directly proportional to size and number of blisters. This explains
why only boats with very large blisters can end up with serious
structural weakness problems.
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| This photo represents a
typical case of extensive blisters, small enough to be called
pimples. They are dime-sized and smaller, but no matter how
many of them there are, they are very unlikely to threaten
the structural integrity. But they do make sanding and painting
the bottom very difficult, and will cause a slight speed loss
on sailboats. |
This is one of the few
examples we've seen where large blisters threaten the integrity
of the hull. However, the problem here was that the builder
used chopped strand mat that was over 1/4" thick on a
foam cored hull. The mat absorbed huge amounts of water, creating
these enormous blisters. Thus the real danger to the hull
was less a matter of blisters than the way the builder built
the hull. In other words, too much of the structure was invested
in a very weak material. This hull lacked strength to begin
with, proven by the fact that it was also badly delaminated.
Vessel: Irwin 65' |
Since the vast majority of boats develop only dime-sized
blisters, the amount of damage or structural degradation resulting
is very small, even when the bottom is extensively blistered. Even
boats with numerous blisters up to about 1" in diameter, usually
show no significant weakening of the plastic. The illustration above
shows the relationship of blister size to the laminate thickness.
Here it can be seen that even if some of the plastic is dissolved
under and around the blister (indicated by dotted line) in proportion
to the overall laminate thickness, its not much, even when the amount
of degradation is above average. The amount of blistering would
have to be truly severe to have even minimal effect.
These conclusions are based on two completely different
types of evidence. First is the fact that physical inspection, probing
and sounding rarely reveals softening or degradation in the area
immediately peripheral to the blister. Second, the fact that significant
structural weakening will make its presence known (before failure)
in the form of delamination, surface deformation and stress cracking.
The good news is that I know of no reports of these conditions occurring
as a result of blistering, unless the blisters are extremely large.
We're talking here blister 4" and larger, at which point the
problem becomes rather obvious. From these facts I conclude that
well over 95% of all hull blistering cases do not cause significant
structural damage to the laminate.
Getting back to our original question, "Should
someone buy a boat with blisters?" can be answered from several
viewpoints. If you insist on a boat without blisters, fine, then
go try to find one. If its an older boat, you may have little choice,
since blistering tends to run in certain builder's lines and you
may have to look at quite a few before you find one. All things
being equal, you'd certainly want to choose a boat without blisters.
Unfortunately, unless the seller is kind enough to tell you, you
can't find out until the boat is hauled for survey, at which point
you've already invested some money in it. Its a fact that most blistered
boats are sold without regard to the blistering, and this is one
of the reasons why. In my experience, the number of cases where
blisters cause the boat to be rejected, or give rise to price renegotiations
is considerably less than 5%.
Its a different story with newer boats, as well
it should be, not only from the standpoint of the higher value,
but from what's going to happen in the future. By the time a boat
is 8-10 years old, whatever is going to happen to the hull has probably
already happened. With a 2 - 3 year old boat, its rather likely
that the blistering process is just beginning. The great unknown
is just how extensive the blistering will become. Whatever your
decision, it should be made in the awareness that the blistering
could get a lot worse, and you should be prepared to accept that,
or reject the boat. Renegotiating price to account for the problem
should be a serious consideration.
Is the Repair of Blistered Bottoms Mandatory?
Based on the foregoing discussion, the obvious conclusion in most
cases is negative. If the blisters cannot be shown to be causing
significant damage, then repair is certainly not mandatory, despite
the many horror stories you may hear from people trying to sell
you a costly repair job. If the blisters are large and numerous,
it would be wise to seek unbiased, professional advice before you
proceed. Bear in mind that blister repair jobs are now big business
for boat yards, so that taking advice from yard managers may not
be a good idea.
Yes, blisters are unsightly and may cause a resale
problem. These are all factors you must weigh, in addition to the
very high cost, when deciding to repair or not. Further, you should
also be aware that the number of failed blister repair jobs that
surveyor's find is very high. No one's ever going to know why blister
repairs fail because no one is going to spend the money to find
out. That there are so many should also play a role in your decision
to repair or not. And even though the repairer may give a warranty
on the repairs, you'd best get it in writing and read the fine print.
Then make sure the repairer is likely to be around years later to
honor that warranty if it becomes necessary.
Related Reading:
Illustration of Water
Absorption From Hull Interior
Moisture Meters
on Boat Hulls
Articles in Hull
Blisters Section
Additional
links - February 20, 2005
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