Offwind hullshapes an VPP prediction...

Discussion in 'Sailboats' started by ErikG, Aug 13, 2002.

  1. ErikG
    Joined: Feb 2002
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    ErikG Senior Member

    Here are some ramblings from my puzzled mind...

    I'm trying to get a better understanding of the relationships between hullshapes, planing (or semi-planing if you prefer) and the results I've seen from VPP's.

    According to Larsson & Eliasson [L&L hereafter] offwind speed and planing ability is promoted by a flat and horizontal aft section. They also say that there is not much information about this in hydrodynamics litterature.
    How than can I (or someone else) be sure of the higher speed predictions given by VPP programs? Either there is some sort of solution or the high speed VPP results are just bogus... are they?

    Howabout submerged or almost submerged transoms, L&L mentions submerged transoms is standard when it comes to highspeed powerboats but that they induce to much drag on a sailingboat.
    What about a "perfect" transom ie. one that ends just at the waterline and at higher speeds "probably" becomes immersed?

    What general tips can you give me regarding hullshapes?
    Light displacment is obvious, flat aft sections according to L&L but what constitutes a flat aft body? how large part of the lwl should be flatter compared to more U or V shaped sections? How wide should it be (impossible question I know)? Should the whole of the wetted surface be wider and shallower than non-planing boats (I guess it should, but how much)?

    And then I have the U vs V question... it's obviuosly a tradeoff between pitching and slamming and I'm sure there's more to it than that too.
    Any comments on this?

    I've rambled long enough for today... :eek:
    Don't fall asleep y'all

    Erik
     
  2. ErikG
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    ErikG Senior Member

    Anyone?

    :?: :confused: :!:

    Oh, and the U vs V question is related to the forward part of the hull shape.
     
  3. Steve Hollister
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    Steve Hollister Junior Member

    My advice is to not send out press releases if a VPP program says that your boat is a breakthrough. The VPP is not that accurate. As I mentioned in another message, The VPP consists of two parts; the Lines Processing Program (LPP) and the Velocity Prediction Program (VPP). The VPP uses empirical equations for things like residual resistance (see pg 75 in the second edition of L&E). Although this formula is based on tank testing results, it still simplifies the complex resistance of a 3D shape into a formula based on dimensions like Cp, LCB, Bwl, Tc and volume. This means that the VPP says that all boats which have those same principal dimensions have the same resistance.

    The VPP is no CFD program so don't expect it to tell you much about slight changes in hull shape or changes in shape that are way outside the range of shapes used in the towing tank. [Actually, CFD programs have their own accuracy and validation problems too.] The VPP is best used as a relative tool and not as an absolute calculation tool - I would never yell at my crew if the boat wasn't making the target numbers from a VPP polar speed diagram. The best way to use the VPP is to compare boat A with boat B, with boat C, and so on, making one type of change at a time. [parametric analysis] You can then look at trends and try to relate them to real life. Also, these changes should be based on those principal dimensions that the VPP formula uses for its calculations - Lwl, Bwl, Cp, etc. and not subtle changes in hull shape.

    Many people call me wanting to know if there is some "high-powered" program that will answer all of their resistance and performance questions. Usually, the answer is not what they expect. It's actually kind of nice to know that there are still very many unanswered questions. Some of these are best answered by personal experience and evaluation. Get out on the boats and tramp through the boatyards in the winter to study the hull shapes and talk to people. Use your common sense. If you spend too much time in the office, you might end up with a "Mariner" transom.

    Regards,
    Steve Hollister

    P.S. I know that I didn't specifically answer any of your questions! I would have to say, however, that everything is a trade-off and a lot depends on how you are using the boat. For sailboat racing, a lot depends on the type of racing and the expected conditions.
     
  4. Fco.Lopez
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    Fco.Lopez Junior Member

    Open 60' boats usually has a transom like you explain. It's because they are design only for surfing at very high speeds, boats with a lot of sail area and very little weight.

    Nowadays, with new open60' with a little focus on upwind performance, transom are a little different, a little up.

    In a boat with an usually sail area/desplacemnt coef. if you put a transom that ends just at the waterline when it pitch upwind, the transom will be sumerged, and it increases drag.

