Osmosis is the GRP boat disease number one


A physically very useful process in many cases is osmosis. Plants can live through them while solids get their difficulties. With glass fiber plastic, the osmosis causes blistering between the top laminate layer and the gel coat. A renovation of such damage is inevitable.

Probability increases with boat age

If an older boat is to be restored, damage and signs of wear must first be determined. For boats with a lifespan of a decade or more, they are very likely. Usually, they are visually recognizable by a "speckling" of the gelcoat surface resulting from air bubbles.

The air bubbles form bubbles a few millimeters below the surface of the seal or the gel coat. They locally "blast" the homogeneous bond between the final laminate layer of the fiberglass and gel coats. Preferably, the osmosis occurs in the underwater area of ​​the boat hull.

Physical cause

Osmosis describes the physical law that attracts higher density liquids to other low density liquids. What plant cells serve to supply drinking water has a destructive effect in the GRP. Smallest trapped air left behind when laminating GRP can cause "full blown" bubbles.

Moisture accumulates in the trapped air containing a mixture of solvents and various resin flooding. This fluid has a higher density than river, sea or seawater. Especially under the waterline, this ratio draws the outside water into the existing bubbles. The gelcoat acts like a membrane and allows water molecules to pass through. As a result, the bubbles grow in size and exert pressure on the GRP.

Possibilities for diagnosis

Osmosis can spread to many small bubbles and be invisible to the naked eye. Whether it becomes necessary to repair the hull can be determined by different measuring methods. Ultrasound and sound emissions try to localize bubbles by differences in GRP tone reflections. All procedures are laborious and the results can only be assessed by experienced boat builders.

Tips & Tricks

If you want to buy a used boat and are not sure if there is damage due to osmosis, there is an indirect investigation. With the boat dry, close all hatches and openings to the cabin interior and leave the boat in this condition for three days. Then open the cabin and immediately do a smell test. A musty sour smell is an indicator of existing osmosis.


Video Board: Fiber glass boat survey on a yacht that need Osmosis Treatment,