Cause I Don't Know Diddley

Discussion in 'Multihulls' started by ElGringo, Mar 19, 2014.

  1. ElGringo
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    ElGringo Senior Member

    I've never built a boat and it's beginning to look like I never will but, I am curious about why on the hulls of catamarans on the bottom, they always have a low point. Like a rocking chair. By looking at the side view it seems that it's lowest point is about where you would find the center of balance front to rear. I have seen it on sail and power catamarans and just curious.
     
  2. hoytedow
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    hoytedow Wood Butcher

    That is called rocker. Think of why a saber is curved. It lessens the shock of impact and makes for a smoother cut. It also aids in maneuverability.
     
  3. hoytedow
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    hoytedow Wood Butcher

    PS. I don't know Diddley either.
     
  4. ElGringo
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    ElGringo Senior Member

    OK, the name makes sense. Is it located at the center of balance or is it a measurement depending on length of the hull?
     
  5. hoytedow
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    hoytedow Wood Butcher

  6. ElGringo
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    ElGringo Senior Member

    Thanks, I understand that part but, is it just placed wherever the designer thinks is cool, or is there a way to determine where it goes.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    You certainly will not see the rockered profile you speak of in all powered cats, only the "displacement" types. It is usual for the displaced volume of these hulls to taper away gradually toward the stern, to avoid an abrupt end that would cause eddies and drag, at the speeds the boats are intended to operate. That doesn't mean you must have rocker though, it could end in a canoe stern, where all the taper is seen in the plan view. But if you have a boat where the keel line rises aft, the usual practice is for it to rise forward as well, so the centre of lateral area is not too far from the centre of the boat, which is important for how the boat sails, or turns. It is also a way of reducing wetted area, for better performance.
     
  8. ElGringo
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    ElGringo Senior Member

    OK Thanks, I just thought it might be some deep secret magic trick and I couldn't find any information on it.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Not the same principle as a rocking chair, but not a secret either. A boat that has excessive rocker, especially combined with tapered ends in the horizontal plane, is a bit of a nuisance inasmuch as they hobby-horse more than others, because the displaced volume is concentrated too much toward the middle, and small shifts in weights fore and aft, or wave contours changing, causes large changes in longitudinal trim.
     
  10. pogo
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    pogo ingenious dilletante

  11. hoytedow
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    hoytedow Wood Butcher

    I stand corrected. :)
     
  12. hoytedow
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    hoytedow Wood Butcher

  13. ElGringo
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    ElGringo Senior Member

    I looked in the book store section of the forum but could not find anything that was about Catamarans specifically other than racing sail catamarans. So, I am wondering about some other things like hull shapes. From what I can find the shape, length, distance apart and many other things have to be considered. If the shape of the hulls is decided by engines, beds, eating, and storage, why not make them the best shape and use a pod type cabin Then, also attach the diesel engine/transmission on the bottom of the pod. The bottom of the engine well could be shaped to whatever works best. Then you have one engine and no holes in your hulls. If a seal or hose leaks nothing bad happens because the pod is supported by the hulls.
     
  14. nimblemotors
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    nimblemotors Senior Member

    A boat like this:

    http://www.boatdesign.net/forums/multihulls/look-ma-no-rocker-42608.html
     

  15. ElGringo
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    ElGringo Senior Member

    Nimble, not quite what I was thinking. Those look like two gas powered outboards in the up tilt position. I'm talking about something like a Yanmar three cylinder diesel with forward and reverse transmission with a prop and pod that stays in the water.
     
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