http://www.yachtsurvey.com/BuyingBlisterBoat.htm



Wilbur, Read down to ***!



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.

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.

Boat hull blisters
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.