Regardless of direct or indirect drive, the physics involved are what makes it best to configure your systems such that the motor is running near its peak power point for your most frequent boat speeds.
The propeller gets its mechanical power (speed x torque) from the motor either direct or indirect. The motor generates that mechanical power thru electromagnetic interaction consuming electrical power (Vmtr * Imtr). For a DC motor, motor torque is proportional to motor current (Imtr). Motor speed is proportional to motor voltage (Vmtr).
But power loss (heat) in the motor is proportional to motor current (Imtr) squared! And so, for a given power target, heat loss is minimized by minimizing current and maximizing voltage, subject to the limiting battery pack voltage. And since voltage is proportional to motor speed, you want to run the motor closer to its No Load speed to achieve higher efficiency.
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
Sent: Saturday, June 15, 2019 8:15 PM
To: electricboats@yahoogroups.com
Subject: Re:[Electric Boats] Re: Albin Vega direct drive?
Actually no running an electric engine below its rated speed(you should read it as most efficient speed) causes the motor to run hot which will lead to it actually using more amps per feet traveled. It needs the added RPMs to make sure the fans move enough air thru it to prevent it from overheating. If you want a direct drive it needs to be wound for the correct RPMs you want to turn the propeller 90% of the time.
Posted by: "Myles Twete" <matwete@comcast.net>
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