Model Boat Mayhem
Technical, Techniques, Hints, and Tips => Other Technical Questions... => Topic started by: RipSlider 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
-
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!!!
-
One motor, two propshafts and a drive belt. Job done. O0
-
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
-
Flippin eck Steve,
Told you I was no expert but that Bluebird knows a thing or two...thats for sure... :embarrassed:
-
what about putting a twist in one of the belts ,or would there be too much friction .
daz
-
If you twist the belts then put a vertical roller where they cross. One bit of tube (lubricated) over another will do.
-
Can one of the 'props' have reverse blades, to counter screw the water?
ken
-
Deans do a unit
http://www.deansmarine.co.uk/html/motors.html
Proteus
-
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?
-
Yep you are correct as the deans do.
Proteus
-
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.
-
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
-
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
-
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.
-
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
-
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
-
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
-
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
-
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
-
Here is the other picture, it got lost in the process
Bob
-
Bob,
Get it patented. a brilliant idea...
-
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
-
Ripslider,
Would love to answer your question.......but.......I have not got a clue what you are talking about!!!! :embarrassed: :embarrassed:
-
Well, all the speed boys use single props....
-
hi all,
I'm a newbie to this, but just a thought ..
if you went to five cogs, with 1 - 2 - 4 - 5 in a row, but with 2 & 4 not touching, then fitted cog three to the motor, and positioned it above this row, and meshing with 2 & 4, you would get the two props, on cogs 1 & 5 counter rotating, and still be able to keep the motor on the centre line of the boat.
depending on how you lined up your shafts the motor could also be below the line of cogs, or in line with them.
on the plan for the psv I'm building, it shows the propshafts as parallel to the bottom of the hull, so getting the motor low enough to be in line may be a problem. the real engines obviously drive from the crankshaft at the bottom of the engine.
I assume cog 3 could be a different size to either slow down or speed up the props compared to the motor.
does this sound right, or have I made a mistake somewhere ?
having a great time reading all the posts, and learning loads !!
cheers
alan
-
As you move out from the centre, the pairs of cogs rotate the same way, so no counter rotation. 2 and 5, and 1 and 4 will give counter rotation. Usually this is good and noisy.
-
ho hum, back to the drawing board !
-
if you fix the two pinions together or find a motor with a long output shaft this does away with having the extra idler as in bobs set up and is prob easyer if you have no facileties as you can get different length belts and by positing the motor you should be able to get the right mesh and tension.
peter
-
OK, another question on this, if you used the gears, would you still need a uj in the system, and if so where ?
I presume that with a belt system, the belts would take care of any misalignment.
cheers
alan
-
I did a couple of belt drive for one prop recently both motor facing and behind and did not use joints as thats the nice thing about doing it this way.this is a heavy duty set up so you would use lighter belts but a similar set up with a longer plate and gears on the other side of the motor.
Peter