Balsa cores... Chocolate tea pots.... Discuss.

[bigwow]

I hadn't realised there was resin between the blocks. I presumed it's supplied as a sheet? Are the blocks already sealed?

Putting the foam sheet over a barrel and rolling with resin usually ensures resin is in the voids between foam blocks

 

[dom]

Worth a look...
http://www.yachtsurvey.com/structuralissues.htm

A highly interesting and informative link; thanks for that oldgit :encouragement:

 

[geem]

Well, Balsa orginally came from the aircraft industry. The Mosquito bomber is probably the best and finest example, faster the the German hunters and still able to carry a nice payload. Everything else with solid skins on a frame was much heavier and slower.

From there it made ist way into boatbuilding. Clear is that cored composites were a novel way, to combine stiffness , thermal insulation and weight saving. Unfortunately boats have never straight surfaces, so they came up with the idea to bond balsa cubes on a scrim closth ind order to coply with the curvatures. In the beginning builders were not competent enough to apply this technique correctly and a lot of delamination failures and water ingress into unfilled occurred. It was not until 10 - 15 years later that the leading builders learned that a balsa core had to be hermetically sealed to the outside and all voids properly filled, reducing the weight saving adavantage. Most of these hulls turned its blasa core into peat.
Sometimes this was not the fault of the original builder, but simply the result of a sloppy euqipment Installation/upgrade by small yards or DIY's.
Then came foamed cores, initially in racing yachts, replacing Balsa. Still Balsa had the highest compression strength, but foams evolved over time as THE preferred core material. Honeycomb cores , again derived from aerospace Technologies, were another dead end street. They were horribly expensive, and had a limited lifetime due to delaminations.
For a relatively flat deck Balsa is absolute ok, for hulls I would prefer a damage resistant foam like Airex R63 over any crosslinked Divynicell or Herex. Newer foams are now based on thermally formable PET polymers and are used increasingly in closed mold boat building techniques.

Our deck is cored with Airex. She was built in 1980. The later boats from the same yard used balsa, presumably to save money. All the Trintella 44/45 had teak decks with screw fixing. Just removed out teak deck. Core in good condition but reinforcing ply under windlass like hot chocolate. Now replaced with new ply and all glassed in epoxy. Two layers of 6oz cloth over deck and deck is stiffer than when she had teak decks. Also with cream deck paint cabin notably cooler than when we had teak and we can walk on deck in the sun with no shoes on without burning our feet!!

 

[Avocet]

Reading this I'm so glad Avocet is just one huge thick lump of fibreglass! I don't mind it being heavy! Actually, that's not strictly true. There are plywood stringers across under the deck that were laminated in to form beams. Some of them have rotted (I know because I drilled through some of them to run cables) but the stiffness really comes from the shape of the fibreglass that was originally formed round them. The only wood I've replaced was the one under the mast step (I could see the coachroof sinking locally) so I had to cut the inner skin away, scoop the remains of the plywood out with a teaspoon, and lay up half an inch of solid GRP under it - to hell with the righting moment!