Thursday, Apr 27, 2006 at 23:47
I don't believe I was being nasty, but I get sick of people bringing up osmosis as if boats will disintegrate & sink when it is a relatively rare & usually little more than a cosmetic problem. If I offended you then I apologise.
I base my observations on 24 years in the marine industry, originally as a boatbuilder & for the last 13 years as a yacht broker. In this time I have never seen or heard of any osmotic reaction in a trailer boat, the major problems in
trailer boats are rotting in the transom core material & in the sub floor under the cockpit sole. Most vessels that suffer osmosis are boats that have been built in Asia in very humid conditions, which traps moisture in the laminate. I agree that gelcoat is quite porous & contributes to the problem. Polyester GRP vessels coated with an epoxy layer over the original gelcoat will be protected. Modern vessels built with epoxy resin will never suffer the problem. Your mention of a 'tiny bubble' could mean many things, as in "all that glitters is not gold" .... "all that bubbles is not osmosis" There are other things that can cause bubbling, including power stirring of the resin causing aeration.
I refer you to an extract from one of my manuals:
By definition OSMOSIS is a permeation through a membrane. This is a natural phenomenon created by pressure trying to equalise the concentrations of
two solutions through the membrane. In the case of fibreglass boats the cause begins at the time of manufacture. Glass fibre stacked or stored in an
environment that does not have a humidity controlled facility runs the risk of transferring moisture contaminated strands into the moulded matrix. When
these strands become encapsulated within the glass structure the moisture settles into convenient voids created by the random distribution of the mass.
(See fig. 1.) To understand this process observe what happens when warm breath meets a cold glass mirror. As the condensed vapour mixes with
residuals of the polyester it produces a pocket of concentrated solution.
At this stage Osmotic pressure is created. Within the moulded matrix there are pockets of very concentrated solutions whilst on the outside, the
seawater (also a solution) is trying to equalise this force. Where these moisture “cells” are close to the outer surface or able to “wick” down glass
strands to reach the gelcoat layers the reaction starts to take place. (See fig. 2). Because the concentration of the solution is greater within the hull, the
direction of pressure is outwards into the seawater. When the pressure exceeds the containment by the membrane (gelcoat), the bubble bursts and
equalises the force. (See fig. 3). At this point the reaction is over. However the seawater is now able to permeate the mass in the same “wicking”
action and could theoretically find other “sites” for further osmosis cultivation. The potential for this is of minor concern and should be put into
perspective. The Polyester resin crosslinking process is waterproof and structurally sound. This mass (surrounding the reinforcing fibres) ensures the
integrity of the structure … In other words the “matrix” of resin and reinforcing cannot be compromised further. The waterproofing property and
adhesion to the glass reinforcing fibres is not compromised and therefore does not represent a structural weakness. In a nutshell osmosis is a cosmetic
condition.
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