Off-setting the outboard motor

Discussion in 'Boat Design' started by richardf, Aug 5, 2013.

  1. richardf
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    richardf Junior Member

    Here is a minor challenge for you designers out there which would yield useful information:

    Off-setting a transom mounted, tiller controlled, outboard motor one foot to the left of centerline and seating the operator one foot to the right of centerline allows a more natural movement of the tiller with the left hand (not so cramped against the back). What is the cost in propulsion efficiency or loss in forward thrust in doing this (assume a flat bottom boat)?

    If you assume a propeller thrust of X lbs acting one foot to the left of the center of gravity (CG assumed to be on centerline and the boat going straight). This would cause a right turning moment of X foot-lbs about the CG. To offset this and keep the boat going straight, a left X foot-lb moment must be applied by turning the motor/propeller the appropriate amount. This is straight forward vector analysis, but I am not up to carrying it out. I was hoping you would calculate it for me. Assume the CG is 10 feet forward of the propeller.

    How much forward thrust will be lost in having the propeller thrust vector off-set enough to counteract the right turning moment. It might be small enough to warrant the more pleasant action of steering an offset motor instead of having the tiller crammed into your back. Also, does this loss of thrust lessen as the boat length (or CG) moves forward (intuitively I would say so).

    Thanks,
    Richard
     
  2. Richard Woods
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    Richard Woods Woods Designs

    Probably more important is whether the tiller arm is port or std of the engine and whether the prop is RH or LH

    But certainly on my own small dinghies I offset the outboard - but then I only go at displacement speeds.

    Richard Woods
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Don't think too many outboards have left-hand rotation in tiller models, but should not make a difference if they did, that's what that adjustable little tab on the cav plate is for, to get neutral steering. Offset engines I have seen on fisherman's net boats, presumably so they can pull nets over the stern more easily. And there is a line of small catamarans with a single engine on one side that seems to work. But it is adding an unnecesssary complication without a very strong reason to do it. Much easier to find an engine with a central tiller if you don't like left hand version.
     
  4. messabout
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    messabout Senior Member

    Yes, an offset engine will introduce a force vector. I have done that more than a few times for the same reasons, without difficulty or excess steering input.

    The Cg of the boat is not as important as the center of lateral resistance for these calculations . Change of speed or planeing angle of incidence will move the CLR one way or the other. Vector analysis will therefore be problematic.

    Tiller steering, I would guess, means not much more than 20 hp. Small boat, planing, lightly loaded, you may notice a small difference in tiller feel. Larger boat, more heavily laden, not likely to notice much effect.

    So what if you lose one MPH of top speed in exchange for more comfort?
    Racing Hydros sometimes deliberately off set the line of thrust to compensate for prop walk. But the offset is usually very small. But they are looking for the last tiny fraction of top speed not comfort.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Things get complicated in waves, because the position of the CLR moves back and forth, then you have to constantly adjust the line of thrust or go off course.
     
  6. richardf
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    richardf Junior Member

    How about using this imperical method of determination:
    While at speed, note the angle subtended between the boat centerline and the tiller handle when the boat is running straight (no turning moment because the thrust line is through the CLR). Then 1-the cosine of the angle will give the percent decrease in centerline thrust. For 10 degrees, it is less than 2%. But because of the side slip component (pushing perpendicular to the boat centerline), you will have to crab the bow to the side of the intended track to make good the intended coarse.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Sounds impressive Richard, but I'm not sure that the term "bullsh*t baffles brains" applies to some of the wise guys here !
     
  8. richardf
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    richardf Junior Member

    :confused:
     
  9. alan craig
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    alan craig Senior Member

    Richard, your post gave me an idea which I will use on my own small boat which might fit your requirements:

    Mount the o/b on the centreline as usual and mount a seperate tiller, pivoting at the transom and offset to wherever it feels comfortable. It could be longer than the existing tiller if this helps fore/aft trim and could incorporate the throttle (possibly). All you need to do then is add a link parallel to the transom from the o/b to the new tiller.

    My own home made electric o/b already has a suitable point for the linkage; arranging this point might be problematic on your o/b.

    Edit: nearly forgot - the link would have to have ball joints or similar to allow the o/b to tilt without having to tilt the tiller equally.
     
    Last edited: Aug 6, 2013
  10. richardf
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    richardf Junior Member

    Alan,
    Great idea but I think overcomplicated for what I am trying to accomplish. I have a 10' punt that uses an electric trolling motor. I can't sit back by the transom or the bow is too high, so I sit amidships with an extension on the tiller handle. I offset the motor quite a bit to the left so that the tiller isn't in my back. It works really well.
    The 25 HP motor which is soon to be mounted on my 14' Seneca has the shifter built into the handle, which is already offset to the left of the motor centerline, but it would be more comfortable to have it even more offset to the left. Based on my calculations, the loss in centerline thrust will be insignificant, but I was hoping someone would back up my conclusion. I know the handle angle to maintain track will change with the boats speed when it is on the plane, but I doubt if it will be more than ten degrees, which isn't much.
    Richard
     
  11. Easy Rider
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    Easy Rider Senior Member

    A tiller extended that far is not very user friendly or even very usable other than going in a straight line.

    Put the woman fwd on the port side and sit normally to stbd aft w normal tiller.

    Rig a center helm fwd.

    Propeller torque is another thing to consider. With the skiff pictured I sit w my but against the stbd gunwale and w my wife on CL fwd. The boat still runs high on the stbd side w wife on CL. Would need wife further to stbd to level the boat. So on my skiff torque is a much bigger issue than asymmetrical thrust. But my engine is on CL.

    I'd be looking at a remote shift and a simple drum and pull pull cable system w a helm. Poor man's center counsel. Then sit off center to trim.
     

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  12. Peter Edmonds
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    Peter Edmonds Junior Member

    I have used offset outboard on my previous trailer-sailer, in common with many others in this boat space. Not an issue - particularly as most steering was by boat rudder.

    I am reminded of an off beat issue with offset outboard and twist grip throttle. I found that if I sat so "wrist up to slow" the manoeuvering was easy; "wrist down to slow" produced all sorts of unexpected results. I spotted this as a carryover from the motorcycle speed control convention.

    Centre of gravity doesn't really come into it. We are looking at propeller thrust force related to hull drag force - on centreline for unheeled hull.
     
  13. philSweet
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    philSweet Senior Member

    Peter, that brings back memories. In Key largo, if it was blowing 0-15 knots it was a motorcycle day. If it was blowing 15-25 knots, a sailing day. And I had set the boat up so I had to wrist-down-to-slow. I used to get in the boat, put the OB in neutral, rev the throttle three times, make a fist and pound my forehead three times, rev the throttle three more times, thump three more times, then take off.:D
     
  14. Gilbert
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    Gilbert Senior Member

    I would suggest you make the transom the right height for the motor all the way across (or nearly so) and you can try it in any spot that you would like to try. You will find the spot you like.
     

  15. jonr
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    jonr Senior Member

    > Based on my calculations, the loss in centerline thrust will be insignificant

    Actual data always trumps calculations. Try mounting it both ways and measure the effect on top speed.
     
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