on the design of ultra slim yachts

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

  1. gonzo
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    gonzo Senior Member

    Paul B:
    Please read the start of the thread. It proposed using one hull off a catamaran. The pix you posted show a beamier hull with wings. Also, please cut off the insults.
    Eric:
    A standing rigging like a Cheasapeake racing canoe with outriggers for the shrouds helps avoid spreaders.
     
  2. Paul B

    Paul B Previous Member

    I think I'll just ignore those who aren't following


    I wonder what the purpose of the boat might be? Is it intended to sail all around against full race boats of similar size? Is it for daysailing/coastal cruising? Offshore distance racing?

    I did a really quick study based on your length and beam. I threw in a freeboard forward of 5' and a stern freeboard of 4'. It looks OK in some views:
     

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  3. Paul B

    Paul B Previous Member

    but then you look at the bottom view:

    Hmmm...
     

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  4. Paul B

    Paul B Previous Member

    from the stern angle.

    The shape is pretty much semi-circular below the waterline for minimal wetted area. Hardening the bilges probably wouldn't help much in a boat this narrow.
     

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  5. Paul B

    Paul B Previous Member

    side view

    This was quick and dirty. After seeing it in a model I really wonder if you want to do it this narrow. Just moving about below, getting around the artucture and mechanicals, is going to be a challenge.

    With your cat rig you might be making the bow a bit fuller, but it is still going to be "different".
     

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  6. dionysis
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    dionysis Senior Member

    Yes Paul, accomodation is the problem: 6.5 ft is very narrow inside for getting about, but only just. If you go to 7 ft all is well, but then you have a boat 91 ft long; to long. I like 84 ft length. I would rather it was a little spartan below that go longer.

    The shape I had in mind is somewhat "different":

    Imagine a circular hull, no flat deck at all, going to a point fore and aft, somwhat like a wave pearcing javelin. Around 6 ft of bow and 6 ft of stern out of the water. The advantage of such a symmetrical design is structural simplicty, low wetted surface, ability to trim for and aft to a marked degree.

    For surfing, trim aft, for ghosting trim forward.

    Put some portholes along the side would be nice too.

    For a deck you could have netting rigged on virtually horizontal stanchions on either side of the hull, meeting a little ahead of the mast.

    My scanner has gone caput so I cannot do a sketch.

    A boat like this, with state of the art canting keel, I imagine would competitively race around the world.

    I would stick to a singe stick for simplicity Eric, but I take your point: two high aspect wingmasts would make this boat sail on rails.
     
  7. Willallison
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    Willallison Senior Member

    Suddenly your LWL has gone from 84ft to 72 - and your length/beam drops from 13:1 to 11:1
     
  8. Paul B

    Paul B Previous Member

    Something like this?
     

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  9. gonzo
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    gonzo Senior Member

    Looking at the wetted surface of the hull it can't have enough ballast. With a deep keel and a large bulb it would sink several times more. It is fancy but doesn't follow the laws of physics.
     
  10. Paul B

    Paul B Previous Member

    from the bottom angle

    Again, quick and dirty. I'm trying to understand your concept.
     

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  11. dionysis
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    dionysis Senior Member

    OK, let's start at the beginning:

    Imagine a circular tube 6.5 ft in diameter and 84 ft long. Draw both ends of the tube to a point uniformly from the longtudinal center. You now have a shape that looks like a javelin only fatter. (a javelin would be closer to 40:1)

    Float said javelin in water and weigh it down so that it has a waterline length of about 72 ft. This works out to be around 8 tons. (If it were floating half in and half out of the water, (with it's two points just touching the water, it would have a waterline length of 84ft and would then displace about 15 tons.)

    Take 60% of this weight and mold it into a lead bulb 5 times longer than it is wide - about 12 ft long) and place it 15 - 20 ft below the waterline.

    The waterplane is symmetrical about the midline. (There is a slight resitance penalty for symmetrical waterline for and aft; the penalty I surmise would get bigger, the wider the boat was relative to it's length)

    The waterline does not cut the water forward, it is rounded. It is more like the AC forward waterline sections. (There maybe some argument here about pounding in a seaway with such a light design. I have not concluded on this aspect yet.). In larger and rougher conditions the bow will pierce waves like a needle.

    I hope this gives you a better idea of what I have in mind. My appologies for not being able to give you a picture, but Paul, you are doing a terrific job. I personally think the design is well worth the effort.

    Those renders are getting closer
     
  12. dionysis
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    dionysis Senior Member

    I should read a little further up the list.

    Will, waterline beam is not 6.5 ft, it is closer to 5.5 ft.

    Paul, the mathematics underpinning the hull shape are for a sears-haack body - used by aeronauticists for minimum resistance of aeroplanes beyond the speed of sound.

    I have Tom Speer to thank for the idea.
     
  13. SailDesign
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    SailDesign Old Phart! Stay upwind..

    Dionysis,
    How many Sears-Haack bodies have to operate on the interface? This is a common problem when borrowing from aeronautics. ;-)
    Steve
     
  14. Paul B

    Paul B Previous Member

    Now I get it. A javelin, like people throw in track and field competitions. I was reading your posts as a Javelin, the new F18 Catamaran.

    Later I'll see about modeling this. It is a very easy one.

    By the way, I looked at the Displacement of the two versions I did yesterday. I was using a BWL of about 5.5 to 6 feet on both. The one with the transom stern, well, I missed just a bit. On the waterline shown it is close to 29,000 pounds. So on the double ender I made the midsection considerably shallower and came in at about 13,000 pounds at the waterline. Add another 1,000 for the keel, bulb, and rudder displacement and it is only about 15% too light! Ooops again.

    Truthfully, I don't think your concept is going to work well in the real world. I'm still not convinced it is buildable, the interior is going to be really difficult to fit things and people into, and it may not sail well. With this hull shape and rig I don't think you are going to go upwind with some of the more recent big boats. Downwind, who knows, but the shape doesn't say "planing" to me.

    Another thing, there aren't any races around the world for 84 footers these days, so you'll either play alone or have to downsize to the new Volvo 70 limit.
     

  15. Paul B

    Paul B Previous Member

    Something like this?

    If this is what you have in mind I think you might want to go back and check your displacement and wetted area calculations. My program is spitting out much larger numbers than you quoted.
     

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