SAILING Mag RSA August 2016 issue 385 to ZINIO

Osmosis Treatment of a 36’ Yacht
By Guy Fabre
The hull being dried out with heat pads which are vacuumed on.
The vessel in question is a Lavranos 36,
built late eighties, and purchased 11 years
ago by the present owner. It was diagnosed
with the dreaded osmosis and treated in the
conventional manner at the time.
The boat has now been out of the water
and in the owner’s yard for about 10 years
now. It is a well appointed yacht which, after
a recent comprehensive refit which included it
being re-engined, all electrics rebuilt and more,
represents a considerable asset.
The classic osmosis treatment done six
years ago included drying out the hull by natural
exposure to the air, followed by two coats of
a good quality epoxy. At the time this was
deemed to be the best solution.
Recent research done last year (2015)
indicated that this was perhaps not the ideal
system nor a long-term effective solution.
Subsequent investigations revealed the
possible true extent of the osmosis plus the
correct procedures to eliminate further risk and
correctly repair the damage to the hull.
It must be understood that the original
evidence of osmosis on this vessel was a
multitude of small bumps that, when broken
open for examination, gave off fluid with a
characteristic smell of vinegar/acetic acid.
Research Findings
Chemists working with polyesters
and peroxide catalysts are not always in
agreement as to the real reasons for osmosis.
The actual degradation process is hydrolysis,
but convention uses the word osmosis. As a
result I decided to combine two theories and
to investigate the present best practices around
Europe. Two theories were the most credible.
Briefly, one school of thought states
that resins cannot adequately cure at room
temperature and therefore require a post cure
to promote a full and complete reaction of the
various components, ultimately resulting in a
totally cured final product. Resin pockets for
various reasons are formed in the laminates,
and when water eventually gets to them, will
start a reverse process (hydrolysis) that breaks
down the polyester, leaving behind the fibres.
When studied closely it actually looks as if there
is resin starvation in the affected area. Thus it
becomes apparent that gelcoats as formulated
then, and possibly now, are not as impervious
to water ingress into the substrate as originally
thought!
The other school of thought I looked at
denied the above, although they did grudgingly
accept that post cured GRP was not as
susceptible to osmosis as the uncured. The
decomposition of the polyester resin was, in
their opinion, attributed purely to the action
of water. Their solution was to separate the
polyester resin substrate, which might contain
some unreacted resin components, from
the water. This was achieved by keeping the
underwater hull encapsulated in an envelope
of epoxy resin which has a far higher resistance
to water ingress. This impervious barrier would
prevent the commencement of hydrolysis.
This opinion also stated that the boat should
be dried out from the inside with fans. I failed
to see how this could be practical as in any
boat most of the furniture will not allow the
air to reach the internal surfaces of the hull in
any significant quantity. Any treatment with a
dehumidifier would also do extensive damage
to the furniture by excessive removal of moisture
from any wood components.
With the onset of osmosis and consequent
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