Osmosis Blistering Guide

Osmosis Blistering in Fibre Reinforce Plastic Hulls

Below is a simple explanation on hull blistering it’s not intended to be a technical document but a pragmatic view on the problem.

A new fibre reinforced plastic yacht hulls will start to absorb moisture as soon as they are launched. Normally this moisture will pass slowly through the laminate and into the bilges, where it will disperse harmlessly as water vapour. The problem comes when this moisture passes into small air inclusion originally built into the hull which is mostly commonly found between the gelcoat skin and the laminate. These small voids often contain chemicals from the manufacturing process and when mixed with the absorbed moisture make a small fluid filled blister containing a strong chemical solution. At this stage the blister is unlikely to be visible on the hulls surface however over time through the process of osmosis this blister gets larger and eventually starts to show itself as a raised blister on the hulls surface.

Since glass reinforced plastics were first used in boat building in the early 60’s the resins being used in the building process have been greatly improved with tighter curing structures. This means that amount of moisture being absorbed through the resins is greatly reduced. In addition to improvement in resins both moulding techniques have been greatly improved and the fibre reinforcements have become more chemically inert. Osmosis type blistering is therefore more of a problem in vessels built before the 90’s however not exclusively.

How to reduce the risk of osmosis blistering developing in older vessels

 The most important thing is to rest the vessel out of water in good drying conditions so that the absorbed moisture has time to dry back and doesn’t gradually build up. This doesn’t mean lifting the boat out in January and putting it back in again at the end of February it must be when the humidity in the air is low and the temperature is high enough to dry moisture below the surface.

The other common practice is to coat the hull with an epoxy barrier coat to reduce the amount of moisture being absorbed. However, this should only be done if the hull is absolutely dry before the coating is applied. Failure to do this will trap any moisture already in the hull and accelerate any small blisters not yet visible to develop. Its not uncommon to see blisters developing in older boats which have been epoxy coated within a few years.

What to do if blistering has developed

 The good news is that blistering can be repaired. However, the bad news is that it is expensive to do so.  It is though very important not to leap into getting a hull repaired immediately when a few blisters have been sighted as the technique of repairing a lightly or heavily blistered hull will be the same. It is not uncommon to find boatyards undertaking blister repairs on lightly blistered hull and not fully removing the gelcoat back to the laminate. This is a mistake and whilst the hull may dry back to an acceptable level to epoxy, the existing problem is still underlying and blistering will re-occur after a few years. Therefore, it is always worth waiting until the blistering is in an advanced state before carryout a repair.  Like hull moulding, techniques of repairing blistering have greatly improved in the last ten years of so and vessels repaired today should be cured of blistering again for at least ten to fifteen years and in a few years’ time the techniques will have again improved and may extend the service life of the repair even further.

The biggest problem comes when blister develops on a boat of low value in that the repair can sometimes be near the market value of the boat. In these cases, one can often carryout localised blister repairs on the large blisters and still maintain the vessel structural integrity for use. Personally, I have never heard of or seen of a vessel that has sunk due to osmosis blistering alone.