Multihulls and Iceboats

Discussion in 'Multihulls' started by rapscallion, Feb 25, 2013.

  1. rapscallion
    Joined: Oct 2006
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    rapscallion Senior Member

    The Gougeon brothers were adapting what they had learned from iceboating to multihulls in the late 60s; with articulating floats, wave piercing bows, reduced windage, ect...

    My question is would there be an advantage to adopt the bendy DN rig to a boat like a Moth or a foiling catamaran?
     
  2. Doug Lord
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    Doug Lord Flight Ready

    I doubt it-you mean this kind of bend, right? Isn't the bend used to flatten the sail to the max or is there another purpose as well?
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  3. CutOnce

    CutOnce Previous Member

    I think Doug is right on this one. The DN was designed for a low drag/low friction ice environment and average speeds far in excess of the fastest water-born boats of it's day. I've sailed on DNs here and found the experience unbelievably different from skiff sailing on the water.

    C-Class Designers like Steve Clark already have access to all the accumulated rig design refinements of the past half century and if DN style rigs had promise, they already would have incorporated them. Both the Moth and A and C Class cats have a great population of risk-taking, innovative designers that are willing to take any design risk possible.

    --
    CutOnce
     
  4. rapscallion
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    rapscallion Senior Member

    Flatten the sail to the max? maybe...

    I was under the impression it optimized the angle of attack for the entire sail vs. just the sections with the max chord length.

    I was just thinking there may be other benefits, like lowering the center of effort of the sail, thus reducing the heeling moment - like a windsurfer who wanted to prevent power loss due to mast bend in the puffs could bow the mast to windward...

    I have no idea... it just is interesting to see tech from iceboats trickle into multis, and I wondered if this would have benefits as well. The fixed wing probably trumps the bendy mast, or does it?

    You guys are right... DNs are way faster than wet water sailboats... My point is foiling moths are catching up speedwise.. and maybe some tech benefits now overlap. I tried sailing a DN, and it was crazy! GPS said my max speed was 43.8 MPH. It felt like I was going over 100MPH. I couldn't figure out if I was thrilled, scared, or whether I loved it or hated it. The only thing I knew for sure was that I wasn't bored, and I was really cold.
     
  5. Doug Lord
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    Doug Lord Flight Ready

    Raps, one of the things with cats/tris and even Moths that is beneficial is quite different and that is heeling the rig to windward(or keeping it vertical on a cat or tri). On a Moth the windward heel of the rig is just a side benefit of Veal Heel where the biggest benefit is a large increase in RM for no change in weight. The pictures and video of Team Australia, the 60' ORMA tri, show the mast canted vertically or slightly to windward to avoid the loss of area that comes with heel.
    It seems that the DN mast bend would definitely have a side effect of lowering the center of effort, like you say, and maybe thats the key: with their great speed the important thing might be dumping power rather than increasing it like a canted mast can do. And I would imagine the rig ,bent like that would have a component that presses the boat into the ice? Heeling the DN rig to windward like a Moth probably would be harmful in unloading the skates where on the Moth it is beneficial in unloading the foils a bit.
    Interesting stuff..
     
  6. John Perry
    Joined: Nov 2003
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    John Perry Senior Member

    This is something I have thought about quite a lot and I actually made a rig with a pre-curved mast for a small sailing hydrofoil. It did work well.

    My reason for curving the mast was mainly to reduce or eliminate twist in the sail. With a straight mast, slackness or stretching of the leech of the sail results in twist in the sail. It is widely accepted that it is beneficial to have some twist in a sail in order to allow for wind shear, however I think it is fair to say that the twist in many mast mounted sails is far greater than needed just to compensate for wind shear. If the twist is greater than needed to compensate for wind shear then there will be a variation in the angle of attack over the height of the sail. Some such variation may be beneficial in that it can reduce induced drag and it can also reduce the height of the centre of effort in strong winds. However, one often sees sails with so much twist, even in fairly light winds, that the flow near the boom is likely to be stalled whereas the upper part of the sail is at such low angle of attack that it is doing next to nothing. In this situation there is also a large vertical airflow component which does not contribute to lift. In order to reduce twist with a straight mast, high mainsheet tension or boom vang tension is needed, applying heavy loads to the rig. If a way can be found to curve the mast to leeward (i.e. make the middle of the mast deflect to leeward) then twist can be controlled with much reduced loading on the rig since the leeward curve of the sail leech can be matched by a leeward curve of the mast.

