Twin Propellor counter rotation?

Discussion in 'Powerboats' started by Louis.howe, Aug 13, 2013.

  1. Louis.howe
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    Louis.howe New Member

    I have a twin screw, outdrive boat which is about to be launched for the first time in my possession after years of work. I am confused as to which way my props should be turning in forward gear and why? Currently port goes clockwise and starboard goes anticlockwise. Looking at the back of the boat when ashore.
     
  2. rasorinc
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    rasorinc Senior Member

    That is the way they supposed to turn. opposite of each other.
     
  3. Eric Sponberg
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    Eric Sponberg Senior Member

    Propellers in twin-engine boats are set to turn in opposite directions so that the torque created by each balances the other out. If both propellers turned in the same direction, you would feel it at the steering wheel--you would have to counter the torque by steering constantly in the same direction. That is, if both props turned clockwise, the stern would always want to walk to starboard, so the bow would go to port, and you would have to apply pressure to the wheel to starboard to counteract the prop torque. The opposite is true if you had two counter-clockwise props, you would have to constantly steer to port.

    It is unusual (although not unknown) that you have the rotation that you have. Usually, the props are to turn with the top of the prop disc going outboard, so the starboard prop would turn clockwise, and the port prop counter-clockwise.

    I hope that helps.

    Eric
     
  4. MikeJohns
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    MikeJohns Senior Member


    Rotation can be either what we call in or out ( viewed from astern looking at the top of the prop). Posting so far has covered prop walk but not the reason for inboard or outboard rotation.


    Generally from memory Outward turning props produce a bit more stern lift, inward turning sink the stern a little. You'll find both configurations and there is no hard rule it really depends on the hullform and usage.

    If you find the boat hard to get onto the plane ie hard to lift the bow then generally turn them inward. There can be exceptions as it depends on the whole wake field/hull form.
     
  5. Carioca
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    Carioca Junior Member

    I recall reading up an article authored by Dave Gerr in Professional Boatbuilder dealing with this subject:

    If the props are to turn in - or out - has to do also with the distance to the keel (and or c/l) of the prop drive shaft and the vertical rudder shafts.

    If my memory isn´t failing me.... should the rudder shafts be 'out' and the prop shafts 'in', the props turn out, for maximum rudder effect.

    And conversely.
     
  6. Eric Sponberg
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    Eric Sponberg Senior Member

    The OP said these were I/O drives, so rudders are not in the problem.

    However, in a twin inboard set-up, if the props turn outboard (as defined above, standing at the stern looking forward, direction at top of prop disk) then the rudders should be set just outboard of the prop shaft centerlines. Dave Gerr recommends maximum 2.2 prop shaft diameters that the centerlines of the rudder stocks be offset from the centerlines of the prop shafts. If the propellers are inward turning, then the rudder shafts are set inboard of the prop shaft centerlines.

    Eric
     
  7. DMacPherson
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    DMacPherson Senior Member

  8. Louis.howe
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    Louis.howe New Member

    Thankyou all very much for your replies. You have been very helpful in explaining this to me in such great detail. Consensus is if I have trouble getting into the plane then change the direction of rotation so they turn out. As the boat has only just been launched and I have a slight pinking issue on my starboard engine I will sort this out and try her at speed, if she struggles I know what to do.

    Thank you all very much.
     
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  9. rasorinc
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    rasorinc Senior Member

    You are very welcome. Isn't it great to have a forum like this with highly experienced people willing to assist all members with their knowledge. I consider it a great resource and am thankful to all.
     
  10. Carioca
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    Carioca Junior Member

    Indeed.
     
  11. mich mechanic
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    mich mechanic New Member

    Twin Engine Boats

    When working on a twin engine boat, where one engine turns one way, and the other engine turns the opposite way, DO NOT USE STANDARD PARTS. the engine that turns opposite has the crank, cam, dist., and a few others ground backwards. This makes a big difference as if you look closely, it'll take a microscope, you'll see what looks just like a file with the teeth bend over. when used in the proper rotation is used they will lay down with use. Use them in a counter rotating motor and they will act like a file and bearing life will suffer greatly. If you need the crank ground, for instants make sure the machine shop can reverse grind. Same with the bore reamers for bearings. All rotating parts are reverse ground. Hope this helps, Randy
     
  12. 7228sedan
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    7228sedan Senior Member

    That depends... Most current production sterndrives handle the prop rotation in the gear box and all use standard rotation engines. Older units may have used reverse rotation engines however. If the OP could post the year, make, and model this could be easily confirmed.
     
  13. Westfield 11
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    Westfield 11 Senior Member

    Ditto. I thought that marine gearboxes did not care which direction of rotation moved the boat forwards: you just selected R/H or L/H props as needed for best performance and handling.
     
  14. 7228sedan
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    7228sedan Senior Member

    There was a time when marine engines were manufactured to turn in opposite directions in order to obtain counter rotating props in twin engine vessels. This posed obvious manufacturing concerns as specific internal components were required in order to accomplish this. In order to lower cost, the gear boxes were modified to handle the LH vs RH rotation leaving all engines to spin in their standard automotive rotations. This allowed the use of standardized long blocks across the board.
     

  15. Yobarnacle
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    Yobarnacle Senior Member

    There are a few advantanges to having inboard turning props. Ie: RH prop on port and LH prop to starboard. The advantages are more speed, less wake, and the ability to walk the boat sideways.
    Before the advent of jet boats, most oilfield crew boats had inboard turning wheels, for the above reasons.

    The ability to walk sideways against beam current and wind is a necessary ability, in order to remain in position under the crane hook. A heavy lift uses the slow block. The boat cannot be moving around beneath the load, or somebody gets killed. This picture is a calm day, but the rig expects to load and offload in 8 ft seas. Common in the gulf during winter months.
     

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