Choosing a motor for your boat.
Probably the best way of doing this is by asking others who have the same or similar model or looking on the internet. You may find someone has built the model and has already gone through the process of choosing motors. If you're lucky you may find lots of information on different motors that have been tried.
If doing that leaves you with more questions than answers you're on your own. So, where do you start?
My approach is to consider what speed and/or power you want for your model. A tug will need plenty of power if it's going to be towing but it doesn't have to be fast. An MTB or similar should be pretty fast. Other considerations are - number of motors, battery type/size/voltage/current capacity, prop size/pitch/number of blades, space in hull, total weight of the model, how long you want to run from a single charge, etc.
Basics of Propulsion
The propellor acts like a screw (hence the reason why aircraft propellors are often referred to as 'air-screws'). The propellor works in exactly the same manner as a wood-screw. As you turn the screw it is 'drawn' into the wood by its 'thread'. Two things have a major effect on the speed of your model. First is the speed (rpm) at which the propellor turns, the second is the 'pitch' of the blade. The 'pitch' of the blade is the distance from its leading edge to its trailing edge. (see diagram) A 'coarse pitch' prop will travel further than a 'fine pitch' prop for the same number of turns (revolutions). In theory, a prop with a pitch of 3mm (fine) will travel 3 metres for every thousand turns of the shaft (pitch * turns) - a 6mm pitch (coarse) will travel 6 metres for every thousand turns. This is only good in theory, it doesn't work like this in practice because water is not a 'solid' medium. This loss is called 'slippage'. The size of the prop and the number of blades affect the amount of slippage - a larger prop with more blades has less 'slippage'
Pitch, diameter and number of blades affect the load put on the motor. Coarse pictch, larger diameter and more blades increase the load on the motor - fine pitch, small diameter and less blades reduce the load.
As load on the motor increases, current draw rises. Attempting to drive a large prop with a small motor will likely lead to a burnt-out motor. However, a small, high-reving motor, run through a gearbox (or other reduction unit) will be able to drive a large prop.
Choosing a motor/prop combination is a balancing act with a large number of variable all affecting the results. There is no simple answer for us modellers - trial and error leading to experience is the only answer.
Some Simple Guidelines
A large, coarse pitch prop running at lower revs can give the same speed as a smaller, finer pitch prop running at high revs.
A reduction unit will improve performance in many situations.
Larger props usually run at lower revs.
Electric motors usually have a 'Maximum Efficiency' figure measured in RPM
Current draw increases with load.
Motor to prop-shaft should be as straight as possible. An out of line coupling increases the load on the motor.
Large motors don't always draw more current than smaller motors.
Brushless motors are more effecient than brushed motors. However, they are more expensive and require an expensive, specialised controller.
Always remember -
No matter what motor you choose, make sure it is suppressed. Some motors come complete with suppression but most don't.
If you have chosen a 'brushless' motor, you don't need suppressors.
A cheap fuse is quicker, easier and cheaper to replace than an expensive speed controller (ESC)
Twin Motors
In my opinion, twin motors are best run from twin controllers. If your budget won't stretch to buying two controllers you can run two motors from one ESC. When running twin motors from one controller they should be wired in parallel - see diagram. It is possible to wire twin motors in series but the motors will run unpredictably. A boat with twin props should always have the props turn in opposite directions to counter the effect known as 'prop walk'.
When connecting two motors to one controller you must remember that the load on the ESC is doubled. For example, two 10 amp motors will put a load of 20 amps on the speed controller.
Twin ESC's can be connected to one channel of a receiver using a 'Y-lead'. When connected like this both motors will run at the same time and at the same speed.
To have independant control of twin motors you will need to plug each controller into its own channel on the receiver.
Mixer untis are available which will give much better control of tyour model. These units usually have two plugs that go into the receiver - one to the rudder and the other to the throttle. The rudder servo and the two ESC's then connect to the mixer. When in operation, motor speed is affected by rudder position such that, at full turn to port (for eample) the starboard motor will run forward and the port motor will stop or even go into reverse.
Motor current rating
For a modeller, there are two figures that are important. These are the 'stall' current and the 'full load' current. Current is measured in amps.
The 'stall' current is always the highest figure. It is the amount of current drawn by the motor when it is stalled, i.e. when your prop is fouled by weeds, fishing line, or other hazards to navigation.
The 'full load' figure is the amount of current drawn by the motor when operating at full load, i.e. in the water with throttle at full power. N.B. - It is NOT the current draw at full speed of the motor. To get an accurate reading of the full load current, the test must be performed with the boat in water. The current draw will be slightly higher when the model is stationary and throttle is set to full than it would be when the model is moving through the water. This is a good thing!
When buying a speed controller, you must get one that is rated at or above the full load current of the motor (double if running two motors from one ESC) at the very least. A margin of error is desirable. E.g. full load current = 10amps, recommended ESC 15amps or higher. Of course, this doesn't take into account the stall current. If the stall current is greater than the ESC rating, i.e. a 15amp ESC and stall current of 20amps, then you must use a fifteen amp fuse to protect the ESC or risk an expensive burn-out of the ESC and, possibly, your model as well.
In a twin ESC set-up, protect each ESC with its own fuse of the correct rating. |