    :cool:
     
  5. gonzo
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    gonzo Senior Member

    I think the size of the boat also has a huge importance. Smaller boats, up to 25' or so, have a large percentage of the displacement in crew weight. That means it is moveable ballast. A transom for a boat in that size range, can be very flat. The crew moves fore and aft to change trim. In larger boats dynamic lift and changes in trim due to wave creation determines the aft section shape. From a different perspective, look at powerboats. instead of sails they have engines. Powerboats go straight downwind at all times. The aft sections of a planing powerboat apply to sailboats. However, they won't be much of an upwind sailor.
     
  6. SailDesign
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    SailDesign Old Phart! Stay upwind..

    I think the bottom line here, to back up what Steve Hollister says (Hi, Steve!) is that the VPP is a comparative tool only. It will provide estimates to real performance, sure, but it is truly useful when comparing one boat to another.
    Steve
     
  7. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Hi Steve and Steve. I think Steve B's question is an interesting one, but in order to get a quantitative answer you'll have to go farther in asking a quantitative question. Is there a particular Froude Number range you're interested in?

    I own the first edition of Larsson, not the second. The first seems to suggest that a boat optimised for surfing to a certain Froude Number may have a lower Cp than a boat designed to be driven at that Froude Number (under power, for instance) owing to a longer, flatter run aft. I have no idea if he's right. I don't have the book in front of me, but I think he attributes surfing to a diminution of the low pressure on the after portion of the hull as a wave moves under the stern.

    When Eric Sponberg was designing Tom Doyle's boat Peter Schwenn did seperate VPP runs for displacement and plaining modes. I'm not clear whether he did this using two different programs, but in any case the results are discussed in Eric's Professional BoatBuilder article. Hydrocomp's software also utilizes different formulae for different froude number ranges. Perhaps Don McPherson of Hydrocomp is someone who could begin to answer your question.

    It seems to me that the semi-displacement range is what's really at issue here, modified by the effect of a wave under the stern if we're talking surfing. One thing I'd like to know: at what speed does water breaks clean from the transom such that transom drag is no longer an adverse factor? Does this scale with Froude Number, or differently?

    One thing that seems pretty consistent in powerboat design is that a planing surface wants to be at about a 4 degree angle of incidence for optimum performance. To get the flat run of a sailboat to that angle typically requires considerable trim by the stern. Also, the bow typically sees more water pressure than the stern. The hull shape I'm contemplating for my Classic Moth is flat down the centerline from about station 1.5 to about station 5.5, transversely and longitudinally, with a positive angle of incidence at static trim.
     
  8. SailDesign
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    SailDesign Old Phart! Stay upwind..

    Steve D says:
    "One thing I'd like to know: at what speed does water breaks clean from the transom such that transom drag is no longer an adverse factor? Does this scale with Froude Number, or differently?"

    Well, I can attest that it scales with all sorts of wierd things. ;-)
    On "Coyote", we tried to keep the transom dry at rest, by a couple of inches, but were unsuccessful when the boat was finished. We had maybe 1.5" immersed, and by 3 knots (Yes, Three) the transom was dry with a little baby rooster-tail type thing happening, just like a model being towed.
    Bear in mind that the LWL was close to 60', but the max canoe boady depth was only 13.5"
    I think the latter was responsible, not Froude number ;-)

    Steve B
     
  9. gonzo
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    gonzo Senior Member

    Do you guys agree that the size of the boat matters? In dighies the weight of the crew changes trim by several degrees.
     
  10. SuperPiper
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    SuperPiper Men With Little Boats . .

    I agree with Gonzo that in small craft the crew can shift weight to trim.

    But, I am also curious to know about getting the water to stream smoothly from the transom. At 3 knots, my little cruiser seems to bury its transom by a couple of inches unless the crew moves forward out of the cockpit. At rest, the transom is a few inches above the water mark.

    Is there a transom shape or detail that eliminates this drag?
     

  11. SailDesign
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    SailDesign Old Phart! Stay upwind..

    "Is there a transom shape or detail that eliminates this drag?"

    If there were such a beast, it would be worth a fortune, and widely advertised.
    Sadly, I don't know of one. The trick is to get the transom clear as often and early as possible, or apply lots of power ;-)
     
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