Look Ma, No Rocker

Discussion in 'Multihulls' started by ImaginaryNumber, Apr 9, 2012.

  1. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    In this post, http://www.boatdesign.net/forums/multihulls/main-less-rig-21274-2.html#post185146 , Triroo mentions that John Hitch's X-IT catamaran has no rocker.

    Further specs from: thecoastalpassage.com/xit.html ,
    L.O.A.-16.25m
    B.O.A - 9.75m
    Draft-0.9m (~3 ft)
    Draft-BOARDS DOWN - 2m

    My question is how well does a catamaran tack if it has no rocker? I assume that an advantage of a rockerless hull is more displacement for a given length, width and draft. But if rocker is minimally useful, why don't more catamarans have no-rocker hulls?
     

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  2. gonzo
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    gonzo Senior Member

    No rocker can work on a very narrow hull. It is also used in barges. You can't take a single aspect of a design in isolation. It is part of a structure designed for certain operating conditions.
     
  3. daiquiri
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    daiquiri Engineering and Design

    If I see it correctly, the stem knee is out of the water - so the keel must have a rocker or else the hull would be levitating above the water surface. ;) See the attachment.
     

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  4. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    Gonzo, could you elaborate under what conditions no rocker can work on narrow hulls? If no rocker can work on both a narrow hull (catamaran) and a very wide hull (barge), why wouldn't it work on every size hull in between?

    Daiquiri, here is a side view of X-IT. I can't tell for sure whether there is a knee at the stem, or not.
     

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  5. daiquiri
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    daiquiri Engineering and Design

    Perhaps we tend to see what we want to see, but... I've magnified the stem and have tried to enhance the view with contrast and sharpen tools. Here is the result:

    Cat2.jpg

    The red line in the bottom pic shows what I see. ;)
     
  6. rayaldridge
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    rayaldridge Senior Member

    I'm a little dubious about the no rocker thing. If the boat draws almost a meter, then without any rocker, wouldn't the transoms be deeply submerged?

    Maybe the hulls are double-ended, but even so, seems draggy.
     
  7. rayaldridge
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    rayaldridge Senior Member

    (Edit) I looked around and saw a couple of other designs by John Hitch, and while they do have some rocker the transoms are deeply submerged and come to a sharp edge. I think the guy is known for unusual ideas, from what I've read. Interesting stuff.

    [​IMG]
     
  8. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    Daiquiri, I can see what you've highlighted in the photos. It is even more apparent when I magnify my original image. However, I'm still not sure whether what I see is a cutaway forefoot, implying rocker, or just a wavelet masking a plumb bow.

    Of course, Ray's photo makes it pretty clear that Hitch did experiment with minimal rocker.

    So I'm still curious how difficult it is to turn no-rocker catamarans. Maybe some of our Australian or New Zealand forum members have more first-hand knowledge about Hitch's designs?
     

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  9. gonzo
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    gonzo Senior Member

    No rocker gives the maximum waterline length, therefore carrying capacity. For example, in flat water conditions it can work well. Barges that are used with large moving loads, like cranes, also benefit from the initial stability of no rocker.
     
  10. Triroo
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    Triroo Junior Member

    imgp-1541
     

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  11. Triroo
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    Triroo Junior Member

    Mvc-698f.jpg (75.6
     

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  12. Triroo
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    Triroo Junior Member

    Mvc-696f.jpg (76.1 Kb)
     

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  13. Leo Lazauskas
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    Leo Lazauskas Senior Member

    That doesn't sound correct to me.
    For the same beam, draft and displacement, a hull with rocker would have to
    be longer than one without. Of course, if the shape of the cross-sections
    was different for the two boats then you might be correct.
     
  14. redreuben
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    redreuben redreuben

    Leo, surely the waterline length is unchanged, the rocker is below the waterline yes ?
     

  15. masalai
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    masalai masalai

    Oh My God, and I thought I was a venal pontificator. . . . . . . . . . . On rereading the thread, - I think Triroo may mean "No CHINES at all as the hulls. . . " - below waterline are in W.R.C. strip plank glassed and the sides are end-grain-balsa manually glassed.

    I have been on board Johns boat "X-IT" and it handles very nicely, tacking sweetly. - - and has someone been raiding my library? Those images belong to John Hitch. - I hope you have learned something therein? - John single handed it around the northern parts of Australia and back and is now building, I am led to understand, a motor only version as at about 76+ he maybe considers that sailing is best left to the younger folk? - "X-IT" is for sale. http://www.thecoastalpassage.com/xit.html

    Many of his boats were more to do with rig and sail handling systems and of the post #7 by rayaldridge, surely you can see the heritage of those hulls and their simplicity of construction. (Wharram Tiki series)

    To have rocker does not mean that the rocker MUST extend above the waterline, Look at most modern monohulls? - - Alternately boats with full length keels, as per the old pearling luggers operating around Broome and Thursday Island - Northern Australia, are more inclined to hold a straight course... - - "X-IT" is not in that category by any means and it has an amazingly shallow draft for its LOA and BOA... His boat, like mine "CNO", have the same waterline when either loaded or lightships Johns, at 16.25M is substantially longer than mine at 11.99M... Both draw about the same water 0.9M or 3ft...

    Mine is designed and built to carry the same sail layout as John's boats (The Hitch-hiker rig) comprising dual flat cut genoa to each bow. All cats should look at this rig for ease of operation, cruising comfort and economic elegance... GRP chainplates are inbuilt on "CNO", each of the 4 points are with a static load capacity exceeding 3500kg...
     
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