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Old 11 May 2002, 05:52   #1
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Osmosis

Hi folks

Do RIB's suffer the same as other GPR boats with osmosis, or is the expected life of a RIB to short for osmosis to be a problem, even if they are kept permantly afloat

From my days with yachts it was quite normay to epoxy coat a new boat to help prevent osmosis or at least delay it.
Does anyone know of any RIB's suffering from Osmosis, or if it is working epoxying a RIB from new.

If its worth doing its best done from new before any antifouling is applied.

Thnaks Gary
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Old 12 May 2002, 02:45   #2
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Osmisis?

As I'm never afraid to show my ignorance, I'll jump in here.

I'm not quite sure what you mean by Osmosis in the case of a fibreglass hull, as my understanding of the word does not seem to fit here. I shall try to avoid an excersise in semantics.

Do you mean porosity, in that the fibreglass eventualy becomes porous?

Remember that I do not understand these things as my boat does not have a fibreglass hull so please excuse my ignorance.

Keith (my brain hurts) Hart
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Old 12 May 2002, 04:02   #3
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Osmosis

Yes, Ribs can suffer the same as any grp vessel be it a sailing yacht or motor cruiser. Most ribs are kept on trailers though which helps in allowing the hull to completely "Dry Out". If the rib is left in the water for long periods it could develop osmosis.
But saying that, modern grp methods are far better than they used to be.
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Old 12 May 2002, 04:55   #4
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Thanks David, do you feel its work epoxying a rib from new that will spend 95% of its life afloat ?.

Keith, what is osmosis or known ad boat pox.

This is where water is drawn through the gel coat then get stuck between the gel coat and the fiberglass itself.
This is caused because gel coat is more "porous" than the fiberglass itself. Once there the water slowly builds up, while it is doing this is desloves uncures styreen from the resin and makes an acidic mixture. This in turn attacks the fiberglass itself.
Osmosis starts as small blisters on th hull, getting as big as 2p,s or bigger.
Its worst in fresh water where the osmitic pressure is greater than salt water.
Its a known common problem with boats, but as it often does not start till 20 years + I was not sure if a RIB could be expected to last that long.

It helps if the hull is coated in epoxy resin over the gelcoat as early as possible as epoxy is far less porous than the gel coat.

It can be sort of cured all be it a major job. They sand blast or grind away all the gelcoat, wash & dry out the hull for weeks the replace with epoxy. sound drastic and it is.

Gary
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Old 12 May 2002, 16:10   #5
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Right then, if you are buying an old second hand rib it would be worth looking out for. Me and my pvc boat eh!

Keith (plastic man) Hart
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Old 13 May 2002, 04:13   #6
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Gary, if coating the underwater part of the boat's gelcoat with epoxy because it is less porous than gelcoat, why do boat builders bother with gelcoat underwater all?

Surely it would be better to gelcoat down to the waterline and then expoxy straight over the glass fibre before covering with anti-fouling?

That should save on both weight and cost.

Or am I missing something, here?
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Old 13 May 2002, 08:29   #7
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Hi folks


Dont worry your safe Keith, no Osmosis problems, just keep a eye out for the tube worm that inflatables can get. They attack the botton of the hull and once inside there no getting rid of then

And yes Mike it would be better to use Epoxy rather than gelcoat.
However epoxy is expensive, say 100/gallon at retail prices as opposed to 7-10/gallon for polyester. This makes it more economical to have a thick polyester gelcoat and a thin coat of epoxy.

Based on things though does anyone know what an expected life of a RIB hull will be, not the tubes just the hull. To work out if it long enought for osmosis to be a problem.

Regards Gary
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Old 13 May 2002, 08:45   #8
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Very interesting Gary. If the below water area of a RIB was ,say, 200 sq ft. How many gallons of epoxy would you need to put a worthwhile protective coat on it?
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Old 13 May 2002, 16:33   #9
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Hi Mike,
Thats a difficult one. The last boat I did with epoxy was a 26 foot bilge keel yacht. That took about 3-4 gallons.
You put as many coats on as you can, but at least 4-5 coats, with no sanding down between. The next coat has to go on while the previous coat is still "green" which means one after the other, yes all the coats in one day!!

Regards Gary
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Old 14 May 2002, 01:39   #10
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Again, very interesting stuff, Gary. Sounds like that for a biggish, say, 10m, RIB it would take about 5 gallons for the underwater part at an additional cost in materials at a very round 500. In the overall scheme of things not a lot - if it works.

My first sailboat was a Tradewind 33, built in 1979. It was supposed to be indestructable, built to a standard, not a price etc etc. For that I paid top wack.

The boat got osmosis within 2 years. In those days, if you got osmosis it was like getting the Big C. Shock horror all round.

Fortunately, the builder was a very reputable old family firm (long gone bust, now!) and they paid for the repair. It took 6 months to store it indoors and cook it it dry it out before the job was finally done.

Interestingly, I eventually discovered why the problem arose in the first place. The designer was a crusty b****r and he owned the plug. He fell out with the moulder, Halmatic, I think, over money and took the plug away and gave it to a company called, again I think, Seaglass, in Winchester.

My hull was the first Tradewind the new company had made and half way through the job they went bust. The administrator cut all costs and minimal materials were used to get the hull out of the factory just to get my payment in.

Nowadays, people don't panic about osmosis like they did. But prevention is far better than cure. Your idea makes a lotta sense to me.
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