Cox's Bay Skimmer

Discussion in 'Sailboats' started by Doug Lord, Jan 11, 2014.

  1. Baltic Bandit

    Baltic Bandit Previous Member

    please stop diverting this topic. this is not about your designs
     
  2. Skyak
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    Skyak Senior Member

    BB
    Selig was the name I was trying to think of. The S1223 foil section trimmed up (the trailing edge is way too fine) is what the latest DSS foil reminds me of. He did an impressive amount of work and is widely published. I suspect the similarity to this profile says a lot about the flow conditions the DSS foil experiences.

    Doug, BB is right I will reply to your latest post about crossbow on your crossbow thread. Regarding the skimmer adding a similar crossbeam ballast setup I think you need to prove it's worth first and there is no timing for your full size manned prototype. DSS is unproven on light shallow dingys and this ballast shifter is unproven anywhere.
     
  3. Doug Lord
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    Doug Lord Flight Ready

    ================
    If you say so. I've deleted the post that you were referring to and reposted it in the Crossbow fl thread.
    I have NOT suggested that Gary's Skimmer utilize the Trapeze Power Ballast System! Only that I would probably use it on my version(post 7,page 1).
    It doesn't matter that DSS is unproven on a small ballasted singlehander-the inventor of DSS has guided the essential elements of the design so that DSS will be very likely to work on the Crossbow fl design. It has been proven on shallow, high performance crew ballasted boats like Brace, Brace, Brace and the Quant 28 with the Quant 28 having an unmatched record against boats her own size and much larger.
    You're dead wrong that the "ballast shifter is unproven anywhere"-it has been used on test models over a ten year period(as shown in earlier in post 147 of the Crossbow fl thread) and is well proven and very effective on test models. http://www.boatdesign.net/forums/sailboats/crossbow-fl-43615-10.html
     
  4. Baltic Bandit

    Baltic Bandit Previous Member

    Doug you are still derailing this topic. This isn't about your Trap-X-Wing-Tie-Fighter whatever.

    Its interesting that when Gary was still active in this thread you were telling him that it was a proven design for small light dinghies. I'm confused.

    in any case doug, I've reported this disruption to the moderator as well
     
  5. Gary Baigent
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    Gary Baigent Senior Member

    BBandit, it's not MY thread - I'm a "democratic" anarchist; if you blokes want to carry on into differing design aspects - fine. All is interesting, new stuff uncovered.
    I would like to fit DSS and try it out on an old laser ... but have a number of things on my plate at moment - and don't really want to butcher the Skimmer because the boat is what it was designed for.
    Need to start afresh imo - after learning from smaller boat.
     
  6. Baltic Bandit

    Baltic Bandit Previous Member

    Well I keep getting corrected by Doug in other threads to stay on topic. Only to have him be a disruptive influence in these threads. I think an old laser is the right place to try out DSS. I suspect it will not work too well. because you just don't get enough immersion and Lasers are slow with more than 5 deg of heel.

    I too would be very leary of butchering a beautiful boat like yours just to try out an unproven concept. I suspect what we will find over the long run is that DSS works great on keel boats where it is a RM Enhancer, whereas in non ballasted boats, its going to be less useful
     
  7. P Flados
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    P Flados Senior Member

    Skyak,

    FYI, someone pointed the S1223 out to me when I was was working on an idea for a "reversible camber" section with both high lift and a thicker section towards the front for structural needs. I remember reading that some tried this section with a more reasonable thickness and less curved trailing edge and were happy with the results.
     
  8. Skyak
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    Skyak Senior Member

    Based on what I have read s1223 should be a winner when you are looking for maximum lift at low speed and sailboats are low speed. The reasons I didn't go further and try to use it are
    -the high angle of attack -a sail needs a plan for lower angles and I am skeptical that my attempt will will not stall early
    -the high curvature of the windward face -I don't know how important it is but other wing profiles have easy flats -this is kind of twice the work.
    -the low volume and high camber makes the structure difficult to execute on a high aspect foil

    For these reasons I didn't plan to use s1223 but I did plan to mess with it in Xfoil to see how much performance I could retain on an easier structure. It appears to me that Hugh might have done exactly that in his DSS foil -which makes it extra interesting. The other interesting aspect is DSS operation at less than 2 cord lengths to the surface and above the surface. I don't have any good data in this region, Hugh must. What I would like is to contrast the DSS profile with a common asymmetric daggerboard from say a mini transatlantic -what is the difference for operating near the surface. They are doing a DSS mini. Maybe it will have both profiles.

