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Author Topic: Propeller Rotation & Configuration  (Read 8080 times)

Marty

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Propeller Rotation & Configuration
« on: June 01, 2015, 11:36:18 PM »

     A few days ago I was having a discussion with a fellow member of my local boat club about what’s the best configuration of prop rotation for a twin-screw scale model tug.
 
I have a lot of ship handling experience from driving fishing boats, twin screw inshore ferry and my day job as Chief Mate of a 102,000 ton LNG Carrier and I wanted to share some of my knowledge to help clear up some of the issues that scale modellers have when deciding what’s the best propeller/motor rotation configuration for your model tug.
 
I recently took delivery of a new model that hadn’t been on the water and it was down to me to set her up. She’s a one 1/32 scale model tug built in Canada for the great lakes towing company. “Handy One”
 
I fitted her out with two MFA 919 series 6:1 reduction gearboxes.
 
Propeller configuration is as follows, 60mm Right hand prop on the starboard motor and a 60mm left hand prop on the port motor. Both motors outward turning.
 
Now when ship/boat handling the most important thing is knowing the characteristics of your propeller and the effect of transverse thrust and the effect it will have on your boat.
 
Before I begin I want to outline the setup on outward turning and inward turning propellers.
 
Outward turning propellers: Right hand propeller to starboard and left hand to port.
Inward turning propellers:   Left hand propeller to starboard and right hand to port

 
If you imagine a right-handed propeller (outward turning) viewed from astern to drive the boat forward, it must rotate in a clockwise direction.  The advantage of knowing the direction of rotation of a propeller is important when astern power is applied. When a right handed propeller goes astern the propeller is rotated anticlockwise and the helical discharge i.e. the wash from the propeller blade is directed to the starboard side of the boat, this in turn swings the stern to port and pushes the bow round to starboard.
 
Now if we imagine a left hand propeller  (inward turning) it needs to turn anticlockwise to push the boat forward when viewed from astern. When astern power is applied the propeller turns clockwise to move the boat astern, the helical discharge is directed to the port side of the boat pushing the stern to starboard and the bow to port.
 
OK with me so far?  Right so what does this have to do with propeller rotation right?
As I mentioned I was having a discussion with a mate of mine as to what was the best direction to have your motors rotate.
 
The answer is outward turning!
The reason I say out ward turning over inward turning is because if you want to turn the boat to starboard in her own length, push the port motor ahead and the starboard motor astern, very easy, and round she swings. Give some starboard helm and she’ll turn even quicker. The advantage of having outward turning props over inward turning is that when going astern on a right hand propeller inward turning it would mean the wash would be directed inboard or the centreline. This can cross over onto the surface of the opposite side rudder and completely mess up the manoeuvre.
I know for a fact that there are very few vessels with inward turning propellers used on tugs and anchor handlers because in all essence there an absolute pig to handle.
 
Another advantage of outward turning props is when coming into berth. Imagine approaching at 45 degrees to the berth, say berthing starboard side alongside, slowly drag your port proper astern and you’ll see that the stern will tuck in nicely and allow you to berth bodily alongside in a controlled manner.
 
 
I want to quote a passage from I book that I used when I was studying ship handling in college.
 
“The Ship Handlers Guide” by Capt. R.W Rowe, FNI
 
Outward Turning Propellers
In relation to each other when going ahead, the blades of these propellers are outward turning in the upper half of their circle of rotation, when viewed from astern. (See fig 56a) If however the starboard propeller is put astern, to assist for example in turning the ship to starboard, it will now be rotating in the opposite direction. (See fig 56b) This propeller is therefore now behaving in exactly the same way as a right-handed propeller on a single screw ship and part of the helical discharge will be deflected up and onto the starboard quarter. The resultant transverse thrust will cant the bow to starboard, not only assisting the turn, but also working in conjunction with both rudders and propeller torque.
 
Inward Turning Propellers.
These propellers when viewed from astern are now inward turning in the upper half of their circle of rotation. (See fig 57a) if once again the ship is turning to starboard and the starboard propeller is put astern to assist, it will be rotating in the opposite direction (see fig 57b). This propeller is acting in the same way as a left handed propeller on a single screw ship so, whilst going stern, part of the helical discharge will be deflected up towards the port quarter! The resultant transverse thrust will attempt to cant the bow to port, not only in the opposite direction to the desired turn, but also working against the rudders and propeller torque. The astern wash from the starboard propeller may also seriously deflect the smooth flow of water from the port propeller onto its own rudder.
 
