2000-01-11 – Further tests of stability

Further tests of stability

Well, further tests on the structure reveal the incredible stability of this system. Each plastic cup holds 240ml, or 240 grams worth of air, whilst only weighing 4.24 grams. The mass of the glue used is very low, and will be reported when I have measured it, but as it is hot glue gun glue, I believe it is the same density polyethylene as the cups themselves.

Allowing 5% as the mass of the “fixings”, this gives a lift ratio of 53.9 times the mass of the structure. This is really very impressive. Indeed, with the 120gram set of weights I had to hand, the structures were virtually unsinkable. The six cups in a triangle when trimmed by pumping air under some cells and removing it from others, supported the full 120grams on one corner. The total mass of the structure is about 27 grams. Stability once trimmed was, surprisingly, not much changed. The main problem was the weight set sliding off under the wave action. This resulted in the platform becoming unstable, and the corner holding the weight lifting from the surface. In a larger structure the surface lifting would be prevented by the mass of the next cell along. In order to ensure that lifting can never occur, the edges of the structure can easily be partly evacuated, so reducing the lift they provide, while increasing their mass. This may not prove to be effective against high seas, and so physically attached masses may be required. The partially filled cells provide better sea anchoring, and will be used for the beach areas anyway, due to the requirement for shallow waters.

Note: The glue turns out to be about 1/2 a gram per bond, so is nearer 10% of total mass. This reduces the maximum lift ratio to 51.4.

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