Osmosis

Despite most people’s assumptions to the contrary, fibreglass mouldings, no matter what resins are used, are not actually totally waterproof. Individual water molecules are so small that they can actually find their way into and ultimately right through the layers of glass and resin forming a boat hull, or GRP pond liner. This in itself is not osmosis, it is simply a minute degree of permeability of the material.

The problems start to occur when the water molecules migrating into the GRP encounter other chemicals inside the laminate, primarily water-soluble materials (WSMs) such as the the emulsion binders used to hold the glass mat together before it is moulded, or pockets of uncured or only partly cured resins in the moulding. The water molecules can then have a chemical reaction with these substances, forming larger molecules of a new chemical, often acidic - which unlike the original small water molecules, cannot carry on passing through the GRP. These larger molecules are then trapped. This is the point at which osmosis actually starts. The process of osmosis is the same mechanism by which plants and trees draw water up from the soil to their branches and leaves, and is described in a dictionary as:-

os·mo·sis (oz-mo'sis, os-)

Diffusion of fluid through a semipermeable membrane from a solution with a low solute concentration to a solution with a higher solute concentration until there is an equal concentration of fluid on both sides of the membrane. The tendency of fluids to diffuse in such a manner.

The important parts are that the hull is not waterproof (it is a semi-permeable material), and that osmosis causes a low concentration fluid (water) to pass through the hull to join the higher concentration fluid (the chemical mix formed by the water plus WSM) inside the laminate).

Pressure is thus built up inside the laminate. If this process takes place in a solid part of the laminate, there is usually no problem as the structure is strong enough to contain the pressure. If however it takes place on the boundary of a small air-bubble in the moulding, or at a point where layers of GRP are poorly bonded, the resultant new chemical compound or compounds slowly fill up the bubbles or the minute gaps between layers with liquid. Almost all mouldings have these air bubbles and small areas of poor bonding, although they should not. Ideally the resin should totally fill the gaps between the glass strands, and every layer should perfectly bond to the next. In practice, however, this is extremely difficult to achieve with conventional moulding techniques.

If this osmosis (using the term in it’s correct manner for once) was all that happened, it would be a very minor problem. Even completely saturated with water molecules, a GRP laminate still retains most of it’s strength, although it does become slightly more flexible. Racers who want stiff hulls with the absolute minimum weight already mostly keep their boats ashore when not sailing, and for any properly built cruising boat 2% or so extra weight and a trace more flexibility in the structure should not be a problem.

Once again, if the air bubble simply filled with this acidic compound, the problem would still be relatively minor. However the nature of the osmosis process is that water molecules keep osmosing through the laminate, and join the chemicals in the bubble, steadily building up hydraulic pressure. Eventually this causes the surface of the moulding to blister.

These blisters are the typical sign of what boat-owners usually refer to as ‘osmosis’. When pierced these blisters will give off a small amount of chemical-smelling (usually vinegary) liquid - which is the juice built up inside the pressure-raised blisters. The term ‘blister juice’ is often used. This ‘blister juice’, which is usually acid, can break down the polyester. This breakdown process is known as hydrolysis, and causes a reduction in strength of the laminate. This is however normally very localised, and the moulding as a whole will still retain most of it’s strength despite blistering. Only if the blisters are very large, or very deep-seated, is this generally a problem.

Two photos of the same blisters on the topsides of a yacht - chosen for the clear view of the blisters

It is rare, but not that rare, to find osmotic blisters above the waterline. They can however occur anywhere on a GRP moulding. More often they are found underwater, often partially obscured by layers of antifouling paint. When looking for them shining a light along the hull surface can help, as can wetting areas of the hull surface to make it shinier. The yacht in the photos above had been given a below-waterline 'osmosis treatment' at some previous date, and the bottom was now in good condition, with low moisture levels by a moisture meter. The topsides however gave very high readings, and had a fair number of blisters.

One scientific study of the problem of blistering was given in the paper "Causes of Boat hull blistering". This 1987 study was carried out by Thomas Rockett, Ph.D. and Vincent Rose, Ph.D., at the University of Rhode Island, and was partly funded by the US Coastguard Service.

Yacht hulls are typically built with a substantial safety margin of thickness and strength, and it is incredibly rare to find a yacht hull that is seriously weakened even by very advanced osmotic blistering. Often, the older the boat the heavier the original construction, and the greater margin of safety. Most blistering occurs near the outer surface of the moulding, usually in the borderline between gelcoat and underlying layers of resin and glass - and the gelcoat is largely cosmetic and not structural.

