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Yacht hull: solid GRP, balsa and foam core explained
Solid GRP, balsa sandwich and Divinycell/Airex foam: what the hull construction means for a moisture meter, percussion, a survey and a deposit.
The query "yacht hull construction" now often lands on a short video about the parts of a yacht. The video answers the curiosity of "what does it look like". It does not answer what solid GRP, balsa sandwich or Divinycell/Airex foam means for a deposit and a moisture meter. This page takes over that fork; the clip Art of Sailing - Main elements #1 stays as an illustration inside it, not as the owner of the intent.
The type of construction is not a brand virtue in a listing. It is a survey map: where to tap, where to read moisture from, which holes in the core are deadly and which zones on a solid laminate behave differently.
Three layups that listings confuse
| Layup | What it is | Strength | Typical failure / risk | Survey emphasis |
|---|---|---|---|---|
| Solid GRP | Monolithic laminate with no core in the skin | No "rotten core"; clearer picture for osmosis/blisters | Heavier; stiffness is gained by thickness and framing instead | Osmosis, repairs, thickness/moisture map on the solid laminate |
| End-grain balsa sandwich | Lightweight core between skins | Stiffness at low weight; balsa soaks up resin well | Water in the core - rot / loss of bond | Percussion; moisture preferably from inside; holes through the core |
| PVC foam (Divinycell / Airex class) | Foam in the sandwich | Does not rot like wood | Weaker bond with resin; water + freeze cycles - delamination | Percussion; look for delamination; do not confuse "dry foam" with an intact bond line |
A historical yard rule that surveyors still keep in mind: core below the waterline is a separate conversation about risk. Many successful hulls keep the core above it; breaches and drilling through the core without sealing are a frequent source of trouble on the deck at the chainplates, the stanchions and the hatches. See also the wiki: deck core, osmosis, blistering, gelcoat, chainplates.
What the moisture meter says - and what it does not
On a solid laminate, high relative moisture after a haul-out reads differently than on a sandwich. On a sandwich the outside reading often lies: paint, metal, a fresh wash. It is more reliable when the inner skin is dry and accessible and percussion has already shown a "dull" area. Balsa can hold water in the channels of the grain; foam can remain an "intact" material with a bond line that is already dead. So a single number from the instrument, with no map and no context of the construction, is not a basis for a deposit.
A steel spider / grid in some production hulls is a separate layer of inspection (the link between the grid and the keel), not a "replacement" for the conversation about the deck core. Aramid / Kevlar in a listing's marketing, with no layup schedule, does not prove the impact resistance of the zone you are buying.
How to read a listing and a model page
In listings "solid glass" and "cored deck" often stand side by side with no schedule. Ask the seller: where the core is (deck / topsides / bottom), what material, whether the core has been repaired, whether there is an old survey with a map. On the model pages of Northern cruisers, balsa zones at the chainplates are a recurring story; check the listing against the model report, not against the general word "GRP".
The video above shows the language of hull parts. This page turns that language into a decision: whether to go, what survey scope to write, which photos to collect remotely before a listing review for €49.
下一步
If the construction is not named in the listing or is named in promotional terms - first a listing-review from the listing link, then survey coordination with percussion and a moisture map for your type of laminate. Ownership range: ownership-cost.
Interactive lot decode and hidden CapEx: CapEx Scanner.
常见问题
Q: Is solid GRP always safer than a sandwich? A: No. A solid laminate has other risks (including osmosis and weight). A sandwich is safe while the core is dry and bonded; it is dangerous when water and delamination are already inside.
Q: Foam "does not rot" - does that mean moisture does not matter? A: There is no wood rot, but delamination and water in the cavity destroy stiffness. A survey looks for the bond between the skins, not just the "dry" label of the material.
Q: Why this page if there is the Art of Sailing video? A: The video shows the parts of a yacht. The page answers how the type of construction changes the inspection and the deposit, and ties together the wiki and the survey scope.
Q: Can one moisture meter reading from the outside be trusted? A: Not as the sole basis for a purchase. You need the context of the construction, percussion, preferably interior zones and a map, not a single point after a wash.