Outboard horsepower calculator

How much horsepower does a boat need?

The honest answer is a range, not a single number. Power needed grows with the total weight the motor has to push — hull, fuel, crew and gear — and with how quickly you want that weight on the plane. A 16ft aluminium boat used to reach a mooring is happy with a small portable; the same hull towing a wakeboarder needs close to its rated maximum.

This calculator works from loaded weight and hull length, then widens or narrows the range according to how you use the boat. Treat the top of the range as a target and the bottom as the point where performance starts to feel slow with a full load.

Never exceed the capacity plate

Every boat has a maximum horsepower stamped on its capacity plate, usually on the transom or beside the helm. Fitting more power than the plate allows is unsafe and in most countries illegal, and it will normally void your insurance. If this calculator suggests more than your plate permits, the plate wins.

Weight to include

Use the loaded weight, not the dry hull weight from the brochure: add fuel (about 6.1 lb or 2.8 kg per gallon of petrol), batteries, the motor itself, and the people and gear you normally carry. Under-stating weight is the most common reason a new motor feels underpowered.