Cored vs solid

Designers and builders of boats know that cored composite fiberglass structures have better characteristics than solid fiberglass structures. The general boating public does not. We tend to believe that solid fiberglass is stronger and less likely to suffer damage from water saturation, and the latter is true. Virtually every high performance racing sail or powerboat is cored.

The mass production of fiberglass boats began in the 1960s and from the beginning the advantages of coring fiberglass panels were known. Fiberglass panels’ strength comes from the exterior plys and the thickness. Coring is the addition of a different material between two thin layers of fiberglass and the resulting panel is better in almost every way.

Builders began coring hulls and taking advantage of the lighter, stronger composites and were able to make the boats go faster and be more efficient, and then they realized the coring process had its own set of challenges, including water intrusion.

Fiberglass coring is generally made of balsa wood, closed cell foam or varieties of honey comb. In the earlier days of coring, balsa was the normal choice. Balsa is actually still better in many ways than any man made coring, but it is organic and with moisture, fungus spores and the proper temperature, it deteriorates. Many production fiberglass boats were made with balsa core without properly sealing the penetrations, including through hulls, port lights, deck hardware, etc…

The boating public became aware of the problem and many builders started advertising “solid fiberglass hulls”, likely beginning the boating public’s opinion that this was a better way to build a boat. It’s not. It is however a way to eliminate concerns about water intrusion into core.

Balsa and foam core comes in sheets sliced on one side to allow it to bend to conform to a curved mold. These open “kerfs” would allow water to accumulate and flow, resulting in significant weight gain and in the case of balsa, fungus deterioration (rot). The boating community had a reason to be concerned. The cored composite was good, but the construction methods were not.

Most builders have come a full circle and are coring hulls but doing it better. They remove the core from around through hulls and port lights in the mold, they fill the kerf lines and they design and build with water intrusion in mind.

To determine if you have water in your boat’s core use a moisture meter, pull a through hull, drill a few holes or weigh it. If you determine you do have water in the core, should you care? Well if it is fresh water and balsa core, yes, it will eventually rot and the two thin pieces of fiberglass separated by rotten wood is not strong.

If you have salt water in your balsa (or on your plywood), it may not rot and of course the man made cores can live in water and not deteriorate. There can be issues with freeze and thaw cycles (expansion) and the added weight is never a positive, but many builders have responded to complaints of water in the core with “well seal the entry point to prevent air from getting in and it won’t rot” or “it is closed cell PVC foam, it will be fine”. And they are right, but as a surveyor, I know it will reduce performance and value. But like blisters, you may have water in the core for years and never know or care.

If you own one of the boats designed to keep water out of the core, be careful when you install a new through hull or piece of hardware, remove the core from around the hole, back fill with an epoxy and then bed the new installation with a good sealant.
p.s. most of those boats with solid fiberglass hulls still have balsa cored decks