OSMOSIS
Osmosis
is a condition that affects GRP boats, cars and water tanks and manifests
itself by the appearance of blisters between the different layers of
the layup.
Polyester
resin is slightly porous. Moisture does after a time travel through
it and will collect in any voids in the GRP construction. These blisters
can be very small, about the size of a half of a hundred and thousand
cake decoration, about the size of a split pea or up to the size of
half a cherry. Any bigger than this is described by surveyors as being
serious delamination.
The
problem is caused by many different factors or a combination of a number
of factors. Osmosis is more common on vessels used in warm waters or
fresh water lakes or rivers. I have seen terrible osmosis in yachts
in yards around Lake Geneva. They keep on using them and apart from
being a little heavy due to water ingress not much else happens and
they are fine to carry on using. Boats used in the Lake District tend
to suffer from Osmosis more than vessels found in the cold salt water
of England. My own Hurley has bad osmosis in its water tank and also
under any areas of Treadmaster decking. This is where warm rainwater
has collected. I carry on using the water tank and don't notice a lot
of smell. I use plastic containers of water from Tesco to drink or brush
my teeth.
Reasons
for osmosis.
-
Failure to mix the resin and catalyst together properly during manufacture
which creates pockets of uncured resin;
-
Leaving
the layers to cure for too long between coats of resin. For example
over night or over a weekend;
-
Poor
attention to humidity levels in the workshop where the boat was
layed up. If condensation is allowed to form between the layers
the gelcoat or resin will not adhere properly;
-
Poor
levels of cleanliness in the workshop. Contamination by dust etc.
So
the moisture travels through the one or two layers of gelcoat (resin
and pigment) towards the layers of matting or woven rovings which are
wetted in with resin. The moisture collects in pockets and creates areas
of high pressure, i.e. the chemicals in the dissolved resin mix with
water to form a mixture that has a higher pressure than that of the
water outside the hull. Osmosis is often most evident when a vessel
is first taken out of the water. If the sun is strong the warmth will
increase the volume of the water in the blister pockets and make it
expand. The blisters will become larger. If you burst a proper osmotic
blister you might be able to smell acetic acid. You should taste it
too by rubbing your finger into the blister. The acetic acid is a product
of the styrene and acetone used as solvents in the resin.
Osmosis
is commonly found on boats from the mid-seventies, which was around
the time of the oil crisis. Boat builders were experimenting with alternative
types of synthetic catalysts to save money. Not all worked. Lucky for
us Hurley used standard isophthalic resins which were of good quality
and the boats were well made.
Modern
boats are built in workshops where humidity is controlled. Boat builders
work throughout the clock to ensure that curing and layering times are
kept precise. Resins are now advanced. However, USA and Canadian boats
can suffer as their governments have forced boat builders to reduce
styrene levels by using alternative resins.
If
you go Emsworth Harbour near Chichester you will see lots of almost
new Trader motor yachts being treated for osmosis under warrantee. Emsworth
is shallow, the water is warm and brackish. Same thing happens to boats
in Christchurch. There are loads of Westerly Centaurs in Christchurch.
Many suffer from osmosis but don't come to come to any real harm. The
owners just antifouled over the blisters.
Alternatives.
What to do.
-
Ask
a surveyor who is passing how bad the blisters are and ask him or
her to check moisture content.
-
If
the blisters are small or the vessel is of low value then just carry
on using the boat as if nothing had happened.
-
Monitor
the blisters each year. Take photographs and record moisture levels.
However, the moisture levels will change depending on atmospheric
conditions and should be taken as a relative reading against the
reading taken of the topsides above the waterline. Typically three
or four times higher below that waterline compared to above is normal.
Typically a Sovereign meter the reading above would average 4 and
below would be between 14 and 19. This would be fine for a Hurley
but wet for a newer boat or a Motorboat. A condition called wicking
might occur where moisture 'wicks' up through the glass fibres to
the topsides and creates small blisters around the waterline. This
is not a great problem.
Cures.
-
Do nothing and carry on using the boat knowing that the value of
the boat might be affected;
-
Remove
the gelcoat by pealing it off by a hired plane type device, by having
it slurry blasted of or scraping it off with a scraper and hot air
paint stripper. This last way is messy, might damage the laminate,
might poison you with fumes or might set fire to the boat but is
cheap.
You
will need to remove all of the gelcoat and allow the hull to dry for
a very long time until the moisture levels fall to about the same below
the waterline compared to what they are above the waterline. This will
take up to nine months but depends on climate. Drying times can be speeded
up by using infra red lamps or de-humidifiers but this is expensive.
It is not a good idea to paint epoxy resin onto a damp hull. This will
just trap moisture in and cause the lay-up to delaminate. If this happens
the structure will be scrap. Epoxy
resin should be applied to a dry hull stripped of gelcoat or you will
be wasting your money.
This
is a simplistic explanation of osmosis. There will be lots of folk who
will disagree as it is a very contentious issue.More about why boats
aren't just built of epoxy resin later. That is a different story.
Nick
Osmosis
treatment of Hurley 24 'Circe'
Click
on a picture to see a larger version
Thanks to Stuart
Shaw for supplying the pictures