Choosing an outboard for a sailboat is different from powering a runabout. A sailboat motor does not need to plane the hull. It needs to push a heavy displacement hull at a sensible speed, keep going into a headwind and chop, and stay reliable after weeks of sitting idle on the transom. Too small and you will struggle against a foul tide; too big and you carry extra weight on the stern for no gain.

Quick answer: a common starting point is about 2 hp for a boat under 1,000 lb, plus roughly 1 hp for every additional 1,000 lb of displacement. Most trailer sailers of 16 to 22 feet end up with 3.5 to 6 hp, and most 24- to 28-foot sailboats run happily on 8 to 9.9 hp, ideally with a long or extra-long shaft and a high-thrust propeller.

Why Sailboats Need Less Power Than You Might Think

A displacement sailboat has a maximum practical speed, its hull speed, set mainly by waterline length (roughly 1.34 × the square root of the waterline length in feet, in knots). A 20-foot waterline gives about 6 knots. Once you are close to that speed, extra horsepower just makes a bigger wave and burns more fuel. That is why a 9.9 hp outboard can push a 5,000 lb cruiser almost as fast as a 20 hp one could.

What you do need is a margin of power for bad conditions: motoring into 20 knots of wind, pushing against a 2-knot current, or clawing off a lee shore. Size the motor for those moments, not for flat calm.

Rule of Thumb: Horsepower by Displacement

Use your boat’s actual loaded displacement (not the brochure light-ship figure) and add some weight for crew, water, fuel and gear. If you are unsure of the number, our guide on how much sailboats weigh gives typical figures by length. The table below is a starting point, not a rule set in stone.

Boat type (typical)	Loaded displacement	Suggested outboard
Dinghy, small daysailer	Under 1,000 lb	2–3.5 hp (or small electric)
Trailer sailer 16–19 ft	1,000–2,500 lb	3.5–6 hp
Pocket cruiser 20–23 ft	2,500–4,500 lb	5–8 hp
Cruiser 24–28 ft	4,500–7,000 lb	8–9.9 hp, high-thrust prop
Heavier or windage-heavy boats	7,000 lb+	9.9–15 hp high-thrust, or consider an inboard

Move up a size if your boat has lots of windage (tall topsides, a dodger, a bimini), if you sail in places with strong tidal streams, or if you regularly motor long distances. Move down if you sail a light racing boat where every pound on the stern counts and the engine is only used to leave and enter the harbor.

Always check the boat’s capacity plate or the builder’s recommendation and never exceed the maximum horsepower or weight rating of the transom or bracket.

Shaft Length: Short, Long or Extra-Long?

Shaft length matters as much as horsepower on a sailboat. The propeller and cooling-water intake must stay submerged when the stern lifts in a swell or when crew move forward. If the prop comes out of the water, the motor over-revs and loses thrust, and if the intake sucks air the engine can overheat.

• Short shaft (about 15 in): small dinghies and low transoms. Rarely the right choice for a keelboat.
• Long shaft (about 20 in): the usual choice for trailer sailers and small cruisers on a bracket.
• Extra-long shaft (about 25 in): higher transoms, boats that pitch in open water, and outboards mounted in a well.

Measure from the top of the mounting bracket (where the clamps sit) down to the waterline, then add depth so the anti-ventilation plate sits several inches below the surface with the boat loaded. Some one-design classes specify a shaft length, so check class rules if you race.

Weight: The Hidden Cost of Extra Horsepower

Weight hangs off the very end of the boat, where it hurts trim and pitching the most. It also has to be lifted onto the bracket or off the transom by hand. As a rough guide:

• 2.5–3.5 hp four-strokes: around 35–45 lb
• 5–6 hp: around 55–65 lb
• 8–9.9 hp: roughly 80–100 lb or more, depending on features such as electric start

Check the current specification of any model you are considering, because weights vary between brands and shaft lengths. If you will regularly remove the motor (for example, to store it below or move it to a tender), weight may decide the question for you.

