Displacement distribution

Discussion in 'Sailboats' started by Wardi, May 12, 2005.

  1. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member

    Hull Form

    While this is not strictly yacht design, a page in Principles of Naval Architecture defines the ideal Fn, Cb, Cp, LCG, Section shape, Entry angle, ect. from the slowest boat to the fastest.

    I use this as a guide when optimizing Hull Form or when choosing which hull would best suit the performance that is being asked for.

    Unfortunately, the simple presentation requires that you know much about the basic formula calculations to arrive at the correct hull form.

  2. Wynand N
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    Wynand N Retired Steelboatbuilder

    Here is a rough "curve of areas (volume)" pencil sketch of a boat I'm busy with now.
    The max distribution of displacement is slightly more forward than usual, and the bow sections a bit fuller. Because of the beam carried well aft the object is to prevent bow from burying when heeled. This curve shows a well distributed displacement over the LWL. At about 20 degrees heel the WPA reduces by a fair margin.
    This boat is designed to sail the coastal waters of the notorious east coast and Capes of South Africa. Thereby it is heavy weather boat with a static CP of 0.62 (keel included)

    No mathematical model was used other than common sence and past experiences. What is paper perfect, is not nesseccary ideal in a seaway, as conditions changes with every puff of wind and wave.

    Attached Files:

  3. DavidG
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    DavidG Junior Member

    Curve of Areas

    There is a Sparkman & Stephens curve of areas in Skene's, I guess that this is probably as good as any (respect I think is the current buzz word).

    I think that the Delft Series works well for dinghies such as National 12's at displacement speeds. at planing speeds more dynamic devices such as chines and trim come into force.

    The Delft research is borne out by the fact that high prismatic 0.6+ dinghies tend to be sticky in light winds, the skill is working out the compromises to be made between the two requirements.

    I would also commend Bob Ames SNAME paper which is quite analytical about I14 designs of the early 80's.

  4. BillyDoc
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    BillyDoc Senior Member

    Eyeing the Pump Effect

    Brian Eiland posted a very interesting paper by Nils Lucander in the Twin-Keel Research forum ("Eyeing the Pump Effect" http://boatdesign.net/forums/attachment.php?attachmentid=2038) which makes the very reasonable suggestion that forward resistance is strongly related to wave production. This paper, in turn, suggests to me that minimizing resistance could be done with CFD --- just tweak the hull until you get minimal waves.

    Has anyone tried this? And a related question --- is there a CFD program up to the task that anyone can afford?
  5. CT 249
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    CT 249 Senior Member

    It's interesting to recall that a few years back (mid '90s IIRC) there was a big hullaballoo in IMS racing, because boats with more volume 3/4 aft got a huge drop in rating for a very small drop in performance. So 40 footers were being rolled over and padded out under the cockpit.

    Basically, the designers' VPPs said adding volume about 3/4 aft would slow the boat down fractionally, whereas the IMS VPP said that adding that much volume in that area would slow the boat down dramatically. In other words, the various VPPs had radically different ideas about the ideal curve of areas.

    It just showed, to me, how hard it is to quantify things, because almost every designer got "caught out" by the fact that their experience and VPPs were so at variance with the IMS VPP which is derived from the Delft series IIRC.

    I know this is not an IMS thread and I'm not looking into the reason for the differences. I just thought it was interesting that the skills and the very highly developed VPPs from the IMS people (on the one hand) and Farr and Reichel/Pugh etc (on the other hand) could come up with such widely-differing results. Therefore, it sort of underlines that there is not some simple way of using a VPP or experience to work out what is best, and it underlines (once again) how using theory or just writing a VPP code from tests does not come up with the "truth".
  6. Mikey
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    Mikey Senior Member

    Quote from Wynard
    Max at 53% something, interesting.

    Do you mean a straight curve of areas of stations or actual volume (or area/Vol^2/3)?

    Last edited: Jun 3, 2005
  7. Paul B

    Paul B Previous Member

    Not only 40 footers, but also a very well known 66 footer, virtually unbeatable before the mods (so why do them?).

    Funny things happened to some of the boats. Designers would draw up the volume increases and sometimes yards would build on the shape, but miss in the laminate somehow. So some boats ended up with increased volume, but decreased bouyancy aft (due to the additions being significantly heavier than the same volume of H2O). This lead to increased waterline length and something less than the designed rating benefit.

    Lots of money spent to sail more slowly. Sad but true, common in IMS.

  8. Mikey
    Joined: Sep 2004
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    Mikey Senior Member


    Would you share Cp without keel and rudder for the boat with us?

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