Variable deadrise powerboats

Discussion in 'Boat Design' started by tom28571, Oct 19, 2016.

  1. tom28571
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    tom28571 Senior Member

    In the 1960's Carl Mosely designed and patented a longitudinally stepped hull to compete with the Bertrams offshore and it was successful. I have been working on a small version and have modeled it although it is not like Mosely's in that I intend it for low planing speed and efficiency rather than high speed and big water.

    Does any one familiar with this hull bottom design have any comments?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    A picture is worth a thousand words, perhaps ? Are you talking of a hull that has greater deadrise at the centreline, decreasing in steps toward the chine ?
     
  3. JSL
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    JSL Senior Member

    Please, yes... a photo showing this hull form would really help. I have designed several boats ( 33' to 64') which operate in the semi-displacement speed range (speed length ratio of 1.4 to 3). Many other designers have as well, and few, if any, have had a longitudinal step in the bottom.
     
  4. tom28571
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    tom28571 Senior Member

    Here is one manufacturer's offering. http://www.fsboatworks.com/Design/
    The original design is from Carl Mosely

    A local dealer is: https://www.facebook.com/Brazdamarine/

    You should ignore the hyperbola claims by some of these as Mosely is the real source.

    These boats have the bottom panel nearest the keel at a steep deadrise angle and the second panel following the first step at a lesser angle with the panel nearest the chine at an even flatter angle of deadrise. The intention is fairly obvious but you can't really say what the effective station deadrise is, as is possible on other "normal" hulls.

    I first read about this in Professional Boatbuilder some years ago. I am interested in applying Mosely's principle in a different way for a different purpose.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    There have been a few attempts along these lines in my part of the world, some more concave curved than steps, the end result seems to be a boat that planes more readily, holds plane and a flatter running attude better with moderate power, but does not ride that great. Off-plane stability is generally improved.
     
  6. tom28571
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    tom28571 Senior Member

    In the early 1950's I owned a 12' Aristocraft runabout that had a single longitudinal step with the chine panel at a higher deadrise which I assumed was mainly for non tripping as the boat was fast. I think that step also reduced both frictional surface area at high speed and increased the effective aspect ratio. I am not sure I agree with all your conclusions but experience trumps opinion if proof is available.

    Just about everything has been tried in one form or another and boat design patents are rarely effective unless the patent owner is well heeled and/or lawyer-ed like Hinkley.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    I think I prefer the idea of a step with the higher deadrise at the chine, at least aft.
     
  8. tom28571
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    tom28571 Senior Member

    The highest aft deadrise on my model is on the chine panel aft of station 6. I understand why the boats in the photos I linked are not that way though.
     
  9. JSL
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    JSL Senior Member

    There is an interesting article (Misbehavioural Analysis, p 68 ) in the Oct/Nov 2016 issue of Professional Boatbuilder regarding handling characteristics of 19' 'Flats' boats with a single longitudinal step.
    Not sure if these 'steps' are really beneficial: I have seen lots of hull designs with strakes like "Bertrams' but none (other than illustrated) with longitudinal steps.
    ps: After reading the subsequent posts (below), the boat(s) in the above article have many of these 'unique' characteristics.... but the best is 'end swapping' or 'spinning out' in which occupants get thrown out.
     
    Last edited: Oct 20, 2016
  10. HJS
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    HJS Member

    You are quite right, Mr. E

    According to my calculations and experiments at full scale, it is the planing bottom width that is crucial, much more than the dead rise. Therefore, goes the boat with double chine much softer, with less trim angle and therefore less engine power than a boat with a deep V-hull.

    Calculate and try it for yourself, do not guess.

    JS

    http://www.youtube.com:80/watch?v=cXDfQqxJ_pM
     

    Attached Files:

  11. daiquiri
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    daiquiri Engineering and Design

    My two-cents worth:

    Theory is one thing, practice is another. In theory, a longitudinally stepped hull should use only the bottom-most part of the hull, like in the first picture on that FS Boatworks page:
    [​IMG]
    - hence, water should see only a very slender and power-efficient effective hull.

    In practice, the third picture seem to show something very different:
    [​IMG]

    IMO, in order for the idea to work, a very lightweight boat is required - or else the water will find a way to hit all the area it can, in order to produce the required lift.

    Having said that - even in the case of a lightweight hull, what happens to the dynamic transverse stability once the hull manages to climb on the super-narrow bottom strip? What prevents it from rolling and tripping the chine-walk?
     
  12. tom28571
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    tom28571 Senior Member

    You hit the nail on the head in my thinking Daquiri.

    Like all my power designs, low speed planing efficiency is the goal and light weight is necessary to get there. Included in that is low bottomloading. One aspect of the variable deadrise with flatter central strake deadrise runs counter to high lateral stability. So, yes that is a concern for me. Less a concern for the boats in the photos though. My way through this presumed dilemma may be to widen the garboard strakes. That, of course, leads to a rougher ride and further compromise.

    Most commercial powerboat builder don't see weight as a big issue and just add more power. Low speed planing efficiency is mostly left to us wooden boat workers who can build very light and strong if good engineering is practiced.
     
  13. tom28571
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    tom28571 Senior Member

    HJS,

    I followed that design when it appeared here some time ago. In the video, the prototype boat appears to have high longitudinal stability, the source of which which is not explained. I also work with low trim angle and high longitudinal stability achieved with aft chine flats. The double chine effect does not seem much of a factor as it is mostly out of the water and appears to provide small lift. Light weight and low trim angle with high longitudinal stability does make for achieving planing at low speed and smoother ride in some waters but can be a disadvantage at high speed sometime.
     
  14. HJS
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    HJS Member

    There are two dimensions that are important, the relationship between planing bottom surface area and the the total weight, and the relationship between bottom width and the CG, center of gravity, longitudinal stability. That's what I optimized in these designs.
    Ap/Disp^2/3 is about 7,0.
    CG / aft Chinebeam 170% in this case
    And it has so far been tested up to Froude volume ratio of 5.5 in rough seas.
    It is shown in the video and described in the report.
    There is of course more background material than that reported here.

    JS
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    You are right of course, there is a significant difference between small planing hulls that have a greater bottom area per unit of weight, and larger ones where rising significantly out of the water does not occur, to be able to run on a narrow central strip. And especially one with extra deadrise. Just a function of hull volume (and weight) increasing on the cube, and bottom area on the square, of length. Runner planks (flat or near flat) work on lighter boats going fast, by reducing the overall wetted area, but don't actually run on the plank alone, the boat would indeed chine walk madly.
     
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