Can anyone help me out regarding design of stern

Discussion in 'Boat Design' started by Hondaen, Jan 11, 2009.

  1. Hondaen
    Joined: Jan 2009
    Posts: 14
    Likes: 1, Points: 0, Legacy Rep: 28
    Location: Norway

    Hondaen Junior Member

    Hello :D

    I need some help regarding the design of the stern of a boat:

    [​IMG]

    You can see the end part of this boat, shaped as a V-hull. Sorry for bad picture. (I do not want to
    show the whole design of the boat yet).

    I shaped the end like this for aerodynamical reasons (high speed vessel). Do not worry about how to add propeller etc. I have managed to get propulsion without the use of regular propulsion technices, but that is another story :)

    I know this shape give a bigger wet area and that = drag
    I have also learned that if I want more speed, I shold cut of the stern direct. Not the way I have done on the picture.

    Can anyone help me out with some design tips? ;) Is this correct?

    How will the wave from this vessel be? I have designed the boat in FREESHIP!. I have found michlet software in order to calculate waves, but it seems like it require some time in order to get usefull information out of that program..
     
  2. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I'm not sure what you're really trying to ask, but the only portions of the after body of your hull you need to be concerned with is the underwater areas. If you have a clean release (step, transom edge, etc.) then the rest of the shape is irrelevant.

    The reversed transom shape is fine, many boats use it. Having it come to a point will increase drag as the stern squats, eventually (as speeds rise) it will begin to cause substantial handling difficulties (it'll probably walk like a hooker wearing only one high heel shoe). Also the crisp turn toward the point will create a fair amount of speed robbing suction. Consider fairing this in for a smooth transition from hull side to stern sections. It'll still cost you speed, but much less in eddies.

    Not knowing what your hull form is, what the bottom, especially the aft sections looks like, the location of the CG, etc. anything else is guess work.

    Working with free and low cost software is wonderful for developing shapes, but the software doesn't tell you what works, nor why some things should be avoided. Model testing is an option, but I think you'll quickly find why more conventional shapes are used in powerboat sterns once you do. Make a model, and tow it across a pool. The wave formation and wake will be similar to the full size model. Of course this will not tell you much without something to compare it to, so make two models, one with a conventional transom for comparison.

    On a "high speed vessel" your aerodynamic concerns can be important, but frankly, I wouldn't get particularly bothered by them. Wave formation and frictional resistance will be much more important considerations once you S/L gets over a certain point. If you can drive this hull form fast enough, you may want to consider air flow over the hull, but a simple wing (or two) can help keep her planted in the wet stuff. Honestly, with the pointed stern as you've shown here, you'll never get the boat fast enough to have to worry about hull aerodynamics. She'll be tossing skipper and crew members out of the boat, long before aerodynamic considerations are necessary.
     
  3. InetRoadkill
    Joined: Jan 2009
    Posts: 61
    Likes: 1, Points: 8, Legacy Rep: 10
    Location: San Antonio, Texas

    InetRoadkill Junior Member

    How much stern squat occurs on a typical sailboat? (35' or so.) I'm still a bit worried that I need more of a rise for my stern since the taper ends just a few inches out of the water. (Trying to maximize the LWL.)
     
  4. Hondaen
    Joined: Jan 2009
    Posts: 14
    Likes: 1, Points: 0, Legacy Rep: 28
    Location: Norway

    Hondaen Junior Member

    PAR:

    Thank you for input.

    FYI this hull is a part of an aeroplane, so once it leave the water it will gain speed.

    Can you tell me abit more why it wil have substantial handling difficulties at higher speeds?

    Plan is to build a 2.2 metre model of this hull in aluminium and test it. I love working with aluminium :)
     

  5. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Once the hull reaches 1.4 S/L ratio, the stern will have begun to squat as a function of moving through it's generated wave train, in relation to it's speed. The squatting stern is of no concern at this S/L but if speeds are increased, which would be necessary of a high speed hull, then the plane surface would terminate in a point, providing no "bearing area". She'd experience "chine walk", which is a condition where the hull would roll (usually quite violently) from one chine to the other, partly as it met the oncoming flow and partly because of general sea state conditions it was in at the time. You'd likely dip a wing (in the water) on a sea plane with this condition, which could be rough on the paint job, to say the least. Wing mounted floats would prevent this, but you'd really bury one eventually, which would break it off, spin the boat or slow it down substantially.

    On a sea plane, you have to accept some "hull" drag in the aerodynamic envelope, to make it work as a boat.

    I would think it would be possible to use an "inclined plane" hull form to get the speeds necessary for take off, which could provide much better aerodynamics on the hull shapes. The only problem is you'd still have to deal with the suction issue at take off speed. It's this effect that causes every single sea plane hull (that I've seen) to employ a stepped hull design, rather then more aerodynamic friendly shapes.

    It might be possible to use flaps or covers to change the hydrodynamicly clean shapes to more aerodynamically clean shapes after you're airborne.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.