Single rudder on a cat

Discussion in 'Boat Design' started by Wand, Jul 17, 2013.

  1. Wand
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    Wand Junior Member

    Am wondering if anyone has tried a single, centred rudder on an off-the-beach cat (12-16 feet approx). Obviously couldn't let a hull fly too high but, apart from those moments, would like to know how it went. Did the single rudder provide enough control, especially downwind? Any other observations or thoughts? Thanks
     
  2. Ilan Voyager
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    Ilan Voyager Senior Member

    What's the interest of such a structural complication? what do you want to obtain?
     
  3. messabout
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    messabout Senior Member

    Way back in time there was a nice little factory built cat called DIngo. It had foils in the center. It was a dog as the name implies. Ventilation was readily apparent if you ever got it going fast enough. A center mounted rudder will have to operate in the converging waves from the two hulls. Not good.
     
  4. Ilan Voyager
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    Ilan Voyager Senior Member

    It's not a good solution. It has ventilation and structural problems.
    Structurally it's a complication as you put stresses in the middle of the rear beam, plus the stresses within the longer and wider govern. That asks heavy structures.
    It's so obvious to use already the mechanical inertia of the sterns to counteract the smaller stresses induced by the smaller governs. Plus the plate effect you can get from the hulls bottoms. The lone "complication" is the link of the two rudders. With 2 rudders you can get also the benefits of a ackerman steering, and even try asymmetric rudder profiles to get a higher stability or better maneuverability.
     
  5. Wand
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    Wand Junior Member

    Thanks to the above posters for their comments. The theory as outlined seems sound and well considered; no quibbles with any of it from me. But sometimes the real world experience differs from the theory (and from second hand reports) and I'm wondering if anyone posting here has gone to the trouble of actually building a single rudder (and maybe single centreboard) system for a beach cat and can report directly from that experience.
     
  6. Steve W
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    Steve W Senior Member

    I actually
     
  7. rwatson
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    rwatson Senior Member

    Attached Files:

  8. Steve W
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    Steve W Senior Member

    I actually asked a similar question a while ago and only got opinions, no actual experience.I was interested for a bigger cat that woulg never fly a hull, I have seen kelsal, a grainger and otherswith a single rudder but in my opinion a beach cat would not be a good candidate due to the likleyhood of lifting a hull and the structural issues though not unsolvable.
     
  9. Ilan Voyager
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    Ilan Voyager Senior Member

    I have some practical experience of the thing, after all I've worked on boats and ships since 1971 including a lot of multis.
    I navigated on one a big pack of years ago (more than 30), a 18 feet beach cat, a modified Unicorn. Apart an horrendous weight and a complicated structure, a central centerboard messing with the boom, a rudder tiller in the way on the mainsail chariot and inefficient ventilating foils (the losses of control were spectacular), the cat was able to float and move, flatly beaten by the worn out Hobies 14 of the Carnac's sail school.
    As naval engineer, I do not see the need of adding torsional stresses on the beams, ie in the middle of nowhere, the added lever of effort of the longer foils and the weight of added structure to withstand these loads, when you have already adequate support for the job, I mean the hulls.
    I won't mention the ventilation problems, and the lack of control when heeling unless using ridiculously big rudder and board.
    If it has had a shadow of efficiency we would be seeing a lot of central boards on cats, cruising and racing. It's not the case.
    On cruising cats it's not better as you want a clean wide platform for the amenities. And calculate the stresses induced by the long lever (more than 2 feet -water clearance of the platform plus the hull draft -on a 25 feet cruising cat) of the boards.

    Because of my age I'm a bit slow; can someone explain me the outstanding advantages of central boards on a cat? So great that overcome all the the structural and hydro worries? Let's be rational.

    Comment on the picture; what a beautiful hydrodynamic brake and I love the tiller just in the way of the mainsail track. I like also the security of the box rudder system, very practical when you want to get it up. Excellent if you plan to drown your rich bachelor uncle. If the rudder hits something, it will be a lot of damages. Why not using pivoting small rudders installed on the sterns going up with simple ropes ??? Wouldn't be safer? Easier to make and lighter?
     
  10. Richard Woods
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    Richard Woods Woods Designs

    I agree with Ilan's comments. Saves me writing almost the same thing.

    Prouts started with a central rudder on their 16ft Shearwater in 1953(?) but quickly changed to hull mounted rudders. The Stiletto 27 had a central board until owners discovered that one in each hull was much better

    It's one of those ideas that is so obvious that if it really worked everyone would use it.

