Boat Engine Size Calculator
Calculate the right engine horsepower for your boat based on hull type, weight and intended use.
How to Use
- Enter your boat length in metres
- Enter total loaded weight including fuel, gear and passengers
- Select your hull type (planing, semi-displacement or displacement)
- Select your primary use case
- Click Calculate to see the recommended HP range
Note: These are general guidelines. Always consult your boat manufacturer's specifications for maximum engine ratings and transom capacity.
Frequently Asked Questions
What size outboard motor do I need for my boat?
As a general guide, planing hulls need 20-40 HP per tonne of loaded weight to get on plane. A 5m tinny at 800 kg needs around 30-50 HP, while a 6m fibreglass runabout at 1,500 kg needs 60-100 HP.
Can you put too big an engine on a boat?
Yes. Every boat has a maximum HP rating on its compliance plate. Exceeding it compromises stability, voids insurance, and is illegal in most Australian states. This calculator gives a recommended range that stays within safe limits.
What is the difference between repowering and upgrading?
Repowering means replacing a worn-out engine with a new one of similar size. Upgrading means fitting a larger engine — which may require transom reinforcement, steering upgrades, and re-certification of the compliance plate.