Single offcenter rudder for a cruising cat?

Discussion in 'Multihulls' started by rayaldridge, Jul 29, 2010.

  1. rayaldridge
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    rayaldridge Senior Member

    How feasible is this?

    Derek Kelsall uses single rudders on his smaller cruising cats, I understand, but I assume this is a rudder on the centerline of the boat.

    What would be the downside to putting one big rudder on one hull instead of two smaller rudders on each hull?

    I've wondered about this for various reasons, and though my intuitive reaction is that there would be a "good tack" and a "bad tack" intuitive reactions are not always correct. For example, intuitively most of us would want a daggerboard in each hull, but John Shuttleworth demonstrated using tank testing that a single big daggerboard in one hull was more efficient. That's the way I went with Slider, my little beachcruising cat, and it worked very well indeed.

    Of course, a single offset rudder would only be feasible for a cruising cat that was intended to never fly a hull.

    This would be for a cat that has a rig on centerline-- in other words, not a tacking proa.

    I can think of a couple of advantages to such an offcenter rudder. For one, it would probably have a better lift/drag ratio than the sum of two smaller rudders-- I think this is probably why one daggerboard is better than two. (An analogy would be .the superiority of a sloop rig to a ketch rig, efficiency-wise.) Another advantage is that you could hang an outboard off the unused transom, and it would be protected from spray and already in the entrained water from the hull. It would be easier to build and cost less.

    So what are the potential drawbacks? It occurs to me that beach cats and other racing cats flying a hull are already using an offset rudder. What problems are associated with steering while flying a hull?
     
  2. CatBuilder

    CatBuilder Previous Member

    You did say *cruising* boat and as a cruiser, my answer would be:

    Lack of redundancy.
     
  3. jamez
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    jamez Senior Member

    I've sailed a GBE with a single rudder (after its twin was damaged) and I was really surprised by how well it sailed even when the single rudder was to windward. This was sailing to windward in 20 knots breeze. These rudders are a dagger style with 3-4 feet of blade in the water at rest. We did lose bite a couple of times when the windward hull flew in the gusts, but you wouldn't (presumably) be doing that in a cruising boat. I think its a great idea, after my experience I'd be surprised if you had to go much larger to have effective control and as you say offers a simpler solution on the other hull for that pesky egg-beater.
     
  4. catsketcher
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    catsketcher Senior Member

    going downwind

    Strangely enough Ray the time I most like deep twin rudders is downwind in a swell. With the swell on the quarter you often get the windward rudder almost half out of the water just before the boat starts to surf down the wave. If the single rudder was the windward one it would be heavily loaded and half out of the water at the same time - bad combination

    cheers

    Phil
     
  5. rayaldridge
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    rayaldridge Senior Member

    Good points.

    One of the reasons I started thinking about this was that I'm drawing a new little cat-- this one a 20 footer that folds and has cabins.

    The ergonomic goals of such a design are quite difficult to achieve, because of the size of the boat. I've been able to shoehorn two good bunks, a galley, a dinette for two, and a tiny space for a portable toilet into the boat, but I was able to achieve that only because I ignored symmetry.

    Anyway, in the course of this exercise, I realized that we sometimes compromise a design for the sake of symmetry. I'm not sure that's a quality to put before other qualities. Certainly the single daggerboard in Slider was a notable success, but I was advised by many folks not to do it.

    Anyway, rough sketch of new boat:

    [​IMG]
     
  6. rob denney
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    rob denney Senior Member

    I put a single rudder on W, a 12m x 7m cat with the rig in one hull. Worked a treat although there was definitely a good and a not so good tack. I also sail my proas on one rudder (which also doubles as a leeway preventer, saving the cost, space, complexity and danger of a daggerboard), except in the light. Well worth a try, in my opinion.

    rob
     
  7. Petros
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    Petros Senior Member

    It should work fine in terms of control, two largest drawback is lack of redundancy and you will have asymmetric drag that will have to be trimmed out, and it will have a small but measurable effect on the performance.

    Once in a regatta with a huge fleet of beach cats in Newport beach, the popular thing to do when going down wind was to pull up one dagger board and one rudder to presumably reduce drag. I got an idea by putting both dagger boards and rudders down evenly, but just barely into the water. Without the asymmetric drag we crept ahead and won that leg of the race. Since it was a one design event (hobie 18), everyone was inspecting our cat real carefully convinced we were cheating. No one figured it out.

    So there is some loss associated with asymmetric configurations.
     
  8. rayaldridge
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    rayaldridge Senior Member

    Petros, I see what you mean by asymmetric drag. But isn't this always the case with a cat? On most points of sail, one hull is more deeply immersed than the other and therefore has more drag.

    I believe that a single large daggerboard is superior to smaller double daggerboards, at least in terms of lift and drag. However, I don't know how that translates to rudders, since the function is different.

