Propellor pitch effect on power needed

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21 November 2011 at 12:48 #32505
Jeremy
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@jeremy15845
As the pitch of a propellor increases, does this require more or less power to turn it? I have a 35mm diameter 30mm pitch propellor on my Chris Craft with a 2880KV brushless motor, 60 amp ESC and 8.2V NiMh battery. It goes like the wind but the motor sometimes stops running in mid lake (help!) which, I am told, is down to the ESC being overloaded. I have been advised to reduce the prop size to 30mm and the battery size to 7.2V. Graupner do a 29mm diameter, 40mm prop (K series) or there is 32.5mm diameter, 28mm pitch (2303 series). Which of these two would require less power?

Any advice would be much appreciated.

Jeremy
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21 November 2011 at 12:48 #5148
Jeremy
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@jeremy15845
21 November 2011 at 14:28 #32506
Gareth Jones
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@garethjones79649
If the pitch of a propeller increases the power to drive it will also increase.

If you imagine a propeller where the pitch of the blades is zero, i.e the angle of the blade at the hub is zero, it will be a plain disc and the power required to spin it in the water would be low

If you now compare that with a propeller where the pitch is infinite, the angle of the blades at the hub would be 90 degrees and the power required to spin it in the water would be high.

It is difficult to compare the two alternative propellers you have quoted. The 2303 series unit has a smaller diameter and pitch than your current propeller so it should certainly absorb less power. The other alternative is smaller still in diameter but bigger in pitch so it is not clear whether it will take more or less power than your current one. Personally I would go for the 32.5 mm diam, 28 mm pitch alternative.
22 November 2011 at 08:00 #32513
ashley needham
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@ashleyneedham69188
Jeremy. If you are overloading a 60 amp ESC something is a bit wrong!

Do the motor specs give an “average” max current draw? Unfortunately stating the Kv of the motor is like saying “I have a 600 motor” of which there are loads of versions.

Is the prop you are using a special or an “X” or “S” pitch common plastic one? (two blade)

Lastly, as I am no expert..what size is the Chris-craft?

Nope..lastly.. the purchase of a cheap ammeter is a valuable addition to anyones toolbox, and can sort out some of these troubles in a jiff. The first thing I would do is to take a current measurement to see if the unit is really drawing such high currents, and check if the ESC is rated for “up 60A for short periods” but 20A for long periods…

Ashley
22 November 2011 at 12:48 #32518
Jeremy
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@jeremy15845
Hi Ashley

The motor spec does not go beyond that is 2881KV to be used with voltage from 8.4 to 18V, sold as a 600 replacement but much more powerful! The Seaking 60 amp ESC with which it is twinned in a combo deal from the supplier is rated at 60amp with a 120amp burst. The prop is a Graupner plastic series 451, 35mm diameter 30mm pitch. The boat is 28″ long.

The advice from the supplier has been – That prop is way too big for submerged drive with that motor. I
would think you are lucky the ESC has not burned out! The ESC will trip out
and if it gets very hot internally it will take a while to come back. Try a 30mm diameter prop and then 7.2volts to see
if it cures the problem. Reducing the voltage will help. The motor is 2,880 rpm per volt and
so on 8.4 it will be trying to run ca 24,200 rpm. To drive a 35mm Graupner
racing prop at 24,200 would take about 1500 watts which is not going to happen
and so the motor rpm is pulled down because the prop is too big and the amps go
up.

Does any of this make sense? I am beginning to wonder if the problem may simply have been a loose connection. Disconnecting one of the three motor leads results in the pulsing which I was getting. I have now taped the leads in place. It goes fine out of the water.

I have a cheap multi-meter from Maplin which measures amps. However, its usage intructions are very basic. To measure amps, I have to link it is series with the circuit to be measured. The battery is linked to the ESC via a Deans connector. Where should I place the leads of the meter to read amps – across the Deans connectors or in line with one of them? Neither gives any reading. Linked across the Deans connectors, it reads voltage OK.

Thanks for your help.

Regards

Jeremy
22 November 2011 at 14:50 #32520
ashley needham
Participant
@ashleyneedham69188
Jeremy..do not use that meter to measure current !!. they normally only go up to about 10A max.. Current has to be measured at the battery, ie with the meter connected in the run of one of the wires (series).

Plastic “x” or “s” pitch props are very cheap and so I would buy a few to try out, small sizes ! X is pitchier than S.

Loose wires…a possibility, but it rather sounds like a lower Kv motor may be required OR gear your motor down, as much better option… gearing down 2:1 may let you run your current prop ok, and let the motor rev in its most efficient manner. really need the input of a fast electric man here

Ashley
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