    For the curved mast rig that I made, the mast rotated with the boom. Viewed in the direction from which the apparant wind is coming, the middle part of the mast did curve to leeward, although the curvature projected in this direction was much less than the curvature as seen from side on to the rig. To gain more effect from a curved mast you could make it 'under rotate'. By this I mean make the mast rotate relative to the boom in the oposite direction to the way that most rotating masts rotate (sometimes called over rotation). Clearly you would not want to use a conventional type of rotating mast section with 'under rotation', but you could use a round section mast, perhaps with a luff sleeve to fair it into the sail. For the rig I made, the mast section was slightly oval but a wide luff sleeve and camber inducers were used to produce a smoothly faired leeward surface to the sail. Another approach to achieving a leeward curve to the middle part of a mast would be to use a conventional wing mast, thin enough to be slightly flexible, and to apply differential tension to diamond stays to produce the required curvature.

    A point to remember with any scheme to apply bend to a mast is that a stayed mast is a compression member - if you deliberately make it bent you need to allow for that when thinking about the buckling load. Also its a good idea to design the rig so that as wind loading increases any leeward curvature of the middle of the mast tends to reduce, this will have the effect of feathering the sail and reducing the height of the centre of effort in strong winds.
     
  7. Submarine Tom

    Submarine Tom Previous Member

    I gotta jump in here and attempt to express the acceleration one feels when they pilot a DN in fresh breezes.

    It was absolutely mind blowing to feel the snap in my neck the first time I sheeted in.

    Of course the full face helmet only adds to the effect but the sensation is one I have never felt on the water.

    I had a 195 pound DN for three years when I lived in the great white north.

    Memories I will never forget.
     
  8. rapscallion
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    rapscallion Senior Member

    I never owned one, but my dad had a DN for 10 years or so. I only sailed it once... but it was awesome. The DN guys travel a lot in search of the perfect ice, and I don't know if I would actually do that... but I probably should... because winters are terrible if you are a sailor.

    I wish a guy like Steve Clark would tell us about his experiments with bendy masts if he actually tried them on a cat... but maybe a solid wing trumps a bendy mast... and I suspect a bendy mast isn't something one would use with headsails. If A cats ever get up on foils a bendy mast might be a benefit...
     
  9. Steve Clark
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    Steve Clark Charged Particle

    The DN rig is quite different to anything used on soft water, and it works well for reasons unique to iceboats. There have been rigs that used similar concepts tried on catamarans.
    The first thing that having the lower panel of the mast sag to leeward accomplishes is to have the leading edge sag match the leech sag. Thus eliminating twist and actually get the sail trimmed so hat it is more or less parallel to center line. The geometry of a DN platform is such that this cannot be achieved by any other end. In the C Class, Roy Bacon tried such a rig for several years on Hellcat. The mast was very bendy and sag was controlled by lower shrouds.
    The curved trailing edge on Austin Farrar's Lady Helmsman wing mast accomplished much the same thing. In that case, sail cloth was not stiff enough not to stretch at the high wind loads, so forcing the leading edge to leeward accomplished the job of getting rid of unwanted twist.
    Another advantage of mid mast sag is that you, in theory, can run the mid span of the sail at a higher angle of attack than the top and bottom. This should result in a net increase in lift coefficient without much increase in drag. I tried it on a uni rigged IC but found no particular joy. I may have been able to win if I had seriously redesigned the sail, but the middle stalled before the head and foot, so I had to revert to a more conventional bend profile.
    The other thing the DN mast sag accomplishes is true only for iceboats, the sail provides down force which adds stability and traction. This is desirable on ice because the drag does not increase significantly with down pressure. On the water, you would be driving the hull deeper into the drink, which is the opposite of what you want to do. I believe on dirt boats, the additional rolling resistance caused by the rig down force is enough to make the concept a net loser.
    SHC
     

  10. rapscallion
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    rapscallion Senior Member

    Did that just happen?

    Thanks Steve for the post! Your experience and insight is much appreciated.

    The downward force is a benefit that interested me. I was curious if it could be used to generate righting moment on a catamaran. If the rig produced righting moment, maybe a cat could carry more sail area in heavier air, and/or reduce the risk of capsize.

    I just find this kind of topic intensely interesting, and I very much appreciate your response Steve, thanks again.
     
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