Up wind sailing

Discussion in 'Sailboats' started by tscanlon08, May 29, 2007.

  1. tscanlon08
    Joined: May 2007
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    tscanlon08 Junior Member

    A general question about sloop rigged monohulls

    What factors in sail design maximize pointing ability and up wind speed?

    One other question, obviously off topic-

    What is faster, an asymmetrical or symmetrical spinnaker and in which conditions is each preferable?
  2. mgpedersen
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    mgpedersen Junior Member

    Upwind ability is not only about sails, but since you didn't ask, I won't tell :)

    I'm assuming you're talking about conventionally rigged boats with big things that stick up in the air and are supported by wires.

    Primary drivers are:

    Aspect ratio (higher is better)
    Windage (mast, rigging, hull, lower is better)
    Headstay catenary (less is generally better as the wind pipes up but the ideal amount also depends on the draft of the sail/sail design)
    Draft of the sails (finer entries are less forgiving but point higher)
    Pitching motion (less is better)
    Deck gap (less is better)
    Planform (more elliptical is better)

    I probably missed some.

    Most of these also apply for wing sails too (with the exception of catenary).

    As far as the assym v. symm, I think for equal area an assym is better on more reachy courses, and the symm for deeper wind angles (sailhandling aspects not considered).
    Last edited: May 29, 2007
  3. tscanlon08
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    tscanlon08 Junior Member

    In addition what hull shapes are most advantageous, narrower, deeper cutting hull or otherwise?

    Plus I may need more of an explanation of these ideas mentioned above as I am not familiar with all of the terms.

    Thanks a bunch
  4. water addict
    Joined: Jun 2004
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    water addict Naval Architect

    Asym spinnakers are "usually" better on light boats, better for reaching instead of running, better in light air.

    There are many other circumstances in the decision of asym vs. sym spins.
  5. mgpedersen
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    mgpedersen Junior Member

    Windward ability

    I thought you might get around to asking that :)

    Narrow hulls generally do much better upwind - the resistance in waves is, I think, primarily how much water you have to deflect sideways from the topsides. The wider the hull the more deflection, and the more energy it saps from your forward motion. That's why you see these extreme bows on modern boats with almost no flare - to minimize resistance in waves. Keep the weight out of the bow on these boats though :)

    One of the tradeoffs for race boats, at least, is the beamier the boat the bigger the effect from the crew sitting on the rail. So what you see is extreme flare in the topsides amidships, so the designer can keep the crew where he wants it but also have a narrower BWL (with less wetted surface). Narrow BWL generally means better handling characteristics. The additional deck beam also makes for a nice working platfrom so the crew can move around, and down below it makes the boat look bigger. Where is the sweet spot - I'm not sure it's well known and that's one reason why boat design is an art/science.

    For cruisers the crew on the rail isn't as big a concern. However more beam below generally results in a more open feeling and sells better. I personallly prefer a boat with a compact interior so when (not if) I get thrown around I don't have far to fall.

    The narrower boat feels more "tippy" and there are a lot of people who don't like that, or maybe I should say they prefer the alternative which is wide BWL the boat feels more solid at low heel angles (even though it's less safe in a big storm offshore).

    An even bigger contribution to windward ability comes from the keel. I know the guy who did all the wind tunnel testing for the 777 and I was asking him about keel design one day. He said it's all about span, and that's true with keels too. Go for as deep a draft as you can tolerate, and that will probably make the biggest difference in your windward perfromance. Shorter chord (fore and aft length) will make you more efficient too. Modern race boats have deep short chord keels and are tacking through 75 degrees or so when the conditions are right (not too choppy).

    Lastly, don't underestimate the effect of righting moment. All things being equal a stiffer boat is going to go upwind better - it can translate wind power into forward motion, while the more tender boat heels more and absorbs some of the wind energy.

    Heres some definitions of my previous post:
    Aspect ratio - ratio of sail height to foot length
    Windage (I think this is self-explanatory)
    Headstay catenary (catenary = sag). When the wind picks up you crank on the backstay, which reduces the headstay sag. When the headstay straightens out it flattens the sail a little bit, depowering the sail and allowing you to point marginally higher.
    Draft of the sails (how much does the sail curve, when looking down from the top). A totally board flat sail would have no draft, while a spinnaker has a lot.
    Deck gap (space between jib foot and deck)
    Planform (looking at the sail from the side, the "theoretical" ideal looks like a spitfire airplane wing)
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  6. mholguin
    Joined: Jan 2005
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    mholguin Junior Member

    Wouldn't the relationship between mainsail area and jib area play a role also? And I dare to add that also the placement of the rig (theoretical center of effort of the sail plan) with regards of the theoretical center of lateral resistance of the hull (including appendages) have a significant effect on upwind ability (based on my own non-scientific experiment with my self designed and built RC sailboat)....


  7. alan white
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    alan white Senior Member

    Balance between centers is important, the sail generally leading by a small percentage based on a lot of hull design factors.
    Upwind performance can be enhanced by a slight weather helm, the rudder slightly angled providing lift to assist the keel. The lead of the CE provides that weather helm.
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