Horrible Osmosis Shock

[Frayed Knot]

A while ago we bought a then 38 year old, 38’ grp boat. Although it had been on the hard for several months we were aware that it had been afloat continuously for five or six years.

We had a pre purchase survey by a well known, well qualified surveyor who stated that, for it’s age it was the driest boat he’d ever seen - he even tested his moisture meter on a couple of nearby boats to check it was actually working.

Fast forward five & a half years & we’ve just had the old, built up layers of antifouling removed & discovered the hull to be not only blistered, but the moisture levels over the entire hull are, quite literally, “off the scale” - even to a point several inches above the waterline.

During our ownership the boat has had two, non consecutive, full years afloat & three winters ashore; twice for about four months & once for seven.

I find it difficult to believe such a rapid deterioration could be possible. Has anyone else had similar experience?

 

[RunAgroundHard]

Moisture readings are sensitive to time out of the water. If the hull is drying, it could take a while for the levels to go down, like weeks or months. Internal water in bilges or captured in stringers will show up as high moisture readings, so check the bilges are dry. Something as simple as heavy condensation will show up. This can be tested. With the moisture meter on the hull, say at a cockpit cave locker, low reading, place a plastic bottle of water inside the locker against the surface being measured and the reading goes through the roof.

Keep taking readings and see if they are coming down over time, which means that the moisture is passing through the hull and drying out. Question, did you apply epoxy coating when you bought the boat new and before launching when the gull was dry? If so, epoxy can trap moisture that would otherwise dry out ad cause blisters. If not, are you in warm water, fresher water than the sea? If the boat ws in cold saltwater, fresher, warmer water can cause osmosis that was otherwise not forming, although the conditions existed for osmosis.

Good news it is not likely to be disastrous, and DIY monitoring and addressing without ripping half the laminate off is a reasonable solution.

My old boat, readings go away over the winter layup, but are high when I take her out the water, no blisters, but in the past she did have, which a previous owner addressed.

 

[Refueler]

What meter was used by 'first surveyor' and by 'second surveyor' ?

If first used the usual Sovereign - then it literally needs to be in a bucket of water to read high !

If second was a Tramex Skipper - then it is far more sensitive and its not unusual for an old boat to have very high results.

I have two meters ... Protimeter for general - which equates to about Sovereign results and a Skipper for use during Osmosis treatment.

 

[Frayed Knot]

Thanks for the replies. The boat has never been epoxied & I’ll look into what meters were used.
I plan to re prime & antifoul for this season then reassess next autumn.

 

[bitbaltic]

Moisture meters are reading resistivity (or conductivity) and the minimum depth at which they read a value is roughly equal to the offset (distance) between the two probes. So if they are an inch apart they are reading the moisture value (Or conductivity) of material an inch behind the surface of the hull. This is why the plastic bottle thing produces a ‘wet’ reading (and assuming there is some amount of dissolved solute in the water in the bottle).

Quite academic if you have blisters and most likely the blisters were always there but too shallow (relative to the probe spacing) to have been detected until you stripped the antifoul and found them.

 

[Frayed Knot]

What meter was used by 'first surveyor' and by 'second surveyor' ?

If first used the usual Sovereign - then it literally needs to be in a bucket of water to read high !

If second was a Tramex Skipper - then it is far more sensitive and its not unusual for an old boat to have very high results.

I have two meters ... Protimeter for general - which equates to about Sovereign results and a Skipper for use during Osmosis treatment.

The survey 5 years ago used a Tramex on B scale & readings were between 18 & 30. I’ll find out what the boatyard used tomorrow

 

[bignick]

this is useful reading…
https://www.passionforpaint.co.uk/product_uploads/15574838191.pdf

 

[Elessar]

Moisture readings are sensitive to time out of the water. If the hull is drying, it could take a while for the levels to go down, like weeks or months. Internal water in bilges or captured in stringers will show up as high moisture readings, so check the bilges are dry. Something as simple as heavy condensation will show up. This can be tested. With the moisture meter on the hull, say at a cockpit cave locker, low reading, place a plastic bottle of water inside the locker against the surface being measured and the reading goes through the roof.

Keep taking readings and see if they are coming down over time, which means that the moisture is passing through the hull and drying out. Question, did you apply epoxy coating when you bought the boat new and before launching when the gull was dry? If so, epoxy can trap moisture that would otherwise dry out ad cause blisters. If not, are you in warm water, fresher water than the sea? If the boat ws in cold saltwater, fresher, warmer water can cause osmosis that was otherwise not forming, although the conditions existed for osmosis.

Good news it is not likely to be disastrous, and DIY monitoring and addressing without ripping half the laminate off is a reasonable solution.

My old boat, readings go away over the winter layup, but are high when I take her out the water, no blisters, but in the past she did have, which a previous owner addressed.

What you just said is maybe true of a wooden boat. For GRP it is very far from true. GRP boats do not dry out on the hard in any meaningful way.

 

[RunAgroundHard]

What you just said is maybe true of a wooden boat. For GRP it is very far from true. GRP boats do not dry out on the hard in any meaningful way.

