Optimization

I think that the most important factor for installing an electric drive for peak operating efficiency is what the gear ratio is. It may also be the reason for many electric drives failing. As a propellor increases in revolutions per minute, the power increases exponentially. This makes it very difficult to pick the right gear ratio. By being able to change the gear ratio, we are able to optimize the efficiency.

Power in kilowatts is voltage multiplied by amperage. The controller varies speed by increasing voltage and current at the same time. Voltage is directly proportionate to motor rpm. If the gearbox is geared to high (lets say 4:1 or 5:1) then the motor will get to its voltage but will not draw enough current. If the gearbox is geared too low, the (lets say 2:1) then the motor will get not enough voltage and too much current.

Too much current is where problems start happening for electric motors. They will overheat. All electrical conductors lose their conductivity as temperature increases. (A good way to remember this is almost everything is a conductor at super low temperatures approaching absolute zero.)

Electric motors draw current proportionately to their load. Since the load increases exponentially to rpm, fine tuned gearing has tremendous benefits in reliability and maximum power created.

The first time an electric motor is installed it needs to be run at full throttle. If it develops less current than what it is rated for then choose a lower number in the gear ratio.( closer to 1.25: 1) If it develops more current than what its rated for then choose a higher number in the gear ratio. (closer to 5:1) The closer you can get to its peak operating current to the gear ratio, the more efficient and reliable the electric motor will be.

Above the Waterline has solved the gear ratio problem associated with electric drive installations by using Browning Gearboxes. We have 10 different gear ratios between 1.25:1 and 5:1. We offer an exchange program for our customers to allow them to optimize to their particular sailboat. All propellors are different and all packing glands have different resistances so this service is very important. No other electric drive manufacturer offers this service as part of the purchase.

If the motor does not reach its peak operating current it will not develop its peak power.

If the motor exceeds its peak power it will cost efficiency. However, it may be desireable to be able to develop more than its peak power. If it is geared to develop more than its peak power, beware of the intermittent rating. For instance, Sevcon's controller is capable of producing 95 amps at 48 volts continuously, however it is capable of producing up to 200 amps for 2 minutes. If pressed to exceed the time limit, either the controller will overheat or a time out circuit is activated, then the controller either substantially reduces power or shuts off completely.

Gearing it to be able to develop the intermittent rating allows incredible acceleration and deceleration. One test showed achieving full boat speed within two boat lengths when geared in this fashion. We recommend if a customer wants to gear their boat this way, that they do so at their own risk, and install an ammeter with a warning of exceeding continous ratings.