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Old 05-30-2009, 12:56 PM   #1
PT Boat
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Gear reduction for twin prop shafts

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Hi Guys,

I'm designing and starting to build the gearbox for my 1/20 scale aluminum PT Boat, and I'd like to ask some experienced R/C boaters a question about gear ratios when splitting the power between two propeller shafts.

First a little background information. The boat is a 45" long (12" beam) replica of a WW2 ELCO PT Boat, scratch-built from 3003 aluminum sheet with a thickness of .063". The final weight of the empty floating hull is expected to weigh somewhere around 12 -16 lbs, so I don't expect to set any speed records with this boat, but I'd like to optimize its performance wherever possible while I'm still in the engineering stage.

For power, I've got a Zenoah G26 gas powered marine engine fitted with a centrifugal clutch. This engine will drive the power forward with a flexible cable to the gearbox I'm designing, where the power will be geared down and split between the two propeller shafts, which then run backwards underneath the engine again and exit the bottom of the hull just behind the engine compartment. The counter rotating props are expected to be in the 50-60 mm diameter range, although I have enough hull clearance to go as big as 65mm if required.

For gearing, I decided to base the gearbox design around replacement gears for the REVO series 4-wheel drive R/C trucks, to ensure that I have a reliable source of replacement parts later on. By using various combinations of these 36, 40, 43, and 53 tooth gears, I was able to come up with a design that gives me six possible drive ratios, which will come in handy later on when I start trying to get this thing dialed in.

So here's the big question:

I'm told that the Zenoah G26 is usually set up to drive a single propeller shaft straight out the back, and that I can expect to have to gear the power down to some degree since I'm splitting it between two prop shafts with good sized propellers on the end of each of them.

These are the six possible ratios I have incorporated into my design that I can choose from. Crankshaft rotation is 1, gearbox output speed is expressed as the decimal fraction in brackets on the right:

1 : (.91)
1 : (.80)
1 : (.70)
1 : (.67)
1 : (.57)
1 : (.47)

Any idea which ratio would you guys would pick for your first run on the water driving twin 50-60mm propellers with a Zenoah G26, given the size (4') and weight (~20 lbs) of the boat in relation to the available power?


Last edited by PT Boat; 05-30-2009 at 02:41 PM.
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Old 05-30-2009, 09:25 PM   #2
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I am: Luc White
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Re: Gear reduction for twin prop shafts

hello these guys may help you out check
out this site lot's of stuff they sell good luck

got to love the water.
IMPBA# 20380 S CD

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Old 05-31-2009, 10:12 PM   #3
PT Boat
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Re: Gear reduction for twin prop shafts

Originally Posted by LoCo View Post
hello these guys may help you out check
out this site lot's of stuff they sell good luck
Thanks Luc,

Those are some neat looking gear boxes they are selling over there, they even have an oil filled crankcase! I figured somebody had to be making those things, but I'd never spotted them yet until you posted your link.

Interestingly, these guys are actually gearing these boxes UP 33%, which runs counter to what the guy at the local R/C shop said, as he suggested that I'd need to gear the box down to handle the twin props. At this point I think I'll aim for the middle somewhere, and set my gearbox up just a little lower than 1:1 for my for run on the water.

I hope you guys don't mind another question, but I'm pretty new to the R/C thing and I'm a bit unfamiliar with common drive setups. I was just wondering what drive ratio people use for a single engine/single prop arrangement, is it always 1:1 from the crankshaft straight through to the prop, or are there times when people want to gear the propeller shaft speed up or down to modify performance?

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