Four-Stroke, Two-Stroke or Electric?

Four-stroke

Four-strokes are now the standard for new small outboards in the US and Europe. They are quiet, fuel-efficient and run on straight gasoline with no oil mixing. Downsides: more weight per horsepower, and they must be stored the right way up (or on the side the manual specifies) to avoid oil getting where it should not. For a sailboat that motors at moderate revs for hours, a four-stroke is usually the best choice.

Two-stroke

Old-style carbureted two-strokes are lighter and simple, which is why many sailors still look for them on the used market. However, new carbureted two-strokes are no longer sold in many countries because of emissions rules, they smoke and smell more, and they need premixed fuel. Direct-injection two-strokes exist but are mostly found in larger sizes.

Electric

Electric outboards from brands such as Torqeedo and ePropulsion are increasingly popular on small sailboats. They are silent, start instantly and need almost no maintenance. The limit is range: battery capacity, not the motor, decides how far you can go. Manufacturers often compare a motor of around 1 kW to a 3 hp gas outboard in thrust, but at full power the battery may last only an hour or so. Electric works well for harbor maneuvers and short hops on boats up to roughly 20 feet; for longer motoring on bigger boats, gas is still the practical option unless you have a large battery bank and a way to recharge it.

Propane outboards are another option in the small sizes; see our guide to propane outboard pros and cons and check current availability before planning around one.

Propeller: Look for High Thrust

Standard outboard propellers are pitched for light, fast boats. A sailboat needs thrust at low speed, which is why several makers offer sail-specific versions, such as Tohatsu’s Sail Pro, Mercury’s Sail Power and Yamaha’s high-thrust models. These use a larger, lower-pitch prop and sometimes a different gear ratio. The result is better push into waves, stronger reverse and more control in a marina.

If your engine came with a standard prop, ask a dealer whether a lower-pitch option is available. A correctly propped motor should reach its rated maximum rpm range at wide-open throttle without over-revving.

Mounting Bracket or Outboard Well

Most small sailboats carry the outboard on a transom bracket. A good bracket:

• Is rated for the weight and horsepower of your motor
• Lifts high enough to clear waves when sailing and drops low enough to keep the prop deep when motoring
• Has spring assistance or a counterbalance so you can raise it without straining your back
• Is bolted through the transom with a solid backing plate

Some boats have an outboard well. Wells keep the motor closer to the centerline and the prop deeper, but they can restrict exhaust and air flow and usually require an extra-long shaft. Make sure the motor can tilt or be removed to reduce drag when sailing.

Other Features Worth Considering

• Charging output: some 6–9.9 hp models have charging coils or an alternator to top up a starting or house battery.
• Electric start and remote controls: useful on larger boats where the motor is hard to reach, but they add weight and cost.
• Tiller length and gear shift position: check that you can reach the controls from the cockpit.
• Fuel tank: integral tanks are convenient on small motors; an external tank gives much more range.
• Dealer support: pick a brand that is well supported where you sail. See our overview of the best outboard motor brands.

Keeping a Sailboat Outboard Reliable

Sailboat outboards often sit unused for weeks, which is hard on fuel systems. Use fresh fuel with a stabilizer, especially with ethanol blends, run the carburetor dry or drain it before long lay-ups, check that the cooling-water tell-tale is flowing every time you start, and inspect the prop and anodes regularly. Our guides to common outboard motor problems and winterizing an outboard motor cover the details.

Summary

• Start with about 2 hp for the first 1,000 lb of displacement and add about 1 hp per additional 1,000 lb, then add a margin for wind, waves and current.
• Most 24–28 ft sailboats are well served by an 8–9.9 hp four-stroke with a high-thrust propeller.
• Choose a long or extra-long shaft so the prop and cooling intake stay submerged when the boat pitches.
• Weigh the motor against what your transom, bracket and back can handle.
• Electric is excellent for short trips on small boats; gas still wins for range.

For more on choosing and setting up a small boat, see our guide to the best small and trailerable sailboats.