Shaft Angle

Discussion in 'Boat Design' started by Mat-C, May 6, 2012.

  1. Mat-C
    Joined: May 2007
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    Mat-C Senior Member

    I've been looking at plans for a little 20-odd foot displacement powerboat. The plans show the shaft angle at something ike 15 degrees, which looks very steep.
    I have read that 15 degrees is acceptable, but that anything much over this will start to have a more serious impact on efficiency. Is this accurate? Is 15 degrees ok? (again - for a displacement power boat)
     
  2. masalai
    Joined: Oct 2007
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    masalai masalai

    Use an outboard, as the set-up, installation and running costs will be generally cheaper overall... Ideal is for the axis of the propeller to be horizontal and well immersed in a smooth water flow... (even for a displacement design - but will you find a high thrust large diameter propeller and suitably geared system (again an easy ideal would be 1000rpm at the propeller), - but most outboards develop peak torque at propeller revolutions triple or more that..... :eek: :p 15 degrees is OK, but 15 degrees from ideal :D :D
     
  3. SamSam
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    SamSam Senior Member

    What boat?

    If an inboard is not actually important, I would do outboard for ease of maneuvering alone, plus beaching, shallow water.
     
  4. PAR
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    PAR Yacht Designer/Builder

    An outboard can be raised, a straight shaft can't. All this aside, anything over 8 degrees and a serious drop off in productive output from the prop, becomes progressively more noticeable. 15 degrees is about the limit of what engine manufactures recommend and substantial thrust loses are occurring at this angle. So, to answer you, yep 15 degrees will work, but you don't want any more and if possible reduce this angle.
     
  5. tunnels

    tunnels Previous Member

    There must be some way of reducing the angle! take motor forward a little ! change the location of the prop shaft !use a vee drive !. But like has already been said fit a outboard and save room and a boat load of hassles . Most out boards these days have a charging circut and a resonably big alternater for charging with . something in the 20 to 30 hp is much lighter than the totall installation of a inboard of the equivlent horse power !!, a ouboard that size is also light enough to be bracketed and could be rise and fall so dosent have to be in the water all the time when not being used !!. theres more pluses then minuses !!:p:D:p
     
  6. Mat-C
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    Mat-C Senior Member

    Thanks guys - yes I agree an outboard would be a logical choice. I'm not planning on building the boat... they're just an old set of plans I cam across and noted the high shaft angle, so was just wondering aloud how this would affect efficiency.
    It was actually in Gerr's Propellor Handbook where I read about the 15 dgree upper limit.
     
  7. philSweet
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    philSweet Senior Member

    Okay, 20 odd feet displacement speed runs 10-20 hp installed. what is the point of this again? You've used more electrical power asking the question online than you will save in a year by perfecting this.:D

    The short answer is 15 degrees would not be measurably less efficient unless the prop is huge, say diameter more than .33 vessel beam. That would get the slip way down and then the shaft angle is more of an issue.

    It's the go fast guys that have to worry about shaft angle.
     
    Last edited: May 6, 2012
  8. Mat-C
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    Mat-C Senior Member

    I'm not trying to be difficult (really!:)) but what's a bloke to take away from these two statements?
    This thing swings what looks to be about a 16" prop on a beam of about 6 ft on the wl, so not overly large diameter. Clearly, a horizontal shaft line is going to be advantageous in terms of thrust efficiency, but what's the loss in having such a steep shaft angle?
    By my rough calcs, about 16% of the thrust is directed down instead of aft. Does that mean that it will thus be 16% less efficient at pushing the boat forwards?
     
  9. portacruise
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    portacruise Senior Member

    I think it should be less than that once the boat is in motion. Some small amount of the misdirected thrust will be lifting the stern, reducing water friction with the smaller waterline. If there is no skeg or a small skeg the drag losses for that part will be less than the lower unit of an outboard. There may also be other favorable water laminar flow effects that come into play at that angle, depending on speed.

    Porta

     
  10. Submarine Tom

    Submarine Tom Previous Member

    The less angle the better, but it may be a trade-off.
     

  11. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Phil is correct. On this boat, the speeds and prop slip aren't going to be significant enough to worry about. Again, 15 degrees is still pretty steep and usually the cut off point for standard setups, unless designed to handle steep angles.
     
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