Three props

[Tony CattParticipant @tonycatt96553]

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 ?

[Tony CattParticipant @tonycatt96553]

[Dave MilbournParticipant @davemilbourn48782]

One of the props has a reverse pitch from the other two.

DM

[Tony CattParticipant @tonycatt96553]

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 MilbournParticipant @davemilbourn48782]

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 HadleyParticipant @tonyhadley]

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 needhamParticipant @ashleyneedham69188]

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 MilbournParticipant @davemilbourn48782]

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 BishopModerator @colinbishop34627]

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 FraryParticipant @malcolmfrary95515]

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 EParticipant @johnwe]

Hi 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).

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

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

[Paul TParticipant @pault84577]

Having three (or two) props all rotating the same way will induce 'prop walk' on model boats but oddly enough the larger the boat the less the phenomena is noticed.

For example 600mm long hull will need almost constant time/distance steering adjustment to maintain a straight course whereas a 1200mm long hull, where you might expect a 50% time/distance steering adjustment, actually requires only 20% time/distance steering adjustment.

When looking at full size craft the need for steering adjustment falls to 5%>.

Before people start pointing to the excellent photos provided by Bluebird that clearly show a R/R/L configuration and say that the phenomena must exist in full size boats I would ask them to consider those craft that have multi outboard motors all of which have props rotating in the same direction and offer the proposition that the Vosper designers were simply being cautious.

Paul

 

Edited By Paul T on 12/03/2017 17:07:25

[ashley needhamParticipant @ashleyneedham69188]

I believe that a lot of wartime torpedo boats had props rotating the same way, simply so as not to have to have a different gearbox in production.

​I think for a model boat, having two and one rotation would be the optimum. It is likely the wind on a large Perkasa will affect the steering more than any combination of proppery…

​Ashley

[Paul TParticipant @pault84577]

Hi Ash

Proppery?

[ashley needhamParticipant @ashleyneedham69188]

Proppery… the collective term for thing appertaining to props, drives, rotation and all that sort of thing..

[Paul TParticipant @pault84577]

Ah……good term.

I agree that wind, wave and tidal force will have a far greater effect on a full size boats performance than a non contra-rotating prop.

Paul

[Empire ParkstoneParticipant @empireparkstone]

My weisel torpedo boat has 3 props all the same hand and direction no problems what so ever

[John W EParticipant @johnwe]

Hi Ashley

You are quite correct, there were several MTBs and MGBs built during the War which had all the props turning in the same direction. One of the main reasons was a short supply of the reverse gear boxes. To cut a long story short, after the War, they did experiment with configuration of rotation of propellers – they found there was a slight improvement in speed with all the props turning in the same direction – but – only a slight improvement.

One of the most notorious vessels that Royal Air Force had built was the 68 ft RTTL It originally started off with 3 props but then was reduced and re-engine to have 2 props (all rotating in the same direction) and this vessel was renowned to have prop walk especially when diving off the top of a wave at speed. On numerous occasions, several of these vessels, split the hull at the bow. So, what Vosper tried to do for one of the fixes was add an extra layer of planking to the bottom and then they experimented with an aluminium hull.

If I can find the pics, they will follow

John

 

Edited By bluebird on 12/03/2017 19:01:10

Edited By bluebird on 12/03/2017 19:01:37

Edited By bluebird on 12/03/2017 19:02:04

[Tony CattParticipant @tonycatt96553]

Thanks gentlemen, I couldn't ask for more info, think it might be better for a newbie to stick to two props.

[Dave MilbournParticipant @davemilbourn48782]

Wise man. This article might be of use to you **LINK**

DM

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