OSMOSIS IS A PROCESS NOT A DEFECT!


BY GEOFF WADDINGTON FIIMS & IIMS VICE PRESIDENT


Even after all the articles and papers raised on the subject it has come to my attention following some recent involvements with other surveyors some of who were IIMS members that there is still an underlying lack of knowledge regarding the process, causes and even identification of FRP blisters and defects. One argued that the hull was to be considered without defect and not suffering from ‘Osmosis’ because the moisture meter readings were low.


In the 1980’s I bought my wife a new crash helmet, not a cheap polycarbonate one like mine but a posh expensive GRP one. Having children led to the helmet being put on the top shelf in her wardrobe where it lay unused for many years. When we took it out it was covered in blisters, having never been wet nor even suffered the effects of sunlight. Later a GRP shower tray I fitted also suffered from blisters, I have seen blisters in topsides, superstructure, decks and of course outer bottoms, the point I am trying to make is that


there are no hard and fast rules to this problem. A wet hull may have no blisters at all but another dry hull could be covered with them. The reason being is that the defects are built into the hull when it is manufactured and later circumstances dictate how and when these defects will come to light.


History – Up until the late sixties wood was the preferred boat building material, then the advantages of moulding boats using GRP (as it was), became the preferred method of boat construction. During the seventies and early eighties problems became apparent in the form of hull blisters and the term Osmosis was wrongly but widely attributed to the problem and the term stuck.


Osmosis – This is the process of a fluid passing through a semi- permeable membrane from a low density on one side (Sea Water), through to a higher density on the other side, (the Laminate or in effect the WSM’s within it).


Hydrolysis – This is the defect caused by the formation of acidic compounds from the chemical breakdown, (decomposition) and splitting of a bond between compounds due to reaction with the addition of Hydrogen such as found in water.


Background – Basically, the laying up of a GRP (FRP) hull involves using a mould into which a coat of release agent or wax polish is laid. Over this a layer of Gel coat is applied, (the membrane), when this is still not fully cured a layer of fine CSM, (Chopped Strand Mat), well wetted out with resin is applied, followed by further layers of well wetted mat to a pre- determined thickness and including reinforcement by layers of woven mat as necessary.


Sandwich construction was also used by some manufacturers using end grain Balsa Wood Core material or foam core, which forms a double skin effect, reducing to single skin in way of skin fittings or timber or other reinforcement to prevent


The Report • September 2019 • Issue 89 | 41


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