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Author Topic: running two props off one motor ?  (Read 6827 times)

RipSlider

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running two props off one motor ?
« on: August 30, 2008, 06:06:56 PM »

Hello folks

I was wondering if anyone has ever come accross, or designed, a device that would allow two prop shafts to be driven off a single motor.

Does such a thing exist?

Ta

Steve
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barryfoote

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Re: running two props off one motor ?
« Reply #1 on: August 30, 2008, 06:15:03 PM »

Steve,

I am no expert, but seeing as most of them are at Llanberis I will give it a go,

Yes. Use a pair of geared cogs, one on each prop shaft, connected via a band to a similat cog on the motor shaft...Hey presto, two shafts working from one motor. Now as for all the technical bits.........Wait for the Llanberis crew to get home!!!
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DickyD

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Re: running two props off one motor ?
« Reply #2 on: August 30, 2008, 06:21:23 PM »

One motor, two propshafts and a drive belt. Job done. O0
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John W E

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Re: running two props off one motor ?
« Reply #3 on: August 30, 2008, 06:29:14 PM »

One motor, two propshafts and a drive belt. Job done.

I am no expert, but seeing as most of them are at Llanberis I will give it a go,

Yes. Use a pair of geared cogs, one on each prop shaft, connected via a band to a similat cog on the motor shaft...Hey presto, two shafts working from one motor. Now as for all the technical bits.........Wait for the Llanberis crew to get home!!
!


The above description is alright and will drive 2 prop shafts - as long as you want both propellers and shafts turning in the same direction.   If you want both propellers to counter-rotate to one another you will have to add - either an idling cog - or an idling pulley - in amongst your drive chain.  To reverse the drive of one propeller shaft.

Years ago some of the I.C. people, when they wanted to drive two prop shafts from one motor, they used to build a gear box incorporating a clutch mechanism.

aye
john e
bluebird
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barryfoote

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Re: running two props off one motor ?
« Reply #4 on: August 30, 2008, 07:51:15 PM »

Flippin eck Steve,

Told you I was no expert but that Bluebird knows a thing or two...thats for sure... :embarrassed:
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portside II

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Re: running two props off one motor ?
« Reply #5 on: August 30, 2008, 08:12:00 PM »

what about putting a twist in one of the belts ,or would there be too much friction .
daz
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Colin Bishop

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Re: running two props off one motor ?
« Reply #6 on: August 30, 2008, 08:31:33 PM »

If you twist the belts then put a vertical roller where they cross. One bit of tube (lubricated) over another will do.
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Tug-Kenny

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Re: running two props off one motor ?
« Reply #7 on: September 01, 2008, 01:11:41 PM »


Can one of the 'props' have reverse blades, to counter screw the water?

ken
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Proteus

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Re: running two props off one motor ?
« Reply #8 on: September 01, 2008, 02:17:19 PM »

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Hagar

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Re: running two props off one motor ?
« Reply #9 on: September 01, 2008, 08:21:56 PM »

I am by no means an expert, but if you had three gear wheels of equal size, motor in the middle, and a propshaft with a gear on one end and a left and a right hand prop on the other, would this not give counter rotating props?
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Proteus

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Re: running two props off one motor ?
« Reply #10 on: September 01, 2008, 08:34:03 PM »

Yep you are correct as the deans do.

Proteus
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malcolmfrary

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Re: running two props off one motor ?
« Reply #11 on: September 01, 2008, 08:54:02 PM »

The motor in the middle arrangement will have both shafts rotating the same way, Dean gearbox has the motor cog driving one shaft, which in turn drives the other shaft.  This allows counter rotating.
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John W E

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Re: running two props off one motor ?
« Reply #12 on: September 01, 2008, 08:58:07 PM »

Hi there all

Just a quick scribble to help explain the directional movement of gears;

In figure 1 – that is 2 gears ‘meshing’ and the motor directly drives one prop shaft – and the other prop shaft is driven by the ‘meshing’ gear.   These 2 shafts drive in opposite directions; the only disadvantage with this, for model boat purposes, is the distance between the prop shafts, e.g. if the distance between the 2 prop shafts were say 50 mm; each cog would have to be 50 mm diameter.   Think about this one.

In figure 2 – you will see 3 cogs driving – the centre cog being the one driven by the motor and how it turns the 2 outer cogs (or the driven cogs) in the same direction. 

In figure 3 – you will see a 4 cog set up; with cog number 3 being the ‘idler’.   The idler cog changes the direction of rotation on cog 4.   

Hope this is of some help.

One last thing; if this is still not clear; get 4 round coins of equal size; place them on a flat surface; all edge to edge (in a straight line).  Turn one coin whilst butting another coin against it, and see which way it turns that coin.  This may help to understand.

As mentioned before, when we are dealing with belts and pulleys; the scenario changes a little, because we can twist the belts into a figure of 8 which does reverse the rotation of the pulley.

Just out of interest; if we think back – or look back – in some cases  :) to the ‘older’ industries and ‘old’ workshops, where there used to be one driving steam engine, in a steam engine house-building; driving what was called a ‘line shaft’.  This line shaft would run the full length of the workshop/mill whichever the case and off this line shaft they would take belt drives off on various size pulleys to drive various machines.   So, the one line shaft could drive as many as say, 100 workshop machines at once.

Aye
John E
Bluebird


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RipSlider

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Re: running two props off one motor ?
« Reply #13 on: September 02, 2008, 09:09:09 AM »

OK - that's that one sorted out - and thank you very much indeed for all the comments.

