(REVERSE) OSMOSIS?

[alisdair4]

I know that a GRP hull will (gradually) absorb water over the years. I have read that this is exacerbated by the boat being left in the water all-year round.

Is this process reversed when the boat is left out of the water, under cover, for a prolonged period?

 

[Danny]

As I understand it the hull will slowly dry out but any delamination that has occurred due to the osmotic pressure within the GRP will not be corrected.

 

[Evadne]

No.
Osmosis is the migration of molecules (water in this case) through a membrane from an area where they are higher in concentration to an area where they are lower. To reverse this osmotically you would have to place the boat in an extremely briny solution or increase the pressure physically on the inside of the gelcoat.
It's easier to remove the gelcoat and then let the water evaporate, or drive it off by localised warming.

This doesn't include non-osmotic absorption, e.g. through the bilges.

 

[tangofour]

Leaving the boat out of the water only pauses the process.

Of course placing your boat, that has already shown an osmotic weakness , in a tank full of very briny solution MAY draw the salts back out of the blistered areas. It will of course provide a stronger drive to the rest of the hull areas that were not previously affected.

When you subsequently lift the boat out the outer 3mm of GRP should remain floating quietly in the water /forums/images/graemlins/smile.gif

 

[Evadne]

Well, I reckon you should patent the process as a new way of removing the gelcoat. /forums/images/graemlins/smile.gif

 

[alisdair4]

The good nes is that the boat, although 25 this year, doesn't have any evidence of osmosis. I was merely curious, given that I have had to have a year off, and she is in a wetherproof shed until next season.

 

[Birdseye]

I'm not a surveyor so this is my layman's understanding of the issues, put in simple words in a n effort to help..

There are two seperate but related issues in what people generally describe as osmosis.

GRP is very slowly water permeable. When water gets into the laminate, it can dissolve some of the constituents from the original lay up. Once this occurs, then you have a situation with tiny lumps of a thick soupy liquid on the inside of the gelcoat, and a thinner liquid on the outside that the boat is floating in. These two liquids try to equalise. So there is increased pressure for water to pass through the gel into the laminate. With no where to go once it gets there, this increased fluid volume shows up as blisters.

This is why osmosis is a worse problem with boats kept in fresh water

But the problem isnt the blisters unless they start to occur deep in the laminate. The problem is that the thick soupy acidic liquid can start to break down the bond between the resin and the glass. Bit like you get rot in wood. So if you open up a long term blister you may find some of the laminate underneath crumbly / pasty and a vinegar smelling liquid oozing out.

The stuff that dissolves in water to give the thick gloop is hygroscopic like salt. So it never dries out totally on its own, and the process starts again as soon as the boat hits the water again. Blisters that shrink as the boat dries a bit on land quickly re-grow when it goes bck in the water.

Blisters are not mandatory and certainly not necessarily large and visible. Best test is to measure moisture with a meter when the boat is recovered to land and then measure is just before re-launch. If the hull does dry out you are less likely to have a problem. If it doesnt, then involve a surveyor who really does know what he's talking about.

In the meantime, dont worry. Someone always comes along to tell you that osmosis doesnt matter, that its all a conspiracy by boatyards and no boat has ever sunk because of it. I always wonder how they know that. But it is true to say that there are lots and lots of boats about that have some osmosis and are happily being used and enjoyed