Outboard Motors for Small Boats: Size for the Load You Really Carry
Boat length gets you a rough answer. The kids, the esky, the crab pots and the tide running against you get you the right one.
Outboard motors for small boats are portable petrol or electric engines, usually 2.5 to 30 HP, that mount on the transom of boats under about 5 m (16 ft) and are chosen to suit the boat’s weight, load and maximum rated power.
The Nguyen family and the fully loaded Sunday
Linh and Danny bought a 3.8 m (12.5 ft) tinnie with an 8 HP already fitted. On their first outing it was just the two of them, and the boat skipped along the bay like a dream. They decided the motor was perfect.
Then came the real test: both kids, two eskies, a bucket of crab pots and a picnic. Four people and all that gear put about 330 kg (730 lb) in the boat, still inside the plate’s load limit but a very different job for the motor. The little motor pushed the bow up, dug the stern in and never got the hull onto the plane. Coming home against the run-out tide, they were overtaken by a kayak.
Their capacity plate allowed 15 HP, so they upgraded within the rating to a Mercury 15 EFI. The next loaded Sunday the tinnie climbed onto the plane with everyone aboard and came home with fuel to spare. The kayaker was not seen again.
What changed was not the boat. It was sizing the motor for the heaviest day, not the lightest. Buy for the full boat, stay under the plate.
How much horsepower does a small boat need for its load?
A simple rule: allow roughly 1 HP for every 25–35 kg (55–77 lb) of total weight (boat, motor, people and gear) if you want a light hull to plane. Displacement use needs far less.
| Typical load | Total weight | Where you boat | Suggested HP |
|---|---|---|---|
| One person, tender or canoe | under 150 kg (330 lb) | Calm water, short hops | 2.5–4 HP or electric |
| Two adults, inflatable | 200–300 kg (440–660 lb) | Bay or harbour | 5–9.9 HP |
| Two anglers, 3.5–4 m tinnie | 300–400 kg (660–880 lb) | Estuary with tide | 9.9–15 HP |
| Family of four, 4–4.5 m tinnie | 450–600 kg (990–1,320 lb) | Bay, some chop | 20–30 HP |
| Small yacht or trailer sailer | 600 kg+ (1,320 lb+) | Motoring, not planing | 4–9.9 HP long shaft |
Never exceed the maximum HP or maximum load on your boat’s capacity plate. If your plate and this chart disagree, the plate wins.
How do you size an outboard for your small boat, step by step?
- Find the capacity plate. Note maximum HP, maximum persons and maximum load.
- Add up your heaviest normal day. People, fuel, eskies, anchor, safety gear. Be honest.
- Decide: plane or putter? Planing a tinnie needs power; pushing a tender or yacht at displacement speed needs very little.
- Check the motor’s own weight. A 25 HP adds around 70 kg (154 lb) at the stern, which small hulls feel.
- Match shaft length. About 38 cm (15 in) transom for a short shaft, about 51 cm (20 in) for long.
Outboard motors for small boats, by load
Pick the card that matches your heaviest day on the water. Starting prices in USD.
Outboard motors for small boats: FAQ
Can I put a bigger outboard on my small boat than the plate says?
No. The maximum HP on the capacity plate is a safety limit for that hull. Going over it can make the boat unstable and may breach local rules. Check your local boating authority.
Is 9.9 HP enough for a small boat?
For one or two people in a light 3.5 m tinnie, usually yes. With more people or gear, or strong tides, 15 to 20 HP will plane far more reliably.
Does the motor weight count toward the boat’s load limit?
On many capacity plates the maximum load includes people, motor and gear together. Read the wording on your plate and add everything up.
Is an electric outboard strong enough for a loaded small boat?
For displacement speeds, yes. A Torqeedo Cruise pushes a loaded tinnie or small yacht comfortably, but it will not plane a heavy boat the way a 20 HP petrol motor will.
Size for your busiest day on the water.
Send us your boat, your plate and your usual crew. We will name the HP.