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Osmosis Evaluation


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Osmosis Evaluation



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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.

There are various small voids within the laminate. The water molecules can collect with in these. Within the GRP laminate and the voids are various water soluble components. These are solvents, by products and uncured resins from the manufacturing process.
The water within the voids dissolves the resin and chemically react with these components. This process is known as "hydrolysis". "Hydrolysis" will continue with the voids enlarging, a dissolved solution is formed, the main ingredients being hydrochloric acid and styrene.

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.

Reason for Osmosis Testing continued 

To determine the full extent of the defects and therefore the detail of the complete treatment required it will be necessary to examine the hull laminate after the gel coat has been removed. In some cases additional laminate repair may be required prior to epoxy barrio coating the laminate.

The "Treatment Process"

As noted earlier there are a number of factors associated with "osmosis". The treatment process has to deal with all these factors.  Simply "drying" the hull and covering with an epoxy paint system will not work.

The contaminants dissolved in the water also have to be removed. As noted earlier some of these are hydroscopic (water attracting) and in general are large molecules. These have to be removed from the laminate when still in solution with in the water.

Simply drying or forced drying using dehumidifiers and or heat lamps will tend to remove the water but leave many of the contaminants behind. For this reason steam cleaning and washing the hull surface is important.

The gel coat, although not water proof is still a very effective water barrier and will not allow the passage of larger molecules. For this reason the gel coat has to be removed. This is most effectively achieved by the use of a "Gel Peeler". This removes a controlled thickness of gel coat and / or laminate leaving an even surface.

The surface left by the "Gel Peeler" however has several problems; firstly the surface does not promote drying well and doesn't provide a good abraded surface for a good mechanical bond for the epoxy coating. In addition the "Gel Peeler" will not remove softer material within the blisters deeper than this smooth surface. For these reasons the peeled surface should be cleaned and dried after peeling.

As the aim of the treatment is to remove the contaminant from the laminate along with the water it is best to initiate treatment, peeling and drying shortly after the vessel has been lifted.

Once peeled and blasted it's necessary to wash out the contaminant from within the laminate. This is best achieved by steam cleaning or hot pressure washing. It is necessary to monitor this process with moisture meter readings.

The exact specification, coating, application procedures etc . will be determined by the product used and the manufacturers specifications.

Does peeling the gelcoat weaken the hull of a boat?

No, removing gelcoat and any chop strand layer has no structural effect and adding the epoxy barrier coating actually strengthens the hull.

Triggers are random chemical processes, which are spontaneously activated in the presence of water. It is the chemical processes that hydrolyze polyester resin in the laminate, producing primarily alkali, acid and neutral substances. Some laminates have no "triggers", while others have many, which explain the seemingly random "osmosis" blistering problem.

Blisters become visible after resin hydrolization has progressed for quite some time. The discovery of just one blister indicates that at least the outer 1-2 laminate layers of the underwater hull surface in the area of the blisters has been affected.

After blisters have been discovered, the resin break-down process spreads into deeper-lying laminate layers causing the type of damage that is often mis-diagnosed as "osmosis delaminations" or "dry" laminate.

Such damages indicate areas where the resin has been completely hydrolyzed, leaving a void filled with liquid and/or a granular mush of solid resin remnants. These voids can vary in size from minute to a dozen inches in diameter or more.

If left unchecked for too long, the structural damage  can be severe. The longer you wait the more damage needs to be repaired and the more expensive the cost. Or from another point of view, the less value the vessel will have on the open market.

A Boat Owner Should Know a Fiber Reinforced glass osmosis evaluation is performed!

Moisture meter

Moisture Testing involves non-destructive moisture readings.  This determines intrusion of moisture into fiberglass laminate, which causes delamination of the structural components.  
Maritime Vessel Surveying, Inc. can provide a detailed assessment of the osmosis penetration in a vessel. This includes  comprehensive moisture meter readings to provide the technical measurements. We will produce a report which will be acceptable to the vessels manufacture for a warranty claim. 

Combined with Hull Sounding, the moisture meter is used to determine condition of wood boat planking and structural components and indicates saturation of wooden structural systems and presence of wood rot.

The vessel will need to be hauled out, all water removed from the bilge and small areas of bottom paint will need to be removed to get accurate moisture meter readings.

Osmosis inspection is included in our pre-purchase condition survey.  It may also be carried out as a separate assignment for a boat  owner who wants their boat checked out. 

Our survey  includes a comprehensive moisture meter readings, ultra sound tests and soundings, using our industry standard equipment. (See Diagnostic Tools Used)

Please contact our office if we may be of service to you:
Captain Stanley G. Konz, A.M.S.
Maritime Vessel Surveying, Inc.
If you require immediate support on your vessel, please fill out the Auto-Responder located on the  Contact Us page and we will get back to you as soon as possible.