    Another reason I started thinking about this is that the new design is trailerable, and with a single rudder, you wouldn't have to take the tiller bar apart to telescope the beams.

    I wonder about the loss of Ackerman effect with a single rudder. Of course, I tend to think of Ackerman as a solution to the problem of twin rudders not tracking together, so maybe that would also be a plus for a single rudder.
     
  9. ThomD
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    ThomD Senior Member

    So you are thinking single rudder and daggerboard? Kendrick has a boat about your size with a single centrally mounted daggerboard, which appeals to me even more. Just not sure how they engineer that, wires? Presumably borrowed from Kelsal. Did Jones have one?

    Anyway, there is also the acceptance thing. The single DB might be good, but if you intend to promote it as heavily as Slider - How about Sidewinder? - It might be easier with a more conventional system.
     
  10. cavalier mk2
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    cavalier mk2 Senior Member

    If you think about it on one tack it is no different than a trimaran - lee ama combo, in fact when you go to tack that is still the setup until you are through the eye of the wind and the new lee ama loads up.
     
  11. ThomD
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    ThomD Senior Member

    On the trimaran all the foils are lined up including the sail. For what that is worth.

    Speaking of cats that fold, whatever happend to that NZ design that folded like a play pen?
     
  12. rayaldridge
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    rayaldridge Senior Member

    The problem with centrally mounted foils, in my opinion, is the lack of endstopping. It's the reason I'm dubious about leeboards on fast multis. As an example, the Stiletto originally came with a central board, and it just wasn't as fast as Stilettos with daggerboards in the hulls.

    Hey, if I was worried about acceptance, I'd have never built Slider. I might call the new boat Slammer-- it's intended to be a much faster boat, since it will have a lot more sail area and beam. Of course, the word has some negative connotations for sailors, so maybe not.
     
  13. ThomD
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    ThomD Senior Member

    The guy building the Ray cat (the other Ray), is in florida somewhere, so you probably know him. :)

    I think I would prefer the endstopping problems to some others. For one thing getting the DB out of the hull is huge, and hugely easier to build. I think Simpson might have also used the central rudder, and maybe Jarcat. These are all pretty low confidence recollections...

    Good luck, sounds like a lot of fun!
     
  14. cavalier mk2
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    cavalier mk2 Senior Member

    On a Horstman or other tri with daggerboard foils they aren't all lined up, on a tri with ama rudders and boards the rig isn't lined up and it works fine. Ed made a convincing argument for the thought that while the centers of effort are important offsetting to the side wasn't. As I recall he said "It's not Mickey Mouse, think about it dummy !"
     

  15. Chindamba
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    Chindamba New Member

    Tacking problems with off centreline mast, leeboard & mast!

    This is the first time I have attempted t use a forum of any kind. Forgive me if I mess up.
    I have sailed all manner of small craft from day boats to hobie cats over the past 40 or so years, and now retired I decided to attempt to design & build something to keep me busy in retirement! I designed and built the attached "Tigger" last winter as a proa. However, whilst sailing on a very small lake, the swinging of mast etc on shunting convinced me that a conventional rig would be easier. At this point I had not fitted any centre board or leeboard.
    The craft design was based entirely on proas I had observed on the utube videos. Since this is an experement for me, I decided to attach what I thought would act as a centre board, in line with the mast and rudder, all set to one side of the main hull for ease of construction! (see attached photos)
    With the outrigger hull and "centreboard" to leeward, all is well. However, to windward ( on the opposing tack) the craft luffs up to the extent that I am unable to bear away at all and am obliged to contrive to shunt! Much to the amusement of onlookers. I think this was happening in the proa configuration. I put it down to trees and wind veer.
    Do you think this phenominum of luffing up, is due to not having a leeboard on the lee side of the main hull, or to something more fundamental, like the position of the mast or seats?
    On the subject of the seats. The idea is that they slide out to allow balance (either the float lifting or digging in, depending on the tack) retained by bungies to draw them in, and foot pressure to push out. This also keeps me away from the boom.
    You may know that the proa uses a prop, placed on the float to support the mast. In the conventional configuration, I have used this prop as the forestay allowing the shrouds to attach foreward of the mast. This was necessary as I need to use the cross member,which is foreward of the mast step, to attach the shrouds.
    The float is also problematic as it is made from 4" plastic tubing. It has been increased from 3 to 4 tubes, the end caps now too bulky. This is next winter's project to make it from ply.
    You may also notice that the main hull is in 3 sections. This is to allow me to stow it away in the space over the cab in our camper van. The whole craft disassembles very satisfactorily. The sailing characteristics are my main cancern.
    Thank you for reading this. I hiope you might be able to focus on what is happening here.
     

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