prop rpm

[ernest2]

hi!
what would happen if the prop spins more than the recommended rpm? like i have a raboesch 60mm prop, designed to spin at 5800 rpm. what would happen if say my motor can spin the prop at 7500 rpm? will it cavitates? will the prop breaks?
thanks!

[Umi_Ryuzuki]

Probably make a great "blender". :D

It will cavitate, and the boat will go practically no where.
You will know when you when this happens.

[patmat2350]

Info-
For those who haven't seen it, www.raboesch.com has tables of max speeds for their props... probably roughly applicable to most scale props (they use the same numbers across their different series of prop forms).
On a graph of rpm vs. power delivered, there's likely a gradual transition from "more rpm's = more power" to "flat, more rpm's does diddly, due to cavitation". But yes, go fast enough, soldered blades might start flying... I don't want to find that point!

Pat Matthews

[ernest2]

thanks for the replies, i'll keep it all in mind.
another question, is the 60mm prop too big or too small or just right for a trawler type yacht about 44" x 14"?

[green-boat]

A 60 mm prop is about the right size for that size of model. I would doubt if you would turn faster than 1200 RPM on a displacement/trawler hull. Anything more would be a waste of power and probably faster than scale.

[ernest2]

the boat im planning on building is a trawler type motoryacht. something like a grandbanks 42, so i guess a little more speed would be ok.
besides mack product, where else can i find a dual output gearbox?
thanks!

[green-boat]

On something that big, why not go with 2 motors. You will have plenty of space for batteries and the extra motor and it will be quieter.

[ernest2]

i am planning on powering it with a trimmer engine, honda 4 stroke. i have the engine already so its cheaper for me than buying 2 esc and batteries. i can also use bicycle sprockets and chain if i cant find a gearbox or if its too costly to build to reduce rpm.
although im still in the planning stage and everythings not set yet, im still studying whats better.

[ernest2]

well, if i go electric, will a single 540 with 6volt gel cell be able to power a 60mm prop direct? or a gear reduction is needed?

[patmat2350]

do gears!
and if you don't like gear whine, shipsnthings.com has some neat belt reducers.
PM

[green-boat]

The Mack unit won't handle the gas engine, they are designed more for an electric motor. The gas engine will produce a lot vibration whereas the electric motor is much smoother. With the chain sprocket idea you will not be able to get your props counter-rotating. Going with electric motors is not that much more expensive and a lot less strain on the hull.

Proboat ESC- $40.00 each, new
6v-4a gelcell- $20.00 each, new
motor- $5-$10 each, salvaged from discarded cordless drill, thrift store, Power Wheels
u-joint- $5.00 each, new

[CG Bob]

Actually the MACK (http://www.mackproductsrc.com/Power%20p3.htm) gear box is rated for a gas engine up to 1.5 cu. in. From the MACK site:
2800 Twin-Drives by M.A.C.K.
This unit allows a single electric motor or gas engine (1.5 cu. in displacement max.) to turn two propellers --- counter-rotating.

[ThrustTugKeith]

Thats what im putting in the Dauntless...motors from Hand-held drills. Supposed to turn 21,000@18vdc. I'm running them @12 vdc, and am trying to guess what rpm they'll be turning. I'm going to run them direct at first, just to see how the boat responds. If they burn....tear em' out and put in new ones with 3:1 reduction. See what that does.

Scott

[CG Bob]

Supposed to turn 21,000@18vdc. I'm running them @12 vdc, and am trying to guess what rpm they'll be turning.About 14,000 rpm @ 12VDC. 21,000@18VDC/3 = 7,000@6VDC.

[ThrustTugKeith]

14,000....eh? Think that'll be too much for the boat running 2" 3 bladers??
Mind you , I've got a 6x4x2.5 inch battery weighing approx.3lbs sitting about 1/3 the way back.
Wont ba able to final balance till glassing is done and get er' in the tub.

Thanxs, Bob!

Scott

[green-boat]

Do you have the gear boxes that came with the drill motors. If so run them with the gear boxes attached. The newer DeWalt motors will pull a fair amount of current leaving you with a shorter run time. Try to find some older 6.0 or 9.6 cordless drills, their motors are smaller, pull less current and are plenty big for your Dauntless.

[ropanach]

I have 3 boats running 9.6v motors from battery operated drills, I run them with 7.2v nicad batterys, and they all run very good, but after about 15 min. they get hot, the lighter boat will cavetate the prop at high speed, it is running a 1 3/4" plastic prop that may be part of the problem with the cavetation, the heavies boat is running a 1 1/4" prop, and it's plastic, haven't changed them to brass yet, the one with the 2 1/2" brass prop seem to work great but it's in a tug, and I don't run it as hard as the other two. My son said that the over heat may be because the voltage is to low, I haven't tried to chang it, tooooo besy building another boat.