46 C.F.R. § 45.77 – Salt water freeboard.

(a) The salt water addition in inches to freeboard applicable to each fresh water mark is obtained by the formula:
Addition=Δ/41T
where: Δ = displacement in fresh water, in tons of 2,240 pounds, at the summer load waterline. T = tons per inch immersion, of 2,240 pounds, in fresh water at the summer load waterline.
(b) When the displacement at the summer load waterline cannot be certified, the addition in inches to the minimum freeboard in fresh water may be obtained by multiplying 0.25 by the summer draught in feet measured from the top of the keel to the center of the load line diamond.