When my surveyor explained to me the significance of the blisters
below the waterline of my '90 Seaward Fox, I think I was too excited
about buying the boat to even pay attention; he gave the boat an
unequivocal "thumbs up", and that's all I needed to know. However,
I do think blistering and gelcoat delamination can be a serious
problem if not treated properly. That being said, I thought I'd
forward a blister explanation to the Seaward list. For those of you
who own 80s/early 90s Seawards that have been consistently slipped in
water, it might explain a thing or two. (Special thanks to Bob
Proctor of the S2 6.7 and 6.9 owners' who wrote the article.)
"Blisters" that occur below the waterline of fiberglass boats are
craters in the resin that vary from the size of a grain of rice to
about the size of a manís glove. The craters are sometimes referred
to as "osmotic blisters" and as being formed by osmosis.
Biologically, osmosis refers to the movement of a liquid through a
semi-permeable membrane to achieve equal concentrations on both sides
of the membrane. Semi-permeable means the membrane allows certain
solutions to pass through it but other solutions cannot pass.
Dried prunes, for example, have a skin (membrane) through which water
can pass in but the sugars inside the skin, even when dissolved in
water, cannot pass out. When the prune is put in water, it swells as
the water outside goes through the skin "trying to achieve" an equal
concentration of water on both sides; the prune gets juicier and
plumper. But if we put the dried prune in a sugar syrup, no liquid
passes through the semi-permeable skin; it wonít allow sugar solutions
to go through. You donít have (as youíll see) water passing through
the boatís hull, so principles other than osmosis are really involved.
But just as you sometimes figuratively use osmosis to refer to a
gradual, diffuse process (as in "he learned by osmosis"), so itís
also used non-technically in this case.
Fiberglass (FG) hulls get almost all their strength from their glass
fibers. A resin is used to bind and hold these fibers in the desired
shapes (hull, cabin, etc.) and to provide a smooth exterior and
colored finish (gelcoat) on the FG. Until sometime in the 1980s, the
resin that was typically used for all FG marine construction was a
polyester resin. It was originally thought that FG hulls constructed
with polyester resin as the binder to the FG (i.e., cloth, mat, and/or
chopped FG) were as impervious to water as is a steel hull. But
after about twenty years experience and the industryís discovery of
the "blister" or "osmosis" problem, it was found that FG hulls are
NOT impervious to water, they are just "relatively impervious".
When there is long term immersion, water very slowly seeps into the
material but itís at such a very slow rate that FG hulls can be
considered waterproof for all monthly or seasonal purposes. As water
migrates in, it sometimes chemically reacts with the polyester resin
under the surface to form an acid (liquid) in various size globules.
The resin binder has dissolved in those spots in reaction with the
water. The acid may further dissolve intact resin that it touches.
Many of these globules eventually grow large enough and close enough
to the surface that they break through the gelcoat. These are the
blisters that are typically found on the outer surface of the boatís
hull below the waterline. They are usually within the gelcoat and
resin layers down to the layers of structural glass (cloth, mat,
Blisters can also form on the inner surface of the hull if that region
is exposed to prolonged water. And on a few boats, blisters have
been found to have penetrated even farther, into the structural FG of
the hull itself, although fortunately, that seems more rare.
There may be many factors that determine when and where blisters will
form. The one that Iíve commonly heard of that promotes blistering
is the contamination of the polyester resin by dust and other
chemicals. When the polyester resin and all FG is kept meticulously
clean, almost surgically sterile, during building, less blistering--
or none--occurs. Any contamination apparently supports the formation
of the acid.
In the early days of boat production with FG, this was unknown. This
contamination theory is one explanation of why some boats of a
particular model may have much blistering while others may have
little. For example, if the bolts of FG cloth or mat came fresh out
of the box just before layup, there may be few blisters; but if
portions of cloth picked up shop dust before layup, then where the
resin came in contact with it on the FG, there may be many blisters.
