Yacht encyclopedia
Osmosis (GRP yacht hull)
On polyester GRP, moisture depth matters more than the word blister. Schneider grades 1-3, and the repair still left, not the photo count.
- Schneider calls that grade 1, usually a local repair.
- Schneider’s grade 2 is more blisters, fluid that is cloudy and sour, layers not yet badly hit.
- Grade 3 is a wider field, more fluid, and the layer may already be weaker.
What this is
Osmosis on a GRP hull is moisture moving into the polyester laminate until the gelcoat lifts into blisters. At a survey the word in the advert matters less than the language: cosmetic, watch, or a repair inside the years you mean to keep the boat.
On the hard, the boat already out of the water, one word is standing in for two hulls. On the first you can count the blisters, and the fluid is clear. Schneider calls that grade 1, usually a local repair. Their scale is not a standard every yard must follow. On the second the field is wide, the fluid cloudy and sour, and the laminate may already be weaker. Call both hulls osmosis, and a discount starts to look like a fair trade. The buyer needs the repair cost before the price is set, not after.
How it develops
Early GRP often went out without full osmosis resistance, or without an epoxy barrier. Winter storage slows the process. It does not reverse it, and it does not dry laminate that is already wet. Blisters shrink within hours of the lift: West System asks you to look at the bottom on the day she comes out. A later look on the hard easily misses what was there at the lift.
Survey language
A blister by itself does not say how far the water went. PassageMaker describes the check: grind a small patch of paint and read the laminate layer by layer. A drop of alcohol on the bare glass: dark, clear resin is one picture; milky, yellow, or white means the resin has already been eaten by water. Without something to compare against, a moisture meter is quick to scare you. Practical Sailor: take the reading on a dry topside first, and scrape the bottom paint; a high number with no blisters is not yet osmosis. The meter does not give an exact share of water. It compares. It does not assign a grade.
Schneider’s grade 2 is more blisters, fluid that is cloudy and sour, layers not yet badly hit. Grade 3 is a wider field, more fluid, and the layer may already be weaker. Grade 4 is laminate that has lost strength and come apart in places; the repair is in sections, not a spot. He writes that this kind of weakening is rare in his own practice. That is his scale, not a rule a yard is obliged to build to.
Buying
Voids in an ordinary layup are normal. The expensive pattern is a wet balsa core, or chopped strand inside the laminate, not every blister on solid glass. These articles did not measure a water share of “1-2 percent.” Separately, the PassageMaker author’s opinion: a finished repair with a layer that keeps further water out can be preferable to a bottom that has not opened yet. That is an opinion, not a yard rule. Bargain the repair still left to do, not the number of blisters in a photo.
Schneider quotes two other people’s bills, not a tariff for this bottom. A Formosa 51, grade 2, Croatia, about €7,000 without sandblasting. An Azimut 39, grade 2, Italy, about €10,000 including that cleaning. A quote for this particular boat does not follow from the articles. He also warns that a yard sometimes offers to strip the whole colored outer layer where a local repair would have been enough. West System is harder: once the resin has started to dissolve in water, the process feeds itself. That is a dispute between two sources, not a verdict.
FAQ
Q: Should I walk away at any mention of osmosis? A: No. Read the survey grade and the estimate if the repair sits inside the years you mean to keep the boat.
Q: Is epoxy paint enough? A: It takes the symptoms off. A lasting repair is drying the laminate and a barrier.
Q: Does winter stop osmosis? A: It slows the process. It does not dry laminate that is already wet.