VPP for Class40

Discussion in 'Hydrodynamics and Aerodynamics' started by Remmlinger, Mar 9, 2012.

  1. Remmlinger
    Joined: Jan 2011
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    Remmlinger engineer

    Since the Pogo 12.5 was voted as the European Yacht of the Year in the 'Performance Cruiser' category, the market will ask for this type of design. To get past the phase of trial and error, it would help the designer a lot if he had a velocity prediction program at hand that includes a realistic assessment of the vertical lifting forces acting on the hull in the high Froude number range. A polar diagram published at http://www.finot.com/bateaux/batproduction/structures/pogo12/vpp/polaire Pogo40.pdf
    shows boat speeds that are equivalent to a Froude number of close to 1. The latest data for the Delft Systematic Yacht Hull Series, which are commonly used in a VPP, end at a Froude number of 0.75 and do not take into account the trim angle of the hull, which must have an influence at these high speeds. I also do not think that the Savitsky-method can be used to predict the hydrodynamic lift since these Class40 yachts do not have a deep V bottom hull. Does anyone have a clue how the polar diagram in the above link is computed? How are you modeling hydrodynamic lift forces in your VPP?
    Greetings
    Uli
     
    Last edited: Mar 9, 2012
  2. fcfc
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    fcfc Senior Member

  3. Remmlinger
    Joined: Jan 2011
    Posts: 277
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    Location: Germany

    Remmlinger engineer

    Thanks for making me re-read the ORC rules. It is quite possible that Finot used this regression formula for the creation of the polar diagram for the Pogo 40, especially since they describe the polar diagram as only theoretical. I did not consider this formula to be a trustworthy representation of the real flow for the following reasons:
    o 59% of the database used for the regression above Fn=0.6 is not measured but extrapolated from test data at lower speeds und therefore more of a guess.
    o The parameters for the description of the hull that work well at low speeds are also used at high speeds where these parameters are not sufficient. Trim and rocker that have a strong influence on dynamic lift and hence resistance are ignored.
    o The ORC-rule states that a sail trimming moment is applied but the amount of it or the height of the center of effort are unknown. A variation of the center of effort and hence the trim is ignored. That trim has an important influence is obvious, because the rule describes a lengthy procedure to determine the wetted part of the transom overhang that is dependent on trim.

    I was hoping that someone in the forum used a more realistic approach.
    Regards
    Uli
     
  4. fcfc
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    fcfc Senior Member

    I would say it is realistic.

    Just check the average speed on minitransat, or vendée globe. And they are expensive racing machines, purposedly built for that downwind race, and driven by world class skippers, with near unlimited budget.

    The time a cruising boat, even a pogo, sailed by its owner, will go near or over twice hull speed, is next to nil compared to total sailing time.

    I would also say people will not ask of this type of design. They will only dream of. As for Porsche. Most will dream of owning one, but daily drive a Golf.
     

  5. fcfc
    Joined: Feb 2005
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    fcfc Senior Member

    More on this.

    Fast sailboats (twice hull speed) are expensive boats. They do need high strengh low weight materails with high manufacturing costs. At this level full CFD analysis (Shipflow, Fluent or other) will not change price a lot. Anyway, these CFD analysis will still be needed for keel / bulb.

    None of savitsky or similar planning analysis do include sailboat appendage, which may have significant effect at theses speeds.

    What would be much more pratical is keel bulb analysis VPP style. Even small boat low cost as the latest Elan 210 (6m length) do have bulb keel. Righting moment effect of this kind of T keel are obvious. Structural considerations are in way to be solved, or at least understood.

    Now the hydrodynamic effect (lift and drag). CFD is overkill, and overpriced for these kinds of boats.
    But no T keel in the delft appendage series. Even ORC VPP is rather crude on that point : One drag coefficient for fin, one drag coef for bulb. That 's all.

    Just look in this very forum how many post ask for bulb design optimization vs high speed sailboat planning optimization.
     
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