Why is Balsa coring used anywhere on a boat instead of all closed cell foam?

[Na Mara]

Originally Posted by Dpackie

.

My specific questions are:

Would replacing the core with solid fiberglass laminate provide the strength I need if the wet wood core was replaced by the same thickness of laminate?

How much weight would I really add in replacing the wood core with solid glass on a 30 foot boat.

I'm not worried about weight, my primary concerns are strength and never having to repeat the job.

Dave

Assuming a couple of cm thickness in the core, replacing that with solid laminate to the same thickness will be rediculously over engineered and heavy. Even getting that thickness of layup to properly cure would be a trial. A better solution would be to replace the balsa with a modern distance material of the same thickness and laminate over. Or just accept varying thickness in the deckhead, if it’s covered by paneling on the interior no one will know about it but you.

[Mara Mae]

I think strength/weight and build processes are the main reason. My last boat had an Airex cored hull. Corbin 39. Mahogany plywood deck. My CD's hull is solid and the deck is balsa core. Coosa board means wetted out balsa can be refitted to no more wood. The C&C Invaders had a solid GRP deck. Cant imagine how long it took to build. Once you start to imagine building a GRP boat replacing the Balsa core deck with a water impervious material becomes a bit of a puzzle. I have repaired both from the bottom and the top and prefer the top. Cut out the topside deck with a multitool and keep the cutout. Dig out all the rotten core to leave a 1/2" ledge of inner deck on the bottom. Epoxy the 'ledge' of inner GRP deck. Fit in a Coosa board of same thickness. Epoxy on your cutout and fair the deck. I like that method but coosa is expensive and plywood epoxy sealed would probably do just as well for the time you will have the boat.

[Dpackie]

Originally Posted by Na Mara

Assuming a couple of cm thickness in the core, replacing that with solid laminate to the same thickness will be rediculously over engineered and heavy. Even getting that thickness of layup to properly cure would be a trial. A better solution would be to replace the balsa with a modern distance material of the same thickness and laminate over. Or just accept varying thickness in the deckhead, if it’s covered by paneling on the interior no one will know about it but you.

Thnks.

[danstanford]

https://www.youtube.com/watch?v=g2YOz_KnZE8

Short video from J Composites about why the decks are cored. This does not reference as I remember anything about foam vs balsa core but it does do a great job showing the strength to weight differences and why cored construction is used.

[wingssail]

Originally Posted by Na Mara

Assuming a couple of cm thickness in the core, replacing that with solid laminate to the same thickness will be rediculously over engineered and heavy. Even getting that thickness of layup to properly cure would be a trial. A better solution would be to replace the balsa with a modern distance material of the same thickness and laminate over. Or just accept varying thickness in the deckhead, if it’s covered by paneling on the interior no one will know about it but you.

As a compromise coring material (when balsa or foam is unavailable) a soft wood such as pine can be sucessfully used. Cut into blocks to fit the area and ensure all gaps are filled with thickened epoxy.

This results in a strong and stiff composite yet far lighter than solid laminate.

The builder of my boat used end grain balsa and thin skins and unfortuately no solid areas anywhere, not even winch bases, cleats, or tracks but we've had very few failures in 38 years where the backing plates pulled through (a couple of pad eyes) or either skin has fractured. We have had several areas where water ingress occured through bolt holes and the balsa core has become soaked and rotted. Consequently I have done several repairs using soft wood as core, some quite large, and even the earliest one, a replacement of 1 square meter of the cockpit floor, which was done in Vanuatu in 2000, has remained solid and dry even after 21 years of heavy crew members working in the cockput and jumping in and out.

Photo: Deck repair around a small hatch.