    As far as what can be done to produce a reversible S1223 profile sail rig it looks like it might be replicated (very roughly) with a mast nose with two soft sections and a single cloth leach. For non reversing I moved on to an easy more conventional profile with a slot to avoid stall at high AOA.
     
  9. Baltic Bandit

    Baltic Bandit Previous Member

    One of my big concerns about using DSS on a high performance small boat is the Yaw couple it drives. On the 49er, it was death to dip the leeward wing at speed, because it would cause you to cartwheel because of the Yaw force couple.

    if you can insure that the DSS stays immersed - as on a keel boat, or if you have a fairly massive gyradius in your yaw direction (WOxI) then its not a problem. BUT, if you are lightweight and the DSS is very very near the surface, as you break out a wave and then hit the next one I can see a nightmare of helming beginning.
     
  10. Skyak
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    Skyak Senior Member

    Diagonal stability is the limiting factor once again in high performance sailing. I imagine that even if the foil comes out briefly it aerates for a significant period. Good point.
     
  11. Doug Lord
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    Doug Lord Flight Ready

    ==========
    Sorry, but it's not a good point at all-it's the same argument people used to make about the lee hull on a trimaran making it hard to steer-just not true*.
    And in all the literature about DSS no such steering problem has ever been reported-at least that I've read. And in all my conversations with Hugh Welbourn he never mentioned it or anything like it. And Michael Aeppli, developer of the Quant 28 and Quant 30 DSS boats has never mentioned it to me or in his book (that I have a copy of). A "real" problem like that would be impossible to keep secret. So it's just uninformed conjecture.....
    *Old argument against flying the main hull of a high performance tri with a daggerboard and rudder on the main hull.
     
  12. Baltic Bandit

    Baltic Bandit Previous Member

    doug - you have never ever sailed a high performance skiff. So you really don't have any basis for making your comment.

    But to offer you an opportunity to learn, consider this. The ideal aspect ratio for a multi hull is 22:1 (derived empirically by the Gougeon bros), And most performance catamarans are damned close to that. Sure a Trimarans main hull typically is more like 15:1 but its still slender and long.

    such shapes in water have a fairly strong anti-yaw couple built into the shape. So much so that many trimarans go with high prismatic in their hull shape just to make tacking easier.

    Now compare that to a 49er (2.8:1) or 5oh (2.7:1) or Musto (2.4:1) or Gary's boat (2.44:1). Notice how there is roughly an decimal order of magnitude difference in the aspect ratios of the hulls Notice also that the wide flat sterns of these planning boats also don't have much resistance to yaw forces.

    Essentially if you are using a multi-hull's yaw resistance as an example of the yaw resistance of a planning skiff - well poorly informed is the term that comes to mind.

    And that the man pitching this approach didn't inform you of this weakness really doesn't lend your argument credibility one way or the other. And the Quant 28 is a keelboat. in case you missed it, we are talking about shallow draft dinghies here.

    Doug if you don't understand the issue and have no practical experience in sailing such boats - I strongly urge you not to make claims that you cannot support
     
  13. Gary Baigent
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    Gary Baigent Senior Member

    Playing with Freeship. Been asked to design and build a slightly larger Cox's Bay Skimmer-type for a well known older Auckland yachting personality, Colin Frankham - so have produced this 580 version. He wants to singlehand and a small cuddy area under which to sleep - but wants speed and shallow draft and likes the old time dreadnought bow. I'm thinking of una rig, small chord wing mast of maybe 7 metres height and square top main - plus water ballast tank for taming it down in fresh conditions.
    Also have done a more extreme DSS type Skimmer 6.50 metres with narrow waterline, two 7.5m wing masts like the original; large sail area, narrow waaterline, DSS power plus wide beam. Just a daydream.
     

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  14. Doug Lord
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    Doug Lord Flight Ready

    Big Skimmer

    Gary, thats fantastic! Will you build it or someone else?
     

  15. Gary Baigent
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    Gary Baigent Senior Member

    I'll do most of the building but Colin wants to help at times.
    Now, after thinking overnight, he's decided doesn't want the reverse bow. No problem, easy change electronically (not so in build reality) to change to vertical - or a variation of both.
    Because there will be a low cuddy with mast stepped on forward top, there will be room enough (for crossing during tacks/gybes) to lift cockpit floor - on Skimmer it is a little too low, can carry some water in lumpy condtions.
     
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