Mal Smith wrote:
I think you're forgetting to factor in the SG of the construction material, e.g. the bouyancy of an all glass boat is not zero. In fact it would not be much less than the weight of the boat. I would assume that Colin's estimate of 60kg for a foam core is over and above the weight of the boat. The only way to be sure is either to calculate for the volume and SG of each material in the boat, or flood the actual boat and see how much added weight it will float.
Good point... I suspect the SG of a monolithic glass laminate is a lot more than 1, but it was wrong of me to assume the buoyancy was negligible.
According to my model the suface area of a Nethercott Hull is about 6.2m^2. Decks around, what, 3.5m^2. I've used about 3m^2 in internals, carriage etc. Lets say 6mm thick layup, that's 12.7 * .006 m = 0.0762m^3= 76kg buoyancy. So maybe not, but I've ignored the seat...
Its amazing how complicated the simplest rule can get if you put too much casual inaccurate thinking at it! At current (slow) progress I have about three weeks before final decision on what goes in mine, so lets have the definitive news by then please measurers!
I suppose an obvious clarification might be that boats constructed substantially of buoyant materials (wood, foam sandwich) shall carry 75kg of extra buoyancy, and that boats constructed of non buoyant materials shall have sufficent extra buoyancy to allow for that. But it does make things awkward if like me you are deliberately using buoyant materials as part of the engineering structure...
If that's not going to count towards my buoyancy then I'm going to have to considerably rework my build, which is a bit distressing having bought the materials, especially the expensive high strength styrofoam. I should have read and understood the rules more carefully I suppose.
cheers, Jim C