    I guess its OK for a small "'fun off the beach toy boat", similar to some of the awful inflatable catamarans you see, but not if you want a real boat

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  11. Wand
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    Wand Junior Member

    Thanks for your comments but are not these issues simply design faults? Could they not all be resolved? That is, I agree that the setup you've described is problematical but also think there is no imperative for the "tiller to mess with the mainsheet chariot", nor for the "centreboard to mess with the boom". Nor, for that matter, need there be horrendous weight or complicated structure problems.

    For example, central boom sheeting with the traveller just aft of the centreboard case (like many mono dinghy setups) would provide adequate seperation of mainsheet & tiller. And I can see no reason why the centreboard should any more mess with the boom than it does on mono dinghies.

    And with respect to weight: maybe there'd be not much difference overall between two in-hull centreboard cases and one constructed centrally. And maybe not much difference between two rudder setup and one constructed centrally.

    This is only a 14 footer by the way...
     
  12. rwatson
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    rwatson Senior Member

    Yes, of course. You dont have to fly a hull on a little boat, and as you say, monohull dinghys have solved these problems over the years.

    The real case against the concept is just the increased build cost and complexity compared to the more conventional approach.

    The case you have to make is the benefits of such a design.
     
  13. Wand
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    Wand Junior Member

    Taking the last point first, that may be true in the perfect theoretical marketplace. But of course the real world marketplace is not that; it is distorted by all sorts of things, like the intertia of doing things the way we are accustomed, and being subject to the surges of fashion: the "I want what they've got" factor. Lotsa good nautical design ideas (mostly old ideas) are rarely seen these days simply because they've gone out of fashion, or the skills required have been lost, or the client or designer just doesn't want to take the risk with being different. I mean, even blue hulled boats sell at a discount to white ones, because these days 'the customer' thinks that boats should be white. It's just all part of the human condition. :-(

    On the examples of design changes quoted above, I'm sure they're accurate and I'm sure the designers had very good reasons for making those changes. But my query would be: what were the critieria by which they reasoned those changes were desirable?

    For example, the Shearwater may have appealed to the young and gung-ho where a hull was always flown; in such case, one rudder would clearly be less than optimum. And maybe the Stiletto design change was just to enhance the living space by banishing the boards to the hulls and nothing to do with performance. (I don't know the boat so I'm just surmising for the purpose of example)

    I guesss what I'm suggesting is that we ought not to be black and white about what is good and what is bad in nautical design, but judge rather on whether the design accomplishes something useful when set against the particular criteria set. And that critieria will always be wide, simply because the folk who sail boats vary widely in age, experience, capabilities and even just what they want from a boat.
     
  14. Wand
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    Wand Junior Member

    Thanks for the cautious nod. And okay, I'll make a case. Consider southern mainland Australia where summertime seabreezes commonly build to 20-25 knots during an afternoon race. What is the most popular singlehander there these days? It's the Sabre, a real dinky toy of a boat. When it was first launched a few decades back, it was ridiculed as a "old man's boat", and for good reason. It has a small, inefficient rig, few sail controls, an ordinary underwater shape and it looks like a bucket compared to the ritzy skiffs. For years, no-one under forty with any self-respect would be seen dead in one.

    What happened though was that sailors in time found that being one of the slowest classes around the course mattered not one dot; it was the closeness of the competition that made absolute speed irrelevant. Further, when the breeze really kicked in, the smaller rig enabled weekend sailors of average fitness to cope while other classes turned turtle or scampered ashore.

    When it comes to off-the-beach catamarans, there seems to be a prevailing view that the only measure of quality of design and build is absolute speed. Screaming reaches. Flying hulls. Pitchpoling. Breaking sticks. It's gotta be gung-ho fast or else what's the point of having a cat?

    Just like the mono Sabre, I think there'd be an interest (well, I have an interest anyway) in a cat dinghy which is a little underpowered with limited sail controls and easy on the knees. Probably the easiest cat around here & NZ is the Paper Tiger, which is a nice boat but easy to cartwheel in a strong breeze in a seaway. And, like lotsa cats, crawling across the tramp on wonky knees and with a shonky back gets less bearable every year.

    I have a couple of old PT hulls and some playtime, and it's that framework (knee friendly, slightly underpowered, limited sail controls, easy access to the important bits) that I'm gonna work within and see what happens...
     

  15. rwatson
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    rwatson Senior Member

    I dont know what Sabre you saw - the description "inefficient rig, few sail controls, " doesnt match any sabre I have found.

    Likewise, cartwheeling Paper Tigers seems a bit overstated.

    Despite stating your design aims, I cant see where single rudder, single board is going to solve any of your 'problems'.
     
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