The effect of inward turning propellers upon a ship can be extremely severe and render it totally unmanageable from a ship handling point of view. In the worst case it has been found necessary, when manoeuvring, to stop one engine completely and work the vessel in the same manner as a single screw ship.
 
I hope this information is useful to anyone mystified by the direction in which propellers rotate. if any more clarification is needed on this topic please drop me a line.
 
Thanks for looking
 
Martin
 
 
 
 
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Capt Podge

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Re: Propeller Rotation & Configuration
« Reply #1 on: June 01, 2015, 11:45:28 PM »

That's an excellent piece of information Martin - thank you very much for the clarification, this will help with "discussions" at the lakeside in future - Nice one :-))
 
Regards,
 
Ray.
 
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Marty

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Re: Propeller Rotation & Configuration
« Reply #2 on: June 02, 2015, 12:08:51 AM »

Its a pleasure Ray! Thanks for the kind comments!!


Need anything just let me know!!!


Martin
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derekwarner

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Re: Propeller Rotation & Configuration
« Reply #3 on: June 02, 2015, 12:12:21 AM »

Hey Marty......considering your day job..."as Chief Mate of a 102,000 ton LNG Carrier"......I am a little surprised you allowed  >>:-( & experienced the differing speed control systems issues in that other thread last week :o

However the text below is always interesting, there are naturally exceptions to the rule, which you may wish to post to give a balanced view of twin propeller inboard rotation  %)........... Derek

   
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Marty

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Re: Propeller Rotation & Configuration
« Reply #4 on: June 02, 2015, 12:24:58 AM »

you absolutely right Derek...


In fact i'd like to hear from people with inward turning screws on there tugs and similar vessels to see what they find. last week I tried the motors on my tug inward rotating after I replaced both esc's that I was having trouble with. It was terrible!! i'll do some digging and run a few scenarios see what I can come up with.


ESC's!!! they've been a real pain the neck recently. Electronics aren't my thing as I'm just returning to the hobby that i left in my teens!! but I'm learning!!


Anyway my tug handles superbly now that all issues are resolved with the ESC. Just awaiting my working winch from Harbour models in the USA to be delivered........


Marty
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NFMike

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Re: Propeller Rotation & Configuration
« Reply #5 on: June 02, 2015, 01:08:59 AM »

Yes, for open props I have always thought outward turning was preferable, though not for the reasons detailed in the OP.
However, many tugs and AHTS have Kort nozzles, which presumably remove these effects. Is that correct? (I have a picture on file of an AHTS with inward turning props in nozzles.)

Marty

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Re: Propeller Rotation & Configuration
« Reply #6 on: June 02, 2015, 02:45:15 AM »

Inward turning props on AHTS are for one reason only that's because there fitted with controllable pitch propellers. Kort nozzles do not remove the above mentioned effects, there simply there to increase thrust. Most CPP propellers are left handed on twin screw CPP vessels (inward turning) this is a term known as a "handed" vessel. This allows them to have the same characteristics of outward turning props and achieve the same results.


Kort nozzles were initially introduced in canals and inland waterways to reduce erosion on the river banks by focussing the wash from the propeller into the centre of the canal/river, however it was noted that this had no effect other than considerably increasing the thrust produced by propeller. Korts are most useful when used at very low speeds under high load, use them above 8 knots and the vessels hull efficiency through the water is greatly reduced.

Using an azimuth thruster/pod system are CPP vessels, basically there just big outboard engines! Awesome pieces of kit if you ask me. I was working in Limassol earlier this year and we had two Damen tugs standing by our vessel. When alongside us i could see both azipods through the crystal clear water spinning away with zero pitch on the props. When the skipper put on the power the reaction was so quick, its incredible just how fast the direction of the pod can be changed all at the same time.

I've seen a lot of scale models using steerable korts and there great for towing by increasing the thrust, however for steering there not bad but could be better.(in my opinion) A large rudder fitted centre line on a kort directing the water flow provides superb means of control. The only other means better than this is the azipod system where you get maximum thrust in any direction you want. My tug has twin rudders on each kort and the minimum amount of helm gets her swinging.






