Osmosis - freshwater vs. seawater

[jrussill]

The Colvic Countess 28 hull will be built like a tank compared to the lighter Virgo Voyager

Yes she certainly has that feel to her!

 

[eilerts]

Only wooden boats dry out ashore.
GRP boats are just not getting any wetter when they are ashore.

GRP with an epoxy coat is as good as waterproof and won't get wet anyway.

The less dense the water the more likely it is to pass through the gelcoat. Therefore, as has been said, fresh is less dense that salt and warm is less dense than cold. A warm lake therefore the worst.

If water has passed through the gelcoat ("osmosis") it desolves into poorly mixed parts of the laminate, becomes more dense and will not come back out again. Bubbles are caused when the reaction builds up pressure and that's what we call osmosis - actually osmosis is the water going in and any boat without epoxy will have it to a degree.

The only way to dry a GRP hull is to peel off the gelcoat and jetwash every day until it is dry - you are not removing water to dry it, that evaporates in hours, you are removing the more dense substance. OK not the only way you can speed it up with steam, vacuum etc but jetwashing will do it after a few weeks.
Then replace the gelcoat with epoxy.

Frankly, a lot of BS served here, but it is not all wrong. The big picture is not far off, but the details are.

When water diffuses through the gelcoat it does so molecule by molecule. The density of water cold, hot, salt or fresh depend on the distance between the water molecules and the weight of the dissolved salt (or blister juice).
The process where water travels through the gelcoat is called diffusion. Osmosis is when a concentrated solution is trapped behind a semi permeable membrane and is diluted by water diffusing through the membrane.

If water can diffuse in it can certainly diffuse out, but it is not fast, and get slower at low temperature. So I too have had my doubt about the amount of drying I can expect during a winter ashore. I have sometimes heard stories about boats being hundreds of pounds lighter in the spring, but I dont believe that. If the weighting cell of the crane is calibrated for 100ton you should not expect great accuracy for a 5ton yacht. It is like using a bathroom scale when baking a cake.

I agree about the importance of washing. For osmosis treatment I think washing is actually more important than drying. I think I have read somewhere that you only need to get it dry enough for the epoxy to cure. The rest of the moisture will travel through the hull and dry out inward (great, if true! Have no idea about the source)

Even epoxy will let water diffuse through, but much less than polyester. All polymers with more or less random coiled molecular chains will have room enough in between the chains so that single water molecules can mingle through. Higher temperature and under-cured resin will result in more movement of the chains and easier migration of water molecules.

I tried to find some numbers for the diffusion of water through gelcoat, I could not find on the fly. It would be interesting to see the actual time it takes a hull to reach equilibrium water uptake. That would be the point when diffusion of water in through the gelcoat equals the rate of drying to the inside.

 

[Elessar]

Frankly, a lot of BS served here, but it is not all wrong.

Not all BS eh? Thanks for that. Tell me specifically what, in my simplified expaination, was actually wrong.

 

[HunterWanderer]

A grp laminator told me that he thought most blisters were caused by air bubbles trapped between the gel coat and the laminate during lay up.
Its actually difficult to avoid because the gel coat is applied with a brush and leaves brush marks. The first laminate bridges across the marks leaving small bubbles.
When the temperature changes the pressure in the bubble is less than the outside and moisture permeates through the gelcoat into the bubble. When it warms up the air is again expelled leaving moisture in the void. This carries on till the bubble contains only water then the warmth causes the water to hydraulically expand enlarging the bubble and distorting the surface.
This continues with temperature change till the gelcoat blisters can be seen.
When stripping off the gelcoat you can see the original bubble under the first coat of laminate.
So usually the problem is only in the gelcoat to laminate boundary and not a failure of the laminate at all and the gelcoat is only a cosmetic layer so these blisters are not of structural significance.
Applying a layer of epoxy works because it is far less permeable than the gelcoats used.

 

[Elessar]

A grp laminator told me that he thought most blisters were caused by air bubbles trapped between the gel coat and the laminate during lay up.
Its actually difficult to avoid because the gel coat is applied with a brush and leaves brush marks. The first laminate bridges across the marks leaving small bubbles.
When the temperature changes the pressure in the bubble is less than the outside and moisture permeates through the gelcoat into the bubble. When it warms up the air is again expelled leaving moisture in the void. This carries on till the bubble contains only water then the warmth causes the water to hydraulically expand enlarging the bubble and distorting the surface.
This continues with temperature change till the gelcoat blisters can be seen.
When stripping off the gelcoat you can see the original bubble under the first coat of laminate.
So usually the problem is only in the gelcoat to laminate boundary and not a failure of the laminate at all and the gelcoat is only a cosmetic layer so these blisters are not of structural significance.
Applying a layer of epoxy works because it is far less permeable than the gelcoats used.

When you pop the bubble, if it has water in it then it can be as you describe.
If it has acid in it it is osmosis. The smell of osmosis is a cross between vinegar and the corroded battery found in a torch at the bottom of a locker. When you smell it you will be in no doubt. And it is desolving the laminate but I’ve only once seen it bad enough for it to have effected it structurally. That was fixed by laminating mat on the hull before epoxying.

 

[graham]

Slight thread drift but I often notice hulls with no blisters have rudders covered in blisters.Anyone have any theories why?

 

[Elessar]

Slight thread drift but I often notice hulls with no blisters have rudders covered in blisters.Anyone have any theories why?

