Stepped Hulls

Discussion in 'Powerboats' started by Ryon Macey, Oct 26, 2001.

  1. Ryon Macey
    Joined: Oct 2001
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    Ryon Macey Junior Member

    I am currently building an inboard runabout with a stepped hull, and I am in need of some engineering information. Can anyone recommend any texts on the subject of stepped hulls. Thank you in advance for any information.
     
  2. Jeff
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    Jeff Moderator

    There was a discussion on step design in the boat design forum a while back that you might find interesting:
    See http://boatdesign.net/forums/showthread.php?s=&threadid=31

    This is a subject that it seems a lot of people (myself included) want to learn more about, but unfortunately there just doesn't seem to be a lot of accessible information. In the coming months I hope to present some articles on BoatDesign.Net on the subject, but I haven't had a chance to follow up on a couple of good leads yet.

    One of which is that the Florida Atlantic University holds a collection of articles by the late Bob Hobbs on the subject.

    22
    Robert W. Hobbs
    Model Test Results for a Two-Step High-Speed Power Boat Hull Form
    R (17)

    1
    Eugene P. Clement, James D. Pope
    Stepless and Stepped Planing Hulls-Graphs for Performance Prediction and Design
    W (1)

    18
    A. M. Lippisch, R. E. Ablton
    Stepped Planing Boats, Some Full Scale Test Results
    Society of Naval Architects and Marine Engineers, May 1970
    R (13)

    16
    John Teale
    Testing Stepped Hulls in America
    Motor Boat and Yachting, 12 August 1966
    R (11)

    17
    John Teale
    A Step Forward
    Motor Boat and Yachting, 15 September 1967
    R (12)

    10
    Robert W. Hobbs
    Model Test Results for a Two-Step High-Speed Power Boat Hull From
    DD (3)

    See http://duedall.fit.edu/zborowski/oe_na_library.htm
     
  3. Stephen Ditmore
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    Stephen Ditmore Senior Member

  4. Jeff
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    Jeff Moderator

    For some reason that boattest.com link doesn't work in my browser - it appears the pg-rdrct asp script is only working if I come from http://www.boattest.com/general-info.asp?ID=213 (which doesn't make sense but seems to be the case - though it might just be my browser too.)

    I like the fact that Regal is focusing on technology - a few years ago I thought they had a nice look but didn't really have anything to set them apart - I think they've gained some ground. Nice marketing graphics too at http://www.regalboats.com/fastrac_flash.html (except for the bubbling water which might need a little work :) )
     
  5. Stephen Ditmore
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    Stephen Ditmore Senior Member

  6. Guest

    Guest Guest

    Is Regal using a Harry Schoell step or is this their own similar version?

    I also found this interesting:

    [7] LAMINAR FLOW INTERRUPTERS (LFI'S) Created to provide smoother, safer and more efficient turns, LFI's work just like miniature steps. While the boat is turning, they aerate the bow portion of the hull, reducing drag and bounce. You get superior handling and tracking ability during mid- and high-speed cornering.

    I had always thought that steps and aeration were the enemies of good cornering as you wanted the hull to keep a tight grip on the water, not be loose and slide?
     
  7. Jeff
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    Jeff Moderator

    To me Regal's Fastrac hull looks very simiar to Schoell's trademark step design - in fact I was surprised to not see mention of Schoell when describing the step. I did some digging and it looks like there was a lawsuit starting in 1997 over this, and the outcome was in Regal's favor because of the wording of the original Schoell patent revolving around whether a 12 degree hull was "generally flat" or not. There is a note that the Schoell patent was revised to include additional claims and if this is the case I wonder if Regal's Fastrac hull design is grandfathered in... In any case, somewhat interesting reading.

    http://www.ll.georgetown.edu/Fed-Ct/Circuit/fed/opinions/99-1511.html

    http://www.ll.georgetown.edu/Fed-Ct/Circuit/fed/opinions/99-1511.html

    http://www.rbbi.com/folders/pat/regal/regal.htm
     
  8. Jeff
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    Jeff Moderator

    In thinking about it, I'm wondering if I should have posted that since it was a delicate situation and I can see both sides of the case to some extent. In one sense, how broad of an idea should be patentable, and in the other, should semantics get in the way of protecting real intellectual property. But I do find this very interesting, both in terms of step design and also in terms of the protection of design ideas, so I think I'll leave it and hope I don't offend anyone.
     
  9. Jeff
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    Jeff Moderator

  10. micael
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    micael Junior Member

    Hi !

    You can look at the design of Ocke Mannerfelt.

