This excel spreadsheet contains handy calculations to let you figure out what speed of motor and battery voltage you are going to need based on the size of your model and its propellers. Please note that all motor/propeller performance figures are estimates only.

This is how I use it:

I want a scale wake. So, I look at the length and speed of the real vessel. From this, I can plug in and find out the percentage of "hull speed" the original vessel reaches. Then, I can plug in the scale of the model. That will tell me the speed I want the model to reach.

Once I have the speed I want the model to reach, I can plug in the prop size, various motor speeds and battery voltages to give me the right combination that will give me the right speed.

For example - I have a 1:28 scale tugboat. Tugboats are powerful and get close to hull speed at full throttle. So, asking for enough RPM to reach 100% of hull speed is reasonable. For a freighter or ferry, 75-80% of hull speed is more likely as a maximum. The tug has a 55mm prop. I need to know how many prop RPM I need for hull speed. So, I just throw in some numbers - jigger them around until I get about 100% of hull speed - and read the prop RPM. It turns out that I need roughly 1870 prop RPM to go as fast as I want. Now, I can try combinations of battery voltage and motor speed per volt to give me the amount of power I want.

The only tool that most people don't have lying around that helps this process is a direct reading tachometer (PT6208A Digital Contact Tachometer - $37 on Amazon) - it has a little rubber cone. I can power up a motor, hold the cone to the end of the shaft, and read the RPM at a known voltage. Divide RPM by volts used and that's the answer.

Anyway, the spreadsheet is "locked" - you can unprotect and modify it under the "review" tab in excel - just hit "Unprotect Sheet".