Osmosis chickens and eggs

[macd]

Musing about this hoary subject the other day brought me to what seems a logical inconsistency. Many posts (not to mention articles on the subject) refer to the text-book osmosis phenomenon, i.e. that a less concentrated fluid migrates through a semi-permeable membrane to dilute a more concentrated solution of fluids. This makes obvious sense when you have a hull with acid-filled voids floating in sea- or salt water. But that begs the question of how the acid-filled void arose in the first place, since without it there’s no obvious mechanism for the process to take place. Surely it’s not enough that the grp simply gets damp, since plenty of damp hulls seem not to develop osmosis.

Anyone know the answer, or another source that might have?

 

[steverow]

I'm no expert, but as far as I am aware the voids usually arise because of incorrect and incomplete curing, and will always have been there since layup of the matting.
It then becomes just a matter of time/temperature/conditions etc. before osmosis occurs.
But as eveyone always points out, no boat has ever sunk because of it.

Steve.

 

[Birdseye]

I wondered exactly the same thing and what follows is my (no doubt imperfect) understanding of the mechanism.


Polyester is water permiable but fairly slow. The water penetration disolves some of the unreacted components of the resin and also the emulsion commonly used to bind chopped strand mat together.. The low water penetration means that this solution is strong and there is osmotic pressure. But even then, blisters only occurr where there are under surface blemishes that allow the fluid to gather in greater quantities and / or where the gelcoat / laminate bond is weak.

Blisters are purely cosmetice, but the liquid does damage laminates. So presumably there is damage going on even where there are no blisters but the laminate is wet and remains so..

When my boat had the lurgi I tried to find some PhD who could definitively say what was going on. Instead I found that the so called experts I spoke to did not really understand the chemistry either. So I'm still not sure.

 

[Stemar]

Have a look at www.yachtsurvey.com/blisters.htm