Stepped hull pros and cons

Discussion in 'Boat Design' started by silentneko, Mar 6, 2014.

  1. silentneko
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    silentneko Junior Member

    Thinking of making a smaller planing hull in the 15-17' range. I see a few boats out there with stepped hulls. What are the pros and cons?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Do you see these stepped boats in the size range you specify, or in the waters you intend to be boating on ? If not, they are probably not going to have any real advantage for you.
     
  3. IMP-ish
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    IMP-ish powerboater

    What speed are you at?

    Stepped con --- can be looser in turns.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Step shapes and locations are speed dependent, so the most efficient applications are targeted, meaning if you don't know where to place them or how to shape them, fooling around with steps will likely just drive you nuts. Simply put, if you have a step hull that is over powered for the configuration, you'll run out of the effective step range and ideally they should be in a different place or shaped differently. The same is true if running below the targeted speeds, the steps where designed to operate within. Unlike a regular V bottom, steps narrow the speed range they're effective at, so lots of thought and experience needs to be put into their shape and placement.
     
  5. Eric Sponberg
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    Eric Sponberg Senior Member

    I second PAR on his advice. There have been extensive discussions on this forum and elsewhere about the science and applcability of stepped hulls. Professional Boatbuilder magazine ran a few articles about them a few years ago.

    For your application, I would recommend against using a stepped hull because they are very easy to get absolutely wrong. The reason for a stepped hull is faster speed for a given horsepower and fuel consumption. But a 15'-17'er can be plenty fast with just about any horsepower engine. If it is not fast enough, get a more powerful engine.

    In fact, for such a small boat, I generally do not use lifting strakes either, except for the chine strake. Small boats do not need the intermediate strakes that are often added to the hull between the chine and the keel. The lifting strakes are to help the boat get and stay on plane more easily, but they can be a devil to build well and reliably. On a small boat, say less than 20' or so, you simply don't need them--there are plenty of power options available to attain any given speed ("build it strong, and then add power" was advice given to me by a well-know powerboat builder a long time ago.)

    Keep it simple--on a small boat, you don't need steps or intermediate lifting strakes.

    I hope that helps.

    Eric
     
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  6. silentneko
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    silentneko Junior Member

    Thanks guys, I have been starting to see some stepped designs on smaller crafts and was wondering. As far as adding more power, well fishing shallow waters it's not an option most times because of the weight penalty. One day when they make a DI 2-stroke that weighs what the older ones did this won't be an issue.
     
  7. jarmo.hakkinen
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    jarmo.hakkinen Junior Member

  8. silentneko
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    silentneko Junior Member

    Thanks, that's an interesting concept. I think they will have some serious challenges to figure out, including how to load and launch from a trailer.
     
  9. SukiSolo
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    SukiSolo Senior Member

  10. rainmaking
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    rainmaking Junior Member

    I have owned two stepped hull boats much larger than you are considering, a 33 and 38 foot intrepid, and the biggest issue is raising the planing speed. When the water gets rough speed is inversely proportional to comfort. Also as others have said stepped hulls are fussy to get right and not easy on one-off designs. Intrepid is possibly the most successful stepped hull builders out there and even they have boats that can miss some of their handling targets.
     
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  11. Kestrel
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    Kestrel Junior Member

    interesting...
    We built and succesfully tested a stepped hull of 12' lenght many years ago (1996), inside a R&D program made just to invetigate and consolidate stepped hull design methods and related software procedures. Even if the prothoptype was intended as about half-scale model of a bigger craft, its behaviour was totally good. The design included one step and camber on forebody and afterbody, 18deg deadrise at step, and was intended to enhance performace at cruise speed, with high loads, in seaway. The design Froude Vol. N. was 5-5.3 (about 30Kt, for 4m, 40kt for 8m craft), with the small 4m model, it was possible to reach a Fr.V.N. of more than 6 (till 35-36 kts). The beahviour of the small model was so satisfactory (at FrV.N.=5 - displ.600Kg, 35Hp -) that we are, now, building a new small boat using that hull.
    So, in our experience, also small planing boats can take advantages from stepped hull architecture. Sure, an accurate design phase must be followed, sw procedures applied. No "trials and error method" is admitted.
    K.
     
  12. fastwave
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    fastwave Junior Member

    He is my simple view on it,
    Stepped hulls are popular/more effective on bigger boats since they have an issue to get the correct planing angle of 3-4deg.
    On smaller boats this can be easiy achieved by ouboard trim and weight distribultion (sit further back)
    This quickly eliminates most advantages of a step hull design
     
  13. Kestrel
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    Kestrel Junior Member

    Yes, You are right, Fastwave, I agree, that's the reason why we made trials on a small model, it was easier to change and test different mass distibutions an so on, but... if You want to adopt a high deadrise angle (for rough waters, sample), You know that optimal trim is increased, so a small unstepped hull could have a lot of pitch and be not so practical in use, this is solved by a single-step configuration. Also, a more central optimal position of CoG, permits a better distribution of masses all over the craft, while keeping unchanged high planing surfaces aspect ratio (Bw^2/Sw) and so their efficiency.
    Last but not least, starting with a small size, You have the possibility to use it as testing model for the up-scaled versions, if this is the target You have in mind....
    K.
     

  14. HJS
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    HJS Member



    An interceptor is no substitute for a transverse step.

    A step has the purpose of reducing the wetted surface and keep the boat in a given trim angle and thereby reduce the resistance at a given speed.

    An interceptor has two functions, to create a dry surface aft and create a large lift in front. Thereby reducing the wetted friction surface to a minimum.

    The interceptor should be adjustable to fit to the current load and speed.
    I can calculate all the forces satisfying after many trials at model and full scale.

    In my experiments, I have not found any problems with either the balance or taking up the boat on a trailer.

    A slightly larger boat with the same concept has recently awarded first prize in the Swedish Cruising Club design contest on future touring motorboat.

    JS


    View attachment An optimized motor boat 20121212.pdf
     

    Attached Files:

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