prop direction

[Tony BurtonParticipant @tonyburton83970]

[Tony BurtonParticipant @tonyburton83970]

Hi Guys.

I know you will be able to settle an argument……..looking from the stern forward which way should the props rotate on a twin screw ship

[Colin BishopModerator @colinbishop34627]

If there’s one thing guaranteed to start an argu – sorry, discussion, that’s it!

 

It does depend on the type of ship. I have heard that on warships the props rotate outwards and on merachant ships inwards but that is by no means a hard and fast rule.

 

If you are going for scale appearance then try and copy the original, If that isn’t quite so important then it’s worth trying both ways to see which gives the most responsive turning. For eample, if you have twin screws and a single rudder then inward turning props mught be best as the water stream will impact directly on the rudder itself, But if you are using a mixer which slows down the inner prop on a turn then outwards turning props could be better. A lot will also depend on the hull hydrodynamics too.

 

It’s very much a ‘suck it and see’ situation I’m afraid.

 

Colin

[ashley needhamParticipant @ashleyneedham69188]

I suggest that it makes absolutely no practical difference on a model. And if it does the effect is minimal  

 

RN ships have them outwards turning as they say its more efficient, and i dare say that thousands of tests in the Haslar hydrodynamic test tanks proves them right. My bath on the other hand….well, its hard to judge really…

 

I have found on my dumpy 1880 battleship (twin screws and single oversize rudder) that after a lot of trials….I couldnt see much difference, and the sharpness of turn was really more dependent on the wind, and wash from faster ducks or other boats 

 

Ashley

[Tony BurtonParticipant @tonyburton83970]

Thanks Guys ……………….I knew you would sort it…….sort of……………

 

 

 

……………

 

 

Tony

[David MeierParticipant @davidmeier28154]

Hi Ashley and Colin.

When you say inwards turning and outwards turning are you referring to the top of the propellers or the bottom? Forgive me if it is a silly question but it is not clear to me.

 

Cheers.

David.

[Colin BishopModerator @colinbishop34627]

David, the received wisdom as I have always understood it  is  that the starboard prop will turn clockwise when viewed from behind while the prop is providing forward thrust and the port prop anticlockwise.

 

But yes, it is easy to get confused and in practical terms, unless you are are a stickler for scale detail it probably doesn’t make much practical difference as Ashley suggests.

 

Colin

[neil hpParticipant @neilhp]

It actually does make a difference Colin,

 

IF you run faster electric boats such as the big Speedline type lifeboats, you get a greater deal of cavitation, and therefore loss of power and speed if you run the props “inward”…( starb anti and port clockwise)

 

That’s why  all RNLI lifeboats [both real and model] have always had outward turning props.

Edited By neil howard-pritchard on 12/06/2010 15:43:21

[Dave MilbournParticipant @davemilbourn48782]

Tony

Why not try both ways – you’ve got handed props anyway? Stick with the one which suits best for that model, then you can come back to this thread and have your bob’s worth!

FWIW I always use outward-turning props – and that’s at the top from behind, DM.

Dave M (OMG! They’re breeding!!)

[Colin BishopModerator @colinbishop34627]

I’m sure Neil is right too, but off the top of my head I can’t think of many high powered twin screw electric boats apart from lifeboats and service launches although having said that I’m sure someone will come up with a list as long as my arm!

 

Colin

Edited By Colin Bishop, Website Editor on 12/06/2010 16:56:41

[Tony BurtonParticipant @tonyburton83970]

Well thats sorted then…….the reason I asked is that I have run a Pilot ship (48th scale ) for 18 years on home made 40mm  props. I decided to buy good quality made ones and they look superb, but when I tried to turn the ship in its own length it was rubbish compared to the old props. However when I took the boat out of the water there was a great cry from the other chaps of No wonder it wont go you have the props turning inboard. the old ones turned inboard said I.. well they should run out board they repeated, so I am going to try  outboard this week and see what happens

[John W EParticipant @johnwe]

hi therejust to add my two penneth worth in – basically Tony, you have the answer above – try the two options inboard and outboard rotation.   If you havent got the facility of controlling each motor independently, in conjunction with rudder movement.  You will find it very difficult to turn the model in its own length.   Even with independent control, some models are reluctant to behave correctly – in other words – turn when they should.  I have several twin-screw models, with various setups as far as prop rotations go, one of them is

 

 an RAF WW 2 rescue launch 2564.

 

This has outboard turning props along with transome slung rudders, with no independent control of motors and it takes an age to turn.

 

I have an MTB with 3 shaft setup controlled off 2 speed controllers but still not independently controlled motors and this will turn in 3-4 boats’ lengths. 

 

Then I have the RTTL 2571 which is built to true scale and both the props turn in the same direction.   Motors are independently controlled.   At slow speed, this will turn in its own length – under speed she suffers terribly from prop-walk – and turning the model can be pretty hair raising.   Just to throw another spanner in the works,

 

I also have HMS Ajax a 4 propped war ship  –  independently controlled motors and when I had the props turning outboard, even independly controlling the motors in the turn she would NOT turn – make a perfect straight runner.  I swapped the props around so that they all turn inboard & the model will spin on its own length.

 

Food for thought.

 

aye

john

 

 

[Tony BurtonParticipant @tonyburton83970]

Thanks John…….The ship turned perfectly in its own length with the old home made props turning inboard and independant motors. I`m going to try it this week with the new props turning outboard and if its no better then the old props will go back on……..

 

 

Tony…………

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