Osmosis - freshwater vs. seawater

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jrussill

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Hi all,
Does anyone know if osmosis is more likely to develop on a boat moored in freshwater or seawater? It's a bit of an academic question but interested in your thoughts.
Thanks
 
If the boat is going to get osmosis, it will happen faster in freshwater than in seawater. But lots of boats aren't prone to osmosis.
 
pvb said:
If the boat is going to get osmosis, it will happen faster in freshwater than in seawater. But lots of boats aren't prone to osmosis.
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Freshwater worse than seawater. Warm water much worse. The freshwater thing makes me smile when I see fair-sized yachts advertised as ".... only used in fresh water" as if it's a big plus. though to be fair absence of salt does reduce some corrosion in alloy fittings etc.

I don't think there are any GRP yachts that are not prone to osmosis - eventually. I have heard a surveyor say that he's seeing modern gelcoat resin boats with osmosis as owners think they're immune and thus keep boats full-time afloat. Overall though even with eventually some osmosis GRP is a pretty good material. And a few blisters doesn't necessarily mean huge bills.
 
jwilson said:
Freshwater worse than seawater. Warm water much worse. The freshwater thing makes me smile when I see fair-sized yachts advertised as ".... only used in fresh water" as if it's a big plus.
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Just think of the lack of anti-foul on its bottom. ;)
 
I am not any sort of expert beyond reading Hugo du Plessis masterwork and being present at a couple of surveys of GRP boats. I don't think that visible blisters are a reliable indication; the problem is high moisture levels in the grp layup which eventually reduce its strength. What you don't want to see are white looking fibres. Certainly grp boats benefit from an opportunity to lose some moisture, annually.
 
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Hi, thanks for the feedback. Very useful, and I guess the exacerbation in freshwater is down to chemistry. Our boat is in freshwater (Cardiff Bay) and fairly newly acquired so we've not lifted her out yet.

Netx question - how long should a boat be out of the water for each year to make it worthwhile with respect to drying out a bit?
 
jrussill said:
Netx question - how long should a boat be out of the water for each year to make it worthwhile with respect to drying out a bit?
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I've never been convinced that it's necessary to allow the hull to "dry out" ashore, and I certainly don't believe any significant drying is likely to occur over a typical British winter. So if you really want to give it chance to "dry out", you'd need to leave it ashore for a summer at least.

Thousands and thousands of boats are afloat all year round without major problems. If a boat is destined to get osmosis, it'll happen sooner or later, regardless of whether it gets lifted out for periods.
 
jrussill said:
Hi, thanks for the feedback. Very useful, and I guess the exacerbation in freshwater is down to chemistry. Our boat is in freshwater (Cardiff Bay) and fairly newly acquired so we've not lifted her out yet.

Netx question - how long should a boat be out of the water for each year to make it worthwhile with respect to drying out a bit?
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Is it an old boat worth not a great deal? If so I wouldnt worry about it and just go sailing. If its a newer expensive boat then theres lots to worry about,osmosis probably being well down the list.
 
Yes, it's an old boat (1980ish Virgo Voyager) and not worth a great deal in the grand scheme of things. It's mainly for my interest. But we are looking at a larger Colvic Countess 28 that has probably been in the water for years too (and also not worth much), so I'm trying get a feel of how much to worry without paying to have her lifted.
 
vyv_cox said:
The only osmosis in our Sadler is in the shower tray and the heads sink.
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Also very common inside GRP water tanks, as fitted in the bilges of many Nicholson. Usual cure is a flexible liner, as you can't really do much to fix it.
 
Here is a rather academic answer to your bit academic question. What is called osmosis consists of 3 coupled processes.

The first is diffusion of water through the gelcoat, and increasing moisture level in the GRP. This process does not depend on the salinity, in all practical sense. The same for pollution. Except, if the level of salt and pollutant is so high that the concentration of water in the water significantly drops off!
The second start when the moisture level in the GRP reach a certain level. That is a slow breakdown of the polyester. The salinity and pollution on the outside of the gelcoat does not influence. Temperature is of great importance. It goes faster with higher temperature. Also, the breakdown goes faster if the polyester is under-cured.
The third step takes place as the products of the breakdown, acid and glycol, build up, and likely migrates to small voids in the lay up. The products are water soluble and attract water, they can not escape through the gelcoat, so water migrates in to dilute their concentration. This is osmosis, and this is what builds the infamous blisters. Here is where salinity comes in. The salt in seawater represent a pull of water in the opposite direction. The blister juice can only build a blister as long as the juice concentration is higher than the salinity outside.
Consequently, you can buy a blister free boat in a seawater marina, bring it to you river marine, and see blisters in no time.

There is an everlasting discussion whether bad workmanship or bad products are to blame, or a combination. It is hard to pick a group of boats where everything is equal - free of variations at builder, and equally used and maintained. Every builder today boast of skilled labor and strict control of every factor. I have my doubts, and if we talk about 30 years ago, I am certain there was a lot of uncontrolled factors.
I know of a yacht the has a 2,5m wide belt full of blisters across the hull, and not a single blister elsewhere. 2,5m hints to 2 rows of mat. The owner allowed me to grind off the gelcoat in two squares, with and without blisters. To my surprise I could not see any difference in the quality of the laminate, and I looked hard.
 
pvb said:
I've never been convinced that it's necessary to allow the hull to "dry out" ashore, and I certainly don't believe any significant drying is likely to occur over a typical British winter. So if you really want to give it chance to "dry out", you'd need to leave it ashore for a summer at least.

Thousands and thousands of boats are afloat all year round without major problems. If a boat is destined to get osmosis, it'll happen sooner or later, regardless of whether it gets lifted out for periods.
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Must say I do not agree with that though it does seem logical. Way back when we were into racing big time, the IOR handicapped boats were always striving to reduce their rating everyway they could. This included weighing and they always weighed on lift out in the firm belief that the boat was heavier then than when going back in in spring, while I was not directly involved as we raced One Design the IOR owners certainly believed it. Even up here wooden boats do dry out in winter so something similar might occur with any porous hull though it will not be a great deal. We have a modern hygrometer on the wall with our barometer and it does show considerable variations in humidity day to day even in Scotland. Today with a Northerly wind the dust is blowing around and my GRP hull is drying out.
 
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.
 
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The only really bad case of osmosis I've seen was in the late 1980s in my Father-in-Law's 15 year-old boat. He kept it on the Norfolk Broads. It had multiple small blisters on the hull and rainwater had collected in the bilge. There were creamy wispy bits floating in the bilges and it stank of styrene. He continued to use it, however, mopping out the bilge every week, and finally sold it on! Maybe freshwater is as bad for GRP boats as it is for wooden ones.
 
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