"CaveLamb" wrote in message
m...
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.



But, the very next paragraph after the asterisks mentions osmosis.

Put it this way. You've heard of reverse osmosis, right?

That is where you force salt water through a membrane where it would not
normally go by using lots of pressure. The salt is too big to pass through
the membrane and only fresh water passes because water molecules are quite
tiny.

Osmosis without the 'reverse' operates the other way. Water migrates through
membrane because on one side there is a denser substance (laminate chemicals
that attract water) and continues to do so until an equilibrium is reached
between the densities inside and out. This migration at the molecular level
is what pushes up the blisters. Once the water gets inside the laminate it
will NOT migrate back out when the boat remains in the water. It will
diffuse and evaporate out when the boat is stored on the hard in a
low-humidity environment. Cold = low humidity so cold storage is a faster
way to dry out the laminate that is pressurized by osmotic action.

Spraying the hull with water will do nothing to release the trapped osmotic
fluids that push out the blisters. Only in areas where the laminate is
compromised will the osmotic fluid ooze out to be washed off. In areas where
the laminate is intact there is also osmotic fluid under pressure which does
not ooze out. So, Skippy is fixing about half the problem given the
assumption that bout half his hull laminate is structurally compromised as
in cracks, fractures, holes, splits, etc.

Wilbur Hubbard