Jan Glete's approximation for displacement works pretty well

Jan Glete created an approximation for doing displacement calculations for sailing ships when he had little information. My assessment is that his formula works pretty well. Compare the results for the ship Den Briel. My calculations assume a block coefficient and use an estimated mean draft, beam outside the planking, and waterline length. I calculated a displacement of 669.1 tons for the Den Briel, completed in 1655. Jan Glete's approximation, also implemented by me, gives 639.1 tons. Given the uncertainties involved, that is pretty close agreement. His formula is metric, so I have a conversion factor. He uses the "lasts" figure in his calculation. This is my rendering of formula, for English dimensions:

   Displacement = Lasts x 2.5 x (39.37/12)^3/35

I use the figure 39.37 inches per meter. I divide that by 12 inches to the foot to get cubic feet. A ton of seawater is 35 cubic feet.