Electric boat propellers: range is a propeller problem

Electric propulsion changes the propeller question completely. A diesel forgives an inefficient prop with tank capacity; a battery bank doesn't. Every percentage point of propulsive efficiency is range you keep — the propeller is the biggest single lever on an electric boat.

Why conversions disappoint

Most electric repowers keep the old fixed prop or accept whatever the motor vendor bundles. But the motor's torque curve is nothing like the diesel's, and the reduction ratio was often chosen for the motor's electrical happiness, not the hull's hydrodynamics. Result: a propeller matched to a drivetrain that no longer exists, and 'range anxiety' that was never about the battery.

What good looks like

Sizing starts from the electric motor's real power, RPM and reduction — then matches blade loading to a torque source that delivers everything from zero revs. The Eco*Star programme was developed with the Insean research institute for exactly this: self-pitching blades that hold the motor at its efficient point. One documented conversion: 6.0 knots on 4 kW where the old prop managed 5.1 on 6 kW — one vessel, reported conditions, and precisely the kind of result correct matching makes possible.

The order of operations

An 80 hp diesel behind a 2.5:1 box and its 10 kW electric replacement do not want the same reduction ratio — the propeller cares about shaft RPM and power at the shaft, not what the motor brochure says. So size the propeller for the hull first, then choose the motor and gearbox combination that swings it. Conversions that run this order report the range; conversions that run it backwards report the anxiety.

Regeneration, honestly

• Under sail, a suitable prop can spin the motor as a generator and put amp-hours back.
• Recovery depends on boat speed, the motor/controller's regen capability, and the installation — it's a system property, not a propeller feature.
• We confirm the whole chain before promising anything. That's the difference between a documented setup and a brochure.