THISTLEWACK FIELD INSTRUMENT № 01

The Buoyancy Reckoner

Tell us what must float. The instrument returns the water the hull must displace, a prudent two-times reserve, and the draft demanded by a rectangular hull footprint.

ANCESTRAL BUOYANCY CALCULUS

Build more boat than the minimum.

The absolute minimum leaves a vessel nearly awash. Capt. Dan specifies twice that volume as a practical starting point so the hull retains freeboard and a measure of forgiveness.

Read the physics behind the numbers →

Include crew, hull, paddles, decoration, and anything else that intends to return.
Recommended enclosed volume · 2× reserve7.05 ft³
Absolute minimum3.53 ft³
Equivalent volume200 liters
Reserve draft at given footprint4.7 inches
Fresh/salt density62.4 lb/ft³

The draft assumes a simple rectangular footprint and calm, level loading. Add freeboard, structural allowance, and the requirements of your race rules. This reckoner is educational—not a substitute for life jackets or competent supervision.