Like Tom, I had expectations of silence with my first electric boat. In practice it was far quieter than any ICE but far from silent also.
I found the drive was very quiet up to about 20 amps (48v) then made a noticeable resonant whine between 20 and 30. Above 30 the resonance was gone but the mechanical noise became steadily more apparent as power went up to about 85 amps.
Most of our time using the boat was at 10 amps, 4 mph, slow cruising in virtual silence or about 35 amps, 7 mph, where after a few minutes I would totally tune out the small amount of motor noise. I should say conversing at a normal level was easy even at WOT. Passengers rarely commented on any noise.
The shaft and prop make noise which an ICE masks. Increasing the prop tip clearance helped a little bit. Mounting the motor on rubber vibration isolators helped a lot. Lining the motor compartment with tar/lead sound dampening foil and a layer of acoustic foam did not help at all.
Having a dead straight shaft is critical. A tiny amount of whip will cause noise and vibration. On a new install I would use a larger shaft dia than required to transmit the hp. I use 3/4", if I did it over I'd use 1".
My experience with the MARS BLDC motor and Sevcon PMAC controller was this:
No noise at all from the controller
Slight constant high frequency whine from the contactor relay, not objectionable at all.
Very noticeable grinding noise from the motor during the first few revolutions of start-up. More apparent with a very slow application of throttle. This wasn't objectionable either as it only happened for < one second at start-up.
Motor made a steady whining noise, proportional to speed and load. Not too objectionable as it wasn't very loud.
Motor made a much louder whining noise that sounded like some resonant effect at a certain frequency range. This was objectionable. John at MARS said it was due to the controller, Joe at Sevcon said it was loose windings in the motor. I tried two 20 turn and one 28 turn motor, exact same noise in all three (all with same controller). Motor made the noise even when held in my hands, not connected to anything but its wires. I replaced the motor ball bearings with no effect.
This drove me nuts and I spent countless hours digging on the web about BLDC permanent magnet motor noise. I learned it is quite a common problem. I did some experiments with a high end BLDC outrunner RC model airplane motor and it had a similar whine that sounded like a resonance at a mid-range speed.
If I built another inboard I'd be tempted to try a brush type motor, maybe even an old fashioned shunt wound motor with a SEPEX controller.
My second launch has a EPIC whisper outboard motor hidden in a well. It is a direct drive BLDC design mounted under water, just like a big trolling motor. It is much quieter (not technically "silent" though) than the MARS inboard setup. It is significantly less efficient though due the extra drag of the 6" dia motor and kort nozzle.
I have an 18' day sailor with a 34# MK trolling motor and an RC car PWM controller. That really is silent (and cheap!).
Denny Wolfe
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