Originally Posted by C-NUMB
No, the weight of a 3 ton steel boat weights in air exactly as much as a 3 ton sandwich GRP boat. But for flotation You need much less volume for the sandwich GRP one....if any.
So for an accurate reply, one needs more specific details regarding the construction/ equipment of the boat
geir's method of calculation is looking good.
Mass does not vary, so the mass of a 3 tonne boat is the same wherever it is, but the weight is different. Weight is affected by the environment it is in, Mass is not.
For example somethings, such as gasses are measured as weght in Air, or weight in vacuum, the former taking into account that the gas displaces a certain volume of air, and therefore has less weight.
Technically a boat that floats has no weight in water.
to find your true amount of bouyancy to float an object you would need to sink your boat in a test tank, to determine it's volume, i.e. all the construction material, and fittings, paint, anchors etc etc. all the tanks/void spaces etc would need to be flooded as well.
that volume would be it's basic displacement, and for arguments sake could be 1.0 cubic meter.
If the boats Mass is 3.0Mt, then you would need 3.0 /1.025 of bouyancy to make it float, or 2.92m/3 of bouyancy.
If the basic displacement was 1.0 M/3 you would then need to increase the basic displacement by 1.92 M/3 by adding closed cell foam.
However the closed cell foam also has Mass, so the mass of that has to be added to the boat.
The bottom line is that 3.0 of reserve bouyancy will keep your boat afloat, with some to spare, but depending where you put it may not stop it capsizing, or floating with the bow pointing vertically.
That is a whole different ball game!