Why can't most catamarans get over the hump ?

Discussion in 'Multihulls' started by tommymonza, May 4, 2014.

  1. pbmaise
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    pbmaise Senior Member

    Sailboat design is separating into two extremes. The very fast and thrille....versus the slow but steady and safe.

    I am in the second group and thought the "hump" you spoke of was the stall of a multi when going through the wind.

    Many big cat and tri owners will engine assist the tacking.

    Just this week I was experimenting turning my big tri in light winds and a single small foresail.

    In my case putting the mast in the back makes using driving sails more difficult.

    To tack I found I had to bring the clue way forward and all the sail area now forward pushed the vessel through the hump. You have to pay attention at the wheel and not do it too far.

    The fix is the lifting sail. I show that on youtube. My lifting sail will come across without moving a line or starting engine.
    However I can't use the heavy sail cloth in winds below 2 knots.
    Another thing, the bigger and more stable the boat the slower it will seem to you.
     
  2. ICBlowboaters
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    ICBlowboaters Junior Member

    Have you tried a pair of these, they improved my speed quite a bit.


    [​IMG]

    You could always swap some pulley for more boost if that don`t do the trick.
    NEXT!
     
  3. tdem
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    tdem Senior Member

    Could you provide some references for the figures and info you posted?

    Cheers,
    Thomas

     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    "Planing Hull Feasibility Model - Its Role in Improving Patrol Craft Design", Hadler, Hubble, Allen, Blount, Symposium on Small Fast Warships & Security Vessels, RINA, 1978,

    Principles of Naval Architecture, SNAME,

    ‘Resistance Experiments on a Systematic Series of High-Speed Displacement Catamaran Forms: Variation of Length-Displacement Ratio and Breadth-Draught Ratio’, MOLLAND, et al, P.R.,Trans. Royal Institution of Naval Architects, Vol. 137, 1995,

    "Practical Design Aspects in the Hydrodynamics of Fast Craft", Warren, et al, Hydrodynamics of High Speed Craft, RINA Conf, 1999.
     
  5. oldsailor7
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    oldsailor7 Senior Member

    ALSO---"Design for Fast Sailing" by Edmond Bruce and Harry Morse. AYRS 82.
    Particularly on Bruces mathematical and test tank data regarding L/B ratios of hulls, and his findings regarding the size and aspect ratios of vertical fins, (Daggerboards, centreboards, surface piercing lee boards and non ballast keels.)
     
  6. tdem
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    tdem Senior Member

    Thank you Ad Hoc and oldsailor7!
     
  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    That report is available online at:
    http://eprints.soton.ac.uk/46442/
     
  8. Richard Woods
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    Richard Woods Woods Designs

    I agree that length and weight are the primary factors determining resistance. I also agree with Ad Hoc that the other factors are minor and only affect resistance by a few %. However, many races are won by less than 1% and most people consider a 2% increase in speed (or a 2% resistance decrease) a "design breakthough"

    Most high speed hull resistance studies are still only "low speed" compared to many sailing multihulls. A speed/length ratio of 2 is only say 12 knots for a 12m long boat - hardly fast

    I've written this before many times. A "planing" hull is a lifting surface, so needs a good AR and an angle of attack to achieve lift. Neither occur in most sailing multihulls

    There is no point in having a boat optimised for high speed if the drag at low speeds is so great it never can reach those predicted speeds.

    I was away racing a 38ft catamaran last weekend, together with a number of racey monohulls. The fastest monohull a J133, second fastest a Prima 38

    From my blog http://woodsdesigns.blogspot.ca/?view=magazine

    "We knew we had a good boat to windward, so were "pleased" with the next days forecast, which called for 20 knots to windward. In the event, it wasn't quite that strong, but the 25 knots apparent was our full sail limit and in the gusts many of the monohulls were overpowered, despite changing to smaller headsails. All the multihulls had full sail.

    As always, the monohulls started first, so were 10 minutes ahead, but we soon began to reel them in. The best moment was overtaking a Farr designed First 34.7 (First 10R in the USA) with new laminate sails. No surprise really, we were doing 10 knots and pointing higher. Just to rub it in we sailed through their lee, luffed up and were away.

    Who says multihulls don't go to windward?

