Have been reading up more about osmosis in GRP boats. I need to be able to diagnose it if I am to avoid buying a boat that has it. The more I read the more reassured I am.
Blisters? yes one or two but is it Osmosis? |
Essentially,
(let’s not get too scientific here),
Osmosis is like a piece of string with one end in a saucer of water, the string
acts as a wick and although most of the string is out of the water, it still
gets wet along its whole length. As far as GRP boats are concerned the standard
gelcoat is not 100% waterproof, and unless a great deal of care and attention
goes into the manufacture of the hull, there may be hollows (voids) within the
layup. So, water can get in. But that’s not all. There are certain chemicals binding
agents and release agents used in GRP hull manufacture which can also end up
within the layup, filling some of these voids. The result is that if you have
fluids either side of a membrane, i.e. water on one side of a gelcoat, and
other less dense fluids on the other side, there is a tendency for the denser
water to pass through the membrane and mix with the lighter, less dense fluid
on the other side. Over time, the water inside the hull reacts with the
chemicals and causes blisters. If the blisters break, more water is admitted
into the GRP layup, causing more blisters, admitting more water … well you get
the picture.
So,
over time GRP can soak up a considerable amount of water making the hull
heavier and weaker. The cure, depending on how badly the boat is affected, can
be to peal off the gelcoat, dry the core and then reapply a new gelcoat. A
small repair might be within the skill set of an amateur. A bigger job might
require professional expertise. A preventative measure might be to coat the
existing underwater aspects of the hull with a more water resistant material
such as epoxy.
Some GRP boats never suffer from Osmosis |
The
good news seems to be that despite the panic which ensued when it was realised
that GRP was not the ‘no-maintenance’ material it was originally thought to be,
there have been few (if any) reports of boats coming to grief because of it.
The
other good news is that, although it is wise to inspect any blistering on a GRP
boat hull, not all blisters are caused by Osmosis and some blistering is relatively
easy to cure. So how to tell the difference?
Osmotic Blistering
Blisters
caused by Osmosis will be found around or under the waterline. They may be
small pin pricks or large swellings as big as the palm of your hand. It would
be unusual to find osmotic blisters on a boat until she had been immersed for
several seasons. Puncture an osmotic blister and there will be fluid behind it.
Fluid in itself is cause enough for concern but if the fluid has a pungent
vinegary smell, the indications are towards osmosis. The smell comes from
acetic acid, due to the hydrolysation of the emulsion used as a binding agent
in the manufacture of chopped strand mat.
You
can confirm your diagnosis by using PH paper. A pH value of less than 7
indicates acid. Readings of between 5.5 and 3.0 are typical when water and
binding emulsion have joined forces. This would be indication enough to call in
the experts or consider whether you really want to pursue this purchase.
Non
Osmotic Blistering
This
can be found in the same areas as osmotic blisters, in clusters along or under
the waterline. They tend to be small pin pricks – rarely large. They tend also
to be much tougher than osmosis blisters. They can be very hard and difficult
to break. Most importantly though, when you do break into them they are dry and
there is no smell.
They
can occur for two reasons, either:
air was trapped between the gelcoat and the inner mat
during manufacture – and the voids have enlarged due to temperature changes;
there is a fault in the gelcoat due to the absorption
of water into the pigment or extenders
used in the gelcoat when it was manufactured.
Unlike
Osmosis, this problem is more likely to surface within a few months of
launching. The cure? Strip off the antifouling, degrease and sand the gelcoat,
fill the holes with epoxy and then apply an epoxy paint and re-antifoul. A
serious job – but not a disaster. – something a DIYer could probably cope with
Seaward
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