Bluebird, as ever, has made me feel jelous at his ability to do highly useful drawings when all I can do is make a mess on a piece of paper.


OK - here is my next question.


Let us say I go for a two shaft setup - so that one motor is driving two props.


Before I do this, I originally have a boat set up with a motor - be it electric or petrol based - to drive a single prop.

The motor produces X watts of motive force. This is used to drive a prop of Y mm in dia.


Now I add in the two props using gears - to make it easy - lets assume these are magic gears which suffer from zero frictional losses


The same motor is still producing X watts.  My question is - by what proportion should the props reduce in size - i.e des Y -> 0.5 Y, a bit more or a bit less?


My initial thought was that it should reduce by half. However, a prop of half the size needs LESS than half the power to produce half the performance. So potentially, I might be able to use 2 props each 75% the size of the original.

Does anyone have any data or maths to throw at this?

And therefore, this brings me to my third question:

Again assuming we have magic gears with no losses - and lets also assume that the props themselves don't cause additional drag, is it to be expected that 1 motor would proide better performance driving two props than one prop?


Thanks

Steve
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malcolmfrary

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Re: running two props off one motor ?
« Reply #14 on: September 02, 2008, 10:36:23 AM »

Assuming that the number of blades per prop and the pitch remain the same it all comes down to the volume of water shifted in a given length of time.  This comes down to the area of the prop, all else being equal, which would mean the diameter would be divided by square root 2.
Thats just me thinking, I can now sit back and let someone who actually knows give us the proper story.
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dreadnought72

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Re: running two props off one motor ?
« Reply #15 on: September 02, 2008, 10:40:56 AM »

A prop provides power due to its pitch, blade area, and revolutions.

Assuming the shape, pitch and revolutions don't change, then:

One prop of radius R is equal to two props of radius SQRT(1/2)*R, three props of radius SQRT(1/3)*R, etc...

(There'd be extra losses with multiple propellor set-ups in terms of scale factors and blade tip vortices, but within a spit or two the above formula should work ok).

Andy

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dreadnought72

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Re: running two props off one motor ?
« Reply #16 on: September 02, 2008, 10:43:35 AM »

I see Malcolm beat me to it by a gnat's whisker.

Given that we're suggesting the same solution ("divide the original radius by the square root of the number of propellors to get the new radius") I think that's probably correct.

Andy
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RipSlider

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Re: running two props off one motor ?
« Reply #17 on: September 02, 2008, 11:11:08 AM »

OK. Slightly confused:

using this maths and a 2 prop system:

1 x 70mm prop would seem to be equal to 2x 49.6mm props.


Is this right?

Does this therefore mean that 2x 49.6 mm props would require the same number of watts to shift a propel a boat forwards at the same speed

Ta

Steve
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dreadnought72

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Re: running two props off one motor ?
« Reply #18 on: September 02, 2008, 11:26:48 AM »

Yes - but bear in mind there will be losses associated with gearing and the relative inefficiency of smaller diameter propellors. Though those shouldn't add up to more than a few percent.

Andy
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Bob

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Re: running two props off one motor ?
« Reply #19 on: September 03, 2008, 11:32:12 AM »

Hi there Steve,
Here is another solution to your query.    Pinions, pulleys, shafts  and bushes are from photo copiers, mounting base alloy angle, toothed belt from tape deck, centre section ex fishboat window perspex, cover plate alloy. Over all around 3:1 reduction with contra rotating output at 80mm centres. Brass props 35mm dia threaded 4mm on 300mm x 5mm SS shaft running in a 200mm x 6mm brass tube with Thordon bushes at each end.  Motor salvaged from a hand vaccum cleaner left lying around looking for an owner. Performance results some way off as build progress is slow.
Bob Ferguson
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Bob

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Re: running two props off one motor ?
« Reply #20 on: September 03, 2008, 11:34:14 AM »

Here is the other picture, it got lost in the process
Bob
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barryfoote

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Re: running two props off one motor ?
« Reply #21 on: September 03, 2008, 12:32:50 PM »

Bob,

Get it patented. a brilliant idea...
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RipSlider

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Re: running two props off one motor ?
« Reply #22 on: September 03, 2008, 12:43:58 PM »

Thats the sort of think I was thinking of.


OK - here is my last question on this topic.


Assume we have a motor - lets say a 540 motor. On paper, this motor can produce Xwatts from a given poiwer source - or maybe Y ft/lbs per second

On paper this would suggest it could drive a prop of Zmm in dia.

However, on the water performance would be a different story, as propellor size doesn't scale in a linear way as Watts available go up - there are log's in there, and the need for power rises at a greater rate than the prop size.

I did have a link talking about this, but I can't find it now.



Anywho - what I am not sure about is at what size this effect will kick in. Would I, for example, be better off using two smaller props on a small model boat than I would using one big boat, if performance was my objective?

In the case of the 90mm prop compared to the 2x 49mm props - at this scale, would I see any difference in performance. Or is the scaling effect not big enough, and it would be drawned out by the frictional losses?

Ta

Steve
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barryfoote

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Re: running two props off one motor ?
« Reply #23 on: September 03, 2008, 03:41:18 PM »

Ripslider,

Would love to answer your question.......but.......I have not got a clue what you are talking about!!!! :embarrassed: :embarrassed:
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Colin Bishop

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Re: running two props off one motor ?
« Reply #24 on: September 03, 2008, 03:49:00 PM »

Well, all the speed boys use single props....
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