Martin
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Martin [Admin]

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Re: Propeller Rotation & Configuration
« Reply #7 on: June 02, 2015, 02:53:32 AM »

That's an excellent piece of information Martin - thank you very much for the clarification, this will help with "discussions" at the lakeside in future - Nice one :-))
 
Regards,
Ray.

Totally agree with Ray, et al. . Nice to hear it direct from the "horses mouth!"  Thanks for posting Marty.

NB.  "inward turning propellers used on tugs and anchor handlers because in all essence there an absolute pig to handle."
       1. Why would ship builders ever do that then?
       2. Is there any case where inward turning propellers could be an advantage?

Martin   :-)
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Fastfaz

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Re: Propeller Rotation & Configuration
« Reply #8 on: June 02, 2015, 03:20:21 AM »

Hi Marty,
    That's a really useful post and has answered a question I was going to ask later in the year when I start the build on my Portgarth. Re the rudder on the back of a Kort nozzle I have a Lowgarth which is fitted with a 75mm 4 blade prop in a Kort nozzle, when I built the boat I did not fit the rudder blade to the back of the nozzle (had major grief with the parts in the kit and decided to use only the hull and running gear the rest scratch built). The boat sails very well and is pretty good going astern i.e. responsive and will sail straight, what I would like to know is will it make any difference, apart from making it hard to remove the prop, that is beneficial to handling especially when towing?
    Thanks again for you really helpful post.
         Cheers,
            Pete/Faz. :-)) :-)) :-)) :-)) :-)) :-)) :-))
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NFMike

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Re: Propeller Rotation & Configuration
« Reply #9 on: June 02, 2015, 11:04:18 AM »

Inward turning props on AHTS are for one reason only that's because there fitted with controllable pitch propellers. Kort nozzles do not remove the above mentioned effects, there simply there to increase thrust. Most CPP propellers are left handed on twin screw CPP vessels (inward turning) this is a term known as a "handed" vessel. This allows them to have the same characteristics of outward turning props and achieve the same results.

Ah. Ooh. Yes. Penny has dropped ... and the the machine gave me an extra bar of chocolate too  O0
Yes, in that AHTS picture I have it does look like CP props.
This effect was explained to me about 50 years ago as 'paddle-wheel' effect which is why I thought that a nozzle would pretty much negate it, but the 'helical discharge' explanation makes it all fit properly (in my head anyway).

Marty

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Re: Propeller Rotation & Configuration
« Reply #10 on: June 02, 2015, 11:20:59 AM »

Totally agree with Ray, et al. . Nice to hear it direct from the "horses mouth!"  Thanks for posting Marty.

NB.  "inward turning propellers used on tugs and anchor handlers because in all essence there an absolute pig to handle."
       1. Why would ship builders ever do that then?
       2. Is there any case where inward turning propellers could be an advantage?

Martin   :-)




I have no idea as to why ship builders, build ships with  inward turning screws, from my own experience in the ship yards, for the company that I work for, it seems to me that accountants build ships these day!! It's all down to money.


I've read that inward turning screws are more fuel efficient over long voyages, but I've no evidence to support that fact,


Martin
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Marty

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Re: Propeller Rotation & Configuration
« Reply #11 on: June 02, 2015, 12:03:45 PM »

     Fastfaz
 
The biggest benefit about Kort nozzles as I mentioned is the extra thrust produced by the nozzles, I’d say your looking around 50% increase in thrust over a vessel built without a nozzle.
 
You mentioned that one of your previous vessels steers well while going astern, that’s great because, on the full scale vessel it’s very hard to find a ship that steers in a straight line while going astern.
 
With a single screw vessel you’ll always have a pull to either starboard or port when going astern with a right or left hand propeller. With twin screws rotating outward this is greatly reduced because the effectively cancel each other out to certain extent, its not a 100% but its considerably less than that of a single screw vessel.
 
Now in my opinion when planning a build with either fixed of steerable kort I’d take the fixed kort over the steerable one for following reason.
 
A propeller housed by a kort nozzle has a smaller opening on the discharge side form the nozzle (i.e. the after most part of the nozzle). This means that the water leaves the nozzle under greater pressure than what it enters the nozzle; I’ve attached a diagram to help you understand what I’m talking about. This is the increase in your propeller thrust.
 
