Load vs Waterline Calculator (Sink Rate)

Archimedes incrementally: an extra inch of immersion displaces (waterplane area × 1 inch) of water, whose weight is exactly the load that caused it — 5.33 lb per ft² of waterplane per inch in salt. Hull shape below matters for total displacement; the RATE of sinking under load is the waterline footprint's business alone, which is why slab-sided barges shrug at cargo that visibly squats a wineglass-hulled yacht.

Look down at the waterline's shape: a rectangle would be 1.0. Sharp-ended sailing hulls with hollow waterlines run 0.65–0.72; ordinary powerboats with broad transoms 0.78–0.84; pontoons and barges 0.85+. Within that band the answer moves ±10% — fine for the freeboard ledger this tool keeps. (Your sister-ship's published TPI back-solves it exactly, if you're fussy.)

Only while the hull's sides stay near-vertical through the waterline: flared hulls gain waterplane (and lb/in) as they sink — nature's progressive spring — while tumblehome hulls lose it, sinking faster per pound just when you'd want the opposite. For normal loading ranges the constant-rate assumption holds within a few percent; past it, you're asking questions the capacity plate already answered.

Swap 64 for 62.4 lb/ft³ — 2.5% fewer pounds per inch, so the same load sinks her 2.5% deeper in the lake. The effect compounds with the absolute floating level (fresh water floats everything lower to begin with), which is why river-and-sea boats notice the transition and why ship load lines legislate it. For this tool's freeboard ledger, the difference is small but free to include: multiply the result by 0.975.