While I don’t do much with balsa, let’s attack this mathematically;
What thickness do you want for the board to get the same floatation? Well , all other things being equal ( they are not, but for a relatively simple answer without mapping out the volume of the entire board ) …
You have a situation like this: New weight/old weight = new thickness/old thickness to get the same floatation. Then, a leetle algebra ----> you knew that darned old algebra would return to haunt you someday, and this is the day.
( multiplying both sides of the ‘=’ by ‘old thickness’ )
( New weight/old weight) x old thickness = new thickness
Ok, so, if the new board weighs twice as much as the old board, you should go twice as thick, right?
Lets assume for this example, that you go 170 lbs in a soaking wet wetsuit, your old board is 12 lbs with dings, leakage and general misery and the new balsa weighs in at 24 lbs finished, just to be on the safe side. Go with the whole you-plus-board unit -
(194 lbs/ 182 lbs ) x 2 1/2 inches = ?? inches… get out my calculator… that comes to 2.66 inches, call it a fat 2 5/8. That extra thickness won’t carry over everywhere such as rails and edges in general, so bump it up to 2 3/4" and that oughtta be close enough. Going a skosh wider or longer wouldn’t hurt either.
Fill in the actual weights you’re dealing with instead of the ones I picked for the example and go from there.
Similar maths apply to width vs length vs weight and so on.
To estimate the new weight of a balsa board, weigh your rough-shaped balsa blank and then weigh an equivalent foam blank. Add to the weight of the balsa blank the difference between the weight of the foam blank and your current board ( to account for glass and fins, etc) , that oughtta be close enough.
hope that’s of use