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a bit 'naval engineering

I'm trying to develop the project for a boat that is made for navigating, obviously made of plastic bottles.
have had in the beginning of the catamaran: the one that seemed interesting was the chance 'to make two bottles of sausages with a bridge in the middle. The sausage I think one of the forms most 'easy to do with the bottles and even more' fluid.
Actually 'but' catamaran and 'designed for sailing and rowing becomes uncomfortable when you have (a bit you can tell from the image above).

So with Andrea we tried to work on another solution that always works with the "sausage" and at the end and 'came out an idea like this:
The interesting thing' in this way the bottles become the only element that can be seen, and at the same time serve to float and sit ...


The other day I showed these pictures to my studio and the guys were told to be careful that there is also sufficient weight to balance the boat down.
At this point I was thinking of filling the bottles more 'at the bottom of sand, and have a sort of drift. But how to get an idea of \u200b\u200bhow much weight you need?

Maybe between us there is' someone who understands a bit more 'marine engineering and I can' give some suggestions?!