They do. Water molecules can pass through the grp, or at least older resin formulas. The solutes of osmosis don’t because the molecules in the solution, post reaction, are too large. Its part of the reason why the blisters form. With older resin formulas, new hulls measured after a season afloat would show increased moisture. A week or so later on the hard, the levels drop away. Water molecules have to pass through to react with uncured chemicals in the resin, otherwise there would be no issues.

 

[Refueler]

They do. Water molecules can pass through the grp, or at least older resin formulas. The solutes of osmosis don’t because the molecules in the solution, post reaction, are too large. Its part of the reason why the blisters form. With older resin formulas, new hulls measured after a season afloat would show increased moisture. A week or so later on the hard, the levels drop away. Water molecules have to pass through to react with uncured chemicals in the resin, otherwise there would be no issues.

Both of you are partially correct ...

The moisture passes through and mixes - increases density. Majority can then not pass back out - causing the blisters.

The resulting fluid is thicker and usually brown in colour with that vinegar smell ...

Its thickened further due to only a small amount of moisture exiting.

The matter is that once moisture has passed through ... it will never exit fully. It can only exit in very small % ....

An earlier post talks about water in bilges ertc. This is true and many surveys I carried out - I have identified internal structure in a hull that have suffered water ingress.... such as plywood frame ends still carrying water ..

 

[dankilb]

When we had our 35 year old Jen blasted back to gelcoat after many years of hard storage we found a bad case of ‘incipient osmosis’ under the old antifoul, despite low to average sovereign meter readings. This took the form of an extensive rash of very small blisters (these had in turn dried out over time and would only have been revealed by blasting - I reckon even sanding would have simply smoothed them over).

Our age/model is known for osmosis and my point is that we simply found it and addressed it just in time. Someone has to have the misfortune of owning the boat at the time the osmosis manifests (these models all seem to get it and I could comment separately on why I think this is!).

Had the boat relaunched with all those small ‘incipient’ blisters concealed under the antifoul, some dried out and slightly cracked open (but almost all smaller than a 5p and barely proud of the surface), but not giving high moisture readings - we would’ve had one hell of a rash appearing after a year or so afloat.

Dry meter readings just means the laminate isn’t currently wet - not necessarily that it isn’t already osmotic!

 

[oldgit]

Bought a home completed boat based on a Aquafibre hull sold in 1973, (moulded possibly before that ) spent much of its early life ashore being fitted out.
No evidence of blisters was found in any following surveys , when the boat was sold on , the purchasers survey did not raise any concerns regards either osmosis or blisters.
As far as I know the boat is still floating 50 years later.

 

[Elessar]

They do. Water molecules can pass through the grp, or at least older resin formulas. The solutes of osmosis don’t because the molecules in the solution, post reaction, are too large. Its part of the reason why the blisters form. With older resin formulas, new hulls measured after a season afloat would show increased moisture. A week or so later on the hard, the levels drop away. Water molecules have to pass through to react with uncured chemicals in the resin, otherwise there would be no issues.

Water levels drop after about 12 hours or maybe less. Like a road drying. After that nothing.

If it’s passed through the gelcoat it becomes denser. The water won’t pass through unless it is sucked through and it quite simply doesn’t get sucked back out.

The OPs situation is a mystery. But your “solution” is simply not true.

 

[ChromeDome]

Anybody ever heard of a boat sinking due to osmosis?

 

[Bobc]

Anybody ever heard of a boat sinking due to osmosis?

Exactly. It's a nice earner for the industry though.

 

[RunAgroundHard]

Water levels drop after about 12 hours or maybe less. Like a road drying. After that nothing.

If it’s passed through the gelcoat it becomes denser. The water won’t pass through unless it is sucked through and it quite simply doesn’t get sucked back out.

The OPs situation is a mystery. But your “solution” is simply not true.

I don't agree with you and I think your understanding is dated and has been superseded by more accurate understanding of the facts. The water does not become denser. if the water contacts uncured chemicals it reacts and forms another compound that is both larger and denser. The process is not osmotic, that has been known for a long time. GRP has pore spaces and permeability, the pressure differential due to hydrostatic head of water and air inside the hull is what drives molecules through the GRP in insignificant volumes as far as mass is concerned. However, the volumes are enough to react with uncured chemicals.

It is fact that my 1974 hull moisture readings decrease over the weeks and months ashore, in winter, in Scotland.

 

[ChromeDome]

Exactly. It's a nice earner for the industry though.

In my view the concern is much bigger than the risk.

 

[dankilb]

Not sure how helpful the ‘never sank…’ point is for the OP. It doesn’t reassure around resale, labour required to make a cosmetic/fair fix, shock of realising condition is not as surveyed/assumed, etc.

We did a completely DIY fix (and a fix was deemed necessary to have a fair, blister free boat we could confidently offer to market in future) and it took several months of solid, sometimes backbreaking, work and several thousand in materials (that was using ‘cheap’ Jotun rather than Int’l etc.).

 

[ChromeDome]

A fair approach.

But unless planing a sale, not urgent or preventing use of the boat

 

[Bobc]

Usually cheaper to sell the boat at a knock-down price than getting the work done.