Placement of single rudder on catamaran

Discussion in 'Multihulls' started by rayaldridge, Aug 24, 2010.

  1. rayaldridge
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    rayaldridge Senior Member

    The new little cat I'm drawing has a central "spine," necessitated by the method of folding the hulls I'm using. As I understand it, there have been successful designs that used a single centerline rudder and board-- some of Derek Kelsall's smaller cats, for example.

    Here's my question: would there be any notable advantages or disadvantages to extending the spine aft past the transoms, so that the rudder was further aft than would be possible using transom hung rudders?

    The thought was partly generated by something the late great Lock Crowther said about Twiggy-- something to the effect that he'd have moved the mast even further aft, except that he couldn't move the daggerboard any further aft without its turbulence affecting the rudder.

    My first reaction is that a rudder set a couple feet aft of the transoms would improve steering, but maybe I'm wrong. It would be a help to have that extra clearance between the board and rudder, because I'm considering a kick-up board-- sort of like a centerboard without a case, also built into the central spine. It would have to be a very long board, not just to compensate for the possibility of ventilation, but also just to physically reach the water from the center deck, which must have adequate clearance.
     
  2. Dryfeet
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    Dryfeet Junior Member

    Chris White did a 29' design early on that had a central board and rudder such as you describe. However, the rudder was still mounted on the aft beam. My thought is that you've got structural challenges both with the rudder further aft of the transoms and with the side loading on the center/dagger board. Certainly not insurmountable but requiring both good engineering and extra weight as a consequence.

    Personally, I am not attracted to this concept in a catamaran for these reasons and for the surface piercing aspect and sculch collecting potential. I am interested in seeing how you work out the details however.
     
  3. rayaldridge
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    rayaldridge Senior Member

    Good points. Engineering will not be the sticking point, I hope. The central spine caries the mast and will be a fairly deep I-beam. The centerboard will not require a case, and will be largely braced against side deflection by the mast structure. The hulls of this design are 19' 8" LOA, so at least the forces are modest.

    It's true that the surface piercing foils will be less efficient than endstopped foils, but to some extent this can be compensated for by larger deeper foils. The compromise here is to top end speed, because of increased drag, but this is a fast cruiser in concept, not a racer.

    I don't think collected crap on the foils will be a problem, because they will both kick-up, so that stuff will slide off, mostly. I like centerboards for cruising, just because of the auromatic nature of the board's actions on grounding, but centerboard slots are a complication. On Slider, my little open beachcruiser, I use a single big daggerboard, and I had to make the case strong enough to survive a grounding at speed. Weight!

    One practical problem occurs to me: access. You'll have to have a catwalk back out to the rudder, in case you need to work on it afloat.
     
  4. catsketcher
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    catsketcher Senior Member

    When will you stop Ray?

    Another cat Ray? You need BA - boaties anonymous.

    For purely theoretical reasons I will continue.

    The rudder set on a bracket hung way aft is seen a lot in skiffs and Cherubs. It gives a longer lever arm. It was even seen on a mates Twiggy. No problem there. Again I have problems with offshore as I have detailed before but it should be no issue inshore. It will give a greater rudder volume which is usually good.

    cheers

    Phil
     
  5. rayaldridge
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    rayaldridge Senior Member

    Yeah, maybe it's a sickness. But they're very small cats. They're sort of like mini-muffins. They can't have that many calories. Muffin vitamins, really. So I'm really not a cataholic. Not really.

    Can I ask why your mate hung his Twiggy rudder further aft?
     
  6. ThomD
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    ThomD Senior Member

    One advantage to putting it out back would be you could also lower that extention, reducing the load on the rudder. and maybe even caping it against ventilation. That might look a little odd, but it might work.

    I once built a 16' cat , sorta a scalled down sailing charter cat, and it used a motor on the aft beam, there was probably 3 feet of hull extending back there. Steering was difficult because it was so far out of line with the radiuses the hulls were moving through. I think the same thing would happen with a rudder that was short or even between the transoms. No ackerman, and positioned a little far forward, so a rearward extension would be useful if not necesarry.

    How do you intend on supporting the board against side loading?
     
  7. rayaldridge
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    rayaldridge Senior Member

    Thom, if you mean the centerboard, I was planning to box the I-beam at the point of rotation, and continue that box forward to the mast tabernacle, which I planned to extend to the bottom of the central I-beam.

    However, I've heard from Mik Storer that he helped build and sail a couple Kelsall cats with a central pod and rudder setup, and he mentioned that using such a pod gave the boat a really good place to put an outboard well, right at the pitch center, where the outboard wouldn't constantly be coming out of the water.

    So now I'm trying to come up with a way to do that. My problem is that any beam I give to the central spine must be subtracted from the cabins, which are already pretty narrow. But having a reliable little engine is a big deal in the anticipated use of the boat-- scooting across to the Bahamas and on to the Caribbean. There are certainly times when the better part of valor is to choose a windless day for the crossing. Gives you more safe options.
     
  8. redreuben
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    redreuben redreuben

  9. Doug Lord
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    Doug Lord Flight Ready

    =========
    Reuben, thanks for that link! That is one of the most innovative small cat designs I have seen anywhere-really cool.
     
  10. redreuben
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    redreuben redreuben

    My pleasure,
    I really like the pod cat concept, big boat sailing small boat fitout, waterline length at much lower cost ! RR
     
  11. ThomD
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    ThomD Senior Member

  12. rayaldridge
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    rayaldridge Senior Member

    Reuben, thanks for the link-- very interesting stuff. I really like the 5 meter podcat. I'm a little dubious about the 9 meter cat, because the central pod has no clearance. Seems as if it might pound some.

    Thom, that's an amazing video. One of these days someone will put fore and aft rudders on a cat and make it work brilliantly, I suspect.
     
  13. redreuben
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    redreuben redreuben

    Ray,
    I suspect part of the "no clearance" is to provide support to the board, along with headroom, spread mast loads etc. Also may add some end plate effect to the board. it's not perfect but as I mentioned I like the concept.
    Cheers, RR.
     
  14. ThomD
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    ThomD Senior Member


  15. dstgean
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    dstgean Senior Member

    Ray,

    I've sailed on a cat that had such a setup--a Malcolm Tennant 36' cat. It had a box in the deck with a second box that allowed the Honda to be lowered to motoring depth. Pretty cool unless a quick start was needed. It rarely came out even in "lively" conditions.

    Having a central spine of sufficient width will start cutting down on your cabins a bit though. The fold over or under hull arrangements begin looking better at some point, but at 20' I think you're still on the right track. You could have quite the cabin with your beam width & a deck like that once in the harbor!

    Dan--who's still deciding on how best to set up his double Tamanu/h18 combo over the winter this year. I'm thinking 8'6" beam to get the trailering and assembly done quickly.
     
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