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"Should I buy a boat
with blisters?"
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 has been discussed for years, and a
great deal of expense and research has been carried out involving hundreds
of boats. Hopefully this article will answer some of your
questions.
Here on the Norfolk Broads an awful lot of boats
have blisters. Whether this is due to the build quality, warm, freshwater
conditions, or the fact that many of the boats are left afloat all year
round, its difficult to tell. Finding a boat that doesn't have some form
of blistering below the waterline, can be very difficult. If you can find
one, great, if for no other reason than the potential expense you may face
in the future. That expense may result from 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 to demand a
reduction in price, or that the blisters be repaired.
However, In my experience, for older
boats, its usually less of a problem, for the fact is that moderate
blistering on an older boats rarely impedes the sale. Unfortunately,
another fact of boating life, especially on the inland waterways, is that
there is a great deal of misinformation and confusion on this much
talked-about subject. One common misconception is that blisters seriously
weaken and/or damage boat hulls. In years of surveying and examining
various hulls with differing types of blistering I have seen very few
cases that have resulted in serious structural degradation of a hull where
it has weakened to a point where some type of failure was immanent.
What is a blister?
Although often argued, all fibreglass hulls
absorb water to some degree because the gel coat finish on the exterior,
and the internal fibreglass reinforced plastic is porous. Even epoxies,
the most waterproof resinous coatings available, are permeable. Since
water is a solvent, it tends to react with the fibreglass, resulting in
the water and solvents in the plastic mixing to create a weak solvent
solution. Moisture continues to be absorbed through the gel coat and if
resulting solutions are not able to escape by condensation on the
interior, hydraulic pressure will be generated within the laminate,
eventually leading to gel coat blistering.
Moisture Meters
Here's a subject I get a lot of
questions about, and one that I want to address upfront. Since I have
already stated that all fibreglass boats absorb water to some degree, and
often without causing blistering, it follows that the use of a moisture
meter is useless. 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.
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
fibreglass that is used to prevent the coarser weave pattern of heavier
fibreglass 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, fibreglass fabric, being made of bundles
of very fine glass fibres, is very porous also, most especially the outer
layer of mat. Once the gel coat absorbs water, the fibres 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 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. 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.
Is the Repair of
Blistered Bottoms Mandatory?
Based on the foregoing chapter, the obvious
conclusion in most cases is no. 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.
My experience has shown that early
treatment of blistering tends to be less successful than treatment of
vessels with more advanced problems. While this may seem to contradict
normal precautionary practice, it has been found that breakdown reactions
in GRP laminates take some time to reach their conclusion; and if
treatment is carried out prematurely it is much more difficult to remove
reactive or hygroscopic compounds from the laminate, with the result that
a recurrence of the blistering is much more likely to occur. The
overriding factor must always be the integrity and safety of the hull.
Osmosis in its early stages is very much a chemical condition, which has
very little effect on hull strength.
This photo (below left) represents a typical
case of blisters, small enough to be called pimples. They are two penny
sized and smaller, but no matter how many of them there are, they are very
unlikely to threaten the structural integrity. These examples were found
on a hull only three years after full osmosis treatment had been carried
out. Remedial works were expensive approximately £200.00 a foot (30ft
vessel). The works included a full gel peel and an extensive washing and
drying out period! More information at this link: www.gelplane.co.uk
This is one of the few examples I have
seen where large blisters threaten the integrity of the hull. (below
right). These larger blisters (a number the size of side plates) were on a
Freeman 23, and after close examination, the blistering was found to
be noticeably deeper, widely spread and containing high volum
es of liquid glycol. In other words,
too much of the structure was invested in a weak material. It should be
noted that the hull was over 30 years old and was considered one of the
unlucky few.
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 I 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 should get it in writing
and read the fine print. Then make sure the repairer is likely to be
around years later to honour that warranty if it becomes necessary.
Having said this, I have recently
been to a CPD seminar in London on the subject 'Hot Vac
Hull Cure'. This really does appear to be the most successfull treatment
for osmosis to date, with a high success rate. Brooms of Brundall use the
system to great success. Pease read more about this great invention
at www.hotvac.com
Fibreglass
Stress Cracks.
The causes for stress cracks are much the
same as for blistering. Stress cracks occur because of voids under the
surface of the gel coat, and or a lack of strength in the underlying
fibreglass or compounds used in the sub structure of the boat. When these
voids are present, or when the fibreglass is lacking in strength because
it was not mixed properly, or because enough fibreglass was not applied it
paves the way for stress cracking. Your boat body will flex some even
during normal use. Fibreglass is strong, and if mixed properly, and if
enough is applied, the fibreglass will take up much of that flexing. But
gel coat is very brittle and has no strengt
h to it. When the boat flexes and
twists, the fibreglass flexes and twists. When that happens, the gel coat
can not flex with the fibreglass, it just cracks. Hence you get these
little hairline cracks. See example photo
(right) taken last season on an Ocean 30 cruiser.
As in this case the condition has no
significant effect on the strength of the hull laminate. The defect though
unsightly, is not structurally significant at this time and repairs may be
safely deferred until a suitable time.
How to Prevent
Blistering in the first place.
Over the years GRP hulls have been
coated in a variety of paint systems hoping for protection from
blistering. Some manufacturers have progressed through many coatings over
the years. Some have been successful at times, but on other boats, they
have had no noticeable effect. The underlying laminate is usually the
reason for success with top quality resins also contributing. If boats are
badly laid up, lacking in resin and riddled with air voids, moisture will
get to the bad laminate despite the coatings.
Many in the building business thought that
epoxy would solve all permeability and blister problems, but its not quite
that simple. Epoxy is not recommend over gel coat on second hand boats as
boats that have been in the water six months or more will have absorbed
too much moisture into the gel coat and two years or so later, off it all
comes again. Epoxy is too waterproof a coating to put on
moisture-contaminated, aerated coatings like gel coat. Success has been
proven over gel coats on new hulls, as long as they are sanded down with
the right grade of paper. If using epoxy it is also imperative that all
the coats are applied wet on wet to eliminate chances of the epoxy
delaminat
ing between coatings.
Getting back to the 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 and try to find one. If
its an older boat you want that has been on the Norfolk Broads for a
considerable time, you may have little choice, since blistering is common
and also tends to run in certain local builder's lines.You may have to look at quite a few before you find
one with out blisters.
All things being equal, you would 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
out 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%.
Important: Please note that whilst every
effort has been taken to ensure accuracy of information contained in this
subject, no responsibility can be accepted for any errors or omissions
that it may contain.
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