The propeller is designed to have a neat fitting inside the nozzle, some propeller manufactures design their propellers to be fitted a particular brand of kort nozzle. When a steerable kort is turned the pressure on the discharge side is decreased because the water flow into the nozzle has been altered. You’ll find that one side of the nozzle has more space than the other; this means that the pressure the discharge side of the nozzle has been reduced on the side that has more space.
 
I hope you follow me on this, it’s not an easy thing to explain online here…
 
When you place a large rudder on the centre line of the prop you get a larger turning lever(larger surface area) and the flow across the rudder is more effective than a steerable korts. A big rudder with at least 35 degree angle, if you can get 45 degrees then that’s even better you’ll have an superb turning lever.
 
Now in my tug the “Handy One I have twin rudders on each kort and she handles incredibly well. To answer you question about removing the prop, I’ve a few linkages to remove to change the prop but its very straight forward. I recently changed by props to large 60mm four blade props and I see a tremendous improvement from that was previously. All you need to think about is gaining access to the linkages on the rudder’s. see attached picture of my tugs back end above and below the waterline.
 
Here’s a little bit of homework for you….. ever heard of a Schilling rudder?? Have a look and I’ll send you a little information later on the benefits of a Schillig Rudder this would be great on your forthcoming Portgarth build….
 
Hope this helps you mate!
 
Marty
 
 
 
 
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TugCowboy

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Re: Propeller Rotation & Configuration
« Reply #12 on: June 02, 2015, 12:20:25 PM »

Great information Marty, thanks for taking the time to put it together.

I've experimented with Schilling rudders before on one of my tugs but found it caused a wild swing to one side when under full astern.
Being a child of Aerodynamics rather than Hydrodynamics I can't say for sure but I'm assuming that my rudder creation skills weren't up to scratch and there must have been a lack of symmetry.

Are there any Schilling rudders available at RC Scales or do I need to keep trying till I get the darn thing right.

Alex
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Marty

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Re: Propeller Rotation & Configuration
« Reply #13 on: June 02, 2015, 12:26:51 PM »

Thanks Alex


I have'nt seen any online for sale, the thing to remember when building them from scratch is the concaveness aft of the pivot, if it's to steep it will greatly over accelerate the swinging effect. you need to find that happy medium, if that makes sense to you?


I'm doing a little research online and I'm going to see if I can mind a tech drawing and scale it down, maybe even make a one to try out myself.


Last year I was at Laird-side Maritime Centre in Liverpool for a ship handling course,we were driving ships, small product tankers with Schilling rudders. they were fantastic ships to handle.


Marty
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Shipmate60

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Re: Propeller Rotation & Configuration
« Reply #14 on: June 02, 2015, 06:15:21 PM »

Marty,
I have found that twin screw models with a single rudder steer better ahead with inward turning props.
I too experimented with a 6 foot Hunt Class destroyer and found the turning circle greatly reduced on inward without using "tank steering".
I have concluded that the propwash is concentrated under the counter and directed on to the rudder blade to give better ahead steering without engine changes.


Bob
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Marty

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Re: Propeller Rotation & Configuration
« Reply #15 on: June 02, 2015, 07:06:56 PM »

Bob


Does a class destroyer with a single rudder on the centre line? If so I'd like to see how she handles under slow speed with berthing? The thing you have to remember with warships is that there extremely narrow so theres every chance that the some of the prop wash will find its way onto the blade. As long you are making sufficient head way she'll steer well, reduce the speed and her handling will reduce considrably.
I'd like to see her berthing beacuse with a single rudder between two screws she'll be an absolute nightmare.


I used to work for Maersk on there boxes boats, in 2002 they purchased safmarine and the legendary big white box boats. I always remember the South Africa masters talking about the helish job they used to have berthing those ships with a single rudder between two screws.


What you have to remember bob is no mater what direction your props turn to have a useful rudder/ steering system you must have sufficient water flow across it to make her turn well


An effective rudder must be placed directly in front of the the propeller. Nowadays you never see that configuration with a single rudder between two screws.


I've often wondered why warships were built that way.


Do you know why? I'd like to know more and if you have any thoughts I'd really like to hear them.


Thanks bob


Marty.







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Re: Propeller Rotation & Configuration
« Reply #16 on: June 02, 2015, 07:16:47 PM »

Bob



An effective rudder must be placed directly in front of the the propeller. Nowadays you never see that configuration with a single rudder between two screws.


