Selecting electric motors and controllers for a boat — sizing, comms, and supplier examples

The motor and its controller decide how the boat feels under power. The first decision is whether you buy them as one packaged drive or source them separately, because that decides who carries the hardest sizing job: matching the controller's current rating to the motor.

→ The spec calculator outputs your required continuous power, DC current and phase current →

Packaged drive or separate motor and controller?

A packaged drive — Oceanvolt, Torqeedo, ePropulsion — comes with its controller matched by the maker. You choose the drive by power and boat size, and the maker answers for the controller. A separate motor and controller gives you more choice and can cost less, but matching them is your job, and so is proving the controller can carry the current continuously.

Packaged drives, from the makers' own figures

Note that makers quote power differently: Oceanvolt gives output power, while Torqeedo and ePropulsion give input power. A 10 kW motor from one maker isn't necessarily equal to another's. Our motor comparison sets out the details and sources.

Sizing the motor: continuous, not peak

Catalogues often lead with peak power. Size on continuous power, the figure the motor can hold for as long as you motor.

The calculator works out continuous power from displacement and your cruising speed as a fraction of hull speed, using a curve fitted to measured fuel-burn data. At hull speed it gives about 2.5 kW per tonne for a monohull, and at 75% of hull speed about 1 kW per tonne. The how-many-kW guide works through six boats, and shows why speed matters more than length.

Then check the maker's guidance for the boat. Oceanvolt's SD8, for example, is listed for boats up to 12 m and 7,000 kg; the calculator gives a 7.5 t 35-footer an 8 kW motor for 5.5 knots. That sits just past the listing, so ask the maker before buying.

Sizing the controller: phase current, for long enough

For a separate controller, the figure that matters is its phase RMS current over a long period. The calculator works it out from the DC current at the pack's minimum voltage, with 25% headroom and a further 1.25 factor for full torque at low speed:

Don't calculate phase current by dividing power by the DC voltage × √3. The line-to-line voltage a controller can produce is only 0.61–0.71 × the DC voltage, depending on its modulation. The DC vs phase current guide explains why, and why the low-speed case sizes the hardware.

What separate controllers can carry

Makers rate controllers for set periods, and the long-duration figure is the one to compare. Turned round through the same calculator margins, each rating corresponds to a continuous motor power:

So at 48 V, a separately sourced controller limits you to roughly 6–10 kW of continuous motor, depending on the brand and how well you cool it. Above that, a packaged drive or a higher-voltage system is the realistic route. Our controller comparison checks each one against your own figure.

Voltage

The motor, controller and battery must all share a voltage class. The table above shows how differently makers build: Oceanvolt's drives are 48 V, ePropulsion's I-Series is 96 V class, and Torqeedo's Deep Blue runs on about 350 V. The calculator stays at 48 V up to 10 kW and moves to 96 V from 15 kW, on cabling grounds. See 48 V vs 96 V for the full table.

Cooling

Continuous power means continuous heat. Makers state how each motor is cooled, and where they offer both, the higher-power models use liquid: Oceanvolt's SD6 and SD8 are air-cooled and its SD10 and SD15 liquid-cooled. ePropulsion's I-10 is air-cooled and its I-20 and I-40 water-cooled. Plan the engine-space ventilation for an air-cooled motor, and the pump and pipework for a liquid-cooled one. For a separate controller, the rating often depends on heat sinking, as Kelly and emDrive both state. Design the cooling to reach the rating; don't assume it.

Regeneration

If you want the motor to generate under sail, the whole drive must support it. Oceanvolt, Torqeedo's Cruise 12.0 FP and ePropulsion's I-Series all list regeneration under sail. Kelly lists regeneration as a default feature of the KLS-N. For other controllers, check the documentation. How much a propeller generates depends heavily on its type; see the hydro regen guide.

BMS and battery communication

The controller should hear from the BMS before the BMS has to open the contactor. Three ways:

• CAN (best): the BMS sends limits and state of charge, and the controller reduces current progressively as the pack nears its limits. Curtis and Sevcon controllers use CANopen. Check that the BMS's CAN protocol is one the controller understands.
• A wired inhibit or derate input: the BMS switches a controller input to cut or reduce power.
• Nothing: the controller sees only pack voltage, and the BMS's contactor stops the boat abruptly.

For a new installation, prefer CAN, and test the low-battery sequence at the dock.

A worked example: the 40-footer

Boat: 11 m waterline, 10.5 t, wanting 6 knots. The calculator says 10.2 kW, which rounds up to its 15 kW size, at 96 V because 15 kW at 48 V is "Very heavy" for cabling. Phase current: 362 A continuous.

• Separate controller? None of the 96 V figures above comes close to 362 A, and at 48 V a 15 kW motor would need 651 A. A packaged drive is the realistic route.
• Packaged options from the table: an Oceanvolt AXC24 shaft drive (24 kW continuous, 48 V modules, listed for boats up to 14 m and 19 t), or an ePropulsion I-20 (20 kW input, 96 V class).
• Or aim lower. At 5.95 knots the same boat needs 9.9 kW, and an Oceanvolt AXC12 (12 kW) comes into range. Oceanvolt lists it for boats up to 10 t, so ask whether 10.5 t is acceptable.

Supplier examples for motors and motor controllers

For the battery side that pairs with these motors, see the LiFePO₄ marine battery guide.

Mistakes to avoid

• Sizing on peak power instead of continuous. Size on continuous power, then check the peak covers docking against a tide.
• Comparing input power with output power. Check which one each maker quotes.
• Rating a controller by its headline figure. Use its long-duration phase current, and note the period.
• Assuming the rating without the cooling. Several controller ratings depend on heat sinking; design for it.
• Mixing motor and controller brands without checking they're supported together. Ask both makers.

Sources

Checked on 7 November 2026. The product figures are from the makers' documents listed with each entry in our comparisons:

• Electric inboard motors compared: Oceanvolt SD and AXC, Torqeedo Cruise 12.0 FP and Deep Blue 25i/50i, and ePropulsion I-Series, from the makers' product pages
• Motor controllers compared: Curtis 1232E–1238E/SE manual (OS 31), BorgWarner Sevcon Gen4 catalogue, Kelly KLS-N user manual V2.9, Emsiso emDrive L30 datasheet V2.5
• Texas Instruments — Space-Vector PWM With TMS320C24x/F24x (SPRA524): maximum line-to-line output voltage
• The calculator's sizing model: power per tonne, motor sizes, phase-current figures, and the controller-to-motor-power conversion (phase current ÷ 1.5625 × 40 V or 72 V × 0.9)