Model Boat Mayhem
Technical, Techniques, Hints, and Tips => Other Technical Questions... => Topic started by: flag-d on April 27, 2006, 06:11:55 pm
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This forum's too big and popular! I read somewhere here about in-board and out-board prop rotation and how it affects turning etc, but I can't find it now! Perhaps I'm being a bit dense, but is in-board clockwise or anti-clockwise, as viewed from the stern? I have a 4 shaft MTB with opposite hand props on port and stbd, but which way should I have them turning? What difference does it make?
Mike
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Hi flag-d,
I have found no appreciable difference whichever way they turn as long as they are opposite rotation,
I believe in real life they turn out at the top blade so thats the way all mine are, Just doing a three propped job so I will have to think some more about the middle one.. :P
Tug [Ken]
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Try this very helpful page at Alans Boatyard, a site with some very useful tips :)
Link to Alans Boatyard twin screw tip page (http://website.lineone.net/%7Ealanann/propturn.htm)
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Dave, that`s absolutely correct as long as you have a boat with "scale speed". Some Boys I use to know, coming from the fast-electrics section do say, that on a twin-screw-boat the fast electrics are a bit better in speed if the props turn "outboard" and a bit better in manoeverability if they turn "inbord".
I wonder how and whay as I never had done such a fast boat, but I take it as a experience from others....
J?rg
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There is also this on Mayhem - Dave's Definitive answer for setting up Twin Electric Motor installations!
http://www.modelboatmayhem.co.uk/Common/Electrics/Twin_motors.htm
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With inboard rotating props the propwash is directed together and over the rudders nearest the ships counter.
This has the effect of more wash over the rudder(s).
This of course increases the frictional drag on the hull and rudders which will have a small effect on overall top speed.
Most warships have outboard turning props, and tugs have inboard turning props.
This is for full size ships and is only a generalisation.
Bob
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Thank you all for your insights into my question. I think, from what has been said, that I'll just try both inboard and outboard and see if I can see a difference. I'll let you know what I find (if anything!)
For the time being, I'll keep flying flag 'D' (1939-45 RN signals) "Keep clear, manouvering with difficulty"!
Thanks again
Mike
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I'm pleased to report that, of course, this forum is right again! In trials with 4 different sizes and pitches of props on the Fairmile, I can confidently say that in-board turning props do give better manouverability (I'm sure that's not spelled right!).
The Fairmile is 55" LWL, 4 shafts, handed props. With in-board turning props, low speed turning was much improved, High speed turns didn't seem any different. I couldn't really see much in the way of an overall speed difference between in-board and out-board rotation.
The props used were:
Rivabo 35mm brass, 3 blade
Graupner 40mm, plastic, 3 blade
Rivabo 45mm brass, 3 blade
Prop Shop, 38mm, scale type 3 blade
The last set, the Prop Shop items give the best performance, not only a nice, realistic scale speed, but also cool running motors (540 type) and a long run-time too.
Mike
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Glad to help.
Bob
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Hi All
Quite right, on most real motorboats it is righty righty and lefty lefty, for improved performance (i.e. outboard turning). This caught me out when I set up a 1/24th Fairmile A this way and decided to use a mixer with two ESCs so that this famously bad steerer could be used in a pool or pond. The mixer just would not work correctly unless the RH prop was on port and the LH prop on stbd, no matter how many permutations of polarity reversing I tried. I then noted that the Robbe props themselves are marked L on the right hand prop and vice versa. It seems therefore to be the accepted norm in the model boat world to use inboard turning props.
Regards,CDSC123
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cdsc123
I suspect that Robbe, being from the other side of the channel, follow the continental convention that prop rotation is as viewed from the front, the UK convention is that props are viewed from astern when figuring rotation. We've been in the EU for some time now, but we are still not that deeply in.
MalcolmF
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From my observation at inland dry-docks, most free running wheels turn outward and most wheels in Kort Nozzles turn inwards.
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From my understanding it depends on location and operators preference. Most vessels here on the East coast have outboard turning props but when precise manoeuvrability is required inboard turning props are used. On an vessel that turns outboard if you increase the thrust on port for example the vessel will turn to stbd due to increased thrust but "prop walk" will also play a role. On pilot boats for example inboard turning props are used since prop walk is counteracted by the increase in thrust so the operator can gently move closer to the ship...without being called "Captain Crunch" ;)
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Hi Malcolm
I'd be interested to find out more about the continental convention whereby propeller rotation is as viewed from in front; I am in the yacht chandlery trade and have been for 20 years and have never come across it (We deal with many UK and European propeller suppliers). Do you perhaps mean engine rotation direction?
Regards,CDSC123.
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cdsc123
The difference was something that was brought out several times on the old MB forum, and has appeared on this one. I have also been reminded of it when buying a replacement prop with appropriate marking and finding the boat going unexpectedly backward. It turned out that the blades had the opposite pitch, but both were marked "R". As it was only a single prop, I reversed the motor connections - cured!
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On a single shaft it is usual for the prop to turn the same way as the engine, this reduces wear in the gearbox as it is a straight drive and the reversing idler only used for reverse.
As most diesel engines turn clockwise from the free end this transmits as as a left handed prop.
Bob
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You know Bob someday someones going to ask a question and you wont know the answer. Every time I see one of your replies it makes me wonder if I know anything about this hobby.
Richard :-[ :-[
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DickyD,
Just getting some advice from Stavros about paint.
No I certainly dont know it all ;), wish I did.
Just been a Marine Engineer for nearly 30 yrs, from Harbour tugs, Ocean going salvage Tugs, Salvage vessels and survey ships so have a vague idea about real ships.
Bob
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Reading this at Dublin airport enroute to Utah.. ;D
From what I remember on the Mersey class lifeboat, The port prop turned Anticlockwise and the Starboard turned Clockwise. It had something to do with the transmitted torque keeping the boat straight.
I have a document from Dave Milburn (Think thats who I got it from) which explained it all.
Declan