Co-efficients - help appreciated

Discussion in 'Boat Design' started by JRD, Dec 17, 2010.

  1. JRD
    Joined: May 2010
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    JRD Senior Member

    I have been working on a design for a 14' single handed trapeze racing dinghey. This is intended to be capable of planing offwind in most conditions and upwind in about 15knots of wind. This is based on SCP and my experince with boats with similar length, weight and rig geometry.

    The hull shape to date is based on what to me looks right, given the design displacement, immersed volume etc. (And years of looking at every kind of skiff I come across and wondering what makes it go) I have a basic understanding on the affect of Cp around hull shape, fullness in the ends and in relation to wavemaking.
    However I am not clear on what numbers I should be expecting to see for a dinghey that will displacement sail as effiently as possible in light winds with a clean transition to planing once powered up.

    I have used Hullform software to develop my design to date. This gives a block co-efficent, and prismatic co-efficent. Additionally it gives a forward and stern prismatic co-efficient. Would anyone please explain how these relate to the Cp and how they each affect the hull performance?

    My design to date is indicating the following numbers:
    LWL 4.1m
    BWL 1.1m
    Draft 0.1m
    Displacement 185kg
    Prismatic co-effient 0.635
    Forward Pc 0.551
    Stern Pc 0.736
    Hard chine, flat in the stern sections, leading to slightly vee'd forward sections, slight curvature of the floor up to the chine. (similar underwater shape to a modern I14 with a bit more stability and not quite as straight aft)

    Any practical advice would be greatly appreciated.
  2. Eric Sponberg
    Joined: Dec 2001
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    Eric Sponberg Senior Member

    The Forward and Stern prismatic coefficients are exactly that, the Cp for only the forward half of the hull and the after half of the hull, respectively. Where the "half" position is located may depend on how you define it--is it the mid-position of the overall length, or the waterline length, or some other arbitrary position that you define in your design input--that you have to check.

    Your values appear to be fairly wide apart, indicating a rather fine entry forward and fuller sections aft. I am not as well versed on racing dinghy design as others might be, but in your researches and comparisons to other designs, you might find how well, or not, your prismatic coefficients compare to those designs with known good performance. However, that is only part of the story, and you may not be able to assess displacement performance from these numbers. Low drag, high speed performance in the displacement mode has much to do with other factors, such as length-to-beam ratio, transom shape and depth, and wetted surface.

    For more information, I discussed prismatic coefficient and the other design ratios at length earlier this year at the following thread:

    For a complete copy of my lecture series on this thread, you can download a copy near the end of the thread, here, and post #264:

    I hope that helps.

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  3. Doug Lord
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    Doug Lord Flight Ready

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  4. RHough
    Joined: Nov 2005
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    RHough Retro Dude

    I think the Taser and 49er hulls were designed with almost the same goal, a humpless transition onto a plane.

    If your shape looks like those boats you are probably close.


  5. JRD
    Joined: May 2010
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    JRD Senior Member

    Thanks for your replies to my question.

    Eric, thanks so much for the technical information, I don't think I have seen such a comprehensive discussion on this matter in any book. I have some serious reading to do.

    With regard to the stern and bow Cp I will calculate the respective values by hand to see what hullform is giving me here. It makes sense but the whole of boat cp is the mean of the bow and stern CP.

    Its fair to say that the sucessful dinghey and skiff designers know what the relavent range of ratios should be but I've never seen them published, their IP, fair enough. My conclusion, is I better just get on with making it look right to me, making sure it floats, build it and see what happens.

    Doug, thanks for the link, that thread was very informative. It will be interesting to see if he has started building his 14footer?

    RHough, My design currently looks more like an modern I14 with more conservative lines. The boat I am currently sailing is a precurser for my hull design whenever it comes to fruition.

    The hull is an Aero5 NS14. I have moved and reinforced the mast step, and moved the centreboard position to suit the mono rig. (Twice actually - first time I based the location on keelboat CE/CLR theory and put it too far aft DUH. Second attempt was 8" further forward and working much better so far). At this stage I'm working on getting my new rig sorted out, square top sail configuration is looking promising so far.

    Photo attached, note I took this pic with the sail reefed so theres more area for lighter winds, I used this first time out this year in 25knots gusting to 30.

    Attached Files:

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