Because rudders are almost always cored and the core is full of water. They leak at the fittings or rudder post where a great deal of strain breaks the seal.

 

[pvb]

A grp laminator told me that he thought most blisters were caused by air bubbles trapped between the gel coat and the laminate during lay up.
Its actually difficult to avoid because the gel coat is applied with a brush and leaves brush marks. The first laminate bridges across the marks leaving small bubbles.

For higher quality, Bavaria spray the gelcoat into the mould, which ensures a smooth surface on which to start laminating. Interesting video of the process here - gelcoat spray is at around 2:40 in the video.

 

[Elessar]

For higher quality, Bavaria spray the gelcoat into the mould, which ensures a smooth surface on which to start laminating. Interesting video of the process here - gelcoat spray is at around 2:40 in the video.

Bavaria gelcoat is as good as it gets. Better than a rassey by miles.
Shame their keels are rubbish (that isn't a misinformed "falling off" quip - they often rust terribly when nearly new) though you used to be able to spec a lead one which is great, I think they dropped the option.

 

[pvb]

Bavaria gelcoat is as good as it gets. Better than a rassey by miles.

Having owned both, I'd agree.

 

[Yorkshire Exile]

"I have sometimes heard stories about boats being hundreds of pounds lighter in the spring, but I dont believe that. If the weighting cell of the crane is calibrated for 100ton you should not expect great accuracy for a 5ton yacht. It is like using a bathroom scale when baking a cake."
Read more at http://www.ybw.com/forums/showthrea...shwater-vs-seawater/page2#7FESUDKBgsHUbzE4.99

Now I know were my cake making is going wrong.

 

[eilerts]

Not all BS eh? Thanks for that. Tell me specifically what, in my simplified expaination, was actually wrong.

I had a feeling that the expression would cause me some trouble. Here we go.

It is hard to not interpret you words about denser and less dense water to anything else than density, i.e. the mass of a given volum of water. The density of water is not a factor in the phenomenon osmosis. In relation to the topic of the discussion it is an covariance. Hot fresh water is not bad because of its density, but because the hull too will be hot, and the absence of salt will not lower the osmotic potential of the blister juice.

When it comes to the importance of washing, the good properties of epoxy, I agree. Whether GRP boats does not dry at all or just dries slowly is debatable. I have no actual data on this.

 

[eilerts]

A grp laminator told me that he thought most blisters were caused by air bubbles trapped between the gel coat and the laminate during lay up.
Its actually difficult to avoid because the gel coat is applied with a brush and leaves brush marks. The first laminate bridges across the marks leaving small bubbles.
When the temperature changes the pressure in the bubble is less than the outside and moisture permeates through the gelcoat into the bubble. When it warms up the air is again expelled leaving moisture in the void. This carries on till the bubble contains only water then the warmth causes the water to hydraulically expand enlarging the bubble and distorting the surface.
This continues with temperature change till the gelcoat blisters can be seen.
When stripping off the gelcoat you can see the original bubble under the first coat of laminate.
So usually the problem is only in the gelcoat to laminate boundary and not a failure of the laminate at all and the gelcoat is only a cosmetic layer so these blisters are not of structural significance.
Applying a layer of epoxy works because it is far less permeable than the gelcoats used.

To my opinion, this is wrong as a mechanistic explanation for osmosis blisters, and there are some evidence that speaks against it. It is correct that it is almost impossible to avoid small trapped bubbles. The most common place to find trapped air is in the chine and in steps. When osmosis occurs the blisters are not particularly clustered at these places. The mechanism also imply that a vacuum moulded hull cannot get osmosis and that using vinyl ester in the outer layers, or doing the complete layup in VE or epoxy would have no effect on osmosis.
When it comes to why a blister is at a specific position, and not 5cm up or aft, I believe trapped air may play role.

When you grind a blister you find a spot of "dry" glass behind. It is easy to think that this comes from poor wetting, but it is more likely that it is created by the blister. The blister juice is acidic and acid speeds up the breakdown of polyester bond. Normally, when gelcoat is peeled off a osmosis boat you stop at the water line. If you continue all the way up, you will see no more dry spots and the laminate get darker and more translucent.

I have also heard suggested that freezing of trapped water during winters ashore was the cause of the blisters. That does not make sense as there are far more osmosis in regions that seldom see subzero temperatures.

 

[jwilson]

snipped - Whether GRP boats does not dry at all or just dries slowly is debatable. I have no actual data on this.

Boat (not mine) lifted out on a nice dry day late last autumn, after 2 years in the water. A few days later (again dry and sunny) the hull was showing typically 50-60 on the 01-100 scale of a Tramex moisture meter on GRP setting.

Last month checked again after a winter ashore, moisture meter readings now typically 35-50.

My feeling is that wintering ashore clearly reduces time in water, and hence absorption, but also some drying does take place in a UK winter even through antifouling.

 

[Kukri]

Boat (not mine) lifted out on a nice dry day late last autumn, after 2 years in the water. A few days later (again dry and sunny) the hull was showing typically 50-60 on the 01-100 scale of a Tramex moisture meter on GRP setting.

Last month checked again after a winter ashore, moisture meter readings now typically 35-50.

My feeling is that wintering ashore clearly reduces time in water, and hence absorption, but also some drying does take place in a UK winter even through antifouling.

I've had a similar result. Martin Evans tells me this is usual but he knows of a boat that has been ashore in Greece for a couple of years and which has not dried out much at all.