Stepped hulls- 50 knots plus. Why?

Discussion in 'Boat Design' started by jakeeeef, Jun 25, 2023.

  1. jakeeeef
    Joined: Sep 2009
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    Location: Hamble

    jakeeeef Senior Member

    I've heard in several places, none particularly trusted, that the tradeoffs for stepped hulls only start to pay with very high performance boats. Normally 50 knots plus is the figure offered.

    1) Where has this come from and how true is it?

    2) What is the mechanism? Is this because high speeds are required to set up the pressure differential that is large enough to draw air into the centre of the he area behind the step, all the way from the sides of the boat? So at slower speeds air cannot be drawn enough into the middle areas of the boat, so not enough air is drawn in to offset the drag of water going over the step. Is this why modern stepped hulls use aft raked step channels? To make an easier path for the air to get the the keel area at lower speeds?

    How does it differ with step design?. If my rough premise in 2 is correct to some extent, then the deadrise of the hull must be significant in this. In a higher deadrise hull you are having to suck air deeply underwater against its natural inclination. In a flat bottomed hull, once it's planing you are only having to move the air sideways ( roughly).

    I'm wondering whether steps that are aerated directly from above will pay off at a lower speed than traditional single hull designs. I'm not talking about pumping air mechanically into steps from above, although I understand this can pay off from very low speeds. I'm taking about the inside, forward part of the step being a slot, entirely open to the atmosphere by mounting the steps on a double hull. Inner hull keeps the boat from sinking, outer hull allows directly aerated steps.

    What my question is, is does the benefit of hull steps come at a lower speed ' than usual', if the step is open to atmosphere from above across its entire width (apart from some small vertical vanes to achieve structural integrity between the steps)?

    With this system I'd also be inclined to incorporate small downturned planing rails in the shoe area of the outer, stepped hull to discourage the air/ water air mix, exiting the sides. Perhaps also a full width interceptor at the transom to keep an air chamber.

    I've got a boat at the moment that can easily accommodate a slip on/ bolt on central stepped hull component that can be easily arranged to obtain all its step air through a wide, open transom, plus perhaps a bit of ram air from the front. But it's a 30 knot boat, so wondering if it's worth bothering with.
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