Nearly 40 years after the first cases appeared, we have learned a lot about osmosis - but it's still a problem. What causes osmosis, how to prevent it and why treatment can fail?
Boat owners were petrified of the word and many would simply not set foot on a boat known to have dreaded "pox". A blistering boat?
OSMOSIS
We have undoubtedly learnt a lot over the years, and most owners are now quite relaxed about sailing offshore on a boat with a few blisters on its bottom.
What is osmosis?
In short, osmosis is usually defined as: "The equalization of solution strengths by the passage solvent (usually water) through semi-permeable membrane", or something similar.
In the field of biology, osmosis is used by plants and trees to draw water and nutrients from soil, and plays essential role in the function of cells in body tissues. The basic principle can be explained by imagining container which is divided into two separate chambers by a semi-permeable membrane. Simply, boats with fibreglass bottom have a few millimetres thick gel coat (usually white, although other colours are present too). Water can eventually break through and reach fibreglass layer, and this is where blistering occurs.
What we have learnt?
Firstly, we know that osmosis is a natural ageing process in glassfibre - in much the same way that steel and aluminium fatigue and corrode, and that wood rots, shrinks etc. On the positive side, glassfibre is comparatively cheap to fabricate, light in weight, yet remarkably strong and can be moulded into complex shapes. Above all, is almost maintenance free.
Will osmosis make me sink?
Boats are unlikely to sink because of osmosis - although they can be difficult to sell.
Don't treat it too soon!
Early treatment of osmosis is rarely beneficial. The reasons for this are not to difficult to understand: osmosis is a very slow process, and we know that it can take anything up to 30 years for the solutes to migrate in sufficient volume to form blisters in the gel coat. Osmosis treatment is best started at the end of the sailing season, as soon as possible after lifting out as the solutes will be most dilute at this time. By contrast, removing solutes from the hull of a boat which has been on a hard standing for a year or so, or which has not even blistered yet, will be a nigh on impossible task.
Could my boat get osmosis?
Unless the boat was epoxy-coated, in a word yes! However, there are many variables that affect how likely anyone boat is to suffer from blistering (early in its life). A boat kept in the warm waters is more likely to blister prematurely than one kept in colder conditions. Fresh water also accelerates the process, but not as much as temperature.
Preventing osmosis
Cliche as it sounds; osmosis is best prevented by keeping boats well away from water. However unhelpful this may sound, but protecting the underwater hull a glassfibre boat with epoxy before she is launched will, in most cases, prevent osmosis from ever occurring. A properly applied osmosis prevention scheme should also resale values.
Osmosis treatment failures
By and large, the owners of glassfibre boats now accept osmosis as an inescapable fact of life, although for newer boats it is not the certainty that it once was. Osmosis treatment has always been a challenge and even today a significant number of treatments fail, usually by blistering. Given correct preparation, an osmosis treatment should last between eight and fifteen years. After this the epoxy coatings may start to break down, and will need to be removed and reapplied. Most of those treatments that fail prematurely do so because glycol is not completely removed from the laminate before epoxy coatings are applied.
Failures in osmosis treatments are usually highlighted by blistering of the epoxy coatings. Blisters connected to residual glycol will usually remain visible for several weeks or months after the boat is lifted, and will contain sticky or oily fluid.
In this regard, it is interesting to see how many boats are treated for osmosis two or three times over, yet in many cases their gel coats are still comparatively intact.
The problem here is that unlike steel (which is the subject of several worldwide preparation standards), there are no formal standards for preparing glassfibre laminates.
Osmosis - Causes and How to Live With It OSMOSIS
No comments:
Post a Comment