Axopar hull style

Discussion in 'Boat Design' started by A B, May 1, 2019.

  1. A B
    Joined: May 2019
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    A B New Member


    I did a search, but I could not find any discussion of the hull style as seen on the Axopar 28. There seem to be quite a few European boat companies creating boats with these similarly shaped hulls. Boattest just did a review of the axopar 28 and came away impressed with its rough water performance.

    It appears that these hulls are narrower and their bow area is sharp without any flair. One company that has done a few of these types of designs is named Navia Design.

    Has this already been discussed on the forum? Is this really a new style?

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  2. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Axopar is a good compromise in hull design for around here.
    Pricey, but good.
    Nothing is new any more in boats...
  3. Ilan Voyager
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    Ilan Voyager Senior Member

    Welcome to the forum.
    Nothing new. This style of hulls appeared beginning of the 20th century on seaplanes as the steps impedes the suction down of the hull by the flow of water, thus the seaplane takes off with less power and/or a shorter distance. Maybe there has been similar designs on fast boats at the end of 19th century when all the big navies were in a frenetic search of very fast boats and ships powered by steam turbines.
    For sure, that had been tried on motor boats since at least 100 years, specially on race and speed record boats.
    There are many different variants on stepped hulls, the number and position of the steps being a lot of black magic. The purpose is to inject air into the water flow thus getting an air lubrication, and getting flow separation.
    Most of these hulls, as you can see on the Axopar, have very full water entries, and that gives very harsh boats if the water is not mirror flat.
    A lot of these hulls have had the lateral track stability of a soap bar sliding in a wet bathtub.

    A summary about hull bottoms.
    Hull Bottom Technology - BoatUS Magazine

    A technical article.
    Stepped planing hull – pros and cons | JP Marine - Marine Engineering


    The 2002 Hydrafoil catamaran by Yves Parlier.
  4. Magnus W
    Joined: Nov 2017
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    Magnus W Senior Member

    The key to getting a seaplane airborne on is to not let the part of the float aft of the step in the water while ”on the step”.
    The aft part is mostly just for stability while going slow and has little to nothing to do with whats happening when on the plane (ie step).
    So the step has not the function of air-lubricating the aft part of the float, as is the case with Axopar. And as you can see on the Beriev, although not a very good example, the hull after the step as angled upwards, to allow pitching up without dragging the tail, while the Axopar runs parallel.
  5. A B
    Joined: May 2019
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    A B New Member

    Thanks for the details discussion. Would that kind of bow entry help reduce slamming in rough water if you keep it at semi displacement speeds, since more of the bow is already in the water? Does the step reduce the semi displacement region?
  6. Ilan Voyager
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    Ilan Voyager Senior Member

    No, these hulls are for very fast boats with lots of power running largely in the planning zone. The bow entry would not reduce the slamming at rather low speeds, and the step or steps becomes really efficient at rather high speed. These boats are not meant to stay at low speeds, displacement and semi displacement. It's not their purpose. Steps hulls are generally specialized for high speeds and ask for generous amounts of power.
    In the case of the Axopar the steps are more "trenches" to generate a suction of air than changes of angulation of the surfaces of the hull thus interruption of the water flow.
    To get out of the hole or pass the hump the keel angle, the planning surface, the center of gravity, the angle of attack and the angle between the propellers and the hull will be more important. That's where outboards, Z drives or surface drives will shine as it's possible to adjust the trim and orientate properly the thrust given by the propellers.
    Hulls have to be designed for the wanted speeds, and become rather inefficient when driven out of these speeds. Also hulls have to be designed also for the sea state that are more likely to encounter, ie the conditions of winds and waves prevalent in their zone of navigation.
    That you win by one side, you'll have to pay it on the other side.

  7. Joakim
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    Joakim Senior Member

    Pricey? Axopar is a cheap new generation of this XO Boats - Design by sea , which cost 2-3 times more.

    Same group of people started both companies and designed the boats. They sold XO company before starting Axopar. Axopar name is a combination of brands they have started before (Aquador, XO and Paragon).

    Long and narrow is the key to good rough water performance.
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