Choosing an outboard engine: power, two- or four-stroke, shaft, servicing

An outboard is chosen less on its spec sheet than on the boat that will carry it. Before comparing brands, you need to know how much power your hull accepts, what shaft length it needs, how much weight the transom can take and what the engine is for: pushing a tender to the beach, getting a loaded RIB on the plane, or bringing a sailing boat home when the wind drops. This guide goes through these questions in order. The boat builder’s manual and the engine manual always have the last word.

1. Start with the boat’s plate

Every recent pleasure boat carries a builder’s plate which states, among other things, the maximum propulsion power intended. That is the first limit: you may replace or change your engine as long as the power does not exceed the figure set by the boat’s manufacturer. Beyond that it is over-powering: in France it counts as a major modification requiring a post-construction assessment (Transport Code), and exceeding the approved power by more than 15% requires a new assessment of noise and emissions. Talk to your insurer before buying anything.

Conversely, an engine that is too small is no saving: a motor boat designed to plane that cannot get there burns a lot of fuel, handles poorly and lacks the reserve to come home against wind and sea. The right target is usually within the range recommended by the yard, taking your usual load into account (crew, tanks, gear).

2. Power and the licence (France)

• 6 hp (4.5 kW) or less: no licence is required. This is the range of tender engines and auxiliary engines for small sailing boats.
• Above 6 hp: the French pleasure-boat licence is compulsory to drive the boat, at sea and on inland waters — see who needs which licence.
• The annual tax: until 31 December 2026, the “engine duty” of the French tax on pleasure craft applies to boats under 7 m from 22 fiscal horsepower, and to boats of 7 m and over from 6 fiscal horsepower, with a specific rule for removable engines. The calculation changes on 1 January 2027 (power in kW, threshold at 120 kW). Check your case on the ministry’s page before buying a more powerful engine.

3. Two-stroke or four-stroke?

At equal power, both technologies push the boat the same way; the difference lies in how they work. A two-stroke is lubricated by oil mixed with the petrol (or fed from a separate tank): it is reputed lighter and simpler, but it smokes and burns more fuel. A four-stroke has an oil sump like a car engine: it is more economical, quieter and cleaner, but heavier, and its servicing includes an engine oil change.

In practice the question mostly arises second-hand. European emission rules have removed traditional carburettor two-strokes from the new market: what is sold new today is overwhelmingly four-stroke, and a few direct-injection two-strokes have been sold. An old second-hand two-stroke can still do the job on a tender, provided you know its condition and can still find parts.

4. Shaft length: the measurement not to get wrong

The shaft (the engine’s “leg”) must put the propeller deep enough not to draw air, without drowning it needlessly. Measure the transom height at the centre, from the top of the transom to the bottom of the hull, then choose the shaft just above it. Lengths are standardised:

• Short shaft (15 inches, 381 mm): transom under about 38 cm;
• Long shaft (20 inches, 508 mm): transom from 38 to 51 cm;
• Extra-long shaft (25 inches, 635 mm): transom from 51 to 63 cm;
• Ultra-long shaft (30 inches, 762 mm): transom from 63 to 76 cm.

Names vary slightly from one brand to another: have the seller confirm. As a basic setting, the anti-cavitation plate (the horizontal plate above the propeller) sits roughly level with the bottom of the hull; only a sea trial lets you fine-tune the height. On a sailing boat, an auxiliary engine on a bracket must stay immersed when the boat pitches: a long shaft is often preferable.

5. Other practical criteria

• Weight: a tender engine is carried at arm’s length from one boat to another. Weigh the actual engine, tank included, and check what the transom and the engine bracket can take.
• Controls: tiller for small powers, remote control (wheel, lever, cables) above that. Changing the type of control is expensive: start from what the boat already has.
• Starting: manual pull-start or electric starter, which requires a battery and charging on board.
• Fuel tank: built in on small engines, separate above that; plan its space and how it is secured.
• Propeller: its pitch is chosen according to load and use; a serious dealer adjusts it after a trial.
• Service network: you keep an engine for a long time. Before choosing a brand, check that a mechanic who services it is based near your port — see the map of professionals.
• Theft: outboards are stolen a lot. A transom lock and engine marking are often required by insurance policies: read yours.

6. Buying a second-hand outboard

• Paperwork: original invoice, serial number legible on the engine and consistent with the papers, service book or invoices.
• Gearbox oil: white, opaque oil (“mayonnaise”) means water has got into the lower unit.
• A cold start and the tell-tale cooling-water jet, which must come out strongly with the engine running — see no water from the exhaust.
• Corrosion: white deposits, blistering paint on the lower unit, anodes missing or completely eaten away.
• Propeller and shaft: impacts, bent blades, fishing line wound behind the propeller that may have damaged the oil seal.
• A test on the water: gear changes, smooth acceleration with no flat spot, no abnormal smoke. A mechanic can also measure compressions before you buy.

7. The servicing that makes it last

An outboard lives longer the more regularly it is serviced, at the start and end of the season, and every 100 hours for heavy users. The routine jobs: flush the cooling circuit with fresh water after a trip at sea if the manual says so, change the gearbox oil once a year or every 100 hours, change the engine oil of a four-stroke (preferably at the start of the season), replace the spark plugs, check the fuel filter and fuel line, watch the anodes, remove the propeller to grease the shaft, grease the steering and tilt. The water-pump impeller also wears: replace it at the interval given in the manual. Before winter, drain the carburettor bowl to avoid deposits — see laying up and recommissioning.

Keep every invoice and every job: when the boat is sold, an engine with proven servicing reassures the buyer. And if one day it refuses to start, begin with the simplest causes of an engine that will not start.

8. And changing the engine?

Replacing the engine of a registered French boat is not just a mechanical matter: the engine is part of the registration. The change is reported to the DDTM, normally within a month, with the old registration card, the invoice for the new engine and an identity document; the procedure is free and done online on the government pleasure-boating portal. Tell your insurer too, so that the new engine is covered at its value. For the budget of a complete repowering, see how much a repower costs.

The Hullteam logbook keeps the engine invoice, its serial number and every service in one place, with reminders for oil changes and servicing — see which proofs count when you sell.

Sources

Pages consulted in November 2026 (in French).

• La taxe annuelle sur les engins maritimes à usage personnel — French Ministry of the Sea (updated 22/09/2026)
• Vous envisagez de changer le moteur de votre bateau : quelques conseils MAIF — Figaro Nautisme
• Quelle longueur d’arbre pour le hors-bord de mon bateau ? — Bateaux.com
• Comment adapter un moteur hors-bord à une coque — Bateaux.com
• 2 temps ou 4 temps, quelles différences mécaniques sur un moteur ? — Bateaux.com
• 6 idées reçues sur les moteurs hors-bord 2 temps — ActuNautique
• L’entretien du moteur hors-bord — Bateaux.com