Depowering racing boat

Discussion in 'Sailboats' started by JackW, Feb 20, 2007.

  1. JackW
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    JackW New Member

    I am not an engineer but I would appreciate some informed opinion on this subject. Maybe you can help me get around the course a little faster.

    Conventional wisdom is that as your boat begins to be overpowered you start inducing twist in the sail plan spilling air from the upper part of the sail. Recently I have heard some advocating 'slopping' the rig, that is loosening the shrouds and allowing the entire rig to lean off to leward. The advantage of this I am told is that you don't need to twist the sail as much which reduces induced drag.

    Fact or fiction? Thanks guys.
     
  2. jim lee
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    jim lee Senior Member

    I donno'..

    When I want to depower my sailplan, I don't twist it. I flatten out the chord of the sail giving a shallower, higher speed airfoil. (Mostly by bending the mast.)

    I was told that you twist your sail to match the angle of attack for the apparent wind as it changes from the surface of the water up to the top of the mast. That made sense to me so it kinda' stuck.

    But of course, whenever someone beats me on the race course, all my theorys are brought into question. :)

    -jim lee
     
  3. national
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    national Junior Member

    flatten the sail first, using rake and cunningham. Remember you should also try to flatten the foresail if possible, by changing the sheeting angle or cunningham if you have/can fit one. Excessive twist is more of a survival tactic or used when in heavy seas .
     
  4. kenJ
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    kenJ Senior Member

    Sail Trim

    Take a look at www.Catalinaowners.com select message boards from the menu bar, then expert forums, then sail trim. Do some reading of the archives and then start asking questions. Or you can just buy his sail trim chart or book from the chandlery or West Marine/Boat US store to get a better understanding of sail trim. Don't feel alone, until the light comes on it is a confusing subject.
     
  5. Eric Sponberg
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    Eric Sponberg Senior Member

    I am not a racer, rather I enjoy cruising much more. But the matter of overpowering and sail twist is of some concern to cruising sailors. I found that playing with twist to spill excess wind is quick and easy to do and gives you some immediate control over momentary gusts. Slopping the rig to me seems more complicated. I would imagine that in racing, the time and ease in which to do something is important.

    One thing that you can notice in controlling twist (pulling up on the boom topping lift in a gust, for example) is that the overall weather helm lessens through the maneuver, the load eases up on the tiller or wheel--you can actually feel it, so overall drag of the boat is less. Boat speed stays up or increases accordingly.

    On my free-standing mast designs, we build the appropriate amount of deflection into the masts so that they will bend to leeward and spill the wind at about the right wind speed, consistent with the boat's stability. My clients have found that when the masts do this--depower through gusts--the handling of the boat is particularly hands-off (don't have to do anything, the masts bend all on their own) and the boat simply powers through, getting a jolt of speed with each gust and no increase in heel or tendency to round up. You can see an example of this on the mast I designed for a Spenser 42 called "Copernicus", which is featured on my website:

    http://www.sponbergyachtdesign.com/Copernicus.htm

    I hope that helps.

    Eric
     
  6. Dan78
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    Dan78 New Member

    Hello all, I am about to start designing a sport dinghy and I am wondering about the implication in using trapeze on a free standing mast?

    Thank you ...
     
  7. Eric Sponberg
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    Eric Sponberg Senior Member

    A trapeze will put a tremendous amount of extra load on the mast, so its strength and stiffness have to be modified accordingly. The mast is always going to bend some amount, and that is definitely going to affect the set of the sails along their luffs (assuming you are going to have a main and jib). A free-standing mast with a trapeze is also going to experience accelerated fatigue at the partners, so it would have to be particularly beefy in that area. On a trapeze rig, with standing rigging, the nice thing about stays and shrouds is that it keeps the mast very stable when sailing, and a lot of the trapeze load is taken by the shrouds.

    Eric
     
  8. water addict
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    water addict Naval Architect

    Unfortunately, every boat is different, there are no definites as to what is faster for a given boat. Some methods are counter to conventional wisdom, but work.
    If you are sailing one-design, it is a game of trial and error, talking with successful class member and pacing with other boats. Practice for ever and ever.

    PHRF- buddy up with your local handicappers. Buy them drinks, give them gifts at Christmas, pay for their kids braces and/or college. Sad but true.
     
  9. Dan78
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    Dan78 New Member

    Thank you for your advise Eric. The boat will have a cat rig and probably a wishbone boom.
    Do you think as well that a trapeze would also affect the ability of depowering under gusts? Because of the way the mast would deflect?
     
  10. Dan78
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    Dan78 New Member

    Thank you Eric for your advise. The boat will have a cat rig and probably a wishbone.
    Do you think as well that a trapeze would affect the depowering under gusts, because of the way the mast would deflect?
     
  11. Eric Sponberg
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    Eric Sponberg Senior Member

    Dan78,

    You would want to attach the trapeze about 5/8s to 3/4s the way up the mast so that the top of the mast is free to deflect in gusts.

    Eric
     
  12. PI Design
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    PI Design Senior Member

    As Eric says, attach the trapeze about 65-70% of the way up and the part of the mast above here will act the same as for a stayed mast. However, if you trapeze off an unstayed mast you will need a very stiff and strong (hence heavy) mast below the trapeze wire attachment point and even then it will bend to windward due to the load in the wire. Sorry, I don't think it will work.
    Caveat - I have a three week old baby so am somewhat sleep deprived. I am therefore not responible for any nonsense that I write!
     

  13. CT 249
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    CT 249 Senior Member

    And of course, you can run into *******'s Law.

    This is where the wind picks up, so you slide out into trap. The compression from the trap wire then bends the mast, depowering the rig. Because you have lost power, you then come in off trap.

    Once you come in off trap, the mast straightens out and powers up the sail. You then, of course, jump onto trap again. This flattens out the rig, and you move in again.........
     
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