Stepped Hull Resistance

Discussion in 'Boat Design' started by tkdchris, Nov 15, 2010.

  1. tkdchris
    Joined: Nov 2010
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    Location: Southampton

    tkdchris Engineer

    Am working on a stepped hull design, LWL 11.7m with twin steps. I have a basis model without steps that when run through savitsky requires 776kW to reach the design speed, however the program give 725 for the stepped model.

    My concerns are around the program i'm using - hullspeed - and the fact that the manual doesn't state that it can deal with stepped hulls. Not even convinced that Sovitsky is the correct method for stepped hull resistance.

    Any help/advice would be greatly appreciated. Also pointers towards literature on the matter would be super :)

    Chris
     
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  2. yipster
    Joined: Oct 2002
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    yipster designer

  3. HJS
    Joined: Nov 2008
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    HJS Member

    Transverse Steps

    Use this excellent paper by David Svahn. It can by extended to two steps.

    js
     

    Attached Files:

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  4. Kestrel
    Joined: Nov 2010
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    Location: Milan Italy

    Kestrel Junior Member

    stepped hulls

    Hi all,
    I'm new in the forum, I've seen a quite interesting item ... , it's since a long time that I''m interested in. It could be interesting to give a look here:
    http://www.idet.it/idra.html

    bye
     
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  5. tkdchris
    Joined: Nov 2010
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    tkdchris Engineer

    Thank You very much, this is a most useful paper :)
     
  6. JorgenBeyer
    Joined: Jan 2008
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    Location: Southampton

    JorgenBeyer JorgenBeyer

    Stepped hulls

    Hey Chris! I am pretty certain that the software Hullspeed does not consider stepped hulls. It also runs Savitsky and the difference you get must be input values...

    Jurgen Sass: how do you suggest you improve svahn's model to include two steps? I feel some of his assumptions are only valid for a single step, and he said the same thing in a meeting i had with him yesterday.

    Has anyone managed to model resistance on two steps? How would you predict the wake after the last step?

    JBS
     
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  7. Kestrel
    Joined: Nov 2010
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    Location: Milan Italy

    Kestrel Junior Member

    Hi,
    virtually it's the same approach: You have to consider each planing surface, delimited by a step, running in the wake of the one foregoing, then compute the equilibrium condition (pitch and heave) for all the planing area together, and force distribution (i.e. 2 steps, 3 planing surfaces), given a Xcg, speed, displacement, it's normally an iterative procedure (in http://www.idet.it/idra.html, that program works up to 4 planing areas - 3 steps, by an iterative method using 3D geometry definition updated each iteration). More steps in the bottom, more local area calcutions must be done, and a bit more complicate to seek global equilibrium. The process must be done by software. Depending on error tollerance wanted, it involves thousands of iteration normally, even for a single speed data.
    K.
     
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