    I think he has a good design on stepped hull ( he has a design patent on the hull but it can give you some ideers i hope)

    you can see it on www.ocke.se


    Br
    Micke
     
  11. Ole
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    Ole New Member

    Step size

    Hi

    I'm a 5.th year student at the Norwegian University of Science and Technology were I study hydrodynamics.

    I'm going to design a multistepped (2 or 3 steps) 30 ft. hull and looking for information to deside the length of the surface between the steps, and the "hight" of the step itself.

    I guess it depends on the speed of the boat. My boat will operate in the area of 50 - 70 knots.

    I'm also interested in different shapes of the step. Does the shape really matter?

    Thanks

    Ole
     
  12. Stephen Ditmore
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    Stephen Ditmore Senior Member

    I'd advocate a single step forward of the LCG sized so that the planing surfaces are at a 4 deg. angle of incidence at the target speed. After towing models in the canals of Ft. Lauderdale Harry Schoell settled on a step that is radiused looking up at it, like a radiused transom.

    Here's a longer explanation:

    Like wings, most planing boats tend to generate their best lift/drag numbers at about a 4 degree angle of incidence, and tend to generate most of that lift along the leading edge of the whetted surface area. The problem is that, once into the planing regime, the faster one goes the more the boat lifts out of the water and the farther aft this leading edge moves, altering the trim. A boat without a step relies mostly on the LCG (defined as the distance of the center of gravity forward of the transom) augmented perhaps by trim tabs or a trimmable outdrive unit, to maintain its proper planing angle. For every hull shape and speed there's an optimum LCG, but as speed increases the optimum LCG moves aft.

    Stepped hulls have two advantages - they can maintain near optimum trim across a wider planing speed envelope, and the amount of whetted area that is not near the leading edge, and is therefore not producing lift efficiently, is reduced. Boats with a step or two aft of the LCG are achieving this by effectively moving the transom forward as speed increases. The type that Schoell, Morley Smith (fifth "Power Boat Symposium," Southeast Section SNAME, 1993) and I prefer has a single step forward of the LCG, so the running trim is more dependent on the size of the step than on the exact location of the LCG. As Smith points out in his paper, these hulls are at their best in combination with a surface drive, and do not work well with a jet drive since the intake would tend to ventilate.

    I think the important issue is optimization of the running trim over a range of planing speeds, and in my experience a single step forward of the LCG works better than multiple steps.
     
  13. tom28571
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    tom28571 Senior Member

    Stephen,

    It's true that a trim angle of about 4 degrees has been found to minimize the combination of wave and friction drag forces. A single step can indeed give a fairly constant trim angle depending on the height and location of the step. That is the good news. The bad news is the same thing. The single step makes the longitudinal stability so high that the boat can not respond to wave action and forces its way through waves. This makes for a bone jarring ride. That is why so few try to build a single step boat for offshore conditions.

    I'd also argue that efficiency of the step hull does not depend primarily on maintainiing the optimum trim angle since some other boats can do that pretty well also and are not as fast as the step hull. The single step creates two leading surfaces contacting the water where hydrodynamic lift is at its optimum and makes it possible to use a higher bottom loading factor (lbs per sq ft of bottom contact area) with less surface area creating fricton drag.

    I suspect that the use of multiple steps is an attempt to get some of the benefits of the step hull and minimize the increase in longitudinal stability (metacentric radius) which will make the hull more sea kindly.:)
     
  14. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Thanks, Tom. Though I stick by my preference, I think your remarks are an important contribution to the discussion. I differ in that I haven't been convinced that encouraging pitching is the right approach to absorbing wave energy. I would instead favor increasing deadrise. (note: I generally favor a flat "pad" along the centerline, so I'm talking about the deadrise on either side of such a flat.)
     

  15. tom28571
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    tom28571 Senior Member

    Stephen,

    I came to be interested in powerboat design after retiring from a career as an electrical engineer so I'm still very much in the learning stage. From what I see in the literature though, there seems to be very little agreement on planing hull design among the various designers and builders.

    For instance, the relationship between keel pads, planing strakes and chine flats. These all seem to have some common goals but which is best and when? Why do you choose pads over chine flats? I think I see some reasons for some circumstances but would like to hear your point of view. I understand your interest in the deep V for its ability to ease the ride in waves while maintaining a fairly high degree of longitudinal stability but this comes at the price of weight, power efficiency, cost and loss of low speed performance. I guess these issues will remain points of discussion and argument for a long time.

    Steps are very interesting but seem to be useful only at high speed and suffer terribly at low speed. In waves the side air entry can be blocked and cause lateral accelerations that are uncomfortable at best and dangerous at worst. Some try what they call ram air ducts and some have tried powered air inlets to create a kind of surface effect vehicle.

    As far as pure performance at speed in waves goes, I think none of our design preferences can better the powercat in waves.
     
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