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Three props

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Tony Catt11/03/2017 17:23:44
3 forum posts

Hi as a new model boat fan can someone please explain how a boat with three props works, with 2 props going in one direction and 1 going in the other ?

Dave Milbourn11/03/2017 18:06:04
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One of the props has a reverse pitch from the other two.

DM

Tony Catt11/03/2017 18:29:35
3 forum posts

Hi Dave, looking at building a vosper perkasa, it has 3 props and 2 rudders, if 2 props drive in one direction wont tha tend to make the boat turn in one direction?

Dave Milbourn11/03/2017 20:57:52
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Forget the rudders - they are not relevant here. Two of the props rotate clockwise to go forward while the other one has an opposite 'thread' (aka pitch) so has to be rotated anticlockwise - but it still propels the model forward. It's not rocket science - just think about it. There's no other way I can explain it, I'm afraid.

Tony Hadley12/03/2017 08:19:33
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897 forum posts
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In the latest edition of Model Boats magazine (April 2017) Dave Wooley is building an OSA 2 Fast Missile launch which has a three propeller set-up. The article's photographs show the three motor and prop installation. This model also has three rudders.

ashley needham12/03/2017 08:35:11
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Tony C. Yes having three props two of which are twisting one way and only one of which are turning `t other will tend to make the boat turn to one way which is what I think you are asking. However this will be minimal and with just a touch of trim set on the rudder will be eradicated, and lets face it, who spends that much time going in a straight line??

​Ashley

Dave Milbourn12/03/2017 09:37:51
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Ashley

Are we talking "prop walk" here? I didn't think it happened with multiple counter-rotating props i.e. >2 - which I thought was the whole purpose of fitting them. Maybe one of our resident naval architects can give chapter and verse.

Dave M

Colin Bishop12/03/2017 10:08:58
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I'm not a naval architect(!) but power boats with triple props tend to have small fast revving props which will tend to direct the water directly astern.

Contrast this with a single screw boat with a large slow revving prop which will throw some of the water sideways with a paddlewheel effect.

Triple screws are normally only fitted because two aren't enough to do the job and the third one obviously has to turn the same way as one of the others. As Ashley says, the effect on the boat when going ahead will be minimal and any tendency to veer off course can be corrected with a tiny bit of rudder throw, either built in or via the TX trim control.

Incidentally, the commonly observed inability of a single screw boat to steer astern is almost certainly caused by the prop in reverse directing the waterflow along one side of the keel instead of equally both sides. It does horrid things to the hydrodynamics!

Colin

Malcolm Frary12/03/2017 10:13:56
893 forum posts

If it is prop walk that is the theoretical problem, the pair that are counter rotating will cancel each other which lets the unpaired one do its own thing, but that will be no more of a problem than gets experienced by a single prop boat anyway, so no real problem in practice.

The really important thing is nothing to do with handling. Having the rudders out of line with the prop shafts means that the shafts can be withdrawn easily, which is good for maintenance. It also means that the inboard end needs to be fixed so that it can't unfix itself. I've seen a complete shaft and prop lost that way. After it had finished unscrewing itself from the coupling, there was enough drag to pull it out.

John W E12/03/2017 10:29:31
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263 forum posts
257 photos

brave borderer(6) (medium).jpgbrave borderer(5) (medium).jpgHi one of the main reasons the rudder is off centre to the prop shaft is that the thrust from the prop isn't even on either side of it.   There is more thrust on one side therefore you place the rudder in this side of the thrust flow to make the rudder more efficient.  Also smaller diameter props are used with coarser pitches to help with cavitation on high revving engines - here are a couple of pics of the real I am (Vosper perkassa).

brave borderer(4) (medium).jpg

Edited By bluebird on 12/03/2017 10:33:07

Edited By bluebird on 12/03/2017 10:33:51

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