Daggerboard turbulence and rudders

Discussion in 'Multihulls' started by rayaldridge, Jun 18, 2011.

  1. rayaldridge
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    rayaldridge Senior Member

    In my new cat, I want to place the daggerboard box through the cockpit side deck, for several reasons. This seems to be possible, since I also want the rig to be well aft-- the mast is stepped at slightly more than 50 percent of the waterline aft. Presently, the leading edge of the daggerboard is about 8 feet forward of the transoms, on a 23 foot cat.

    I'd like to hear opinions on whether or not the turbulence from the daggerboard will have an adverse effect on the rudder on that side (I'm using a single daggerboard, offset to lie along the inside of the hull.)

    As I understand it, Lock Crowther was a big advocate of keeping multihull rigs aft, and in fact, I read somewhere that, on Twiggy, he would have moved it even further aft, but for the fact that the daggerboard would have been too far aft and caused interference with the rudder.
     
  2. gonzo
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    gonzo Senior Member

    Does this mean you will put one daggerboard only and it will be on the side of one hull?
     
  3. rayaldridge
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    rayaldridge Senior Member

    That's right, Gonzo. That's the setup I used with Slider, and it worked very well. It's on the inner side, which makes it a bit easier to handle.
     
  4. redreuben
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    redreuben redreuben

    Put it in the centre in a pod that also houses outboard, ice box etc with prodder pole on the sharp end like Kelsall and problem solvered ! I like the idea of using a kick up dagger board, that is, the board operates in a case vertically like a dagger but case and board pivot back like a centreboard. Bit of extra weight but best of both methods. If bottom end of the case is below waterline and has similar profile to board you will get endplate effect like putting it through hull ! but of course kicked back off the breeze no drag from empty slot !
    RR
     
  5. cavalier mk2
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    cavalier mk2 Senior Member

    I put the daggerboard in my sailing canoe at one side and you don't notice any difference between tacks + you can still stretch out.
     
  6. rayaldridge
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    rayaldridge Senior Member

    redreuben, I like that idea too, but because of the folding scheme I'm using-- a variation on the kind of folder that is hinged at armpits and centerline, if I make the pod strong enough and wide enough to carry that stuff, I cut substantially into the beam of the cabins, and they are already a little claustrophobic at 44 inches max beam.

    But even if I went with a central board and rudder, the problem of daggerboard turbulence affecting the rudder is the same, given that I want my rig well aft.

    Looking at a profile of Crowther's 29' Shockwave, the spacing is proportionally somewhat similar to what I'm suggesting. The mast looks to be about 50 percent of the waterline aft, and the daggerboard is well aft of the hulls' midpoint. But my drawing has less distance between board and rudder-- my board is further aft.

    Here's a profile of my new cat, though the mast is not shown as far aft as it would have to be for the indicated daggerboard location to work:

    [​IMG]
     
  7. rayaldridge
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    rayaldridge Senior Member

    You know, I'm starting to feel a little dubious about this. I wanted to avoid the complication of having the daggerboard case exit the cabinside, but I'm beginning to think I've just gone too far aft with rig and daggerboard. It's true that the Shockwave is a pretty old design and some of Crowther's later designs went to even greater extremes, but still, the Shockwave daggerboard's position is not nearly as extreme as Slinger's. Above all I want a handy well-behaved boat, and I'm worried that the turbulence at speed might cause some squirreliness with the steering.
     
  8. cavalier mk2
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    cavalier mk2 Senior Member

    I think it needs to go forward a bit. Offset with the back edge even with the end off the cabin should work. The furthest back would be with the board center even with the back of the cabin but that would be harder to build.
     
  9. rayaldridge
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    rayaldridge Senior Member

    cavalier, I think you might be right. In that position, the back of the case would be against a major bulkhead, so would be stronger.

    I'm just wary of trying anything too extreme, even if the CoE and CLR line up properly. The mast can be stepped anywhere along the center spine, so that's not a problem.
     
  10. redreuben
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    redreuben redreuben

    Ray with central board I would keep a rudder on each hull
    RR
     
  11. catsketcher
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    catsketcher Senior Member

    Stop Ray Stop

    Ray I love your work but unless you want this boat to be a bonfire later don't do this.

