Gap between luff and mast, effect on efficiency?

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Aug 29, 2020.

  1. dustman
    Joined: Jun 2019
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    Location: Tucson, AZ

    dustman Junior Member

    Does the gap between the luff and the mast have a significant effect on efficiency? Does it matter enough to try to eliminate or minimize it?
     
  2. messabout
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    messabout Senior Member

    That question implies a lot of variables. In general the answer is yes, no and maybe.. But all that depends on the sail itself, the shape or the mast, and the extent of the gap between the two.
     
  3. dustman
    Joined: Jun 2019
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    dustman Junior Member

    Let's say 4" round mast, 2" gap. Not sure what part the sail itself would play, but planning on going with a gaff rig if that helps.
     
  4. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    2 inches is more gap than I've ever seen. One half to one inch is typical.

    I believe that the turbulence behind a round mast is two to three times it's diameter.

    Your well close enough
     
  5. philSweet
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    philSweet Senior Member

    There will likely be additional spar drag at low wind speeds. But as long as you aren't in a race with a sail area measurement rule, I wouldn't worry about it. The sail efficiency isn't much changed, and you now have an additional side of the sail that can be tweaked via robands, lashings, or hoops.

    With loose luffs, it's better to let the entire sail set on the lee of the mast and not get pulled aft with an outhaul. Loose footed sails supported by low sprits or wishbones are helpful in getting the luff 'round to the lee. The mainsheet on a wishbone or sprit usually pulls forward so that the angle to the sprit/wisbone is less than 90 degrees. This is to counter the outhaul effect of the sprit/wishbone. The adjustable sprit/wishbone lift line allows you to manage this in different conditions. Remember, the sail is trying to pull the boat forward, it wants to run past the mast even when sailing to windward. Let it do that. That's the easy way. It lowers the overall load on the mast and lets you use a smaller and lighter one.

    From the days before social distancing - these guys know what they are doing -

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  6. philSweet
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    philSweet Senior Member


  7. dustman
    Joined: Jun 2019
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    Location: Tucson, AZ

    dustman Junior Member

    Thanks for the good input guys. What I think I'm hearing is that it's not a big deal.

    Sail size, shape, and aspect ratio seem to be the most important factors to consider as far as performance.
     
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