Do not worry about picking on me, I'm good for it. I appreciate the discussion and all that I have learned in this crazy process. While I am not an engineer I spent a big part of my professional life working with them (mechanical engineers), learning from them and from time to time explaining things to them.
Today I was out trying to bond myself quite literally to my boat. Doing overhead glass work is not easy. But we are seeing the light at the end of this tunnel.
This morning after thinking a bit more about my very basic test I wondered if the carriage head itself was contributing to the less than stellar results of either material because of the point load of the hex under the head. I glanced briefly at the SAE C365 spec and saw that they are using plattens for their test. So I went back into the shop this morning I dug out a standard 5/16" bolt and nut and 4 washers, two for each side and retested the balsa and the Cousa material. The Cousa did get to 10 Ft. Lbs. this time and the balsa did better, but fell way short again. I did think about doing the same thing with plywood, but got busy on what I was supposed to be doing today.
This is an important discussion to have because understanding what balsa does and does not do is critical to making these boats go another 30 years. I do understand that there are different grades of balsa and am real sure that there are multiple grades used at least in my boat (not intentionally). The lighter less dense balsa will dry out with reasonable effort, the more dense stuff does not dry easily and has generally been replaced in my boat. Balsa is a good engineering material. It does have very good compressive strength and very good flexural strength when the skin surfaces are held by the resin and glass and it is crazy light. As we all know its big weakness is what happens when it gets wet. But it does not do point loading and by itself has almost no ability to take a lateral point load, so the discussion about how to best manage the barrel of epoxy we all rely on is important.
The reason I am using this Cousa board is because I got a really big box of core materials from my brothers boat plant, mostly the Divincyl. If I had to go buy this stuff I am not too sure I would use it. I think the 4 x 8 sheets are $285 wholesale. But the properties are really good and even if you could get it wet, it would still have nearly the same properties. I would have to ask my brother how the G10 compares to the Cousa, likely similar. He is a very high end boat builder and generally uses the best materials in his boat based on the required spec, not the just the cost of the material.
The next kind of interesting test that I may think about is how to put a big lateral load on one of these bolts to see what happens to the hole itself. Again, my interest is not in an actual failure, but if gaps develop under lateral loading then the leaks start sooner or later
So here are the images from this morning. At 10 ft. lbs. the 1st washer on the Cousa was halfway embedded in the material. The balsa was burying washers on both sides well below the 10 ft. lb. torque value. Again if there were any additional skin on either of these two samples I am very certain the torque values would have gone higher.