negative leeway vessel

Discussion in 'Boat Design' started by david ingles, Mar 1, 2014.

  1. david ingles
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    david ingles Junior Member

    I've had this idea for a very long time. Basically it involves a keel or centreboard which can rotate relative to the centreline to reduce leeway or, with high rotation, make it negative. The boat must be designed to reflect this ability. Instead of a canoe hull (which has least resistance at zero leeway) we use a fat arrow shaped hull with maximum beam carried almost to the transom. Double rudders are necessary. When such a hull is heeled on one tack the shape of the waterline is a lobsided canoe shape splayed out from the centreline at 15 (say) degrees. If the boat can be made to travel along the long axis of this canoe shape its form resistance will be considerably reduced. This is achieved by having the keel and rudder at say 20 degree to the centreline - negative leeway. (I assume the foils will operate at a 5 degree lift angle relative to the flow of water.)
    I call this boat the monomaran as it is a cross between a monohull and a catamaran. It has huge form stability, and the keel weight need only be modest. Water ballast can be used to great advantage. Sails will need to be sheeted 20 degrees wider than normal when the keel is rotated.
     
  2. david ingles
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    david ingles Junior Member

    Note that these huge sheeting angles mean that the boat makes a track through the water similar to a conventional vessel. It crabs up to windward but points much lower. Big sheeting angles mean that mast staying angles are wider (less compression loading) and the headsail slot is opened right up.
     
  3. rwatson
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    rwatson Senior Member

    This has been a point of discussion for a number of reasons over the years.

    The flaw in the logic is that when you rotate the keel, the whole boat gets 're-aligned' in relation to the wind, and the effect is you end up dragging the hull a bit sideways for no net windward gain.

    Thats the only reason why no racing boats have this feature - not because no-one has thought about it.

    eg
    http://www.boatdesign.net/forums/multihulls/catamaran-centreboard-location-10302.html
     
  4. daiquiri
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    daiquiri Engineering and Design

    David, I am not sure you are using the term "leeway" in a correct manner here. The leeway is the sideways drift of the ship respect to the centreline of the boat. In other words, it is the difference between the true boat course and the heading, due to wind action.

    It seems to me that you are using the term leeway to describe the angle of incidence (or mounting angle) of the keel to the centerline, which would be incorrect.
     
  5. david ingles
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    david ingles Junior Member

    negative leeway vessal

    The point is that the boat is specifically designed to have its lowest drag when it is 'dragged sideways' , as you put it. The concept does not apply to a conventionally designed yacht with a canoe body.
    Lay an arrow head on a table; tilt it up at an angle of say 30 degrees. What is now the direction of lowest drag? Its along the side of the arrow head, not on its centre line. The concept is applicable to a heeled yacht, not an upright one. When sailing off the wind, and mainly upright, the keel would be centred.
     
  6. tom28571
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    tom28571 Senior Member

    As said above, this has been hammered around several times and the conclusion is always the same. Many of the new offshore racers, especially those that don't follow a restrictive rating rule, that have a very wide stern and sail at a heel angle that does what you propose. There is apparently some advantage to it but it is probably not how you propose. Tom Speer is the local expert on these matters and he may drop in again. Suggest you search for the very exhaustive thread on the subject.

    There is always the remote possibility that you have thought of a new and novel concept to eliminate leeway but most will have serious reservations until it is proven. Reducing leeway is always possible but it cannot be eliminated as there would then be no side force to counter the sails. Even the most efficient sailplanes have a negative glide ratio and the best boat is a relative slug compared to these.
     
  7. Baltic Bandit

    Baltic Bandit Previous Member

    the advantage of wide sterns are three fold
    • Upwind they give you a lot of RM if you can move weight (crew and water) into them
    • Off wind they give you a large area with which to generate dynamic lift (ie planning force)
    • Off the wind they give you a clean truncated stern separation that allows you to plane more easily rather than digging yourself deeper in to the water

    And they deal with the excess drag such a stern can cause upwind by having an "immersed profile" design that looks like a canoe body. so this is nothing new.

    The "twisting keel" also is nothing new. "Gybing Centerboards" have long been a feature in skiff's like the 5o5 and I-14. as well as in "bilge boarders" going back to the late 1800s and on through modern VOR boats (which also have that wide stern)

    NONE of them are either
    • negative leeway boats
    • Nor do their keels work the way david imagines
    essentially what these "twisted" keels do - because they do improve performance in some boats - is to allow the fore-aft axis of the hull to align more closely with the CMG through water, thus reducing the drag caused by pushing the hull sideways.

    But for this to work, you have to have a hull that is fast enough that it has relatively little leeway generated drag
    and
    a keel that is efficient enough that removing the lateral resistance of the hull does not increase leeway made.

