Electric motor for a boat or tender: power, range, batteries, maintenance

Silent, with no petrol to carry and no fumes, the electric motor has won over tenders and small lake and river boats, and is starting to power sailing boats and slow motor boats. But it is not chosen like a combustion engine: the real question is not only power, it is the energy on board, and so the range, and how you will recharge it. This guide takes the questions in order. The boat builder’s manual and the motor manual always have the last word.

1. What is it for?

Electric is at its best over short trips at moderate speed: getting from the anchorage to the beach, cruising on a lake, manoeuvring in harbour. Battery weight quickly penalises fast boats: the faster and further you want to go, the more batteries you need, and the heavier the boat becomes. Before comparing models, write down your typical trip: distance, target speed, number of people on board, and how often you will be able to recharge.

2. Watts and horsepower: comparing like with like

Electric motors are rated in watts (W) or kilowatts (kW). As a reference, one horsepower is about 735 W. Many manufacturers also give a “petrol equivalent” in horsepower, meant to say which petrol engine their motor matches: it is a marketing indication, useful to place the range, but each brand calculates it its own way. To compare two models, look instead at the actual power in kW, the thrust where it is given, and the tests.

As an order of magnitude, Bateaux.com estimates that a 3 m tender with four people aboard needs about 1.5 kW to move at 3 to 5 knots, the normal pace in a busy anchorage. Do not forget the limit on the boat’s builder’s plate: the maximum power it states also applies to an electric motor — see choosing an outboard engine.

3. The licence

In France, the Ministry of the Sea sets the threshold for the pleasure-boat licence at a propulsion power above 4.5 kW (6 hp), without distinguishing combustion from electric. Below it, no licence is required; above it, one is, at sea as on inland waters — see who needs which licence. Go by the power in kW written on the motor’s plate and in its manual, not by the “petrol equivalent”; if in doubt, ask the maritime authority (DDTM) for your port before buying. Other countries have their own rules.

4. Working out your range

A battery’s capacity is given in watt-hours (Wh), or in amp-hours (Ah): you go from one to the other by multiplying the Ah by the voltage (a 100 Ah battery at 12 V stores 1,200 Wh). The theoretical range is that energy divided by the power drawn: a 1,000 Wh battery feeding a motor drawing 500 W lasts about two hours. In practice it is less, because a battery is never fully emptied and conditions vary.

• Speed changes everything: on most motors, range roughly doubles when you go from full throttle to half throttle, and doubles again from half to a quarter. Slowing down by a knot or two is the best way to go further.
• Current, wind and load: going against a tidal stream or a headwind, or sailing with a full boat, takes more power for the same speed.
• The margin: if you are torn between two capacities, take the larger one. A second battery on board can also serve as a reserve for long outings.
• The display: a screen showing remaining range in time or distance helps a lot to avoid being caught out.

5. Batteries and charging

• Built-in or separate: small tender motors often carry their battery on top of the motor, sometimes removable, which avoids any installation. Above a few kilowatts, the battery is installed in the boat. On a small motor, the battery often accounts for a third of the weight: being able to remove it makes carrying it much easier.
• Charging time: it depends on the charger. Figaro Nautisme gives the example of an empty 2,300 Wh battery recharged in about ten hours with a 230 W charger, and in less than half that time with a more powerful charger. Check where you will plug the charger in: on the pontoon, at home, on the boat’s battery bank.
• Solar: a panel can keep the charge topped up between outings; estimate its output with the solar simulator.
• On a sailing boat: some systems recharge the batteries under sail, the propeller being turned by the water; output becomes worthwhile from about 6 knots, according to Figaro Nautisme.
• The cost of batteries: it is often the biggest item in the budget — see how much switching to lithium costs.

6. Lithium battery safety

Most recent electric motors use lithium batteries. Treated well, they are reliable; damaged or badly charged, they can go into thermal runaway and catch fire. Marine insurers recommend a few simple rules:

• charge with the original charger, under supervision, preferably during the day, on a non-flammable surface, and unplug as soon as the battery is full;
• do not charge on a multi-socket extension, nor in the cabins;
• protect the battery from knocks, water and heat, and secure it so it cannot fall;
• inspect the casing before each charge: a battery that is swollen, cracked or has taken on water is not recharged, it is taken off the boat and checked;
• if there is smoke, stop charging immediately, and remember that a lithium fire can reignite after being put out.

If you travel by air with a tender motor, check with the airline beforehand: high-capacity lithium batteries are subject to strict transport rules.

7. Shaft, salt water and mounting

An electric outboard is chosen with the same shaft length as a combustion outboard: measure the height of the transom, and the propeller must stay submerged when the boat moves. Check that the model is designed for salt water if you sail at sea, and that the transom or motor bracket can take its weight, battery included. For a fixed motor (shaft line or pod), installation, wiring and electrical protection are a job for a marine electrician or mechanic: currents are high and a wiring mistake can cause overheating.

8. Maintenance

An electric motor has no oil change, no spark plugs and no fuel filter: its maintenance is much lighter than a combustion engine’s, without being zero. After an outing at sea, rinse it with fresh water if the manual says so. Keep an eye on the propeller (impacts, fishing line wound round it), the anodes if it has any, and the condition of cables and connectors, which must stay clean and dry. For the winter, store the battery somewhere dry, protected from frost and at the charge level given by the manufacturer, and top it up from time to time if the manual requires it. A battery losing capacity is diagnosed like the boat’s own batteries — see the boat’s batteries keep going flat.

9. Budget and repowering

To buy, electric costs noticeably more than a combustion engine for the same use, mainly because of the batteries; Bateaux.com estimates that on a tender it pays for itself within a few seasons, thanks to no fuel and little maintenance. The gap widens with power. Replacing a sailing boat’s inboard with an electric motor is possible, but it is a real project: space and weight of the batteries, charging, motoring range reduced compared with diesel. Have it studied by a professional, declare the change of engine as for any engine, and tell your insurer — see how much a repower costs. To find a specialist near your port, see the map of professionals.

The Hullteam logbook keeps the motor invoice, its serial number, the dates the batteries were replaced and every job in one place — see which proofs count when you sell.

Sources

Pages consulted in November 2026 (in French).

• Le permis plaisance — French Ministry of the Sea (updated 19/04/2023)
• Adopter la propulsion électrique pour une annexe, un choix logique ? — Bateaux.com
• Moteur hors-bord électrique, tour d’horizon des solutions proposées — Bateaux.com
• La motorisation hors-bord et in-bord électrique en navigation de plaisance — Figaro Nautisme
• Guide d’achat : ces hors-bord électriques existent pour l’annexe — Yacht
• Guide de prévention pour l’utilisation d’appareils alimentés par des batteries Li-ion à bord des yachts — Helvetia