"What is universally agreed is that osmosis discovered within the very early years of a vessel's life is bad .... " from 'Surveying Small Craft' by Ian Nicolson

Once blisters in the gelcoat have appeared, a period of storage ashore, particularly in warm dry weather, may cause them to apparently disappear, as most of the water in the blisters dries out. What is left behind, though, is a highly concentrated solution of the 'blister juice', which will usually rapidly re-absorb water once the hull is put back into the water.

The process of osmosis is fairly well advanced by the time visible blisters start to appear on the bottom of a hull. It must be realised however, that the process starts the minute a new yacht is craned into the water, or even when it’s hull and deck first gets rained on as it is wheeled out of the factory.

Another facet of water absorption into a hull is known as ‘wicking’. This refers to the ability of water molecules to creep along the boundary of the individual strands of glass within the moulding. A totally dry moulding, if moulded with clear resin, will be virtually transparent. If you can see individual strands of glass as whitish threads, what you are seeing is not the glass strands themselves, but water around the strands. This ‘wicking’ is an indication that there is a significant amount of moisture in the resin, and is often a precursor to or accompanies blistering.

Just as the tests for astronauts in the space programme proved that anyone would get motion sickness if shaken around enough, all GRP yachts, from the day they are built, suffer from osmosis. Manufacturers now typically offer five year hull warranties, and it has been said - cynically but probably accurately - that their main concern is not to build yachts that do not suffer osmosis, but to do just enough that they don’t get visible blisters within the warranty period, which for most yachts is 5 years. Several US manufacturers of small power boats, however, now state in their warranty conditions that the boats are not to be left afloat for more than 2 weeks, or the warranty against blisters is invalid! At least one US builder will not offer any warranty at all against blisters.

Historically, there have been some batches of boats that have suffered severely from blistering. Often this was due to changes in layup specification, and use of new materials. For example, it is now known that the use of PVA emulsion bound glass mats is bad practice. Emulsion bound mats were introduced in the 1960s as an improvement, and accepted by Lloyds and other classification societies for standards of hull construction for almost 20 years. Emulsion bound mats are now regarded as bad, as the PVA is water soluble, and tends to encourage wicking. Mats used now should be powder bound, especially in the outer layers of a laminate. Nevertheless, plenty of builders still use emulsion bound glass mat in inner layers of hull layups.

Whilst some builders, including those who produce some very expensive boats, have had runs of boats prone to blistering, they have also turned out apparently identical boats that have not blistered. Current thinking is that cleanliness, temperature and humidity control in the moulding shop, and precision of the mix of resins, are the key to building boats that will not blister. Nevertheless I know of one builder who works in a dirty corrugated iron shed where the climate control is dependent on whether an easterly wind blows through the gaps in the door. His boats actually have a rather good reputation for ‘not getting osmosis’.

Whilst all GRP boats slowly absorb some water, it should not be fast. Visible blisters or wicking are an indication of a well developed absorption of water, and if they occur in the first few years of a boats life are an indication of a moulding problem of some sort, whether it be poor materials, poor workmanship by the laminators, or any other quality control problem ranging from sawdust getting into the moulding to a prolonged delay (factory holiday - several people off with flu?) between laminating up the various layers that form the hull. If the builder is still trading they will normally repair this under warranty. Unfortunately whilst the original bare hull moulding of a £100,000 yacht might only cost £12,000 to mould, it will cost another £50,000 plus to fit it out even if using every removable part from the faulty hull. So the builders will ‘treat’ the ‘osmosis’ by the standard current method, namely stripping off the external gelcoat, drying out, and recoating with epoxy fillers. As this new external coating is essentially ‘glued on’, and not chemically part of the original moulding as was the original gelcoat, it can be argued that the boat is substantially devalued by this repair.

By ten to fifteen years age it is common to find that yachts hulls have a moderate to high moisture content. Some may also have developed a few blisters. This is absolutely typical, and not necessarily a sign that there is anything terribly wrong with the boat.

If a yacht reaches twenty years of age without high moisture content or visible blisters it is actually a bit of a surprise.

These timescales assume a standard mass production yacht, given average use of perhaps seven or eight months afloat a year, with just antifouling paint on the bottom. Painting with polyurethanes or epoxy coating the bottom can considerably, but not totally, slow down the rate of water absorption, and some builders do this from new (including my local tin-shed boatbuilder referred to earlier). Opinions vary as to the effectiveness of epoxying or painting once there is already some moisture in the moulding.

If you are keeping the boat yourself you can look at these options and decide which way to go depending on inclination, temperament and bank balance.

