Motor rpm,efficiency Vs prop size

[Penny LeeParticipant @pennylee76979]

I see some have noticed that when you drop propeller size your boat sometimes goes faster.
This link goes a long way to explaining why http://www.johnsonmotor.com/Performance-Charts.266.0.html
The performance chart is plotted with a motor at constant voltage and subject to variable load torque(different propeller size to you and me)

I corrected a minor typo error on the chart but the green power output curve should converge at stall on X-axis along with the efficiency and speed curve.That was too difficult to rectify.As a result it its difficult to see that….

1. You get 50% efficiency,50% no load rpm and 50% stall current at maximum power with best propeller size(A).
2. With propeller a fraction above optimum load size(A) you can easily reach the stall region(C)
3. Instead of (C) you may as well put an undersized prop(D) to run at peak efficiency and get
4. the same thrust for a third of the current drain at x3 the efficiency at 1/9 the temperature rise.

MTB and fast electric modellers can see how to optimise using rpm or current drain measurement

If you learn how to manipulate the performance chart you can then see how …..

• adding a reduction gear can sometimes result in increased model speed .
• a similar looking motor with lower no load revs actually rotates faster once you immerse the propeller in water.

Hope this helps to clarify rather than confuse what is a tricky notion to grasp.

Lee Penny

[ModelmanParticipant @modelman]

my fleetscale hull uses electronise mtrs rated at 40mm props i use smaller props it works very well .minimal current drain motors run cool who am i to complain?speed not compromised.

[Penny LeeParticipant @pennylee76979]

The graph is courtesy of  Johnson Motors which I saw in the thread "motors and overheating" 

The aim of posting it is to clarify how dc permanent magnet motors perform.

There are 4 critical points in the performance chart and they determine what speed a shaft turns at.

1. No Load Speed
2. No Load Current
3. Stall Torque
4. Stall Current 

The power and efficiency curve arithmetically derive from the Torque and Speed points.

The difference between a high drain and low drain motor can be expressed in terms of these parameters

No unethical vendor can sell junk or "mutton dressed as lamb" in the face of four figures.

The use of  the term "low drain motors" is misused by sales reps and Electronize is a major culprit

[60wattParticipant @60watt]

If two matching opinions makes a consensus then a Monoperm is a 385 with a fancy price tag  or "mutton dressed as lamb"

"Super" and "Special" only reinforces the point

[Penny LeeParticipant @pennylee76979]

A motor performance chart

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