Please explain hull knuckle on power catamarans

Discussion in 'Powerboats' started by GalenP, Jan 16, 2013.

  1. GalenP
    Joined: Dec 2012
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    GalenP Junior Member

    On many displacement type power catamarans there is a hull knuckle close to the water line. What is it's purpose? It does not look like enough volume to be reserve buoyancy. On some of these boats they also have another knuckle higher up in the bow area that looks like it would be for increased displacement. Is the lower knuckle just for knocking down spray?

    In the video below of the Maine Cat P-47, you can see that it does effectively knock down the spray.

    http://youtu.be/E2Xdg9Mh4WA?t=59s

    How come we don't see this on sailing cats?

    Thanks for helping me understand.
     

    Attached Files:

  2. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Those hulls are basically displacement hulls, and when running at high speeds, the water tends to follow the sides upwards. The "knuckles" or spray rails are there to detach this flow. In this process, they also produce some lift, provided they are correctly placed.

    You will find similar spray rails, both integrated, like those shown, and external ones, on high speed round bilge monohulls as well.
     
  3. GalenP
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    GalenP Junior Member

    Thank you. Do you know at what speeds these spray rails become valuable? The displacement hull I am considering would be in the 10-15 knot range, spending most of it's time at around 12 knots.
     
  4. GalenP
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    GalenP Junior Member

  5. Alik
    Joined: Jul 2003
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    Alik Senior Member

    This knuckle is not only for spray but also for extra volume in the hulls.
     

  6. HJS
    Joined: Nov 2008
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    HJS Member

    You ask why these steps are not on sailing catamarans. These steps were on some boats in the early sailing multihull development. They were removed because they are inappropriate and may cause the to boat dive. If the boat goes into a wave there is a risk that the water flows from above over the step. This leads to a strong negative pressure in the bow, which in turn may cause the boat capsizes forward.

    js
     
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