If you are trying to sell the boat, other factors come into play. Buyers almost always prefer boats without blisters, and even if there are no actual blisters, once a surveyor puts a moisture meter on the hull and says there is moisture in there, most buyers think “.... if I buy this it’ll get blisters soon and then I’ll have to spend thousands on an osmosis treatment” - so a ‘wet’ hull, even if unblistered, is more difficult to sell.

Unfortunately it is a common perception that any boat with a high moisture content or blisters, MUST be treated, and that the only treatment is the full Monty - strip off all the gelcoat and recoat. Boatyards like doing the work: it is profitable, and can be time-scheduled in to when staff have some free time. Some surveyors like to recommend it as it means they’ve ‘covered their backs’ against a later claim that they didn’t pick up a defect.

It is certainly easier to sell an boat with no blisters and a low moisture content - so particularly on larger boats (an £8,000 hit on a £100,000 sale is not too disastrous) getting an ‘osmosis treatment’ done is often the way to go. You do however have to allow time for the work to be done - ranging from about 4 weeks if forced drying or a process called ‘hot-vac’ is used, to three to six months or even longer if the hull is left to dry out naturally in the open air, although shielded from rain.

If you are buying a boat, obviously it is preferable to have one with no blisters and a dry hull (low moisture levels on the magic meter). If this condition is achieved by the original hull surface, with no repairs, it is clearly better than a similar boat that is also dry and with no blisters, but achieved by having recently had an ‘osmosis treatment’. The treatments are not cures - they simply 'restart the clock' on a progressive period of absorption of water again, as even epoxy coatings are not totally waterproof.

Occasionally buyers will happily accept a boat with high moisture content or blisters - on the ground that they can haggle down the price as a result. They then may or may not then get some form of treatment done - perhaps just before they sell it on a few years later.

If a seller is genuinely unaware of the moisture content/blisters, a situation which is not uncommon, as many blisters are small and can be invisible under antifouling, and this is discovered on survey, a common compromise is to knock about half the cost of an ‘osmosis treatment’ for that size of boat off the price. It is unfair to expect the owner to take the full price off, as if the gelcoat is replaced the boat is more saleable, and hence more valuable, as it then has a known low moisture content and no blisters.

Moisture meters

The first thing to know about moisture meters is that they don’t actually measure moisture. They measure conductivity. My Tramex meter (a popular model with surveyors and boatyards) invariably shows a very high moisture reading on the solid and absolutely dry glass top of my living room coffee table. It’s measuring the presence of minute traces of carbon (an electrical conductor) in the lightly smoked glass.

Tramex meters read quite deep into the moulding - deep enough to be fooled by materials inside the hull. I have been told by a surveyor that the bilge keels of a Seadog ketch had a surprisingly high moisture level compared to the rest of the hull. I told him (although I’m sure he would have worked it out for himself eventually) that these hollow bilge keels were the water tanks in this design, and currently near full. Bilge water in compartments inside the hull, metal reinforcements, wiring harnesses, even condensation inside inner hull linings, can affect the readings of meters.

Sovereign meters, another common type, generally read to a much shallower depth. This can be either good or bad, depending on what you are trying to measure - surface moisture or deepseated moisture. The newest Tramex meters have an extra mode switch to reduce the depth of the reading. To make matters still more complex, the Sovereign and Tramex meters give results on different scales - Tramex from 0-100% and Sovereign from 0-25. So a reading of ‘20' could be quite good or quite bad, depending on which meter was used.

None of these moisture meters give an absolute reading of the amount of water in a structure: all they do is tell you that if readings in one area are higher than another, then that area might be wetter. It also might not. You could be reading the presence of something else that conducts electricity.

It is common for surveyors to take moisture readings on hulls during a brief (often just 1 hour or so) lift-out. Particularly with older boats, built using orthopthalic resins, high readings will almost always be obtained immediately after hauling out, but these will drop greatly if left ashore for a few days. This effect is less pronounced with isopthalic or vinylester resins - they are better at keeping out the water in the first place, but do not let it dry out again as easily.

The International Institute of Marine Surveyors Code of Practice for marine surveyors recommends that "... ideally the vessel should have been out of water for at least 24 hours". It also sets out a complex form of statistical calculation to reduce randomness of readings and take into account local temperature and humidity when measurements are taken. I have yet to see a yacht survey report that actually used this calculation.

This page is written by John Wilson of Yachtsnet Ltd., from the point of view of a very experienced small boat sailor, former owner of a yacht "with osmosis" (the boat was purchased knowing it had blisters), and yacht broker.