Outboard Motor Size Calculator

How it works

Planing (Crouch): speed (mph) = C ÷ √(lb per hp), C ≈ 150 runabouts / 125 pontoons → hp = weight ÷ (C ÷ mph)²; displacement: 3–4 hp per tonne reaches hull speed 1.34 × √LWL(ft)

Sizing an outboard starts from the fully loaded weight — boat, people, fuel and gear — because that is what the motor must lift onto the plane. Crouch’s formula, a century-old planing-hull relationship, links speed to pounds per horsepower through a hull constant: about 150 for typical runabouts and 125 for pontoons, so 25 knots on an ordinary hull needs roughly 27 lb per hp and getting on the plane at all needs under about 40 lb per hp. Displacement hulls are different: they cannot exceed hull speed no matter the power, so 3–4 hp per tonne is plenty. The capacity plate’s maximum is a legal and structural limit set by the manufacturer; running near 75–100% of it gives the best all-round performance, while a small motor on a heavy boat wallows and burns more fuel than a right-sized one.

Worked example

A 900 kg runabout with four people and 150 kg of gear (1,390 kg, 3,064 lb) aiming for 25 knots: Crouch gives 27 lb/hp → 113 hp, so a 115 hp outboard, within a 150 hp plate; about 77 hp is the minimum to plane.

Frequently asked questions