Osmosis - Blister on the hull

Select Language Chinese Arabic French German Italian Japanese Korean Portuguese Russian Spanish English

Osmosis is the condition of visible blisters on a boat's hull, caused by water absorption. The process of osmosis is the same mechanism by which plants and trees draw water up from the soil to their branches and leaves..

Main causes of Osmosis:
* Long periods afloat without lay-ups
* Warm tropical waters
* Fresh water is worse than salt water
* Coloured resins (including white - the most common) are worse than clear resins.


The process of Osmosis:
The fibreglass mouldings on a yacht hull are not completely waterproof. Individual water molecules are so small that they can find their way into the layers of glass and resin which form the boat's hull. Problems start to occur when water molecules merge with other chemicals inside the laminate.
These are chemicals such as water-soluble materials, emulsion binders for example. The water molecules can then have a chemical reaction with these substances, forming larger molecules of a new chemical, often acidic. Unlike the original small water molecules, these cannot pass through fibreglass and become trapped. This is the point at which osmosis starts.
Waterproof Fibreglass Mouldings:
A fibreglass, or glass reinforced plastic (GRP), moulding is made up of thin glass strands. On their own the glass strands are strong but flexible. When fused with resin it results in a more durable substance. Ideally the final moulding should have lots of glass and only enough resin to hold it together and seal in the glass strands, but most conventional mouldings have much more resin than this.


A typical moulded GRP yacht hull will have a thin coating of a special resin called gelcoat, on its outer layer. This forms the hard outer surface of the hull. Most yacht builders mould the hulls layer by layer, allowing each layer to set before putting on the next. The one exception to this process is vacuum bagging or resin infusion moulding. This is where dry layers of glass cloth and mat are laid in a mould, and held in place by either an inflated plastic balloon, or another close-matching mould. The resin is then drawn into the fibres by vacuum suction.

The process is more expensive, but is capable of producing very light, strong laminates with a high glass content, and most importantly, very few air bubbles.

Treatments for Osmosis:
The best option in most cases is to cut or grind open individual blisters, repeatedly wash out with hot water or steam, to remove the 'blister juice' from any blisters, dry thoroughly and fill with epoxy paste.

On older boats, if there are no blisters, and even if a moisture meter shows high readings, one option is to do nothing! If there are blisters but they are small, they are not likely to have any significant effect on the structural strength.

Go to your local 'Osmosis Treatment Centre' and have the gelcoat removed, the hull washed and dried out, and the hull recoated with epoxy. This is the most expensive course of action and should be taken as a last resort..