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Problems Found in Marine Surveys


Osmosis is often found in marine surveys and is a big worry for GRP boat owners and buyers.   

What is Osmosis?
Osmosis occurs 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. 

Why 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

Yacht and Boat surveys around Neyland and Milford Haven including :-
·         Neyland Marina
·         Milford Marina
Rudders Boatyard

To request a quote for a marine survey in Neyland (or elsewhere) click the link or telephone 07709 376 588.



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 forgotten. 

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 danger.

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 to repair.

Major failure!

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 have sunk. 

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

The 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 was found.


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

Osmotic Blisters


A Burst Osmotic Blister


Large Osmotic Blisters


Problems found in the Hull Structure

 Faults in GRP hulls can be due to :-

poor moulding

de-bonding between components and
·         poor repairs  


The example opposite shows the inside of a central locker under the well deck of an angling boat. 

The external hull is vee shaped and a flat panel has been laid over the vee and bonded to the hull at the sides.  The gap between the hull and the panel is filled with foam. 

The bonding between the sides of the panel and the hull has failed and water has leaked into the gap and soaked the foam.  The wet foam will need to be replaced which is likely to involve repairs to most of the inside of the bottom of the boat. 

This relatively harmless looking crack is likely to be an expensive repair job.

Debonded Plywood Deck

The picture opposite shows the well deck on an angling boat which is constructed from GRP sheathed plywood.

The areas marked in chalk are where the GRP has de-bonded from the plywood and allowed water to get in and rot the ply. 

This is a common problem with GRP sheathed plywood and most of this deck will need to be replaced.

Badly Repaired Hull

The picture opposite shows a section of a GRP hull that has been badly repaired. 

The repaired section is de-bonding and cracking away from the main structure.  The chalked area will need to be cut out and the repairs done again. 

Four small screw holes were also found on this hull which were at least 50mm deep.  The holes went right through the GRP shell and into foam buoyancy on the inside.  It was only because the foam slowed down the leak that the boat had not sunk.

Moulding Fault

The picture opposite shows a spray rail on a motor cruiser with the marine surveyor’s spike through the gel coat and in at a depth of at least 30mm.  

Hammer soundings on the hull had indicated hollow areas on the spray rail and over an area 58cm x 8cm the gel coat was found to flex.  There is a foam core in the hull and poor moulding has left voids between the foam and the gel coat. 

The gel coat was thin enough to get a spike through easily and if it had cracked it would have allowed water into the foam which would have been very expensive to repair.

Perished Petrol Supply Pipe
Missing Exhaust Clamp

Plastic Fuel Pipe


Out Drive Completely Sheared Off!


Leaking Hydraulic Ram on Out Drive


Dezincification in Bronze Propeller

Rigging, Stanchions and Boarding Ladders

A wide variety of problems can be found with rigging and associated items such as guard rails and boarding ladders.

This picture opposite shows a turnbuckle used in a tie rod for a main shroud.  The bottom of the turnbuckle is screwed into the tie rod anchorage but as the picture shows very little of the thread is engaged giving serious doubts as to its strength.


A bent swaged terminal on a shroud. Together with kinks in the wire this could seriously weaken the standing rigging.


This picture shows the inside of a transom and the cluster of 3 bolts are supposed to fasten the boarding ladder.  Not only are there no washers or pads to spread the load but there are no nuts to secure the bolts!

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