Charles, you are right, I ballsed up the figures.
When I double-checked them, it turns out Allan was spot on with his estimate of 3 cubic ft to float the boat.
What confused me was him using cubic feet and kilos in the same equation!!
So, 1 cubic ft supports 28kgs, and there are also 28L in a cubic foot
ergo, you're correct, in that 1L supports 1kg.
So if H-A's boat is 27.5kg, plus 19kg for motor, plus 25kg for 20L fuel and remote tank etc, then that 71.5kg would need approx 72L of foam, or 2.5 cubic ft.
Ergo, H-A's estimate of 3 cubic ft would be plenty. But HE would still have to swim!!