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Author Topic: Prop Rotation  (Read 1253 times)

CGAux26

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Prop Rotation
« on: June 09, 2018, 05:37:52 PM »

There are frequently threads with the central issue being which way props should turn, especially on twin screw boats.  For those who only go ahead except to stop, you may stop reading here.  If you do a lot of maneuvering, towing, docking, etc. this is for you.

Prop walk must be considered in setting up prop rotation.  An article is included that explains prop walk rather well.  So I am saying that outward-turning (going ahead, facing forward looking at the props) provides better handling.  I am not good at diagrams, so just imagine a twin screw boat with outward-turning props that needs to spin left on its beam ends.  Put the rudders hard to port, starboard engine ahead, port astern.  The bottoms of both props are now moving left, acting like car tires on the road to move the stern to the right.  Voila! A much tighter turn than if you did the same thing with inward-turning props.

https://www.sailmagazine.com/cruising/walking-the-prop

To make yourself a believer in prop walk take your single screw boat out and stop it dead in the water.  The give it a quick goose of throttle in reverse, then stop.  the boat's stern will swing left if it has a right-hand prop, or it will swing right with a leftie.  Again, the effect is caused by the bottom blades act like car tires.



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meechingman

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    • Andrew Gilbert
Re: Prop Rotation
« Reply #1 on: June 10, 2018, 02:03:09 PM »

Agree 100%.


My father's 1:1 tug Meeching was twin screw, twin rudder. Screws turned outwards. As a youngster I spent days and days on board in the holidays and Dad, the other skippers and the mate taught me how to handle her, how to tow and much more. (It was assumed that I'd follow in the family tradition and probably end up as a tug skipper!) You could turn her on the spot with ease as described above, but you probably didn't want to use full rudder. You got to a point where adding more rudder seemed to weaken the turning motion. How that relates to model sized craft I don't know, I'll leave that to those with degrees in hydrodynamics!


I have two twin screw model tugs with independent control of motors. The bigger one has outwards turning screws but they're in Korts and I'm sure that negates some of the prop walk. But she'll still turn on the spot with one motor half ahead, one half astern and half rudder in the direction of the turn. The smaller one is Meeching, scratch built (very well!) by a modeller going from photos only - so he had no information of what she was like underwater. The screws are inwards turning on her but she still manages to turn well. As the shafts and props are out for a clean and lube, I'll swap the props over to match the real thing and see if it makes a difference.
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Admiral of the Haven Towage Fleet.

CGAux26

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Re: Prop Rotation
« Reply #2 on: June 10, 2018, 03:19:19 PM »

I will want to hear how your Meeching turns once you reverse the rotations.  I have no experience with korts, but I figured they would largely cancel prop walk.


My experience is a lifetime in 1:1 pleasure boats, up to a twin screw Grand Banks 42.  She had small rudders so more turns of the wheel were necessary to get a tight turn.  I also had an Albin 32, single screw with a long keel.  I chose a slip to keep her in so I could dock on the starboard side, since she had a big left hand prop.  In models, I have Springer that will turn on its own length with single screw and a fishtail rudder.  It backs severely to the right in reverse (LH prop), being short with no keel.  So I love the starboard side docking in steering competitions.
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meechingman

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    • Andrew Gilbert
Re: Prop Rotation
« Reply #3 on: June 10, 2018, 04:01:56 PM »

I'll let you know, but with students' exams coming up it may be the summer before I get her back on the lake.


Single screw tugs - usually J P Knight's Kent or Kite - were always supplied as relief vessels when Meeching was up for refit and survey. So I got to handle them as well. My Dad was well used to them. You knew what they would do, ie left hand screw - ship's head will go to port when going astern. So you could take advantage of that when you wanted to do a snappy 180 in the river. Dad and I would usually do it as a 3 point turn. One of the other skippers had never handled a single screw ship in his career (drafted in from the passenger and car ferries) and Dad and I had to hold in the giggles as he took 9 goes to turn Kite around, fighting against the natural turning movement every time! :)


I haven't been a member of the local club for some years (more work commitments :( ) but did enjoy the steering contests. I have a small single screw tug with a Kort and fishtail and she's marvellous. 3rd place on my first go and 2nd on my 2nd. Would have got 1st on the 3rd contest, apart from one late entry with a small lifeboat that I'll swear had rudders and props all the way around, he was so darned good! :D
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Admiral of the Haven Towage Fleet.

dodes

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Re: Prop Rotation
« Reply #4 on: July 25, 2018, 07:56:55 PM »

We only had one vessel in the RMAS with a Kort nozzle and that was the Newton, one of the best handling single screw boats we had. When you went astern you simplyput the wheel hard over opposite to the direction you wanted the stern to go. The nozzle concentrated the direction of the prop wash, rather than relying on paddle wheel effect or rudder wash.
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