| How Osmosis Affects GRP Hulls | | | | or resin infusion moulding. This is where dry layers of |
| Osmosis is the condition of visible blisters on a boat's | | | | glass cloth and mat are laid in a mould, and held in |
| hull, caused by water absorption. | | | | place by either an inflated plastic balloon, or another |
| The process of osmosis is the same mechanism by | | | | close-matching mould. The resin is then drawn into |
| which plants and trees draw water up from the soil | | | | the fibres by vacuum suction. |
| to their branches and leaves. | | | | The process is more expensive, but is capable of |
| Main Causes of Osmosis. | | | | producing very light, strong laminates with a high |
| Long periods afloat without lay-ups. Warm tropical | | | | glass content, and most importantly, very few air |
| waters. Fresh water is worse than salt water. Resins | | | | bubbles. |
| that are coloured (including white ones) are more | | | | Types of Resin. |
| affected than clear resins. | | | | The earliest resins were orthopthalic polyester resins, |
| The Process of Osmosis. | | | | used in the 1960s. By around 1980, isopthalic |
| The fibreglass mouldings on a yacht hull are not | | | | polyester resins started to be used as well by some |
| completely waterproof. Individual water molecules are | | | | builders, as they were more water resistant. Most |
| so small that they can find their way into the layers | | | | builders used these more expensive isopthalic resins |
| of glass and resin which form the boat's hull. When | | | | only for the first outer coats, and then saved money |
| molecules of water mix with chemicals inside the | | | | by using the cheaper orthopthalic resins for the inner |
| laminate, problems start to happen. These are | | | | layers. |
| chemicals such as water-soluble materials, emulsion | | | | Doubling-up on the gelcoat layer is one method to |
| binders for example. The water molecules can then | | | | reduce the risk of osmosis. |
| have a chemical reaction with these substances, | | | | The latest type of resin, and the most expensive, is |
| forming larger molecules of a new chemical, often | | | | called vinylester, and is even more waterproof than |
| acidic. Unlike the original small water molecules, these | | | | isopthalic polyester. |
| cannot pass through fibreglass and become trapped. | | | | Treatments for Osmosis. |
| This is the point at which osmosis starts. | | | | 1) The best option in most cases is to cut or grind |
| Waterproof Fibreglass Mouldings. | | | | open individual blisters, repeatedly wash out with hot |
| GRP (glass reinforced plastic), or fibreglass hulls are | | | | water or steam, to remove the 'blister juice' from |
| comprised of thin strands of glass. On their own the | | | | any blisters, dry thoroughly and fill with epoxy paste. |
| glass strands are strong but flexible. When fused with | | | | 2) On older boats, if there are no blisters, and even if |
| resin it results in a more durable substance. Ideally the | | | | a moisture meter shows high readings, one option is |
| final moulding should have lots of glass and only | | | | to do nothing! If there are blisters but they are small, |
| enough resin to hold it together and seal in the glass | | | | they are not likely to have any significant effect on |
| strands, but most conventional mouldings have much | | | | the structural strength. |
| more resin than this. Generally, the outside of a | | | | 3) Go to your local 'Osmosis Treatment Centre' and |
| fibreglass hull is coated in a resin called a gelcoat. This | | | | have the gelcoat removed, the hull washed and dried |
| forms the hard outer surface of the hull. Most yacht | | | | out, and the hull recoated with epoxy. This is a costly |
| builders mould the hulls layer by layer, allowing each | | | | process and therefore should only be used if all else |
| layer to set before putting on the next. | | | | fails. |
| The one exception to this process is vacuum bagging | | | | |