How to Make a Small Outboard Motor Faster: Free Speed You Are Leaving Behind
Most slow boats are not short of horsepower. They are short of the right prop, a sensible trim setting and a clean bottom. Fix those first.
How to make a small outboard motor faster: fit a propeller that lets the motor reach its rated full-throttle rpm, set the trim pin correctly, cut excess weight, clean the hull and keep the motor serviced. Never exceed your boat’s maximum HP rating.
Trent’s tinnie that “just wouldn’t go”
Trent had a 4.2 m (14 ft) tinnie and a 20 HP that the previous owner swore would do 20 knots. Trent was lucky to see 14. His mates at the ramp had advice ready: “Chip it.” “Drill the restrictor.” “Bolt on a 30.” Trent’s capacity plate said 25 HP maximum, and he had no interest in a motor that voided its warranty or a boat that handled like a shopping trolley.
So he borrowed a tachometer instead. At full throttle the motor was 700 rpm short of its rated range. The prop was a hand-me-down with too much pitch, so the motor was lugging like a car stuck in fourth gear up a hill.
A lower-pitch prop fixed most of it. Moving the trim pin one hole out lifted the bow a touch. Shifting the fuel tank and the anchor forward stopped the stern squatting. Finally, a scrub removed a season of weed from the hull. His GPS read 19 knots on the next run, and the fuel bill went down, not up.
No shortcuts, no risk. Most speed is found, not bought.
Why is my small outboard slow?
The most common cause is a prop that stops the motor reaching its rated full-throttle rpm. Weight, trim and hull condition come next.
| Symptom | Likely cause | Fix |
|---|---|---|
| Motor below rated rpm at full throttle | Prop pitch too high | Drop 1–2 inches of pitch |
| Motor hits rev limiter, little speed | Prop pitch too low or prop slipping | Raise pitch, check prop hub |
| Bow high, stern squats | Trim out too far or weight aft | Pin in one hole, move load forward |
| Bow ploughs, spray everywhere | Trim in too far | Pin out one hole |
| Prop “breathes” air in turns | Motor too high or wrong shaft | Set anti-ventilation plate near keel level |
| Slower each month | Weed or growth on hull, old fuel | Clean hull, fresh petrol, service |
How do you make a small outboard faster, step by step?
- Check full-throttle rpm. Use a tachometer or the motor’s display. Compare with the rpm range in your owner’s manual.
- Correct the prop. As a rough guide, each inch of pitch shifts full-throttle rpm by around 150–200 rpm. Pick the pitch that lands you in the upper half of the rated range with a normal load.
- Set the trim pin. Aim for a hull that runs flat with the bow slightly raised. Try one hole at a time and test on the same stretch of water.
- Lose weight and balance it. Take out gear you never use and move heavy items, like the fuel tank and battery, toward the middle.
- Clean the hull. Weed, slime and barnacles add drag you can measure in knots.
- Check motor height. The anti-ventilation plate should sit about level with the bottom of the hull. Too low adds drag; too high loses grip.
- Service it. Fresh petrol, a clean fuel filter, new spark plugs and a healthy water pump restore lost power.
- Never fit a motor above your boat’s maximum HP rating.
- Do not remove restrictors, alter emissions parts or override the rev limiter.
- Check your local boating authority for speed limits on your water.
Already maxed out? Upgrade within your rating
If your motor is well below the plate maximum, a lighter, modern four-stroke is the honest speed upgrade. Starting prices in USD.
How to make a small outboard motor faster: FAQ
Will a different propeller make my small outboard faster?
Often, yes. If the motor cannot reach its rated rpm, a lower pitch helps. If it over-revs, a higher pitch helps. The right prop lets the motor make its full power.
Can I derestrict a small outboard for more power?
We do not recommend it. Modifying restrictors, ECU settings or emissions parts can void warranty, breach local rules and push a boat past its safe rating.
Does trim really change speed on a small boat?
Yes. The wrong trim pin hole can cost several knots by making the hull plough or squat. One hole in the right direction is often a noticeable gain.
How much faster is a 20 HP than a 15 HP?
On a light 4 m tinnie, typically a few knots, and it planes more easily with a load. Only upgrade if your capacity plate allows it.
Find your lost knots before you buy new ones.
Still slow after the fixes? Send us your boat and motor details for a second opinion.