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We're frequently asked why our systems use such high voltage, typically 300 to 500 volts. At a given resistance, higher voltage means that the current is lower. In a 100 kW propulsion application, a 12 volt motor would be feet in diameter and very heavy because of the thickness of its windings. To power the motor would require cable as thick as your arm to carry the 8000 amps of current. At 400 volts, the wiring is a more manageable three-quarter inch jacketed cable. Cost of components for switching high power is related both to voltage and current. The systems provided by Hybrid Propulsion typically operate at less than 500 volts DC and 200 amps current.
We're also asked how much power is required. There are many misconceptions that a boat requires much less power from an electric drive than from a conventional engine. Unfortunately, this is just myth. To propel a boat of a particular displacement and hull shape at a specific speed requires a given power at the propeller shaft. This "shaft horsepower" (shp) can be easily estimated given displacement and waterline length of the vessel. Typically a 15 ton boat with a 30 foot waterline will require 66 shp to move the boat at it's "hull speed". Hull speed is the speed at which a displacement (non-planing) boat encounters major resistance from the water.
The required power dramatically increases as the boat approaches hull speed. Our 15 ton boat has a hull speed of 7.3 knots. To move the boat at this speed requires a shaft horsepower of 66.6 shp. To move the boat at 6 knots requires half that power and to move at 4 knots requires only 10 shp (7.5 kW).
Electric motors have one major advantage over a direct engine drive. Electric motors can provide their
maximum torque at any speed. You can think of this has having many gears available in a car versus having a single high speed gear. This provides for very efficient low speed operation and tremendous acceleration. All electric motors, and particularly induction motors, can be smaller and lighter if they operate at high speed, generally 3000 to 5000 rpm. A propeller in a displacement hull will typically operate at less than 1000 rpm. This mismatch in performance can be handled either by gearing down the motor, by using a lower speed electric motor with larger diameter, or by using a larger electric motor than necessary and only operating it at lower speed. Hybrid
Propulsion can assist with the understanding the tradeoffs in motor selection for marine propulsion and generator applications.
Most of the motors we recommend are advanced, light weight motors designed for high peak torque in electric automobiles. They're designed to provide very high performance at a very light weight. If your boat can afford a few hundred extra pounds, a much less expensive motor derived from industrial designs can save about half of the motor cost or more. We can supply a 1500 rpm 30 kW nominal/60 kW peak water cooled motor, weighing about 170 kg (versus 40 kg) for under $3000.
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