Knowledge leads to efficiency
Having a motor that has sufficient power to turn the optimum propeller for your boat will ensure efficiency.
Observing water
Stand by a river and watch how fast the water moves in the middle and how much slower it flows at the edges, you may even see small eddies at the rivers edge. This is as a result of friction caused by the riverbank.
In a similar way when water is thrust out of the back of a propeller the faster moving water is subject to friction or drag from the adjacent stationary water. The faster the water leaves the propeller, the greater the drag. Save this thought for the moment.
Two kayakers are passing by going up stream at the same speed. One kayaker has a full bladed paddle, the other’s paddle has half the surface area and therefore needs to paddle twice as fast to keep up. This is thought number two.
Combine these thoughts together. The smaller paddle is pushing the water twice as fast as the larger paddle. With the adjacent water effectively stationary the faster speed creates greater drag and a large eddy will form behind the paddle. In effect, this is cavitation behind the paddle and leads to inefficiencies.
There are other inefficiencies to consider. Small paddles have a less favourable ratio between ‘blade perimeter’ and surface area. The result is more water slipping over the paddle blade. So not only are they spilling more water because of the less favourable ratio they also create more drag.
Waste not
Propellers are like paddles. Their purpose is to move water. And in this way, a slow moving large propeller is more efficient than a fast moving small propeller.
Closing the circle
Just like the strong kayaker is very capable of using a large bladed paddle, the Törkmar motor is capable of turning a large propeller.
It is a PMAC motor wound specially for low speed applications, sub 1000 RPM. It has the highest torque rating of any motor in its category. It’s a cool customer too and rarely needs cooling.
A 13″ prop has only 43% of the surface area of a 20″ prop.
Back in the classroom
What really matters is trying to match the velocity of the boat with the velocity of the water leaving the propeller. We use the simple formula above to illustrate. Let’s assume that the velocity of the boat is 4mph and both motors are drawing 10kW.
In fig. 1 – Boat speed 4, Prop exhaust 5 ——- 2/(1+(5/4) = 0.88 ——- Propulsive efficiency 88%
In fig. 2 – Boat speed 4, Prop exhaust 10 —— 2/(1+(10/4) = 0.57 —– Propulsive efficiency 57%
Interestingly the large cargo ships that cross our oceans have huge propellers that rotate very slowly 60-120 RPM and reach speeds of 20 knots. These ship have a propulsive efficiency of over 95%. The sailors call the the missing 5% ‘slip’ and they can use it together with the number of revolutions of the propeller to determine distance traveled when navigating using Dead Reckoning.
15kW Motor Characteristic at 48V
| Description | Unit | Value |
|---|---|---|
|
Duty |
- |
S2-60mins |
|
Nominal Speed |
RPM |
1000 |
|
Frequency (Number of poles) |
Hz |
50 (8) |
|
Instant Torque |
Nm |
245 |
|
Rated Torque (S2-60mins) |
Nm |
143 |
|
Rated Current |
Amps |
350 |
