Ship shape Vs Chain

Discussion in 'Boat Design' started by LePrince, Sep 25, 2022.

  1. LePrince
    Joined: Apr 2011
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    LePrince Junior Member

    Hi all

    Sorry if my question seems to be a trivial one.

    what I know is chain hull shape (or knucke) is used for high-speed planning hulls and is not to be used with large vessels since it will cause a lot of fatigue problems.

    My question is what about small displacement vessels such as ferries and small workboats, are there any preferences for the hull shape for them (Shipshape or Knuckles)?

    how much will it affect the hull resistance for these small boats?

    Is there any reference that can guide me on this?
     
  2. Kayakmarathon
    Joined: Sep 2014
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    Kayakmarathon Senior Member

    I've heard of stepped hulls for high speed planing hulls. They look like fish scales and have vents to allow air to be sucked under the hull as lubrication at the end of a step. Is that what is meant by chain/knuckle hull?
     
  3. Heimfried
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    Heimfried Senior Member

    I don't know but I guess LP means perhaps "chine hull".
     
  4. LePrince
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    LePrince Junior Member

    yes I mean that hull which has a discontinuity on its surface in the transverse direction
     
  5. Barry
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    Barry Senior Member

    I believe that the purpose of the step is to create another high pressure stagnation point within the length of the boat.

    On this forum, other threads, there has been discussion about air lubrication on the second stage and am putting this out for scrutiny
    I have not seen information that shows the introduction of air in to the last stage of the hull. The first stage would pretty much run on green water, ie non aerated so there is little documentation that the second stage, Ie after the first step,
    would automatically introduce air bubbles onto the second surface. The second stage has another stagnation point, so I would question if the bubbles can get past this as compared to just working their way transversely
    In order to produce the second stagnation point, the first stage transom has to be ventilated. Not for air entrainment but rather to establish conditions to permit the second stagnation point

    There has been discussion on other threads that the ventilated portion behind the first step reduces wetted surface drag. It appears that the first step is often close to the halfway point back on the first stage. This area of the hull should
    be a relatively high pressure lift area. So while the ventilated area would reduce wetted surface drag, it would also reduce an area of higher pressure lift.
    Hard to say which is more effective

    I owned a Cutwater 30 for a couple of years which had some short steps in the hull. The step did not run the width of the hull so questioned the effectiveness of the close to chines steps. Ie the hydrodynamic pressure as it heads to the chines,
    from the keel decreases. To me this was more of a marketing gimmick than sound engineering. At the Seattle boat show, I questioned the sales manager as to fuel savings with the mini steps and he could not offer anything concrete as to
    their effectiveness.
     

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  6. LePrince
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    LePrince Junior Member

    I mean the difference between round bilge and hard chine (Deep V) hull shapes
     
  7. Kreso
    Joined: Aug 2004
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    Kreso Senior Naval Architect

    In general, the hull form with hard chine (knuckle) is indeed used for the planning hulls, but it can, and it is used in semi-displacement and displacement hulls as well. For displacement hulls, it is usually used as a developable surface to decrease the building costs; if the knuckle is not suitably designed, the consequence is discontinuity of waterlines/buttocks, and this results in occurring of vortexes and increased resistance, so for this round bilge would be a better choice for displacement hulls.
     
  8. LePrince
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    LePrince Junior Member

    Thanks alot for your answer, now it is clear for me
     

  9. Kreso
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    Kreso Senior Naval Architect

    You are welcome!
     
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