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Osmosis Test

Sub-gelcoat moisture and early-stage osmosis measured with two independent instruments; delivered as a moisture map of your hull.

  • Blistering graded by extent and depth
  • Moisture reported as a map, not a single value
  • Skipper 5 to scan, Wetsoon Pro to confirm
  • Osmosis reaching the laminate is separated from surface blistering
  • A cost range to support the repair decision
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An osmosis test establishes the extent and depth of blistering beneath the gelcoat of a GRP hull, and whether it has reached the laminate, using moisture readings and a hammer survey. The boat must be ashore and dry.

A small blister, a large question

A few millimetres of blistering under the antifoul can be one of two things: a harmless paint defect, or the first visible sign of osmosis advancing through the laminate. They cannot be told apart by eye — only by moisture measurement. The difference is expensive: early osmosis is a local repair, late osmosis can mean a full hull peel.

How we measure

The hull is scanned on a grid with the Tramex Skipper 5, reading moisture up to 30 mm deep through paint and gelcoat, calibrated for salt or fresh water as appropriate. Every area above threshold is then re-measured with the analogue Wetsoon Pro — two independent technologies must agree before a finding enters the report. Visual examination and percussion testing run alongside, and thermal imaging extends the scan across large areas where useful.

Moisture meter scanning the topsides of a boat ashore for osmosis
What matters is not whether a blister exists, but how widespread it is, how deep it goes and whether it has reached the laminate.

Osmosis is not a yes-or-no question

Most osmosis reports come down to one sentence: “osmosis was found.” That sentence tells a buyer nothing, because three separate things decide the outcome.

Measured Why it matters
Extent Blistering confined to one area and blistering spread across the hull differ in cost by an order of magnitude
Depth Confined to the gelcoat is surface work; into the laminate is structural repair
Moisture distribution An old blister in a dry hull and a wet laminate are not the same thing

An osmosis verdict given without measuring all three is a guess. Our report carries all three, as numbers and as a map.

Wetsoon Pro osmosis and moisture measuring instrument
The Wetsoon Pro is the verifying instrument: every point the Skipper 5 flags gets a second reading.

What you receive

A moisture map of your hull: which zones read what, which are clean, which need watching. Where osmosis is found, its extent and depth are graded, and the realistic options — local repair, epoxy barrier, or monitoring — are laid out with cost context. If you are buying, this map is the most concrete document you can put on the negotiating table.

Moisture readings written directly onto the hull during the survey
Reading points are marked on the hull itself and carried into the report as a map.
Point-by-point moisture sweep along the topsides
The topsides are swept from the waterline down; osmosis most often starts in the first band below it.

Duration and fee

The osmosis test is carried out together with the hull survey — the hull has to be ashore and dry for both, so separating them wastes time and money.

Scope Fee
Hull survey + osmosis test 10 € / ft
Within a pre-purchase survey No separate charge — full survey rate 26 €/ft

Fees vary with the length of the boat, where she lies and the season. For a firm price call us — tell us about the boat and we will give you the number on the phone.

Estimate your survey fee

From the field

What this work looks like on board

5 frames

These frames were taken during real surveys. None of them are stock: each one is a measurement or a check described on this page.

Frequently asked questions

Common questions

What is osmosis and why does it happen?

Water migrates through the gelcoat of a GRP hull and reacts with soluble compounds in the resin, producing acidic fluid-filled blisters beneath the gelcoat. Caught early it is a cosmetic matter; left to progress it degrades the laminate structurally.

Does the test damage the hull?

No. The measurement is entirely non-destructive: the Tramex Skipper 5 reads moisture through paint and gelcoat by impedance, penetrating up to 30 mm into the laminate - no drilling, scraping or sampling.

Why two different instruments?

A single reading can be skewed by surface conditions. Every area flagged by the digital Skipper 5 scan is re-measured with the analogue Wetsoon Pro. When two independent technologies agree at the same spot, the finding is certain - no false alarms, no missed blisters.

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Tell us where the vessel lies and your approximate date — we reply the same day.

Serkan Kotel — yacht surveyor, portrait

Serkan Kotel

Yacht surveyor · IIMS · ABYC · MCA — the person who answers the phone, walks the boat and signs the report is the same person.

Service areas

Bodrum · Yalıkavak · Turgutreis · Milas · Göcek · Fethiye · Marmaris · Datça · İstanbul · Ayvalık

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