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Quite
a few people have written or called to tell us that they know of
some proven methods of repairing bottom blisters that constitute
a full and final solution. Each of the systems and products offered
by West, Ashland Chemical and International Paint were mentioned.
These, of course, are the most widely used products, but are also
the most frequently involved in the failures, if only by virtue
of their widespread use.
Some of the people who suggested that the information
offered on this site was incorrect were surveyors, stating that
they were sure that these methods and products worked. We pointed
out that not ALL blister repair jobs fail, nor did we suggest that
to be the case. Only that far too many do fail. To those who
stated that they knew of foolproof repair methods, we posed the
question, "How long after the repair jobs did you conduct follow-up
inspections to ensure that the repair was, in fact, effective?"
This threw quite a wrinkle into their arguments.
It is time-consuming and costly to perform follow-up studies and,
as expected, none of those who differed with our views had done
so. They were basing their opinions merely on the fact that they
hadn't heard about the repairs failing, and so their assumption
was that it worked out fine. This is rather typcial of what passes
for knowledge in the boating business - mainly a lot of hastily
conceived conclusions generated from hearsay and assumption.
It was stated in another essay on this site that
effective blister repairs could not be guaranteed because coating
the hull from the exterior could not insure that it would
not once again absorb water from the interior. At the time
that essay was written, we hadn't yet any good photos revealing
just how much water can be absorbed from inside of a hull. That
has now changed. Just recently we came across a boat that yielded
up some pretty good photographic evidence. Now we have the photos
and they're shown below.
Background The boat in this example
is a Trojan International 10.8 meter, ten years old and a one-owner
boat that had never had blisters on the bottom. It had spent its
entire life docked on a canal in South Florida where summertime
water temperatures are as high as 92 degrees. This is a very well
built hull of solid laminate, of conventional roving reinforcement.
It was so thick and hard that when we sounded it with a steel hammer,
the hull "rang." No dead, dull thumps on this one. And
there was not one blister on the bottom. Now take a look at the
photos we took of the interior hull.
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As with most hulls, its painted or gel coated throughout
most of the interior. But up in the bow section we found some areas
that weren't. Here's what we found:
Photo #1. This photo is taken in the forward cabin
just above the point where the sole is taped into the hull sides
and below the waterline. When I first lifted the carpet here, I
was taken aback because my first impression was that the cabin sole
tabbing had been laid over painted fiberglass, since the aft section
of the interior hull was painted. As you can see, the tabbing is
a pink color while the hull laminate seen at the top of the photo
is not translucent like the tape but completely opaque. (Tape or
tabbing are the strips of fiberglass used to join parts together,
such as a bulkhead to the hull in this case.)
Photo #2. This is one of the most graphic
illustrations you will ever see of differing rates of water
absorption in various areas of one hull. This is looking straight
down at the centerline bilge in the forefoot where a bit of black
water lies at the bottom of the vee (the black vertical line). The
yellow section at the center is a separate layer of fabric which
has absorbed far more water than the surrounding laminate and turned
yellow. Whereas in the upper part of the photo, you can see a different
layer of fabric that is pinkish and is not as opaque, meaning that
it has absorbed less water.. Notice that the coloration is the exact
opposite of that in photo #1, where it is only the tabbing that
remains pink and translucent. Since both sections of laminate are
equally hard, most likely what this is telling us that two different
kinds of resin were used in this lay up. Because of the hardness,
cure rate does not seem to be a factor as it is in many cases.
Photo #3. This photo was taken a few feet further
aft. Its the hull bottom between the keel and the stringer (top
of photo). Here the laminate has a blotchy appearance - its whiter
toward the left and center, while a more translucent area is seen
at right. At the bottom is an oily bilge high water line. The stringer
has absorbed very little water and remains translucent, whereas
the bottom laminate has turned opaque. The differing colors,
which are not just surface contamination, suggest that chemicals
in the bilge water have also played a role in the discoloration.
What is most distinguishing about this photo (#3)
is the progressiveness in the change of color of the laminate down
toward the keel; the deeper in the bilge, the more opaque the laminate.
