Motor and propeller

The motor system will consist of:

  • Battery 6 SC cell (7.2V 4000mA)
  • Brushless motor app. 1200kV
  • Brass 6 blade propel 40mm (I am not able to find a 7 blade in that size) (max 8800 rpm)
  • Speed controller

Propeller

The first job was to find the right propeller. The original sub has a propeller with 7 blade, but I haven't been able to find a similar propeller with a diameter of app. 40mm. So the choice was a 6 blade brass propeller. 

Here are a few pictures of the propeller:

Propeller

Propeller

Motor

Next step is the motor selection. If you fly model airplanes you known that there are a lot of tables that match motors and propellers, but that is NOT the case for model boats! Here there is really no guide lines.

It seems that most just use a motor that their either happened to have or just buy a 540 motor size. On the other hand it seems that a sub on my size will only use app. 1A (8W), so why use a 540 motor?I my sub I don't have room for a 540 motor and I don't want to use a gear, so therefore I have looked at a brussless motor, where you can get a motor below 1000kV.

To try to get a better guess, I made a motor test setup, where it is possible to measure the power and the rpm for a motor.

Motor test setup

I just happened to have a small brushless motor with 1800kV with I mounted on a drive shaft. On the shaft I also mounted a flat piece of metal so a tachometer can measure the rpm. On the power lines between the battery and the regulator I placed my power probe (see projects). Now it is possible to see how much power is needed to drive the propeller in idle mode and in the water.

RPM  Idle (A)
Water (A)
 30000.25
-
 50000.65
0.9
 75001.1
1.8

The numbers are so small that just a little more tension on the shaft has an impact. The other aspect is how much is the impact on the RPM? Further tests have showed, that at low speed the impact is higher than at high speed. The logic is that the motor has much more power to work with at higher voltages.

Motor: 1800kV at 7.5V gives app. 13500.

I removed the propeller and made some more tests, which showed that the motor is able to run 11700 RPM in the setup.

So the conclusion is, that the motor should have some overhead in order to handle the water resistance and some overhead to handle the shaft resistance. On the other hand how much speed to you need? I have chosen a brushless motor with 1200kV with gives a max free running speed of 9000 RPM, which in the water should give app. 7500 RPM which hopefully will be enough.

The total power of the motor in insignificant, as you can't get a motor below 2A!

The motor should preferable be mounted in waterline with the shaft to minimize friction, and that means that the motor must be mounted in the top of the WTC with the center 18mm from the top. This mean that the motor should have an outer diameter of no more than 32mm. So the requirement to the motor is:

  • App. 1200kV
  • Outer diameter max 32mm

A quick search on Ebay and I found a 1200kV motor that is 29mm in diameter and can handle 12A continuously! 

Here is a picture for the new motor:

New motor

It is quite powerful and runs app. 1300kV instead of 1200kV.