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The motor used in Terrapin is a LEMCO 24-volt unit - the LM200.
David thinks that this unit is a major factor in the success of his vessel. It is a multi-pole permanent-magnet, low speed, high efficiency design, capable of delivering power to the propeller from 100 rpm up to around 1200 rpm (estimated).
Note that the upper voltage line of each pair shows the maximum allowable current under ideal conditions. The lower voltage line shows an acceptable average current consumption.
The fact that Terrapin cruises at 3 knots with a 10A consumption suggests that the LM200 will be able to power a significantly larger vessel.
Note: Briggs & Stratton license LEMCO technology to power small motive devices with the E-TEK motor.
The use of 24 volts is at variance with the proposed Norfolk Broads standard of 48 volts. However, a 48 volt motor would have meant only being able to fit four or eight batteries. Choosing 24 volts means that two, four or six batteries can be fitted, depending on the design range and cruising speed.
But what choice of motors are there? What are the main considerations? Can any electric motor be used? Use this link to get some answers to those kind of questions.
The propeller is a vital part of an electrically powered craft, given the need for efficiency - in this case the efficient transmission of motor power to the water. When building Terrapin, David opted for a propeller diameter of 12 to 13 inches. In order to determine the best pitch, he carved a series of differing pitch wooden props. and did some time-trials between two fixed points with fixed current consumption. The propeller giving the lowest time was the one chosen to be fitted to Terrapin, but in a more durable material than wood. Unfortunately, David didn't record the actual pitch selected and the best guidance that can be given here is the generally accepted view that the propeller should be as large as practically possible (18 inches is suggested!!) with a pitch 80-90% of the diameter. This is illustrated in these two links to .pdf documents by Cedric Lynch that describe propeller dimensions: Design of Electric Drives for Boats (27K) and Diesel-Electric Drives for Boats (238K) [right-click to save to disk - needs Adobe Reader]. These are valuable design documents in their own right, but each contains useful data about propeller dimensions.