I think we need to get to the bottom of this.
Simply put, buoyancy of a water craft is the volume of water displaced. For the sake of easy calculations lets do this based on fresh water (1 litre = 1 kg)
Lets say a hobie kayak displaced 3000cm square and the weight of the craft is 30kg
Here is the calculation
For every 10cm square you have 1 litre of volume which equates to 1kg of positive buoyancy.
So a 3000cm square hull would be 300 litres which means 300kg of buoyancy.
You then subtract the 30kg’s the hull weighs and you then have 270kg of buoyancy.
Obviously this is for a neutral buoyancy and you don’t want that, you want your craft sitting out of the water by lets say 50% of its height you then subtract 50% of the buoyancy (the 300kg) you get 150 then remove the 30kg to get 120kg
Now, as you have noted above there is a risk of the craft taking on water and thus loosing buoyancy. This is fixed by adding foam or another light weight substance that displaces water.
So, by adding lets say 10 kg of foam you have a craft that can only take 110kg person(with gear) or it rides lower.
Foam ect can fix problems but it also adds problems, if you want your craft to never sink even if it has been fully filled with water you will want to fill your craft with a little over 300cm square of foam so as to offset the weight of the craft
i would say an air tight sealed yak is your best bet, short of a break in the hull it will never sink.
a yak that displaces less water is also (usually) faster (less drag)
hope this helps
foam that floats, placed inside the unused hull, contained within your yak...
, it also holds water a fair bit so choose of foam will be important. You would also need to be careful using air if its under any form of pressure as in a tyre tube etc, it could also expand and cause deformation.