When a laminate, or plastic, absorbs water it turns cloudy or opaque.
We take this as clear evidence of just how much water a hull can
absorb from the interior.
We don't often get to see examples like this because
the interior of most boat hulls are painted. So what is the meaning
of these examples of differing laminate layers absorbing more or
less water? For one thing, it is a dead giveaway that different
batches of resin were used, or that the same resin was handled differently,
such as catalyzing, accelerating or hardnening agents. Secondly,
that some laminates absorb more water than others, some from the
interior and some from the exterior.
Further, these photos also demonstrate (as we already
know) that water migration through a laminate follows the fiber
bundles via the capillary effect. And that there is much less tendency
for water to pass from one layer or lamination to another. Why is
that so? Because the fibers don't extend from layer to layer, but
only horizontally within a layer.
Summary
Here we have an excellent example of a hull that
is fully saturated with water and yet it has not blistered. Not
one. It also proves that hulls can absorb a great deal of water
from the interior, and the reasons why recoating the exterior so
often fails to solve the blistering problem IF a hull is prone to
blistering.
So why didn't this boat blister? We have less than
a complete answer to this question, but we did obtain some indicators.
One is that the resin used is not so superior that it resists water
absorption*, yet it has displayed no tendency to blister. Another
is that the layup quality is well above average - we found no evidence
of void spots or incomplete wet out at any point where the laminate
was not painted. As you can see in these photos, there are no areas
of unwet fibers visible, nor did we find any in other areas.
Although this boat was in the process of being
sold, and we could not attack the bottom with a grinder, a little
digging with a knife determined that the skin out mat on the exterior
is very thin, probably less than 1/8". What this means is that
it does not have a thick layer of mat (which is very difficult to
wet out) that is full of voids and unsaturated fibers. This would
tend to confirm our belief that incomplete wet out is a primary
factor in the blistering process.
This is yet another example that leads us to conclude
that the quality of workmanship in the layup process plays a major
role in blister prevention. Yet that alone cannot explain why this
boat did not have any blisters, for it is entirely unreasonable
to assume that there are no voids or unsaturated fibers in the bottom
outer laminate: that is impossible. Even though the resin is highly
permeable, no chemical reactions occurred to result in blisters.
Clearly, there must be something about the quality of the resin
that prevented this.
But it is equally clear that, if a hull can absorb
water from the interior, recoating the exterior is no fool proof
solution to blister repair. Due to the fact that water does not
migrate as easily through a laminate as along its length
*, recoating with a less permeable resin can have a major
effect on the extent of blistering that can occur. But these examples
should make it pretty obvious that no repair process is going to
guarantee a permanent fix.
* Laminates tend to conduct water along the
longitudinal axis becasue the fiber bundles, which are never completely
wetted out, conveys water readily via the capillary effect. Water
absorption by the plastic resin is a much slower process, possibly
involving hydrolysis. The term "water absorption" means
the induction of water into the laminate by any means.
Useful
Terms
Capillary Effect: The tendency of a fluid
to conduct itself or flow through narrow passages, e.g.
a capillary. Adsorption, absorption, catalysis, diffusion,
osmosis and permeability are all terms that are closely
related.
Permeable: The ability of a fluid
to pass through or penetrate a solid; porous, porosity,
passable, penetrable.
Hydrolysis: A chemical reaction in which
water reacts with another substance to form two or more
new compounds.
Osmosis: The flow or diffusion of a fluid
through a semi-permeable membrane, initiated by differing
concentrations of that solution on each side of the membrane.
It should be noted that osmosis does not occur through a
membrane where the solution exists only on one side. The
membrane, or material, must first be permeable for osmosis
to occur.
Permeate: To pass through pores or interstices.
Semipermiable: Partially but
not freely or wholly permeable; of or constituting a natural
or artificial membrane that is permeable to some, usually
small molecules (as of water or inorganic salts) but
bars the passage of other, usually larger particles. |
1997
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