Electric Boat Motor Basics

Posted June 16 2014



Electric motors are available in a variety of voltages in both AC (alternating current) and DC (direct current). Their amp draw is based on the available voltage and the horsepower of the motor. Electrical definition of one horsepower (1HP) is equal to 746 watts of electrical power. From the basic formula of Watts = Volts x Amps, we can see volts and amps are inversely related. For the same horsepower motor, as the voltage goes up, the amp draw goes down.

Electric Motor Horsepower
Motors are rated in horsepower (HP) or fractions of horsepower (1/4, 1/3, 1/2, etc.)

Horsepower mechanical definition, one horsepower (1HP) is equal to 33,000 pounds being moved 1 foot in 1 minute (or 33,000ft-lb/min).

Electrical definition of one horsepower (1HP) is equal to 746 watts of electrical power.
Watts = Volts x Amps

“Pounds of Thrust” is a bit more arbitrary and is not accurate for calculation of power and storage requirements. Used by manufacturers, this number does offer a helpful basis number for comparison of motor performance.

Motor horsepower is just part of the bigger system of transferring stored power to usable propulsion. System inefficiencies and losses must be reduced for maximum performance on any size boat motor.

Power Consumption
DC motors used for boat propulsion requires a constant high amp draw on your battery and electrical system. Batteries and wiring must be designed for peak performance. The old 10% loss in wiring is unacceptable. Maintaining a stable voltage allows the motor to run at its peak efficiency and substantially reduces wear. Bigger is better for both batteries and wire. See both Battery & Wire Libraries for additional information.

See “Motor Reviews” of leading manufacturers and find real power and consumption.

Electric Motor Types
The two basic types of DC motors "Brush Motors" and "Brushless Motors" have many of the same components. The location of the electro magnetic windings, determines the design difference.  

Brush Motors
Offer the advantages of low initial cost, high reliability and have simple motor control systems. Drawbacks include high maintenance and shorter life-span in demanding applications. Maintenance of brushes, springs and commutator are required because power is transferred from the stationary outside of the motor to the spinning wire windings of the rotor inside the motor.

Brushless Motors
Offer the advantages of long life span, minimal maintenance and high efficiency. Drawbacks include high initial cost and motor controllers are more advanced. Brushless motors have a permanent magnet in the rotor, and electrical coil magnets are stationary on the motor housing. Brushless motors advantage is in its design, by using the electrical power on the stationary magnets and not having to transfer the power to the spinning rotor magnet.This eliminates the need and maintenance of brushes.


Captain Joe

eBoat Marine