Osmosis is often found in marine surveys and is a big worry for GRP boat owners and buyers.
What is Osmosis?
when water is absorbed through the gel coat of the hull and finds voids in the GRP layup. Here it mixes
with chemicals in the GRP and forms a solution which attracts in more water thus increasing the pressure in the void and causing
the gel coat to bubble and form a blister.
is it a problem?
Osmosis rarely develops so far that it needs to be treated on the grounds
of safety and many boats can show signs of osmosis for years without needing special attention. That being
said, many boat owners feel the need to treat it and a full cure is expensive. For this reason any physical
signs of osmosis such as blisters or indications that osmosis might develop such as high moisture readings will lead to a
significant decrease in the value of the boat. The marine surveyor will advise you on how far the condition has progressed
and the likely consequences.
Should you buy a boat with Osmosis?
It is quite feasible to buy a boat with osmosis provided the purchase price is reduced sufficiently to allow for
a full osmosis treatment if required. It may be that you never need to treat the osmosis but when
you come to sell the boat the buyer will expect a similar reduction in price if osmosis is still present. If you are buying the boat with a loan the finance company may regard any suggestion of osmosis as a problem that
needs to be corrected before they will lend you any money.
What conditions are required
for osmosis to develop
The process needs a permeable gel coat, small voids in the hull
and the boat to be in the water for a long period without a chance to dry out. The resins used in early GRP boats were relatively permeable but over the years they have improved and on most modern
boats the gel coat is much less impermeable so in theory they are less susceptible to osmosis. Small voids are formed in the hull during the layup process and the better the quality of the layup the fewer the
voids. In older boats they tended to use much more resin which reduced the chances of voids but as resin
prices increased the amount used, decreased. With modern boats poor quality control can allow areas of
the laminate to be starved of resin which creates small voids. Many boats are kept in marinas all year
long whereas in the past they might have been on moorings during the summer and stored on land during winter.
Fresh water can also cause more problems than salt water. Such long periods in the water promote
osmosis. As there are a number of factors affecting osmosis it is difficult
to predict if it will occur and how quickly it will develop.
What does the marine
surveyor look for
Surveyors look for blisters which show that osmosis has developed and
they use a meter to look for high moisture levels in the laminate as an indicator that osmosis might develop.
High moisture levels show that the GRP is absorbing water but until the water can collect in a void the osmotic process
will not start and the hull will not blister.
Why are high moisture levels emphasised in a survey
A marine surveyor cannot
determine the quality of a GRP laminate and whether it has sufficient small voids to allow osmosis to develop.
They can however measure the moisture content of the hull and if this is high then it is advisable to assume the worst
and warn that there is a significant risk of osmosis developing.
Marine Surveys in West Wales
Crack Indicating Debonding of Floor Panel
GRP Sheathing Debonding from Plywood Deck
A Badly Repaired Hull
Voids in a Spray Rail
Problems With Engines and Related Systems
The visual inspection of the engine installation during a marine survey can show poor maintenance
and a disregard for regulations when installing components.
Perished Petrol Pipe
The picture opposite shows the perished fuel feed hose on a petrol engine. Any
leak in this hose would be a major fire hazard.
Missing Exhaust Clamp
The picture opposite shows an exhaust hose which has been joined but whoever did the job ran out
of hose clamps. No doubt they fully intended to go back and rectify the problem but such things are easily
Hose clips on all types of systems are frequently found to be missing in marine
surveys. Whilst the hose may seem to be tight, over time they can be loosened by vibration and become a
Plastic Fuel Pipe
In the picture opposite we see a professional installation of a fuel system for a twin engine
motor boat with pipes leading to the fuel filters.
At some stage someone has renewed one of
the hoses with plastic pipe which is a fire hazard.
All fuel pipes should be metal or confirm
to the standard set out in ISO 7840. This has probably been done by an owner who simply did not appreciate
the risks involved and was unaware of the regulations.
Out Drives and Propellers
Out Drives are a significant source of problems found in marine surveys. They are in an
exposed position and should be serviced regularly as poor maintenance can cause serious mechanical damage that is very expensive
The picture opposite shows the Out Drive shield and part of the forks; the rest of the drive
has sheared off and was only held in place by the bellows.
If these had failed the boat would
This happened for no discernible reason when the engine was put into gear and the
most likely explanation was that poor maintenance caused the system to seize up.
Split Exhaust Hose on Out Drive
Dezincification in Bronze Propeller
picture opposite shows dezincification in a bronze propeller. This is a process whereby zinc is leached
out of the bronze leaving a pit containing a copper deposit. The pitting itself reduces the strength of
the casting and further weakness may be caused by the changes to the chemical composition of the metal. Poor
quality bronze and porous castings are the usual cause for such problems.The pits were 1-2mm deep and found on the blades and blade root. Scraping the
blade revealed areas where the pits had joined to form areas over 1cm in diameter and in one of these depressions a 1cm crack
Turnbuckle Nearly Unscrewed
A crack in a rigging toggle on a shroud which could lead to a de-masting
This picture shows a crack in the aluminium base of a stanchion.
This is a common fault and if a crew member fell against the stanchion there is a good chance that it would fail and
potentially, someone could fall overboard