Steps for a slower planing hull

Discussion in 'Boat Design' started by jakeeeef, Apr 23, 2022.

  1. jakeeeef
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    jakeeeef Senior Member

    The 'rule of thumb' for steps on planing full sized powerboat hulls is they only work efficiently upwards of (insert 40/50/60 knots number of choice here).

    What's happening here? My assumption is that at slow speeds, but still planing, say 25kn, the suction developed behind the step is not high enough to pull air all the way in from the sides, where much of the air must be coming from, all the way down ( in the case of a V hull) to the middle regions. So this means the changes of water direction over the steps at slower speeds introduce more drag than the limited levels of air lubrication can take away.

    I'm aware of pumping air into steps mechanically, and I don't want to do that on the build I'm thinking about next as that's been done a lot. But, if I was to put a bunch of channels from the deck, through to the vertical plane of a step, would that give it access to more air at lower speeds? Perhaps with a Venturi on deck too to direct some of the fast moving deck air through the channels to the step?

    Somebody must have done this as it's pretty obvious. Will I be wasting my time? Introducing the additional strength between deck and steps to prevent the hull snapping at a step when it's effectively got a great big diagonal gash through it at each step will entail some significant composites work and a little increased weight.

    I guess if pumped air lubrication works on displacement ships ( it does), then making a much easier path for air into the centre of my steps should increase the air lubrication at lower speeds?

    Will the suction behind the step, combined with a Venturi at the deck cause air to be pushed/ sucked through? Impossible question to answer unless I say speed, size of tubes angle of tubes, size of Venturi, Venturi's access to fast moving deck air, distance between deck and hull, etc.

    Just wondering what folks have done in this direction before at this stage.

    My channels will be square and voluminous and multiple, effectively a structure similar to the cellular roof panels you get on conservatory roofs, only in carbon/ epoxy for the appropriate strength.

    Isn't this effectively what we are trying to achieve with more modern scooped out diagonal steps? More air at lower speeds getting to the middle of the hull?

    And would my method also increase the amount of air pulled into the hull at higher speeds, thus allowing a shallower step to be used ( so improving efficiency at lower, even displacement speeds.)
  2. HJS
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    HJS Member

    Last edited: Apr 23, 2022
  3. Barry
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    Barry Senior Member

    It appears that you are assuming that the physical shape of the step will not allow the air to reach the center of the hull behind the step. Is there some information to support this?

    You say suction developed behind the step. ( suction for my comments for this thread would be a pressure below atmospheric pressure, less than 14.7 psia) Are you saying that a pressure less than 0 gauge exist behind the step?

    Is it your belief that main drag reduction for introducing a step is the possible mixing of air and water into the hull behind the step, reducing skin friction?

    And do you have some parameters on your boat, length, speed, hull shape etc that you can provide?
  4. DCockey
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    DCockey Senior Member

    Rob Kaidy made an excellent presentation on stepped hulls at IBEX in 2013. His slides from that presentation can be downloaded at

    His list of advantages:
    Primary Advantages of Properly Designed Stepped Planing Hull over non-stepped:
    •Reduced Resistance
    •Increased Speed
    •Improved Efficiency
    •Improved Seakeeping
    •Compelling Marketing / Hull Story​

    Kaidy's explanation of how steps provide benefits:
    • Reduced Wetted Area
    - Reduced Viscous Resistance​
    • Optimal Trim Angle
    – Planing Surfaces Operating at Best Lift/Drag Ratio​
    • Higher Efficiency Planing Surfaces
    – Multiple high Aspect Ratio Lifting Surfaces versus One Very Low Aspect Ratio Surface​
    • Favorable Effects from Planing Speed to Max Speed
    • Trim Angle Remains Optimal at nearly all Speeds, and constant at max velocity​

    Potential disadvantages and problems listed by Kaidy:
    •Higher off plane trim angles and resistance
    •Dynamic instability / Porpoising
    •High Speed Maneuvering Instability
    •Potential for Hooking
    •Surge in Seaway
    •Structural Discontinuities
    •Potential for improper or incomplete ventilation
    •LCG Sensitivities
    •Off Design or poorly designed craft with higher Resistance than Conv. Hull​

    Kaidy's list of myths about stepped hulls:
    • Air Lubricated
    • Air Bearings
    • Air ... pretty much anything
    • Ram Air Lift
    • Big Steps = Fast Boat
    • Little Steps = Fast Boat
    • More Steps = Fast Boat

    Jimboat, HJS, fallguy and 1 other person like this.
  5. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Any reading that I have done on stepped hulls cite 2 MAJOR contributors to the characteristics.

    1) increased lift due to another high pressure stagnation line, and

    2) somewhat reduced wetted surface.

    Breaking the longitudinal streamline, ie the step, creates both the additional stagnation line and the reduced wetted surface.

    stepped hull pressure distribution.jpg

    Attached Files:

    Last edited: Apr 25, 2022
  6. fallguy
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    fallguy Senior Member

    Glad you posted all that info by Kaidy because as a casual lover of boats, my Rorshak here was slow speed steps = hook.

  7. jakeeeef
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    jakeeeef Senior Member

    Thanks, I'll take a look at that Kaidy stuff, it seems that I have improperly understood what is happening with steps. If they are not about introducing air, then structures to introduce more air are something else.
    fallguy likes this.
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