How long does it take to dry a hull?

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mdonnelly

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Was planning 6 months on hard standing to dry out my hull. Has only been out 3 months in last 2 years and at that time moisture content was checked by a surveyor. Was told moisture content was higher than average but not a problem. I was planning to either apply coppercoat or VC tar. After being quoted £1400 for 6 months out I am wondering what the minimum period is that would dry out the hull.
 
That's like how long a piece of string. depends on weather, how wet the hull is, and many other factors. Out of doors over winter, it will reduce considerably but if you really want to dry it out fully you need to tent it and preferably have some form of heating. In the summer months it will dry out much more quickly but then you can't sail! Suggest you check moisture when she comes out. Compare reading above the WL with below. Then check periodically. The longer you leave it the better but 6 months over winter outside I would think minimum if she is wet.
 
Doesn't it also depend on whether the hull has been gritblasted or the gell coat ground off?

I also understand that pressure washing or ideally steam cleaning the hull periodically is a very good way of speeding up the process. I was told that it washes away the 'chemical soup' that is attracting the water tat as built up or remains in he GRP.

No depth of knowledge here - just repeating what a yard and a couple of surveyors have told me.
 
as I understand it 3 or 4 weeks if there is no osmosis. If there is osmosis, it wont dry completely without treatment if you keep it out forever. In fact, thats a reliable way of telling whether it has the pox.

I'm assuming you are not having the hull blasted or peeled first.
 
Not advisable. Answering your point and Johnny_Morris at the same time, if the moisture is just water then I think you might get satisfactory readings quite quickly. Don't epoxy while wet though or you will lock in the moisture and cause a bigger problem than you will solve. If there are blisters this usually means a cocktail of chemicals that can only be removed by gel-coat peeling and blast washing repeatedly. From the sound of it you don't have a problem so if it ain't broke don't mend it. Water alone will dry out naturally. Take boat out, remove antifoul, preferably tent it with heavy polythene, but on warm dry days ventilate underneath. In fact if possible tent over it but leave about 12" clear of the ground all round to let air circulate. In the spring, when the weather is improving, prepare thoroughly by sanding for a good key before applying at least 6 coats (yes 6) of Gelshield or similar before re-antifouling. All assuming that moisture readings are acceptable.
 
How fast a hull dries out is affected by the type of resins used. Old ortho resins absorb water faster but let it dry faster. Modern iso resins are better at keeping it out, but tend to keep it in better too.

Don't expect water molecules that have taken many years to migrate into the GRP through the water resistant but not waterproof gelcoat to migrate out again in a few months.

Even with the entire gelcoat planed or gritblasted off it typically takes 3-4 months (plus regular washing) to dry out a hull. The washing bit seems counter-intuitive, but it does help, by washing off chemicals other than water. If you don't strip the gelcoat off I doubt if washing makes much difference.

Opinions vary wildly as to epoxying a non-dry hull. Some say it makes things worse, others that it stops it getting any worse.
 
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