Brushless DC motors (BLDC) were invented in the 1960s, rely on an electronic closed loop controller to switch DC currents to the motor windings producing magnetic fields which effectively rotate in space and which the permanent magnet rotor follows. Sort of like a mini circular railgun.
There are many advantages of a brushless motor over brushed motors:
- high power-to-weight ratio,
- high speed,
- nearly instantaneous control of speed (rpm) and torque,
- high efficiency,
- low maintenance.
Gearboxes vs direct drive
Keeping power fixed, there is a trade off between speed and torque (what is the relationship exactly?)
Motor KV describes how quickly the motor spins unloaded given 1V of input. RPM = V * KV
A lot of hobby motors have very high KV since they are designed for a small propeller which doesn't have too much drag on it.
To push a heavier boat (with a human on it) you need a larger propeller which will incur a lot more drag. A high KV motor will not have enough torque and require a gearbox for reduction (speed to torque). A motor that spins too quickly but doesn't have enough torque will fail to actuate the propeller because of the water resistance. It will just overheat and draw too much current.
In summary, there are two options:
- Direct drive with low KV motor (advantage: no gearbox!)
- High KV motor with a gearbox for reduction.
Motor naming convention
Fortunately, most manufacturers in our hobby have settled on a standard naming scheme for their motors. It will normally be a 4-digit number that looks something like “2205”. The first two digits of this number is the diameter (in mm) of the stator, and the second two numbers is it’s height (also in mm)
For larger motors, this is often a 5-digit number. eFoil motors are often called 70120, which means 70mm diameter x 120mm length.
References:
- Elco Motor Yachts FAQ, basic physics explainer
- Brushless motor buyer guide (mostly air drones)
- Understanding Kv ratings
eFoil motors
- BLDC FS65161: POWER=6kW, KV=195KV, CURRENT=200A, TORQUE=9Nm, VOLTAGE=20S (fully waterproof)
- Flipsky 80100: 7kW, 148A, 17Nm, 14S (not waterproof)
- Reacher Tech D65L161-270: 9kW, 150A,
80 kg*cm=7.8Nm, 60V=16S
I ended up with FS65161.