Originally Posted by C-NUMB
" 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."
I don't get what is the disarrangement here...If we scale a steel boat in air and the figure is 3 ton and then scale the sandwich GRP boat in air with a equal weight of 3 ton, think its fair to say that in real life applications the weight (and mass) is same for both boats(in air)?
Not really, We are not weighing the boat in air, but in water, or more specifically, fully sumbmerged in water.
If you take the 3 tonne boat made from steel, the density of steel is approx 8tonnes per M/3, therefore the volume of the steel will be 0.375M/3, which when multiplied by seawater density of 1.025, will give a displacement of 384Kgs of seawater. This displaced water will give 384Kg's of bouyancy to the 3tonne boat, so it will "weigh" 2.616 tonnes underwater, requiring over 2.616 tonnes of bouyancy to make it float or 2.55M/3
Now do the same thing for a 3 tonne fibreglass boat of density 2.5 tonnes per M/3, and you get a volume displaced of 1.2 M/3, and a displacement of 1.23 tonnes, meaning the boat will "weigh" 1.77mt underwater, and require over 1.72 M/3 of bouyancy.
The weight of the boat will be 846 Kgs lighter in water.
It is why all ships are measured by their displacement, not by weight.
However it is all useless theory, as a boat is made of many materials, and trying to find the correct average density is almost impossible, so as a basic rule if the reserve bouyancy in M/3 is the same as the boats displacement it will float, as the materials themselves will add a small amount of bouyancy, even if it was made from lead.
So to answer the OP, yes 3 cubic meters of reserve bouyancy would float your boat, but as other posters have pointed out, it might well make it handle crap, be tender, unstable, and be rubbish! it might make if float upside down, on its side, bow in the air or stern in the air....