Drying Hull

[30boat]

My boat is now 21 years old and the bottom laminate is giving readings of very high moisture content .She was previously treated for the same problem in 1994.I am having the epoxy coating removed now but I would like to know if there is any faster way of drying the hull.I remember reading about a method involving heath and vacuum .Does anybody have any ideas?

Nuno Santiago

 

[johnlilley]

Many people become confused regarding moisture meter readings on grp. When grp hulls become "active" for want of a better word there is a chemical solution that builds up gradually in the laminate. This solution is not water but the meter cannot differentiate between this and water hence the confusion. Water will dry out quite fast, but the other solution will never dry out naturally, and this must be flushed away using water as the flushing medium. It is not possble to flush this away in one simple wash, as it migrates constantly to the surface until no more remains. Unfortunately, epoxy coatings and gel coat can seal this in making flushing virtually impossible until the laminate surface is exposed. High readings on their own do not instantly suggest that the vessel is due for Osmosis treatment, other factors come into play. Suggest you visit my Web Site www.turbolink.co.uk & look for survey page where technical info is available in plain english on Osmosis. John Lilley

 

[Jcorstorphine]

Many years ago I came up with the idea of using vacuum on small (couple of square feet) pads to assist drying out GRP hulls. My actual profession is a high vacuum engineer working on all sorts of applications for vacuum technology and it seemed to be a fairy straitforward application of this technology. I never did take it to any sort of conclusion other than a "thought design"
If you want to try it out all you need is a small single stage vacuum pump of about 2 m3 per hour and a length of plastic hose and some form of pad. The pad can be sealed with plasticene round the outside but you need to fit thin plastic rods to prevent the pad sealing flat against the hull. The vapour pressure of water is around 15 mbar (ish) at room temeprature, below this pressure the water will change state to a vapour which can be removed by the vaccum pump.

There is a commercial organisation who have developed this idea and have included local heating to help stop the water freezing, sorry but can't remember their name but are based on the south coast.

regards

john corstorphine

 

[rogerroger]

how do you measure the moisture level in the hull ? my boat is the same age and is coming out of the water in a couple of weeks - I'd be interested to check. Is this something I can do myself ?



Roger Holden
www.first-magnitude.co.uk

 

[Stemar]

Have a look at www.yachtsurvey.com. It's a site run by an experienced surveyor, with interesting (scathing) views about osmosis and its treatment.

The bottom line is that in his view, unless you've got a lot of blisters, or they're soup plate size, any treatment is of questionable value. If the layup was done right, you won't get blisters, even in grp that's saturated with water. If it wasn't, the blisters are likely to come back eventually whatever you do. The good news is that they're very unlikely to be structurally significant unless they are huge.

Genuine delamination is another problem altogether, but thankfully much rarer.

The approved way of measuring moisure content is to use a moisture meter, but it has to be calibrated for use with grp. A B&Q special won't do, it's not sensitive enough.

If you do decide to treat your hull, the approved way seems to be to remove the gelcoat and leave the boat, ideally in a heated shed, for several months, hosing the hull down regularly with clean water to wash off the styrene that is the cause of the problem.

 

[rogerroger]

as far as I'm aware the hull is fine - there are no blisters but I'd be interested in the moisture content of the hull. I'm taking her out of the water this winter but might not do so every year.

I guess the boatyard where I'm laying up might have a meter ?

Roger Holden
www.first-magnitude.co.uk

 

[charles_reed]

Save your money

Unless you've got actual osmotic blisters - a high moisture reading is not necessarily the precursor of death.
One of the problems is that the boat has been epoxied previously and any water is now trapped in there.
The only certain way of getting it dried out is to take it to somewhere like S Spain where you have a low atmospheric humidity.
A number of specialists "cocoon" the hull in pvc sheet and run dehumidifiers in there for about 3-4 weeks, but winter is not the best time to try and dry a hull in the UK or anywhere else in the N Hemisphere.

 

[Guest]

Re: Save your money

unless you are particularly keen on the cosmetic appearance of the underside of your hull, your only concern need to be the possibility of structural deterioration - not blisters or water content per se. as someone has said above, there is a world of difference between blisters and delamination. you need to make a judgment based on location, symptoms, hull thickness etc, best done with the advice of a good surveyor.

if you then decide to treat, bear in mind that the osmotic fluid is hygroscopic. yes you can force dry just like you can force dry table salt, but if there is any moisture about then it will be picked up once again and off you go. so you need to flush as much as poss of this fluid out of the laminate. funny though it sounds, the more you wash the exposed hull surface (pref with warm water) the faster it will eventually dry when you stop washing. a purist would say that if it doesnt dry naturally, then you havent washed enough.

the sad thing is, it is likely to return. epoxy is not completely impervious to water though way better than polyester. so water will eventually penetrate even an epoxy coat.
one well known resin manufacturer maintains that all polyester boats with a gel coat will get some degree of osmosis eventually since the chemical structure of the resin is easily broken down by water, and gel coats are not impermiable.

but then wood rots, steel rusts, aluminium corrodes and concrete is brittle. the funny thing is thatwe expect boats to last forever, but not our cars, bikes etc

 

[30boat]

Roger
The moisture level of the hull was measured with a moisturemeter by the person who does all the osmosis jobs around here.The readings where taken too soon after hauling out.Ideally one should wait at least two weeks before doing it to allow the laminate to dry.All laminates absorb water to some extent and it does not necessarily mean trouble .The readings have to be interpreted and for that you need experience I'm afraid.

regards

Nuno

 

[30boat]

Re: Save your money

Actualy I am in the algarve .I live here.The boat has had the protective coating stripped during the weekend and the laminate is in rather good condition with no delamination apparent.I believe that with frequent hosings I will get the job done soon.It won't last for ever but the repair should be good for many years, I hope.

 

[charles_reed]

Hit it in one Howard

Couldn't agree more