If me bleating on about this seems familiar It's because I've said this all before.

https://www.modelboatmayhem.co.uk/forum/index.php?topic=56555.0The difference here is that Unbuiltnautilus has come up with a figure to work with.

Tabular data for the RE 385 is in pdf form

https://www.mfacomodrills.com/pdfs/RE385.pdf (It says Stall Current: RE385 at 12v = 5.55A)

Ignoring nonlinearity due to the presence of iron, 8 volt stall current (6x1.33v NiMH)will be (8/12) x 5.55 = 3.7 amp. For 7.2v it's 3.33 amp and 6.0v it's 2.8 amp

Six freshly charged, cooled down NiMH cells has a no-load voltage of (6x1.33=) 8 volts. The voltage out the battery terminals drops under the motor load so take your pick of these three for a lower voltage. For simpicity's sake I'll assume it is measured at 7.2 volt.

Look at performance curves of

https://www.google.com/search?q=dc+motor+performance+graph or look closely at and to the left of the 55 turn, 555 graph.

What you see is:-

* (drain) current at peak power output is half the stall current. (It is just above 50% because of iron loss)

* rpm at peak power is half the no-load rpm

* efficiency, because of iron loss at full load, is slightly under 50% at peak power

(In a coreless motor there is little or no iron loss and the efficiency peak is high and well to left of the graph but quickly drops off.)

If you increase prop load such that tacho measurement is below 50% of no-load figure or ammeter is greater than 50% of the stall current you pass the point of diminishing returns and larger or coarser props slow the model down to burn off most of your power as copper cosses in the motor.

In this theoretical case, the RE 385 at 7.2 volt is overloaded beyond 0.5x3.33= 1.67 amp. That is to say, if a 30mm prop actually gives a current drain of 2 to 3 1/2 amp then you would be well advised to drop to a smaller prop.

You could carry on with the larger prop and use an appropriate reduction gear to take it nearer the peak efficiency at (7.2/12 x 0.99)=0.6 amp then raise the voltage for more power and speed. You'd be even better advised to switch to a 555 motor instead.

^{*}If you still don't believe me then you will see it yourself in your results. The 555 and published data for the MFA com drills motors is correct and performance is referred up to 75 Celsius.