    So David - this is nothing new. Its how the VOR and Mini 6.50 Proto (vs Classe) hulls are largely designed today and have been for a long time
     
  8. Doug Lord
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    Doug Lord Flight Ready

    Cbtf

    ====================
    Have you looked at CBTF designs? They use twin rudders: one forward and one aft. The rudders and be turned "collectively"(both the same direction) or normally: both opposite directions. The twin rudders supply all the lateral resistance for the boat so there is no need for a "normal" keel. They use a canting keel strut that is shorter chord and thicker than a normal keel because it is not designed to generate lateral resistance. These boats can theoretically "crab" sideways into the wind by using "collective" steering but in practice that is slow(except in certain short term tactical racing situations).
    There are numerous threads buried here on this forum including rotating keels,
    gybing boards and more.
    Take a careful look at this site: http://www.cbtfco.com/Home.html
    --from the "Technology" section of the site:

    The second part of CBTF is the use of the twin foil configuration, which addresses maneuverability and lift. Enhanced maneuverability over the conventional system is quickly attributed to the handling of the steering by two turning foils and lighter displacement as opposed to a conventional single rudder boat. For optimized lift efficiency (Cl/Cd) the foils are placed in the high-pressure zones created by the hull. Not only does this increase the underwater appendage package lift efficiency, but also reduces the boat’s overall wavemaking drag.
    Another benefit of the twin foil system is the collective control. This adjustment allows the boat to sail with virtually zero leeway upwind and superior directional stability downwind due to its ability to shift the CLR fore and aft, balancing the hydro and aero forces.
     
  9. Baltic Bandit

    Baltic Bandit Previous Member

    Well if you "look at" the CBTF designs, you will end up having to pay them royalties for anything you do. CBTF is a patented approach. And once you look at how they have done stuff, you would have to go to a "white room isolation" approach to be able to create something that doesn't violate their IP.

    But fundamentally the issue here is that Leeboards or Bilge boards are a much simpler and cheaper way to do what David is talking about.

    Notice the "twisted" angle of the leeboards on this open 60 sailboat. and the wide stern

     
  10. Baltic Bandit

    Baltic Bandit Previous Member

    Here's a better video of of it in action
     
  11. TeddyDiver
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    TeddyDiver Gollywobbler

    The flaw I think your monomaran concept has is it, compared to catamaran, has to sail 'flying' the other hull ie windward side of the hull constantly. This makes the it extremely difficult to control in changing sea state and weather.
    BR Teddy
     
  12. philSweet
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    philSweet Senior Member

    Continuing on with the theme that this is all but commonplace, tabs on the aft edge of keels can be a very simple and effective way to gybe a keel. Another way that this is being accomplished is with the canting keels, where the pivot is not horizontal, but instead makes and angle with fore-aft waterline. When the keel is canted to the side, the board also gybes, or at least doesn't point in the wrong direction due to the bow-down trim that happens when a dart hull heels. This basically requires the cant angle to be carefully matched to the heel angle and boat speed in order to work properly. I agree with BB that this is best suited to fast boats. And anything other than keel tabs makes for a substantial design challenge as far as analyzing the effect of the thing over the entire operating range of the craft.

    Then there's the comment about opening up the slot. You make it sound like that's a good thing.
     
  13. david ingles
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    david ingles Junior Member

    negative leeway yacht

    Just a few comments in relation to the above.
    I agree the VORs and similar wide-sterned vessels are logical candidates for negative leeway foil configurations. However explicit use of negative leeway would allow the optimal beam/length ratio to increase.
    The 'monomaran' which I propose would, as one writer points out, need to be constantly heeled to reduce form resistance. I believe this is doable with water ballasted yachts and in dinghies, with crew weight.
    There are difficulties in combining canting ballast with negative leeway. Unless the ballast keel is nearly horizontal, it will act counter to the lift being generated by the other foils. (This is one reason boats will stall out if the variable foils are pitched at too steep an angle. The other reason is that sail sheeting is too tight. Sails must have a vector of force forward of the foil pitch.)
    I have seen a design for a canting keel which incorporates a variable angle of attack, but the mechanism looks quite complicated (and expensive). That is why I suggest an approach based on a pivoting keel (no canting) and movable water ballast. The wide hull shapes envisaged make this a feasible approach.
    For dinghies negative leeway could be implemented using twin daggerboards in the bilges, with a small central daggerboard for downwind. Leeboards are also possible, as someone noted. Another option is to have a cylindrical mount for the centreboard which is rotatable around the vertical axis. My emphasis is not however on the mechanics, which are soluble, but the theory.
    In general the theory is that in terms of form drag every hull shape has an optimal angle of leeway which depends on the heel angle and the hull shape. All I have done is point out that for some hulls that optimal angle is negative, possible steeply so.
     
  14. gonzo
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    gonzo Senior Member

    Can you explain what "explicit use" means?
     

  15. david ingles
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    david ingles Junior Member

    by 'explicit use' I mean as a design feature. Some current boats may accidentally experience degrees of negative leeway by virtue of using canards, trim-tabs etc set at a slight angle to the centreline. Normally these features are meant to reduce or eliminate leeway.
     
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