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Twin screw handling question?

Discussion in 'General Yachting Discussion' started by Fishy, Sep 7, 2011.

  1. Fishy

    Fishy New Member

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    Hi , I have a question for the seadogs that have more varied twin screw handling experience

    I have served on a small number of vessels for long periods , some single screw , and am inter4ested in why "walking" some twin screws are almost opposite

    EG wanting to walk to port ,rudders 5 stbd , Port eng ahead , Stbd eng astern with slightly more refs on Stbd

    On another vessel to walk to port , rudders stbd 15 , Port eng astern , Stbd eng ahead , again slightly more refs on the astern engine

    On both vessels amount of bow thruster use was similar
    I am purposely not divulging information on the type of vessels as i am trying to understand what is the influence in general and not on these specific vessels

    Is the latter a more common technique on commercial vessels?

    They were vastly different vessels , but how do you approach a new vessel with a technique that can be applied to a specific type of vessel ?
  2. K1W1

    K1W1 Senior Member

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    Hi,

    Which way did the shafts turn when going ahead on each vessel?
  3. Fishy

    Fishy New Member

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    Hi Kiwi

    Putting me in a corner so quick , the second one was outward turning props
    Unfortunately I am hazy as to the first one , was a busy charter boat that had to many issues to focus on , needless to say I didnt last long there

    But you have me scratching my head , could it be such a simple explanation, Feeling kinda silly for not considering it:eek:
  4. Fishy

    Fishy New Member

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    Which brings up another question , I was under the impression that outward turning props are more efficient , what would be the benifit of having inward turning props?

    Either way it brings up a whole new avenue of goooogling
  5. AMG

    AMG YF Moderator

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    I think it is when both props are rotating clockwise, the boat walks to starboard, and the opposite of course.
  6. Fishy

    Fishy New Member

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    I was referring to purpose walking
    After kiwis response and some searching it seems the consenses is that outward turning props will walk (the vessle not just stern ) to port with STBD engine astern and PORT ahead

    This is counter to what I have experienced but I am assuming the fact that it was variable pitch proppellers have something to do with it and my initial confusion
  7. AMG

    AMG YF Moderator

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    Me too.
  8. Fishy

    Fishy New Member

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    Sorry getting what you refering to and now makes perfect sense
  9. Capt J

    Capt J Senior Member

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    Hehehe, I used to do this all of the time on a few 95-105' Browards I used to run as well as others. But on the Browards, I could walk the boat perfectly sideways using just the motors and no bow thruster (they didn't have one on one, and the other wasn't operational). It only works on hulls that have mostly flatter bottoms. A really deep draft/ deep vee vessel won't do it. Here's the way it works.

    If you want to move the entire vessel to port. You would turn your steering/rudders to starboard, put stbd engine in foward, port engine in reverse. What's happening is that splitting the engines in that fashion pushes the bow to port, but the propellor WASH going over the rudders is also pushing the stern to port because the rudders are directly the propellor wash to starboard therefore pushing the stern to port. It has to do with the wash over the rudders, not the rotation of the propellors. I usually turn the rudders nearly hard over when I do it. If the rudders are straight, only the bow will spin, not the entire boat.
  10. AMG

    AMG YF Moderator

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    So how do you explain the difference in post nr 1?
  11. K1W1

    K1W1 Senior Member

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    Hi,

    I am interested to know that too.

    Inboard turning going ahead is actually slightly more efficient than outboard turning. It does make slow speed manoeuvres a little different till you get used to it though.

    A lot of naval and commercial vessels are inboard turning going ahead, some will also have a Counter Rotating engine so the crankshaft turns opposite to the prop shaft direction of rotation going ahead.
  12. Kafue

    Kafue Senior Member

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    Great thread, thanks to all.
    I have prided myself in landing the boats I have owned without any problems by keeping the rudders straight ahead, not touching the wheel, and just using the throttles and PATIENCE. No bow thrusters on any except one, which failed too often to count on.
    Ofcourse, these are not the size vessels you guys are referring to. Average probably 50ft.
    Now all these years later I learn about the rotation of the props.
    Headed to my Hatteras shortly to check the rotation and the method.
    BTW, with the big keel and heavy weight of the Hatt, she is a pussycat to manoeuvre, unlike my Ocean Alex 50 Mk1 with 120 Ford Lehmans which was a b i t c h! She had to be steered to dock.
  13. Fishtigua

    Fishtigua Senior Member

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    For a real learning curve, try going stern-to in a 1960's 88 foot steel trawler yacht with 2 old Gardeners with about as much power as an asmatic hampster fallling of its wheel. Then for added fun, just have one center rudder.

    No thrusters, no big dabs of power and a hell of a lot of planning.

    Ooh, and lots of fenders. :D
  14. Capt J

    Capt J Senior Member

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    I cut my teeth on a 97' Canoe stern MY just like you're talking about. No bow thruster, 8.5' draft, steel, center rudder, and twin 250hp Gardeners. Lot's of planning and using the current to your advantage, and don't forget the mandatory spring line.
  15. Capt J

    Capt J Senior Member

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    The depth of the V if any will account for how much rudder you need and how far foward the running gear is. You can do it with the rudder hard over on any of them, but then the stern might come in faster than the bow etc. The 103' Broward I ran had 3 extensions put on her and started life as a 76' I believe origionally. with the running gear near midships and the entire back half of the boat was flat bottom, she liked the rudder hard over. The 98' Broward still had a relatively flat bottom in the back half but the running gear was near the stern, she liked less rudder........the correlation of how far foward the props and rudders are, rudder size, and draft and bottom configeration have a lot to do with it. If the boat is a deep V, it generally won't maneuver like this because it cannot overcome the depth of the V and also draws the water from foward more than from the side.......I'm not totally sure.......I do know that with a flat bottom such as those Browards, and something like a dinner party boat, will walk completely sideways without a bow-thruster using this method.
  16. AMG

    AMG YF Moderator

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    OK, but the question was why some boats walk to port with the props running ahead and astern and other do it with the props running astern and ahead?
  17. Capt J

    Capt J Senior Member

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    without a bow thruster- the only way to do this and move the entire vessel to port would be to have the rudders to starboard, stbd engine in foward, port in reverse.

    With a bow thruster- rudders neutral, port engine fwd and starboard engine reverse would move the stern to port and the bow to stbd (basically twisting the vessel in a clockwise motion), and you would use the bow thruster to port to overcome the propulsion twisting the bow to starboard and move it to port using the power of the bow thruster. With a deep v or full keel, this would be the only way to move the entire vessel to port.
  18. AMG

    AMG YF Moderator

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    Well, the Original Poster had a bow thruster and got this working on one boat and not on another where it was the opposite, so obviously nobody knows why...
  19. Fishy

    Fishy New Member

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    Thanks for all the info , the difference in the 2 vessels was the variable pitch props on the one

    Anyone with more experience on VPP ? But sounds like the most plausible explanation for the difference in manouvering

    That being said , googling will give you many different oppinions , I have come to this conclusion , opinions welcome

    Wanting to walk to port on outward turning props -Port ahead Stbd Astern
    Inward turning props Opposite?
  20. K1W1

    K1W1 Senior Member

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    Hi,

    I have sent your question and CPP bit to a former master I have worked with a lot.

    He has a huge amount of ship and yacht handling experience with inboard, outboard and CPP Systems.

    If he answers I will post that here.