Sizing the Outrigger Float of a Single-Outrigger Canoe: Ama Volume, Crossbeam Length and Heel Limits

A single-outrigger canoe depends for most of its stability on one small float, the ama, held away from the hull by a crossbeam. Builders size that float by inheritance and by eye, and the penalty for undersizing is paid in a capsize or in a boat that cannot be worked. This paper derives a sizing rule from statics and applies it to three crew and load cases. The rule has two parts. First, a reserve-buoyancy requirement: the righting moment available when the ama is pressed to full immersion, equal to water density times gravity times ama volume times the distance from hull centreline to ama centreline, must reach twice the stated heeling moment, the factor of two being a stated dynamic allowance. Second, a heel check: the equilibrium heel under the stated heeling moment, from the combined stiffness of the hull and the immersed ama, must stay below a stated limit of 10 degrees. For a two-crew canoe of 260 kg total mass facing a heeling moment of 715 N m, the rule requires an ama of 48.6 litres at a centreline distance of 3.0 m, of diameter 0.153 m over a length of 3.3 m; the corresponding volumes are 58.3 litres at 2.5 m and 41.6 litres at 3.5 m. The one-crew case requires 24.3 litres and the three-crew case 80.4 litres at the same 3.0 m distance. In every case the heel check is met with a wide margin — equilibrium heels range from 0.54 degrees to 1.47 degrees across all cases and beam distances — and a float sized by the heel check alone would be under 2 litres, so it is the reserve-buoyancy requirement, not heel stiffness, that sets the size of a safe ama. The capacity when heeled away from the ama, so that the float is lifted rather than pressed, is smaller than the pressed capacity in every case and is tabulated as a warning. The study is simulated, not measured.