I've often wondered why warships were built that way.


Do you know why? I'd like to know more and if you have any thoughts I'd really like to hear them.


Thanks bob


Marty.


I wonder if where full speed was needed, the single rudder caused less drag than two?


Another Mike


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Re: Propeller Rotation & Configuration
« Reply #17 on: June 02, 2015, 07:58:10 PM »

IMO warships were never meant to enter steering competitions, they only have to steer a course on the open sea, one rudder dose this adequately, they seldom if ever berth themselves in a port.
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Re: Propeller Rotation & Configuration
« Reply #18 on: June 02, 2015, 08:25:48 PM »

Thanks for the article as it validates what I have been try to get across to people for some time now. However, in saying that I have noticed when looking a pictures that over the last few years, that a fair percentage of modern tugs and support vessels do appear to have inboard turning screws in nozzles. I also know that the vessels used on the Birkenhead - Belfast ferries Mersey/Dublin Viking etc., are inboard turning and have deep, narrow rudder blades. I always assumed that this configuration was to make berthing at the respective terminals easier. I noticed further, that the screw operated below the level of the keel which, I would assume, by what you have stated, would mitigate the effects of any thrust against the hull.

I would conclude by saying that the latest Maersk box vessels have inboard turning screws for no other reason than this configuration apparently improves fuel consumption by a small margin.

LB who trust that nobody finds anything insulting, rude or unfair in the above.

 
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Marty

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Re: Propeller Rotation & Configuration
« Reply #19 on: June 02, 2015, 08:30:12 PM »

Thats correct liverbudgie, fuel economy is the only reason I can find that benefits from inward turning screws.


Marty
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Re: Propeller Rotation & Configuration
« Reply #20 on: June 02, 2015, 08:39:23 PM »

Marty,
Chief Engineer from Tugs to Armament Vessels to Salvage vessels to Cablers.
The world of models is so different from the full size ships.
Almost all models are vastly overpowered (mine included), Tugs can spin a model supertanker in about 10 seconds.
The average shaft will spin about 5000 rpm as opposed to 250 ish on a medium speed diesel.
The rotation of a single screw shaft is usually designated by main engine direction so as to give a direct drive through the gearbox, astern given by gearing in an idler.
As Joe has said warships usually have outward turning props for speed as Warships tend to rely on tugs to berth.
The handling characteristics of a warship are designed for full speed where the positioning of the hull to give maximum Turret Coverage of a target. Not cost efficiency or low speed maneuvering.
With tank steering or mixers the effect of the overpowering on each shaft tends to negate any  low speed handling shortfalls.
This effect cannot be ignored, neither the efficiency of the propulsors which can vary from abysmal to fairly good.
You might have a good discussion with "Dodes" on this forum as he is a retired Master starting under sail (Thames Barges) through Steam to Motor ships.
His soapbox subject is overpowered models and handling.


Bob
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Re: Propeller Rotation & Configuration
« Reply #21 on: June 02, 2015, 10:04:07 PM »


LB who trust that nobody finds anything insulting, rude or unfair in the above.

                  :-))

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Re: Propeller Rotation & Configuration
« Reply #22 on: June 02, 2015, 10:24:16 PM »

I also know that the vessels used on the Birkenhead - Belfast ferries Mersey/Dublin Viking etc., are inboard turning
As they probably operate normally without tug assistance I'd think they might have controllable pitch props, in which case post #6 is applicable.

In fact CP lets you eat your cake and have it. You (can) have inward turning props for economy in forward, but also have inward turning in reverse for manoeuverability.

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Re: Propeller Rotation & Configuration
« Reply #23 on: June 02, 2015, 10:53:20 PM »

As they probably operate normally without tug assistance I'd think they might have controllable pitch props, in which case post #6 is applicable.

They are fixed pitched, scimitar shaped blades.

LB 
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Re: Propeller Rotation & Configuration
« Reply #24 on: June 02, 2015, 11:18:50 PM »

Well Marty.... :D....,20 responses later........it was an article on the Wartsilla  two stroke engines & fuel consumption of the latest generation of smaller Maersk vessels that included inward turning twin props............I guessed that fuel economy was the grail......but the article didn't cover the logic behind the propeller directions ........ Derek
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