Other factors also encourage blistering. Long-term immersion is an
important factor; the less time in the water, the less opportunity
for blisters to form. Mooring in warmer waters seems to increase
blistering, colder water to retard it. Fresh water is said to foster
blister formation more than salt water. As older FG boats accumulated
years (time) immersed in water, the FG marine industry gradually
became more aware that there was a widespread, generic blistering
problem and it was not, as first reported, isolated among just a few
boats or brands. This awareness slowly dawned mainly in the late 70s
and early 80s.
This was an issue affecting boats moored in the water-- larger
sailboats and powerboats. The thousands of smaller, trailered boats
didnít show the symptoms. It also greatly affected the chemical and
FG industrial suppliers and, of course, builders and dealers of
larger boats. Happily, some vinylester and epoxy resins were found
that were suitable for marine FG fabrication and that were also much
more water resistant than polyester resin. However, those
alternative resins were also more expensive and were limited in their
capacity to bond to other plastic resins.
A number of companies revised their manufacturing techniques and
switched from using polyester resin to vinylester or epoxy resins for
building their hulls. Since prolonged immersion was necessary to
produce blisters, typically the industry has continued to use
polyester resin for molding smaller boats and those parts that wonít
be continually in the water.
Boats with hulls made with polyester resin that will be wet slipped
(moored in the water) should have a barrier coat of epoxy (or other
suitable water resistant material) applied to the bare hull before
the usual bottom painting. The hull should first be suitably dried
and any blisters properly repaired.
The following references will give some information about repairing:
The company below is still in business and while their info doesnít
seem to me to be quite as knowledgeable or well written as Osmocureís
was, itís still worthwhile:
And thereís also a book about blisters (~ $20) that I havenít seen,
but itís described at:
For a credible discussion of hull blistering go here:
Sailing Lake Pen díOreille, Idaho
It's been a long time since I posted a question to the Rhodes list,
but a discussion about blisters, delams and "osmosis" in used boats on
another list to which I subscribe has made me a bit uneasy. Needless
to say, the topic generated a lot of discussion. My understanding is
that prior to the early '80s, the boatbuilding industry used polyester
resin extensively (I think vinylester may be used now, but I'm not
sure.) Hulls that were produced too quickly--i.e., mass produced, and
cored hulls in particular--did not have the resin cure properly, and
that lead to voids and acid production when water infiltration
occurred. The end result is blisters and delaminations. The folks at
gbi address this on the website, where it says that only two owners
of R22s have reported any blistering. Given that R22 hulls are solid
and not cored, and that a LOT of care and time is taken in the lay-up
of the hull, I guess that I shouldn't be suprized that only two
owners have reported any problems.
So...my questions to your collective wisdom, and mostly to those of
you who have recycled R22s:
- 1. Have you ever had a blister problem?
- 2. If so, how were the repairs done, and how costly were they?
- 3. What type of resin was used in the R22 around '85-'90 (&
should it even matter to me)?
I hope to ask Elton about this on Saturday at Strictly Sail when I
see him. I don't remember that this topic has come up in the time I
have been a list subscriber, and I am really quite interested in it.
Thanks for your help!
29 Jan 2001
I am the proud owner of one of the two boats with blister problems.
The reason being, the previous owner did nothing to protect the
fiberglass, not even an anti-fouling coating. My understanding is
that fiberglass hulls without any coating were meant for dry sailing
or boating, not to be left in the water. My boat has been in the
water year around for almost eleven years. The correction isn't all
that bad. A little time out of the water to dry once the blisters
are cut off, repair the holes and two finish coats. One to protect
the fiberglass and the other is an anti-fouling. Total cost I've
been quoted is around $600.00. NOT BAD.
29 Jan 2001
We have a 1995 Rhodes and keep it in a fresh water lake here in sunny
California from April to December. We also trailer to salt water
locations for short trips of two to three weeks. We have found
blisters on the hull and rudder when we pull the boat for the past 3
years. Not many but definitely there. Interestingly, the rudder had
more then the hull this last Dec. We have not had any repairs done.
The blisters tend to "fade" pretty quickly sitting out on the trailer.
Our previous boat was an O'Day 23. It was the first "big" boat we
owned and we didn't recognize the blisters on the hull when we bought
it in 1987. The bottom looked more like the paint job had gone bad or
the boat had a bad case of chicken pox.
02 Feb 2001