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Boat Maintenance
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1
The following is a brief explanation of the condition usually referred to as
“Osmosis” or the pox and a general outline of how I see the subject.
The original definition of osmosis is the biological process where by a liquid
(usually water) will pass through a semipermeable membrane ie in this case the
gelcoat. The gelcoat is permeable to the water molecules but not the various
compounds dissolved within the moisture of the hull.
The term “osmosis” was adopted by the boat industry in the early 70’s to
describe the blistering found on many GRP boat hulls and is now in common use.
The osmotic process probably does occur within the blisters but is not the
only process involved and is not the full story.
A GRP (glass reinforced plastic) boat hull is a mixture of (usually) polyester resin
reinforced with glass fibre matting, built up in layers. The final hull laminate
will have an approximate ratio of 30% glass to 70% resin. This laminate is not
homogeneous, even within a well built GRP hull there will be small voids, air
pockets and microcracks within the Hull lay up and at the interface between the
resin and the glass fibre.
Water can enter into, and through the gel coat and into the laminate
(the polyester resin and the glass fibre mats) as water molecules, not the liquid
form. So a boat hull can absorb a maximum of approx. 25% water in this way on
a sovereign moisture meter . Water may pass slowly through a GRP hull in
this way and disperse in the bilges as water vapour. The moisture content of a
new hull will slowly increase during the first few seasons that she is afloat,
the moisture content will similarly reduce slowly when she is out of the water.
As mentioned earlier there are various small voids within the laminate. The water
molecules can collect and condense within these. Within the GRP laminate and
the micro-voids various water soluble components will be found. These are
solvents, locked in during the boats build process and manufactures constituents
from the resin manufacturing process.
The moisture within the hull matting voids, is able to dissolve and chemically
react with these components. This process is known as “hydrolysis”.
“Hydrolysis” will continue with the voids enlarging, a dissolved solution is
formed, the main ingredients being, acetic and hydrochloric acid and glycol.
These products give “osmotic fluid” its characteristic are a vinegary smell black
or grey in colour and it has a greasy texture.
The glycol in particular is “hydroscopic” (water absorbing). Once this is released
into the voids it will accelerate the rate of water absorption into the laminate.
This process will now continue and will not be reversed by simply taking the
boat out of the water. Moisture content will drop slowly if left ashore but will rise
again fairly rapidly when immersed again. The various hydrolysed products
cannot pass back out through the polyester gel coat / laminate but the water
molecules can.
As this process continues, at some point, the concentration of the vinegary
substance within the voids will become greater than the concentration of the
water the vessel is floating in (sea water). At this point the “osmotic” process
occurs and more water is drawn in and the apparent blistering will occur .
The interface between the glass fibres and the resin can also be broken down.
The binder used on the glass fibres (particularly emulsion bound mats with
polyvinylacetate binder) are water soluble. This can allow liquid water to pass
along the fibre bundles, producing some swelling at the fibre ends and the
characteristic “wicking” or “fibre aligned blisters”.
As this continues the voids are increased in size by “Hydrolysis” and the
pressure within is increased by “Osmosis”. At some point the pressure may
become too high for the surrounding material to support and a blister will
be formed.
This all sounds fairly alarming, however it is important not to over react.
Many boats are used for years in this condition, at this stage the processes
are chemical with very little loss of mechanical strength of the
laminate / or hull structure. Many boat hulls may take 10 - 20 years or
longer to reach this stage.
As this process continues, moisture continues to be absorbed, the
laminate break down accelerates and more blisters are formed. In time
some larger blisters may develop within the laminate as well as those
more commonly occurring between the gel coat and laminate.
Eventually at this stage, treatment will be required
“Osmosis” or the pox and a general outline of how I see the subject.
The original definition of osmosis is the biological process where by a liquid
(usually water) will pass through a semipermeable membrane ie in this case the
gelcoat. The gelcoat is permeable to the water molecules but not the various
compounds dissolved within the moisture of the hull.
The term “osmosis” was adopted by the boat industry in the early 70’s to
describe the blistering found on many GRP boat hulls and is now in common use.
The osmotic process probably does occur within the blisters but is not the
only process involved and is not the full story.
A GRP (glass reinforced plastic) boat hull is a mixture of (usually) polyester resin
reinforced with glass fibre matting, built up in layers. The final hull laminate
will have an approximate ratio of 30% glass to 70% resin. This laminate is not
homogeneous, even within a well built GRP hull there will be small voids, air
pockets and microcracks within the Hull lay up and at the interface between the
resin and the glass fibre.
Water can enter into, and through the gel coat and into the laminate
(the polyester resin and the glass fibre mats) as water molecules, not the liquid
form. So a boat hull can absorb a maximum of approx. 25% water in this way on
a sovereign moisture meter . Water may pass slowly through a GRP hull in
this way and disperse in the bilges as water vapour. The moisture content of a
new hull will slowly increase during the first few seasons that she is afloat,
the moisture content will similarly reduce slowly when she is out of the water.
As mentioned earlier there are various small voids within the laminate. The water
molecules can collect and condense within these. Within the GRP laminate and
the micro-
from the resin manufacturing process.
The moisture within the hull matting voids, is able to dissolve and chemically
react with these components. This process is known as “hydrolysis”.
“Hydrolysis” will continue with the voids enlarging, a dissolved solution is
formed, the main ingredients being, acetic and hydrochloric acid and glycol.
These products give “osmotic fluid” its characteristic are a vinegary smell black
or grey in colour and it has a greasy texture.
The glycol in particular is “hydroscopic” (water absorbing). Once this is released
into the voids it will accelerate the rate of water absorption into the laminate.
This process will now continue and will not be reversed by simply taking the
boat out of the water. Moisture content will drop slowly if left ashore but will rise
again fairly rapidly when immersed again. The various hydrolysed products
cannot pass back out through the polyester gel coat / laminate but the water
molecules can.
As this process continues, at some point, the concentration of the vinegary
substance within the voids will become greater than the concentration of the
water the vessel is floating in (sea water). At this point the “osmotic” process
occurs and more water is drawn in and the apparent blistering will occur .
The interface between the glass fibres and the resin can also be broken down.
The binder used on the glass fibres (particularly emulsion bound mats with
polyvinylacetate binder) are water soluble. This can allow liquid water to pass
along the fibre bundles, producing some swelling at the fibre ends and the
characteristic “wicking” or “fibre aligned blisters”.
As this continues the voids are increased in size by “Hydrolysis” and the
pressure within is increased by “Osmosis”. At some point the pressure may
become too high for the surrounding material to support and a blister will
be formed.
This all sounds fairly alarming, however it is important not to over react.
Many boats are used for years in this condition, at this stage the processes
are chemical with very little loss of mechanical strength of the
laminate / or hull structure. Many boat hulls may take 10 -
As this process continues, moisture continues to be absorbed, the
laminate break down accelerates and more blisters are formed. In time
some larger blisters may develop within the laminate as well as those
more commonly occurring between the gel coat and laminate.
Eventually at this stage, treatment will be required
