Unclear results from VPP of a Scow Mini 650

Discussion in 'Sailboats' started by Air De, May 14, 2020.

  1. Air De
    Joined: Feb 2015
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    Air De Junior Member

    Hello everybody,

    I would like to tell you about something I have been working for some times now and with which I am now kind of stuck!

    I am a big fan of these “scow ” sailboats like the Mini 650 Maximum from David Raison or the latest Pogo Foiler. I find these boats fascinating and wanted to know a bit more about the physic behind them and why they are faster than their “pointy” opponents. I also wanted to try to get some numbers on how much faster they are.

    To achieve this I started first to draw in DelftShip one “pointy” Mini 650 and the first “scow” Mini 650, the number 747 following the lines from Chevalier Taglang which can be found on Chevalier Taglang http://chevaliertaglang.blogspot.com/ in order to get the hydrostatics of each boat.
    Then I develop a VPP in Excel, based on what I found in this forum, the book Principles of Yacht Design (from Eliasson Larsson and Orych) and the documentation on the ORC VPP.
    When the VPP was finally working I put all the hydrostatics and sail data in my VPP and to my surprise the pointy Mini 650 was always faster than the scow Mini 650.
    I know that my VPP is simplified but I was expected that the scow would be at least better than the other one in strong downwind (e.g. more than 15 kts).

    My first thought was that I might have done a mistake in the VPP and I started to look at the different equations in detail to understand the reason for this.
    However everything seemed correct and the reason why the scow is slower is the following:
    For the same boat speed and same wind speed and direction, both sailboats have the same total Lift and Drag coefficient and the Centre of Effort, as the 2 boats have the same rigs and sail area. Then the driving force is also the same for both sailboats.
    However the scow Mini 650 has a higher total resistance than the pointy 650, coming mainly from the fact that the residual resistance of the scow is almost double so high than the one of the other Mini.
    This means that for the same resistance the pointy Mini can go faster than the scow Mini.
    I also tried to increase the sail area of the scow (which possible because of the better righting moment of the boat) to see if I could get a better speed. But even a 30% increase in the sail area of the scow do not compensate the higher resistance and even if the speed increase it stays always under the speed of the pointy Mini.

    Knowing this I started to look at other VPP to see if I had not missed something in my VPP.
    I first found PCSail on the forum and lately the one from Avalon Offshore (VPP - http://www.avalon-routing.com/en/polars/ )
    Both of them estimate that the the scow Mini 650 is slower in all wind force and direction than the pointy Mini 650

    So now as I said in the beginning I am stuck. I would like to understand why I come to these results but don’t really know where or what to look for.

    Does this mean that my VPP and the 2 other ones are not “advanced” enough to take into account the extrem shape of the scow boat? For example the scow has a Cp which is outside of the Cp range allowed for the calculation of the residual resistance based on the Delft Series.
    Or is it because I have missed something really important?

    Maybe some of you can help me?

    Regards,

    Erwan
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The Mini's have developed a form optimal for reaching and downwind sailing. Those are the conditions for the transatlantic race. Is your prediction taking into consideration that the boats will plane? Also, when the boat heels over, the wetted surface is reduced, and the CG moves far to windward from the Center of buoyancy, which increases stability
     
  3. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    How can the CoG move if there is no movement of the weights? And the second question, assuming there is a variation in the relative position CoG vs center of buoyancy (not a CoG move), how and why does that increases stability? Thanks in advance for your explanations.
     
  4. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    Might be better expressed as the centre of buoyancy moving well to leeward of the COG. Same effect.
     
  5. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    No, if the CoG is moving, that means a big danger, a very likely reason for shipwreck.
    Having clear concepts, in this as in any other topic, is very important, in addition to demonstrating the degree of knowledge that one has of a certain topic.
     
  6. Air De
    Joined: Feb 2015
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    Air De Junior Member

    Really good point Gonzo, I have not taken into account planning!

    Actually, I thought about this when I started but could not find any information on this and when I was verifying my VPP I was doing it with a displacement boat, then I forgot about it!

    Now I searched again for information on how to evaluate the resistance when planning for a sailboat but could only found the estimations from Savitsky. However, the equations seem to be made for motorboats with deadrise and I am not sure if this can be applied to sailboats without deadrise.

    Do you know where I can find information on this?
     
  7. Air De
    Joined: Feb 2015
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    Air De Junior Member

    So, anybody with an idea how to implement planing in my VPP?
    Or any other idea what could be the reason for the results of the VPP to not correspond to the reality?
     
  8. Doug Halsey
    Joined: Feb 2007
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    Doug Halsey Senior Member

    You might be having non-uniqueness problems: the VPP is finding a solution, but not necessarily the only one, or the best one.

    This has happened to me in dealing with hydrofoils, but I think it is possible for planing boats as well. Planing boats' drag-versus-speed curves often have humps in them, causing there to be two or more intersection points with the sails' thrust-versus-speed curves.

    This is not just a theoretical oddity. It is often possible to go faster at a given heading by first sailing lower or higher, getting on a plane (or foils), then returning to the original heading at a much higher speed.
     
    Last edited: May 26, 2020
  9. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    In that case, do you treat it like a piecewise function? I suppose the transiton (hump) is a very narrow speed range and may be ignored for simplicity.
     
  10. Doug Halsey
    Joined: Feb 2007
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    Doug Halsey Senior Member

    The cases with multiple solutions that I've dealt with have been foilers, with drag-versus-speed curves similar to the figure below.
    It's been a while since I did this, but I think I could get either the low-speed or the high-speed solution by using different initial estimates of the speed.

    More recently (but still a few years ago), I've estimated the drag (but didn't do a complete VPP solution) of a more-or-less complete Moth configuration (with hull and both sets of T-foils) , and obtained curves like below.
    In my original answer to this thread, I was speculating that a planing hull could have a similar hump & cause trouble for the VPP.
     
  11. gonzo
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    gonzo Senior Member

    The curve with the hump is typical of planing hulls. I think using solutions for two ranges of speed makes it much simpler.
     

  12. tspeer
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    tspeer Senior Member

    When you have a hump like in the second figure above, there are actually three solutions. Two of them are stable, and one is unstable. The unstable solution is where the resistance is decreasing with speed. The faster you go, the less resistance, so you go even faster. The converse is also true - slow down a bit, and it takes more thrust so the boat slows down even faster. You experience this in a powerboat when dropping off the plane as the boat suddenly slows down and the stern wave comes rushing up to the transom.

    In landsailing, sailors call transitioning to the higher speed solution, "getting hooked up." It's why in landsailing, one is allowed to get out and push the yacht to generate apparent wind so it can accelerate to the high speed solution. Otherwise, the yacht just rolls along slowly.

    When I was programming a landyacht VPP, I found the convergence to be better if I solved for the apparent wind angle instead of the speed at a given course to the true wind.
     
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