Triple Prop Question

[Iain MurrayParticipant @iainmurray67939]

Should the centre prop be left-handed or right-handed?

[Iain MurrayParticipant @iainmurray67939]

A friend has just asked me a question about propeller selection.

 

He is building a model with three props for the first time and asked me about left-handed vs. right-handed props. He assumes you should fit one of each on the two outer shafts, but is there any convention for the centre shaft? Does it matter?

 

I have never build a model with three props, and it has never occured to me to wonder about it. Can anyone help with this?

 

Many thanks,

 

Iain.

[ashley needhamParticipant @ashleyneedham69188]

Iain.

No idea.

 

HOWEVER, it is unlikely to make much difference, and it would be easy to see if there was a difference by swapping over props and motor connections to suit.

 

One would have thought that diligent surfing could trawl up from somewhere what the originals used ..?

 

Ashley Would mention my Titanic which has three props but I dont suppose the comparison would be much use.(I followed the original rotations)

[Iain MurrayParticipant @iainmurray67939]

Thanks Ashley. I’m glad it’s not just me who has no idea. I was feeling a bit of a haddie. As indeed I was as soon as I’d posted this and noticed I’d managed to put an extra “P” in “Triple”. Still, I never was very good at typing – that’s why I work in IT.

 

My friend is building a PT Boat of some sort, so i would hope it’s not too difficult to find out what the original used. Thanks for the tip!

 

Iain.

[Kimosubby ShipyardsParticipant @kimosubbyshipyards]

Iain,

 

On my Sea Crusader (Celestine) I’ve two props and convention is to have them rotating outwards, so the starboard prop is left handed and the port right handed. It also makes steering easier.

 

 

Our Mersey lifeboat is fitted the same way.

 

 

The reasoning being that two props rotating the same direction will cause the boat’s stern to step in the direction of rotation (as in a single prop), but more pronounced with a fast boat having two props, and then requiring opposite rudder to counter the turning moment with subsequent loss in speed and wasted energy.

 

As to the centre prop? maybe change it to a water jet.

 

I’ll ask my fellow modellers on Sunday, some have these three screw craft, and they might have the answer – maybe the reasoning too!

 

Kimosubby

 

 

 

 

[John W EParticipant @johnwe]

hi there

 

The rotation of the propellers of triple-screw vessels is dependant on several things.

 

First of all, what type of vessel it is?

 

if you are talking about fast small boats as in MTBs and air-sea rescue vessels, especially of the period of World War II – both British and American design, you will find the vast majority of these vessels’ props rotated in the same direction.

 

This was due to two reasons:

1) you get a slightly increase in speed

2) during the war reverse gear boxes were difficult to come by and also reversing the engines was difficult

 

I have built several MTBs with all three props rotating in the same direction; and the way I overcame the turning difficulties was I have the ability of shutting the 2 outboard props down and running on a centre prop for manouvreing at slow speeds. This is done with the aid of a modified mixer from ACTion which incorporates a mixing of the rudder movement to the propellers when operating all three props together.

 

Hope this helps.

 

aye

john

[John W EParticipant @johnwe]

[Ken ChurchillParticipant @kenchurchill26427]

I am currently building the 3 Propeller model of the “Titanic”.

 

After a lot of research I’ve found that the outer propellers rotate in a contra rotation. That is to say, the wash is directed upwards in the centre because it’s needed on the rudder which was a little too small for the job. Even at 101 tons it was inefficient because the surface area was too small.

 

The centre propeller rotated clockwise, viewed from the stern, but only started turning when she was under way and could not be reversed. It’s blades was also slightly smaller in diameter to the outer props. Two reasons why she had difficulty turning to avoid the Iceberg.

 

I cannot help you with your question of which way to have the centre prop turning as I have no experience yet. It might be worth experimenting with the centre one but the outer ones should be contra rotating to keep her steady as mentioned above.

 

Regards

 

Ken

 

[ashley needhamParticipant @ashleyneedham69188]

Ken, correctamundo, the centre prop was smaller as it was driven by a steam turbine, which was run on exhaust steam from the outer reciprocating engines.

 

I made my rudder on the Titanic MUCH larger than scale…and so at full lock it impinges on either sides propwash, and it turns reasonably well.

I think you would have a serious manouvering issue with the titchy rudder as fitted, on a model.

 

I recon seeing the `berg in good time might have been the real problem !!

 

As per Johns post, dead right I think. However again a non-scale increase in rudder area would sort you out.. these boats had very small rudders.

 

Ashley

[Ken ChurchillParticipant @kenchurchill26427]

Thanks Ashley.

 

The rudder is tall and thin design.

I guess I’ll make a ‘clip on’ extra bit of blade when sailing, for those tight bends.

 

regards

 

Ken

 

[Iain MurrayParticipant @iainmurray67939]

Everyone: Many thanks for your help and advice!

 

Iain.

[Mark Beard 1Participant @markbeard1]

I have a ‘parked’ Perkassa build which has 3 props. There is a photo on the interweb of a sister-ship during build, from which it is possible to ascertain the handedness of props. Unfortunately, from other features on the boat, the photo appears to be a mirror image. Doh!

 

So I’m coming to the conclusion that 3-prop boats are the Devil’s work, and all boats with more than 1 prop should have an even number of them!

[ashley needhamParticipant @ashleyneedham69188]

i think that there appears to be no hard and fast rule with this one.

 

Stick with what was fitted originally if building to detail, otherwise fit whatever and perhaps experiment to see what suits your particular boat.

 

Ash

[Father PavlvsParticipant @fatherpavlvs]

In my experience it makes no difference. If the outboard screws counterrotate, the center becomes like a single screw. The only reason I can think of to favor one way over another is if the vessel is used for oval course racing where it makes only right or left turns then you will want the torque to favor the turn direction i.e., clockwise for left turns and vice versa. I, too am building a model of the Titanic but I am going to leave the center screw unpowered and just run her on the outboard screws. I plan on running her on seperate throttles so I can play at manoeuvering her with seperate speeds and directions as well as her rudder which looks awfully small for a ship of her size but I will build her first with the scale rudder and see if I need to enlarge it later.  Fair winds and following seas!

Edited By Matthew Richardson on 20/06/2011 17:10:55

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