6m low power (10 hp) cat

Discussion in 'Boat Design' started by BrianPearson, Jun 24, 2016.

  1. BrianPearson
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    BrianPearson Junior Member

    Bernd Kohler is building a 6m low power (10 hp) cat at tthe same time as the Cataproa.

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  2. ElGringo
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    ElGringo Senior Member

    I like these kind, can't afford big motors. If it takes more than two people to move a hull, I can't afford it.
     
  3. BrianPearson
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    BrianPearson Junior Member

  4. messabout
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    messabout Senior Member

    The drawing and pictures do not indicate that the boat has aft rocker sufficient to clear the transom above the waterline. If that is the case then the boat is going to be a bit slow with low horsepower. Given enough power the boat will probably plane with its flat after sections.

    If one is to pursue the lowest hull resistance, then a vee bottom is inferior to a three panel bottom because of meaningful differences in wetted surface.

    The pictures show a very workmanlike build. The build is looking good so far.

    Please post more pictures of the continued progress.
     
  5. BrianPearson
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    BrianPearson Junior Member

  6. Manfred.pech
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    Manfred.pech Senior Member

    To understand the design philosophy of Bernhard Köhler it is necessary to have a look into the past. Bernhard has studied Physics and Mathematics and has worked in the aeronautical industries and he knows of course what he is doing.
    The 6m "low power" catamaran seems to be an update of his P5000 http://ikarus342000.com/P5000page.htm which based on the wing theory of Prof. Dr. Ludwig Prandtl ("Führer durch die Strömungslehre 11.Auflage,pp 263ff - Tragflügeltheorie-", hope there is an english version). But this is not the whole truth.
    The wing theory of Prandtl was the base for the research of Prof. Dr. Herbert Wagner and Prof Dr. Fritz Weinig (best in "Über das Gleiten von Wasserfahrzeugen", pp 205-227; Jahrbuch der schiffbautechnischen Gesellschaft, 34, Berlin 1933). Wagner and Weinig proved the validity of the wing theory/airfoiltheory (Tragflügeltheorie) physically and mathematically for the shape of the bottom of a ship/boat. And it was the birth of the hydrodynamic foil.

    [​IMG] Drawings from German "YACHT" Vol.1/1988

    Simply said: When you take the concave underside of a wing/airfoil and use it for the bottom of ship/boat it will deliver lift. If you take the upper side of the wing it will produce negative lift/suction. Rocker is O.K. - but please note - rocker is suction or negative lift and it will eat speed.


    These scientifical results have been proved in a long history of testing and boatbuilding.

    But --- this is only the first step. Every hull and every boat concept has to undergo further research and development. For instance look at the lines of Oracle72:
    [​IMG] or ARKEMA [​IMG]
    Francois Chevalier et JacquesTaglang

    The most research I know of has gone into the designs of Dr. Paul Mader. His last design maderform585 is able to be faster than a TORNADO catamaran.
    http://segelreporter.com/pressemitteilungen/segeljolle-zoom-585-fotos-vom-maderform-rumpfkonzept/
    Discussion: http://segelreporter.com/pressemitteilungen/segeljolle-zoom-585-mit-maderform-rumpfkonzept/
     
    Last edited: Jun 27, 2016
  7. ElGringo
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    ElGringo Senior Member

    Brian Pearson, Do you know if this is going to be like his 5 and 7.5 Power Cats? Or is it more like a Coach Boat?

    I love the size, weight, and simplicity of his power cats. I have spent many hours comparing the small power cat designs found on the Internet. The only thing I would change on his would be a better way to get from the back to the front. When boating alone, it pays to be quick getting from one to the other and going over the top is not easy for old guys.
     
  8. Manfred.pech
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    Manfred.pech Senior Member

    P5000e

    Found an old MULTIHULL INTERNATIONAL from March 1989 with an article about P5000E from Bernhard Köhler. The boat had been built and sailed already.

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  9. BrianPearson
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    BrianPearson Junior Member

  10. ElGringo
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    ElGringo Senior Member

    On the bottom of Bernd's catamarans you can see a slight rocker but at the aft end he puts a dove tail or slight down turn. Is there a name for that down turn? I have looked at many small cat designs and his are the only ones I remember being this way. Is there a reason for rockers on catamarans of less than 30 feet? I am asking about power catamarans only.
     
  11. ElGringo
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    ElGringo Senior Member

    Anybody?? I really thought somebody around here would know.
     
  12. Manfred.pech
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    Manfred.pech Senior Member

    ElGringo, if you want your cat plane you do not need a concave bottom at the stern.
    If you want to understand the design, please read post 6 again. The design is an adaption of the Ludwig Prandtl theory of airfoils.
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    The underside of the airfoil produces lift. If you use the same shape for the bottom of a watercraft, it will give lift and reduce skin friction for more speed or lower fuel consumption. This had been proved by Herbert Wagner und Fritz Weinig (Scientists) and a lot of tests with models and boats.

    The Atkins (N.A.) used a similar shape on some of their designs to reduce draft and found out that the shape had additional benefits:

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    Athol Burns (N.A.) had a design with a concave underwater part:

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    And an amateurbuilder:

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    But it is not easy to find the right shape to distribute lift and suction on the underwater shape and there have been objects of disappointment: Increase of drag!

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    The best results have been found by Dr. Paul Mader (maderform) of hycom. His latest sailboat design ZOOM 585 is faster than a TORNADO catamaran.

    [​IMG] and http://segelreporter.com/pressemitteilungen/segeljolle-zoom-585-fotos-vom-maderform-rumpfkonzept/

    Dr. Mader has done a lot of research and hundreds of tests within 25 years to find out the best underwater shape for sailboards, sailboats, motorboats and ships (ROROs).




    Bernhard Köhler claims that the underwater shape of his designs is derived from the floats of a seaplane. You can ask him on facebook: https://www.facebook.com/ikarus342000?fref=ts
     
  13. Steve W
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    Steve W Senior Member

    Its amusing that a cat designed 50 years ago is still a benchmark, just goes to show how little improvement has really been made.
     
  14. ElGringo
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    ElGringo Senior Member

    Thanks Manfred, you are correct that I missed almost everything you said in post 6. I have been guilty of doing that quite often in the last few years. I know that people with the most knowledge get tired of answering questions. There should be a special area on the forum for us to ask those things which have been answered many, many times. The "Search" part of the forum is really not very good. Thanks again.

    Now, on a power catamaran of 6.5 to 9 meters why would you need a rocker?
     

  15. Rurudyne
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    Rurudyne Senior Member

    I seem to recall reading somewhere, in relation to planing hulls, that a chine angle that is slightly downward benefits a boat by permitting it to ride at a lower pitch while on plane.

    Now, as I understand it, slender sailing hull craft (trimarans or cats) tend to be displacement yet being light weight they also have characteristics of planing hulls, so perhaps the negative chine in that design is an attempt to regulate excessive pitch at speed?

    But with power cats intended for power only rocker should have the same effects on performance as any other planing craft, arguably resulting in a bow pointing more skyward.

    Here we get into another innovation I've seen discussed where the presence of a hollow like is being talked about, followed by a negative chine angle, is used to maintain a low bow angle at speed. As I understand it, in that design they wrote of it like a multi point hydroplane and may have used a step rather than rocker as well. I'm sure there's multiple someones hereabouts that can give a much better account of the idea than I just did.
     
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