Savvy Boat Owners Know...
Osmosis is best defined as a chemical breakdown in a laminate due to moisture ingress which ultimately results in blistering of the gelcoat and dilution of the resins.susceptibility to blisters.
Often boat owners first hear the term "osmosis and or blisters" when his or her vessel is being sold. The buyer and often a lending institution will require a surveyor to examine the vessel.
It is during the time of the survey that the bad news is broken to the seller. Due to the exceptionally negative picture a positive blister report creates, the seller is often saddled with a severe devaluation of the vessel.
As rumors of more and more blistering problems filter through marinas, savvy boat owners become more and more proactive.
Ignoring an "osmosis problem often results in a severe blister problem along with the very expensive prospect of restoring the vessel to a sound and saleable condition.
Many boat owners are opting early on to have their vessels tested for early signs of the developing crisis. As in many other deteriorating conditions, initiating protective measures can save a great deal of worry and expense later on.
What's the difference between "osmosis" and "blistering"?
Just as wood rots, and metal corrodes, the process of deteriorating resins in the hull of a boat is the chemical process coined, "osmosis".
While "osmosis" is the chemical activity causing the deteriorating process, "blisters" are the most visible of the physical manifestations of the process of "osmosis".
The hull gel coat surface is sounded and visually examined for signs of blisters or delamination. The liquid content of any blisters is examined and tested.
The History of the "Words"
The term "osmosis" originated in trade publications in the early 1960's as writers attempted to explain the blistering phenomenon. Early theories suggested that water entering through the gelcoat by osmosis causes a pressure resulting in "osmosis blisters". Although technically incorrect, the name stuck and we'll continue to use the term "osmosis" to describe the overall process resulting in blisters.
Very little true osmosis was found to actually take place in the process. Instead, "osmosis" is the result of specific "trigger" mechanisms that cause resin to hydrolyze. Residues of this process are spread throughout the entire laminate causing a weakening of interlaminate bonds and possibly leading to structural failure.
The term osmosis was originally used to describe the biological process where by a liquid (usually water) will pass through a semi permeable membrane. The membrane is permeable to the water molecules but not the various compounds dissolved within the water. The overall flow of the water molecules under pressure will be from the solution of lowest concentration towards the solution of higher concentration. This flow can be reversed by the application of an external pressure as used in "reverse osmosis" water treatment systems. The osmotic process does occur within the blisters but is not the only process involved.
A GRP (glass reinforced plastic) boat hull is a matrix of (usually) polyester, vinylester or epoxy resin reinforced with glass fibers, built up in layers. The final laminate will have an approximate ratio of 30% glass to 70% resin.
This laminate is not always homo-geneous, that is even within a well built GRP hull there will be small voids, air pockets and cracks within the resin matrix and at the interface between the resin and the glass fibers. Water can diffuse into, and through the gel coat and the laminate as water molecules.
A boat hull may absorb water in this way. Water may pass slowly through a GRP hull in this way and disperse in the bilges. The moisture content of a new hull will slowly increase during the first few seasons that she is afloat; the moisture content will similarly reduce slowly when she is out of the water.
These products give "osmotic fluid" its characteristic vinegary small. The result is "hydroscopic" (water absorbing). Once this is released in the voids it will accelerate the rate of water absorption into the laminate. This process will now continue. Moisture content will drop slowly if left ashore but will rise again fairly rapidly when immersed again. The various hydrolyzed products cannot pass through the gel coat / laminate but the water molecules can.
As this process continues, at some point, the concentration within the voids will become greater than the concentration of the water the vessel is floating in (sea water). The glass fibers and the resin matrix are also broken down. This will allow liquid water to pass along the fiber bundles, producing the characteristic "wicking" or "fiber aligned blisters.
As this process continues, moisture continues to be absorbed, the laminate break down accelerates and more blisters are formed.
In time some larger blisters may develop within the laminate as well as those more commonly occurring between the gel coat and laminate. Eventually at this stage, treatment will be required.
Reason for Osmosis Testing?
Diagnosis of the "osmotic" condition and the decision as to what level of treatment if any is required, and when, is made by considering a number of factors.
The moisture content of the hull is gauged using a moisture meter.