on the design of ultra slim yachts

Discussion in 'Boat Design' started by dionysis, Dec 16, 2003.

  1. dionysis
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    dionysis Senior Member

    Hi all,

    Take one hull of a big racing cat and put a deep bulb keel under it: The sail area would be halved and the cost quartered.

    Ok, accomodation would likewise be quartered, but then you could go to a longer boat.

    If AC yachts were radio controlled would they end up being super slim?

    We do not see super-slim yachts, yet they have a lot of advantages in terms of sailing efficiency and cost.

    Apart from the need to have a very deep keel, what is so wrong with the idea of ultraslim yachts?

    I look forward to any comments. Cheers
     
  2. henrikb
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    henrikb Senior Member

    I think it sounds like the JS9000, aren't they building a 42 ft version also??
    www.sportzboat.com
     
  3. dionysis
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    dionysis Senior Member

    thanks henrikb,

    rs9000 is 6 to 1 length to beam. I was thinking 13 to 1.
     
  4. Eric Sponberg
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    Eric Sponberg Senior Member

    dionysis,

    You give yourself two reasons why you do not see superslim boats--accommodation and draft. People don't like longitudinal spaces, they like boxier spaces. Vessel beam is one of the most important dimensional factors for the interiors of boats. The more beam, generally, the more comfortable and usable the space. This adversely affects performance to a degree, and that is why there is constant compromise between accommodation beam and vessel beam.

    As for draft, most harbors where boats are kept have draft restrictions. Here on the east coast, if your draft is over 6', your cruising and gunkholing grounds are restricted. Of course, you can design a lifting keel to get around that, but not everyone wants a lifting keel.

    There is also the stability factor. Stability comes from two places, low center of gravity and hull form, because stability is dependent on the distance between the center of gravity (weight) and center of buoyancy (submerged volume) in relation to the metacenter. In a very slim hull, you negate the effect of the submerged volume. To a point, wider beam, to gain stability, can help performance.

    Eric Sponberg
     
  5. dionysis
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    dionysis Senior Member

    You are right Eric: people can use beam better than length.

    Nevertheless I am interested in the principle of the idea.

    If this hypothetical boat was to be 8 ft wide, for accomodation's sake, 100 ft long, and has a lifting keel that can range from say 20 ft to 6. What then?

    (I know it sounds strange, but beam has been taken to extreams too, and lifting dagger keels are common.)

    I wonder what the sailing characteristics of such a long and slim boat would be? I think they would be very good. With a canting keel you would not be sailing on your ear all the time too.
     
  6. Eric Sponberg
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    Eric Sponberg Senior Member

    In general, narrower is usually more efficient than wider when sailing to windward. That is, you can make speed to windward with less energy, as you surmize. However, I am sure there are limits to beam, draft and length that are practical, and there may be disadvantages that appear when sailing on other points of the compass. That is, at what point is the boat so long in relation to its beam and keel draft that you no longer gain any extra efficiency, or that you reach adverse effects at other sailing angles? I don't know the answers to those questions. Sounds like it might be a good study for a naval architecture student at college.

    E.
     
  7. Paul B

    Paul B Previous Member

    Buildable?

    Sail boats need a certain amount of stiffness to perform. Trying to make something as long and narrow as mentioned probably isn't buildable with current materials. At least not of you want to make it stiff enough and light enough.

    The current AC boats, at about 80+ feet length and 3.5 meters beam (mixing units?), are becoming a structural problem.

    The other thing to remember is the boats need to be sailed. You need correct sheeting angles, width for shroud base, etc. The Farr (Oracle) boat in the last AC had bulges at the sheerline to get the sheeting angle they wanted. You could argue the use of deck spreaders for the rigging, like some of the Open 60s used, but these seem prone to failure.
     
  8. gonzo
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    gonzo Senior Member

    You are talking about the old plank on edge british designs. They have several disadvantages. The narrow beam makes them heel initially which makes the sails loose a lot of power. Because they heel so much the hull shape is very asymetrical. The wetted surface is much greater than in a shallow hull. The displacement is much higher than in a shallow hull. Also, they are wet and unconfortable.
     
  9. Paul B

    Paul B Previous Member

    Think modern

    No, that's not what is being discussed.

    The initial post said, "Take one hull of a big racing cat and put a deep bulb keel under it..."
     
  10. gonzo
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    gonzo Senior Member

    What is the significant difference?
     
  11. Paul B

    Paul B Previous Member

    Pull the other one.

    I really can't tell if you are serious or not.

    To give you the benefit of the doubt, the old Plank-on-Edge types were extreme high deadrise, deeply veed, heavy displacement, full keel "things".

    A modern racing cat is lightweight with shallow, semi circular sections under the water.


    The two types couldn't be any more different.
     
  12. Willallison
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    Willallison Senior Member

    As a powerboat guy - I'll ask what I consider to be the obvious question - what holds up the mast? With so little beam, surely stays won't ?
     
  13. SailDesign
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    SailDesign Old Phart! Stay upwind..

    Will, since the keel foil will be extremely small in cross-section, you would just keep going and call the top end the mast. Install a rack and pinion, and you have a reefable mast with a variable-depth keel for those big storms ;-)
    I know, April 1st is not for a while, but I like to get ready well ina dvance.
     
  14. Guest

    Guest Guest

    Have you patented that?

    Max draft/min sail for the breeze, min draft/max sail for the runs.


    To answer Will, lots of spreaders is the solution. Using the hypothetical of an 8 foot beam, that would give a chainplate width of probably no more than 3.5 feet (Bmax will not be at the mast).

    I doubt you would want a shroud angle less than about 11 degrees, so the lower spreaders would have to be about 18' above deck. So a 100 foot "I" would require no less than 5 sets of spreaders, plus jumpers for the fractional topmast (assuming masthead kites).

    Depending on the displacement and wetted area you probably need more sail than that (taller rig), so you might be up to six sets of spreaders.

    If you did use a 100' "I" and a 3.5:1 aspect headsail you would have a "J" of about 28.5'. Sheeting a non-overlapping jib to the rail (about 3.5') would result in a 7 degree sheeting angle. That's AC boat angles. If the rig pumped up to a 120' "I" and you used the same 3.5:1 aspect ratio, the sheeting angle would be less than 6 degrees. That's not going to work.
     

  15. Paul B

    Paul B Previous Member

    Timed out again?

    The above post is mine. It seems if you don't write quickly and post right away you time out and, even though your message still shows your name, you are shown as "guest" when you submit the reply.

    Can this be fixed?



    I should also add that the rig mentioned in the post above better have a very stiff structure supporting it. If there is much flex the angles will drop below the point of no return, leading to a big mess.
     
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