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[chris_suffolk]

I'm running a Pitman 24v (running on 12v) geared 5.9:1, with propshop 25275 three blade props. I can do all the relevant calculations, but try as I might can't find the rpm of said motor. There is no label (that I can see as it's in the boat), so I don't know the exact model number, but does anybody know a good source to look at for pitman specs. I've tried googling, but can't find my motor.

Also, when calculating rpm / pitch / hull speed, if I run 2 props, do I effectively double the pitch over running a single prop, or increase their combined efficiency - and if so by how much? Or is that too simplistic?

Thanks in advance

Chris

[DaveB]

Hi Chris
I don't know the spec of the motor but they used to be sold by Alistair Graham at model motors direct it may be worth getting in touch with him
Dave

[chris_suffolk]

Thanks,

I've sent him an e-mail, but just hoped somebody might know where to look

Thanks

Chris

[Subculture]

Have you thought about purchasing an RPM meter. Quite inexpensive and very worthwhile, and takes the guesswork out. e.g.

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=10783&aff=214504

[mudway]

I use a few but have stripped the gearbox off and they run 3,900 rpm at 12 volts no load, so with the box on and at 24 volts would be about 1,300 rpm no load.
At that low speed, you would need a big prop for them to be of use.
One of mine minus the gearbox  runs a 50mm Type 12 5 blade petal prop on 18 volts to propel a Tribal Class frigate.

[chris_suffolk]

Thnx Mudway,

Reckon that means I've got about 650 ish RPM with 12V, running dual 2" x 2.75" pitch brass props. Guess that's a little low but can't fit anything bigger without hitting the hull. May investigate taking the box off. Does the drive shaft then come out of the centre? If so (and I guess it will), I'll have problems aligning the coupling, so may just have to live with a slightly slower than ideal top speed.

Chris

[mudway]

The end with the decoder has a  long shaft ot it which I have always used.
Did do some larger prop tests for Alastair once.
 

Pittman 9234GM (12/24v)
  
  
 

In the water

 
 

Prop Type

Size

R.p.m. @ 12 volts (no load)

Rpm

Efficiency (% rpm)

Amps

[font=]Type 22, 23 & 42[/font]
 

62

3,900

1,500

38.46

3.56

Ticonderoga
 

72

3,900

1,800

46.15

2.95

[font=]Arleigh Burke[/font]
 

[font=]72[/font]

3,900

1,600

41.03

3.77

[Subculture]

I aim to get a loaded shaft speed of no less than 75% of unloaded speed, otherwise the motor is working very hard and pulls a lot of watts, quite a bit of which gets wasted in heat. If you can't match a prop/motor combination direct drive, then use reduction.