    I loved my Twiggy but Lock's idea of putting the centreboard so far aft was a silly one in retrospect and one that no one has followed since.

    The Twiggy had about the same proportions as you have drawn. The idea seems fine - that you move the rig aft to give more forward buoyancy and consequently pull the board back too. Seems good on paper.

    But there are a couple of assumptions that have slid in here. The first is that the centreboard will be where your longitudinal CLR will be. Now when the boat is moving and the board has flow it will produce lift very well and so the CLR is on or near the board but you aren't always doing this.

    When you tack or get hit by a wave or the autopilot takes too long to adjust for a gust the board stalls - all boards do but a Twiggy then does something no other boat I know does. It then weathercocks like a wind vane. Just slows down and then bang goes up into the wind and no rudder can stop it.

    What I theorise happens is that at stall the board develops pretty much the same lift as the hull being dragged sideways. Therefore the stall CLR of the boat moves forward to the position near the centre of the underwater area. The rig is far back and so you round up drastically. Then you also get hit with the reduced distance between the rudder and centreboard. Just when you need it the most the rudder volume (rudder area times lever arm) lets you down as it is very low.

    I really did love my Twiggy and was ready for this quirk. As soon as she slowed on a wave and I lost concentration she may weather cock so I sometimes had to leap for the tiller and drop a few feet of mainsheet.

    My thoughts are that after sweet cats like Slipper and Slider you would be unhappy with a boat that behaved like a Twiggy. I don't know of any term that details both a lift derived CLR and a drag derived CLR but I would like them to be both about the same spot or the boat will do something strange when the board stalls.

    I loved Lock's work but he didn't always produce boats without flaws. The Twiggy was a great boat designed to an old rule. If Lock were alive today I don't think he would design a similar boat. His later tris weren't.

    Better still why not follow that other great tri designer and do what Jim Brown's windriders did. Jim put the drag CLR BEHIND the lift CLR. The Windrider would bear away until after a tack no matter what you did. Great boat.
    cheers

    Phil
     
  12. cavalier mk2
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    cavalier mk2 Senior Member

    Well put. Another problem some of Lock's early boats had was having the bows blown to leeward in strong wind/storm conditions where waves and wind are against you. The only way to make far aft main boards work is to have a forward balance board Like the 1st Newick 3 Cheers and Rogue Wave/Gulfstreamer but the complexity doesn't seem to fit the idea of a simple beachcruiser.
     
  13. rayaldridge
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    rayaldridge Senior Member

    Phil, excellent points. As you may have noticed, I was smitten with doubt about the idea, once I compared my profile with the Shockwave's profile. The underwater profiles are not that different, in spite of Shockwave's big overhangs.

    Maybe the reason Slider is such a well-behaved little boat is that the hull CLR largely coincides with the daggerboard's CLR. I suppose one approach, if you wanted to have the rig that far back would be to design hulls that had their CLR far back too, but that's way too experimental for dabblers like me. In any case, my hull panels are already cut out and drilled for wiring, so... not gonna happen.

    I spent a lot of time thinking about this since I posted last night, and realized that my cabintop is so low that having the case emerge there won't be so bad, and getting it out of the cockpit side deck will simplify the sheeting set-up.


    I don't know about emulating the Windriders. I discovered last summer that with an almost identical amount of sail, Slider was faster to windward than a couple of Windrider 17s, even though they are lighter, and have modern rigs with fully-battened mains and rotating masts. But I attribute this to Slider's good foils-- the molded-in skeg of the Windrider must not help them much to windward.

    It sounds like Twiggy was a high-maintenance girlfriend in every sense of the term. I was really hoping you'd chime in, since you actually have experience with one of Lock Crowther's way-aft rigs. The fault you describe is truly a horrendous fault for a cruising boat. I would have to have a Viking funeral if my boat turned out like that, and I'm not quite ready to go.

    Anyway, new drawing:[​IMG]
     

  14. bruceb
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    bruceb Senior Member

    board position

    A friend has one of the new NACRA 20s with the curved/lifting boards. They and the mast are set about 50% aft and don't seem to disrupt the flow to the rudders, he said it is very controlable at 22+ knots. B
     
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