John Perry wrote:you can usually prove what you want to prove by choosing suitable test specimens and test procedure.
Yes, that was downright cheeky. That wood seemed to be not plywood, but a plank with the grain parallel to the supports, not going across from one side to the other. Of course it bloody broke. Unidirectional glass with the same fibre orientation would break about as easily.
My initial approach would be to use a fireman's axe on an extended handle, so I can drop it from whatever height the working space allowed, and a sledgehammer of equal weight. Use both the cutting and spiky side of the axe. Let the impact be at both 90 degrees and at 45 degrees to the surface. That gives six conditions. Raise the height of the drop until damage is visible on the inside. In sandwich structures, record both first damage penetrating the outer laminate, and damage penetrating the inner laminate. If there are more fibres in one direction, test with the predominant direction either bridging the supports, or parallel to the supports. That would mean 12 test conditions.
I don't know what effect the spacing between the supports is likely to have. On the one hand, stress increases with distance. On the other hand, ability to flex also increases with distance between supports. I don't know how these balance. If all effects are linear, the relative performance of different materials should not be sensitive to this parameter. I don't know whether all effects are linear. Testing two diferent spacings would increase the number of test conditions to 24, so that could be the number of samples needed for each material.
John Perry wrote:So, although its a rather academic question, what hull material does have the best specific puncture resistance, i.e. for a given weight of panel, what kind of panel will best resist puncture from a pointy rock? Many people would say steel, but that is heavy.
I am pretty sure the Dashews ranked materials in the order steel, aluminium, glass composite, wood, ferrocement. I think that was not based on any testing or systematic data collection, but on how boats fared that they happened to know to have gone aground. But I think a lot of the data points came from Cabo San Lucas, where quickly rising wind put a lot of boats onto the same beach, in the same wind and sea conditions. They did acknowledge, though, that there was a lot of luck involved in whether a boat landed on top of another.