    What surprised everyone was that we had the highest freeboard and tallest cabin, yet, of the 38ft cats, we were the quickest to windward, while the lowest freeboard cat was slowest.

    We had taken 2hr 38min for the 20 mile beat. The first monohull, the 5ft longer J133, took 2hr 44min.

    The race results showed we were 3rd, 6th, 1st, for 2nd overall. I was happy with that, especially as I steered for half the first race, all of the last, but none of the second.


    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  9. tommymonza
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    tommymonza Junior Member

    What was the cat you were sailing on for this race series Richard?
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Well, perhaps you have not read those by Molland, his testing is up to Fn = 1.1, or those Series 64 up to 1.4 or those by Muller-Graf up to Fn = 1.6. That's 34 knots on your 12m boat. A S/L ratio of 2 is only Fn = 0.56 that is circa prismatic hump. All the high speed cat hull forms have been away past prismatic hump. We test up to Fn = 2.0, or 42 knots on your 12m boat.

    Hardly slow...!
     
  11. oldsailor7
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    oldsailor7 Senior Member

    The original question was, "Why can't most Catamarans get over the hump."
    The simplest answer is ---most of them do.
    Starting with an L/B ratio of 8 and going right on up to 20, reduction of wave making is continually more effective until you reach a point of diminishing returns, caused by increased form drag and skin friction, --unless displacement is reduced.
    That is why if you have a very long skinny hull it has to be kept light.
    The Lagoons and Gun Boats are slim enough, but being filled with Dick Newicks "Modern Inconveniences" they are a bit heavy for their slim hulls. (not that I wouldn't like to have one).
    The "Tornado" catamaran, at an L/B of 20:1 is a perfect example. It is very light ---and very fast.
    Load it up with three or four big guys, an anchor and lots of chain, a couple of motors, a flush toilet and it would become a pig. :eek:
     
  12. tommymonza
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    tommymonza Junior Member

    So you are saying Old sailor if we had a stripped out Lagoon 44 which is right around your 8 to 1 ratio that it would break out of it's displacemnet speed that they all seem to pretty much sail at.

    Another thing that confuses me a bit is the need for a large underboddy and rocker in these modern designs. I can see it in a very slim hull such as the Spronks and Wharrams where they have a clean sharp transom for the wake to leave. But in a wider lb ratio such as a Lagoon and many others it seems like the bottom is essentially the same design as most older non planning monohulls.

    Why not have a flatter rocker in the bottom with a and carry the draft back and straight such as the Tornado hull?

    Tacking ability because they do not have centerboards being the reason for this bottom design?
     
  13. Richard Woods
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    Richard Woods Woods Designs

    Tony Molland was my head of department when I studied yacht design at Southampton. I did say "most" and thus meant in all time, not just recent papers.

    However another point, a sailing boat has to perform well at many speeds and in many different conditions.

    Many fast commercial craft are designed to run on a specific route in a set sea state and at a certain speed

    Again I have said it before. A hull is a 3D shape. You cannot say "this boat has too much rocker" without looking at the overall hull shape. So a deep V, canoe-sterned Wharram hull will have more rocker than a round bilge hull. And retro fitting a ram bow to an conventional overhanging bow will do nothing, if indeed it can be built at all

    Richard Woods
     
  14. hump101
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    hump101 Senior Member

    You need to be careful when using these results. All Patrick's models used the same hullform (actually of a warship form, NPL round bilge series, so not a typical catamaran hull), just different proportions, and hence of course his results do not show a significant influence of shape.
     

  15. oldsailor7
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    oldsailor7 Senior Member

    Tommy Monza said,
    "So you are saying Old sailor if we had a stripped out Lagoon 44 which is right around your 8 to 1 ratio that it would break out of it's displacement speed that they all seem to pretty much sail at."

    Pretty much YES.
    At least it's displacement speed would be very much extended.
    The late Edmund Bruce showed with his tank and sailing tests that the displacement speed limitation pretty much disappears altogether as the waterline L/B ratio reaches 12/1.
    For instance the Buccaneer 24 has an L/B of 8/1, as it was designed as a pocket cruiser.
    However it has a very good cross sectional hull shape and is very light. This enables it's excellent performance in any conditions, despite it's very rounded rear end.
    So this merely shows that it's a careful combination of design features by the designer which achieves the design goal. :cool:
     
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