Sailrocket 2 set to launch

Discussion in 'Multihulls' started by P Flados, Feb 19, 2011.

  1. markdrela
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    markdrela Senior Member

    Now that I think about it, a ventilated base bubble open to the atmosphere must have Cp=0 or nearly so. In reality, the bubble will assume whatever shape it has to in order to achieve this Cp=0 over its length.

    The closest way to simulate such a bubble in XFOIL is to essentially turn off the wake viscous dissipation with a very large Kdl (I'm using 8.0 now), and shape the rear of the airfoil to get very nearly Cp=0 in the near wake.

    I player around with this idea, and came up with the attached BV02. It's heavily based on transonic airfoil technology, with a deep center "spar" section with little loading, and loading in the front and back. The L/D is a ridiculously huge 20:1. This is even with a generous 6.8% thickness, and min(Cp) = -0.18, which should be good to 65 knots, or 80 knots with some modest sweep. Yowsah!
     

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  2. projetdared
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    projetdared Junior Member

  3. projetdared
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    projetdared Junior Member

    Sorry, i wrote too fast. THe theory was something like "Why are you obsessed by cavitation when you can ventilate"
     
  4. Doug Lord
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    Doug Lord Flight Ready

  5. tomas
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    tomas Senior Member

     
  6. Doug Lord
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    Doug Lord Flight Ready

  7. tomas
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    tomas Senior Member

  8. tomas
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    tomas Senior Member

    image © Helena Darvelid/ Sailrocket
    [​IMG]
    image © Helena Darvelid/ Sailrocket
     
  9. Corley
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    Corley epoxy coated

  10. Doug Lord
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    Doug Lord Flight Ready

    SailRocket

    Good find , Corley-thanks!
     
  11. Karl Wittnebel
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    Karl Wittnebel Junior Member

    I actually went back and forth with Paul Larsen and co a bit on their blog comments about this idea. Essentially, back when they were using their larger foil, before they started hacking it down, I told them that they would incur too much drag if they could not control ventilation. In my head, this is because the atmosphere will just keep pouring air down into the wake as fast as the foil can shed it, and this is slow for some reason.

    I only say it is slow because on a moth, if you suck air down onto the foil, it climbs up into the low pressure area and sits there until, little by little, it gets scrubbed off. In the interim, lift is reduced, and drag markedly increased.

    So I could not see how any foil with a continual connection to the atmosphere would ever go very fast without a LOT of power pushing it.

    In the end, they put a fence on it, and it went fast, which I took as vindication for my position. I realize my thinking is very simplistic, and poorly founded in theory, but in interviews Paul has since said that the fence made the difference, and they never changed it a bit after glomming it onto the foil because it worked.

    I have no idea whether the base ventilation has anything to do with the lift producing part of the foil, or the fence. But bubbles assuming shapes in low pressure areas is pretty much what kept them from going fast for a long time, I think.



     
  12. boradicus
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    boradicus Senior Member

  13. AlexanderSahlin
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    AlexanderSahlin Junior Member

    Base-ventilated foils for sailing

    As a member of another speed-sailing team (http://www.trampofoil.com/speedsailing/index.html) I think I have to comment on this.
    I designed base-ventilated foils for our paravane-speedsailer 2001 to 2004, and the sections I came up with were much like those suggested by Tom Speer above. But my design had 9 % thickness and were designed for cavitation-free operation up to 65 knots at a cl around 0.14 (This was long before Sailrocket were exceeding 50 knots, so we thought that top-speed would do).
    Torsional divergence is an important issue at speed above 50 knots, especially when you build in carbon and not steel. To get not too much negative moment, and enough section-area I designed with cp near zero on the entire pressure-side and cp -0.17 on the front 70% of the suction-side.

    I think also Sailrocket uses something like this, because they have an even longer unsupported span on their foil than we had. So torsional divergence is even more critical for them.

    I use the 6.3 version of X-foil, so I can't adjust the wake-parameters. But the inviscid mode seems to predict the pressure-distribution quite OK. As a validation-check I added a 3-chords long wake on the foil-section, where the wake was adjusted so cp was just slightly below zero. The pressure-distribution on the base-ventilated foil agreed well with that with the long zero cp bubble.

    We also measured base-pressure and drag-angle for the paravane at 40 knots (the highest speed where the towing boat could handle the paravane-force). First test the air-supply to the wake was not sufficient, which resulted in a wake-cp at -0.08 and a ventilation-bubble shorter than one chord. After modification the negative cp in the ventilation bubble was between zero and -0.02, and the bubble was longer than 5 chords. The overall drag-angle went down by 20 %. The profile-drag, that stands for just some 1/3 of the overall, of course went down much more %-wise.
     
  14. P Flados
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    P Flados Senior Member

    Thanks for the post.

    Even though 90% of your discussion is beyond the comprehension of 90% of the readers (myself included), we like to read it anyhow. Also, some of this may actually help in this area where there is so little readily available published info.

    I was following your team while the web page was being updated. It kind of just ended with no real closure. I felt your boat concept was the only one other than the Sail Rocket concept that could really hit the kind of speeds that SR2 eventually achieved. The biggest challenge seemed to be that you did not have enough financial support to enable travel to an adequate location.
     

  15. AlexanderSahlin
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    AlexanderSahlin Junior Member

    thanks for the nice words, P Flados.
    You are right that our project kind of ended up in nothing. We have not been sailing it since 2006. Even though we got support from many people, like carbonfibre and access to autoclaves from Göran Marström, only I and Dr. Jens Österlund were active working in the project.
    But the boat is still able to take out on the water. And I am still convinced it can do some 60 knots peak top speed, at least if we can mobilize a similar effort as the Sailrocket team did. Remember that they passed 40 knots first when they came to Namibia. One problem with sailing such boats is that you need at least 2 persons helping with the launch and running the towing boat, so you get back to the starting point. And they don't get the pleasure of sailing 2 to 3 times the windspeed. After being out on the water two or three occasions waiting for wind it is usually hard to convince them that the next time will be the day we pass 40 or 50 knots. My next sailing boat will be something I can handle on my own!
     
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