Here is a math problem:

Unloaded the motor's kv is 485. Using 10S we have a max rpm of 20,370

When used, max speed was 57mph using a prop pitch of 4.43 which means (assuming 0% slippage or loss) an rpm of 13,587 (how many times 4.43 inches needs to be multiplied to reach 60,192 inches that 57mph moves per minute).

If the math and logic above is correct, can someone tell me how to calculate the amps drawn due to the drop in rpm from unloaded 20,370 rpm to a loaded 13,587 rpm?

...look for fecalc, you could get the results you're after.

In case you don't have it, this is the most up to date version. If your motor is not listed, open the text file and add the data. The instructions are quite clear.

a good thing i do if not using a data logger, time your run then recharge battries
see how many mah have been used, then go to this link http://www.offshoreelectrics.com/conversion.htm at the top of the page you will see average amp draw calculator. put in how many mah used and run time this will give you a average amp draw threw that run,if using this its best to have a short 60 sec run flat out out, its important the timing be exact