The Trolling Motor
These are electric driven outbourd motors which are commonly used to troll for fish at low speeds of up to 5 knots. We bought a Sevilor 12 lbs. thrust weighing only 2 kg. Which runs on 12 volts and about 12 amps. It will push the unloaded boat at 4 km. per hour and the fully loaded boat at 3 kph. It is attached to a small stainless clip at the back of the boat with steering being done mostly with Rod's paddle. We also bought a larger Minkota 44 lb. thrust to try which used 36 Amps at full thrust and propelled the unladen canoe at 6.8 kph. By measuring the speed for different thrust setting along with the current draw a curve of electrical power used against speed was obtained which showed that speed increases with the square root of the power, as to be expected for a displacement hull where the friction increases with the square of the speed. It has something to do with vortex shedding. Rod found the thrust numbers confusing and dimensionalloy incorrect so back to square one. Force equals mass times acceleration and work is done by a force moving a mass over a distance. A horse power is defined as the work done per minute by a Clydesdale horse (a bloody big horse) pulling 30,000 pounds up one foot against gravity. After considerable mucking around with dimensions, kilderkins per firkin, this turns out to be about 750 Watts so all is well if you remember to include the acceleration due to gravity in the thrust. So at the end we can say that a horse power is 56 lbs. thrust and that our little trolling motor was somewhat over 1/5th of a horse power at maximum. Taking 12 volts and 12.5 Amps we arrive at a similar number siggesting that the motor was very efficient, at least in the figures supplied. With the propellor it is quite tricky as you need to know the maximum speed you sish to troll and then purchase an appropriate propellor which allows the motor to turn at maximum revolutions, and hence power, at that speed. We could have optimised the prop but it seemed about right and the bushfires were heating up in the communications department.