Do plastic boats really soak up water.

Vara

Vara

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Its that time of the year, and on a number of posts there has been mention of the "fact" that taking a boat out of the water for six months is good for it as it allows it to dry out.

Now I consider this to be cobblers and have always kept my boats in the water for as long as possible.

Can any body prove me wrong?

I'm quite happy to be proved wrong but remember evidence is not relating the experience of your best friends mate, and that faith based theories are a no/no.

I struggle to find any research on the subject although I'm fairly certain it must be out there somewhere!
 
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Vast amount of literature and research on the subject. Google osmosis and boats, pour yourself a stiff whisky and get stuck in!
 
GRP will allow moisture to move through. It is however at a molecular level and not free water. Comments about weight increase due to GRP soaking up water are well wide of the mark and best ignored.

As I understand this the level of moisture in a hull will be relatively stable when a hull is in sea water, after it has absorbed water / moisture. A good quality GRP lay up will allow this moisture to evaporate off, or to a much lower level, when the hull is out of the sea over a winter period.

In poorly laid up GRP, moisture levels remain elevated because the chemical reactions that cause so called Osmosis combines the water in a solution that has a larger molecule size, hence does not evaporate off.

Thats the best I can do as an interested amateur that owns an old GRP boat.
 
One of the treatments for mild osmosis is to dry it out then apply epoxy so it is a fair assumption that keeping it out of the water will reduce hull dampness.

If the hull is modern and/or well protected against osmosis, lifting out will be of little benefit. If it is more than 30 years old and untreated it is probably worth doing.
 
I reckon many yacht owners are just not concerned enough about getting maximum performance to notice the increase in weight and floppiness in GRP that comes with age.

If you race dinghies however you will be well aware of the increase in weight as the boat absorbs water and the floppiness as the polyester resin loses grip on the fibres. So water is definitely absorbed in GRP (non-epoxy) yachts, but whether that matters is down to the individual.

What will be interesting is the fact that old GRP boats tended to be over-engineered so water probably made less of a difference. Modern boats don't have the same tolerances, but do have isophallic/vinylester resin barriers to minimise the ingress. Water absorbed more slowly, but likely to have a bigger impact on the boat.

Epoxy is the only way to go as far as water absorption goes.
 
Vara said:
No, osmosis is different, I'm talking about the vast majority of boats that don't have osmosis or have had it and been treated.
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No, it is not different. Water gets into the hull through the process of osmosis. Unfornately the word is misused to describe some of the consequences of the process, rather than the process itself. Both gelcoats and epoxy coatings are permeable to varying degrees so the hull will absorb water if it is immersed.

Your original question made no mention of "treatment", but the answer is still the same. The only issues are the rate of absorption, whether it is measurable, and the impact it has on the integrity of the structure.

As already noted there is plenty of research and literature on the subject.
 
I don't believe anyone who tells me my hull will dry out on the hard between the end of Oct and the end of March on the west coast of Scotland.
 
Vara said:
Its that time of the year, and on a number of posts there has been mention of the "fact" that taking a boat out of the water for six months is good for it as it allows it to dry out.

Now I consider this to be cobblers and have always kept my boats in the water for as long as possible.
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+1

Boat sails very badly on dry land - and sailing is what boats are made for.

Leaving your boat on the hard 'to dry out' is cobblers IMO.
Guapa recently spent 2 years on the hard whilst in refit.
Moisture readings were taken when she came out of the water and again just just before she just went in. The readings were virtually identical.
 
ratbag said:
i've often wondered why an epoxy coat is'nt put on new boats before antifoul ??
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It often is. However, improvements in resins and particularly moulding processes have reduced the need for extra protective coatings. "Osmosis" - that is blistering and partial breakdown of the laminate is largely a thing of the past, or rather is less common on more recently made boats.
 
Tranona said:
No, it is not different. Water gets into the hull through the process of osmosis. Unfortunately the word is misused to describe some of the consequences of the process, rather than the process itself. Both gelcoats and epoxy coatings are permeable to varying degrees so the hull will absorb water if it is immersed.

Your original question made no mention of "treatment", but the answer is still the same. The only issues are the rate of absorption, whether it is measurable, and the impact it has on the integrity of the structure.

As already noted there is plenty of research and literature on the subject.
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I agree that about the effects of osmosis. Acid and blistering is a result of the fact that water got in there in the first place; the actual mechanism by which it got in there is irrelevant. In fact it's not until water has actually permeated the hull that osmosis can take place as it's the water in the hull that creates the water solution that is required for the osmotic process.

Absorption and osmosis are totally different mechanisms though. A membrane may be semi-permeable to fresh water (i.e. no water is transmitted into or through the membrane, except very slowly at a molecular level) but add salt to the water and osmosis will take place and the flow will increase dramatically - and a considerable pressure builds up to encourage the flow. If you could make the boat totally impermeable or keep it totally dry then osmosis could never occur. Like-wise, if there were no chemicals in the grp to create a solution (i.e. the water stayed as water) then no osmosis would occur despite the fact that the hull had absorbed water.
 
ratbag said:
i've often wondered why an epoxy coat is'nt put on new boats before antifoul ??
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Why dont all premium boat bulders like HR build the whole boat out of epoxy. Its stronger and much more water resistant.
Epoxy is more expensive than polyester, but the hull cost is only about 1/3 of the total cost so the aditional expense is not enormous.
No teak decks and an epoxy hull would make it a much better boat for the same cost, IMHO.
 
noelex said:
Why dont all premium boat bulders like HR build the whole boat out of epoxy. Its stronger and much more water resistant.
Epoxy is more expensive than polyester, but the hull cost is only about 1/3 of the total cost so the aditional expense is not enormous.
No teak decks and an epoxy hull would make it a much better boat for the same cost, IMHO.
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Why would they need to do that, modern resins are not vulnerable to osmosisis and epoxy resins, while they have some significant advantages, are not the miracle that they have become on here.
Back in the eighties when competitive boats were always having their rating optimised it was usual to have the weight verified on the load cell at lift out rather than return because of the difference, I presume with vinylesters etc. it does not matter now.
 
Quandary said:
Why would they need to do that, modern resins are not vulnerable to osmosisis and epoxy resins, while they have some significant advantages, are not the miracle that they have become on here.
Back in the eighties when competitive boats were always having their rating optimised it was usual to have the weight verified on the load cell at lift out rather than return because of the difference, I presume with vinylesters etc. it does not matter now.
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I don’t share your faith that modern resins are not vulnerable to Osmosis. I have seen a lot of reasonably modern boats having Osmotic peels. Last time I was on the hard the 5 or 6 year old Jeanneau next to me was having it done.
 
One wonders what the real life expectancy is of a epoxy barrier coating-years or decades for example?

One things for sure. Once a boat gets osmosis, its development can be directly related to salinity, water temp and time left in the sea.

Once osmosis or wicking starts to climb the topsides or retrousse stern, it is pretty ugly!
 
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