Erwankerauzen

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About Erwankerauzen

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  1. Erwankerauzen

    The new sailing twin skin setup

    Random, Try to googlize the following and you will get the full workpaper, it is too heavy to be downloaded. You can bet confortably that, if there is some theorical advantage to have a bell-shaped lift distribution (with negative lift at the tip), the AC design team have put it at full use. These "rocket scientists" are professionnal you know! NASA/TP—2016–219072 On Wings of the Minimum Induced Drag: Spanload Implications for Aircraft and Birds Albion H. Bowers, and Oscar J. Murillo Armstrong Flight Research Center, Edwards, California Robert “Red” Jensen, and Brian Eslinger Jacobs Technology, Inc., Edwards, California Christian Gelzer Logical Innovations, Inc., Edwards, California. Good reading
  2. Erwankerauzen

    14' Stunt S9 Foiling Cat

    Good feed back, thank you very much. Everybody is happy to see that the boat is still in her development phase.... since 2012. Wish you the best
  3. Erwankerauzen

    The new sailing twin skin setup

    I doubt you can observe a -3° AoA of the sail top on video. I am not sure (because of my Alzheimer), but it is possible Glenn Ashby has mentionned it in an interview, a few months ago. Remember, in addition to his skipper role, he was also the wing trimmer. Happy week end Random
  4. Erwankerauzen

    The new sailing twin skin setup

    The rocket scientists behind the Godzilla wing of the AC 34 used to say that the thick wing sail allowed tacks and gybes with continuous air flow and no separation (very draggy) which is not the case for a classic teardrop mast. I guess the engineers have considered this point seriously as fast tacks and gybes are a significant part of the whole performance in the cup. In addition, if top main sail negative lift is also a part of efficient trim, in order to achieve the so called Bell shaped lift distribution, it is necessary to have a thick sail in order to fit some "inverting devices" in the middle. Cheers
  5. Erwankerauzen

    The new sailing twin skin setup

    Hi Dorox, Your examples of wing sections look like Javafoil outputs ??? Anyway, thanks for making serious homeworks for the benefit of the "sailing geeks" community. In order to check your CFD model (just in case :garbage in/ garbage out issue) here is how I would make a check For 2D drag (wing section drag/ friction drag/ pressure drag) My benchmark is Minimun drag coef=1.7% or 0.017 for a significant Lift coef around 1 It is what I found on different workpapers in the case of a fully turbulent flow over the sail. With 10m/s apparent wind speed for an A-Cat, the lift coefficient should be around: Cl (2D)=1 and Cd (2D)=0.017 so Lift/Drag = 1/0.017= 59 That is very theorical of course but it provides an idea . For AC 50 Wing, you can consider a 20% to 25% relative thickness for Element 1 and 9% to 12% for Element 2. Mark Drela (MIT Prof) has posted somewhere (probably on BDN) a file with such wing section and the drag coef was around 1% but AFAIR it was with NCrit= 9, so not fully turbulent assumption. Cheers
  6. Erwankerauzen

    The new sailing twin skin setup

    https://patents.google.com/patent/DE102014103999A1/fr Hi Everybody, Please find above a link for a patent, which has been put at full use with the boat Pi 28, and the GONET Company, a few years ago. AFAIR, after "sailing the patent", Mr Hugues de Turckeim, was mentionning in a video, that the performance of this double skin sail windward, was beyond his expectations. The boat equipped with this new sail concept, was a foiler and AFAIR her name was Pi 28. As my Fluid dynamic knowledge has its roots in forums & books, I am far from a CFD guru. But just like Basilicus, it is difficult to see the actual performance pick up with the double skin, but a tiny drag section advantage by eliminating some separation drag. It still sparks a candid question: Can a little drag/separation advantage at the begining of the flow along the main sail chord could lead to a thinner turbulent BL along the whole main sail chord and hence a little friction drag advantage?? And everything else equal, could this thinner BL be less sensitive to TE separation, with its positive consequence on lift ?? Best regards Erwan
  7. Erwankerauzen

    The new sailing twin skin setup

    Nice discussion, even if I am a bit late, I d like to add 2 remarks: 1-According to its very low aspect ratio, I think it is tricky to conclude anything definitive about the relevancy/ efficiency of a wing/ double side sail .... for a Moth. (Induced drag is so big compared to sail section drag) 2-CFD analysis might be the reason for the choice of a double skin sail: In other words, they optimized the global efficiency of the rig (probably drag minimization is more the issue than power maximization) for a design speed of apparent wind. So, once this apparent wind design speed is identified, The CFD could have concluded it was necessary to lower sail section drag, in order to achieve a global drag optimum*. * For an airplane, typically this global minimum is achieved when: induced drag= friction drag as one is function of 1/V^2 and the other one a function of V^2. A double skin sail could provide an opportunity to host a second vertical spar, just like for a slotted wing (at 60% cord). In combination with a double skin sail with ( double battens= good mechanical inertia), it could address the leech tension a bit differently than a double skin "classic sail" and remove a lot of compression stress from the main spar. That is just an idea... Cheers
  8. Erwankerauzen

    Sailing anywhere between Perpignan and Montpelier

    Be carefull with your stuffs, your car, anything on the boat trailer, and so on.... This part of south of France is the "Thief Coast" a bit like in Italy, everybody will try to steal your stuff It is not a special treatment for foreigners, it is the same for frenchies coming from the north of the country. Avoid "Etang de Ber" which is in the middle of the oil reffineries and has been holding for decades the gold medal of the most polluted area in France. Happy Holidays Erwan
  9. Erwankerauzen

    Foiling Monohull - what would it look like?

    Hi, These last few days, I have been traveleing to the sea, and noticed similar features on Hydrofoils from very different machines 1-A big all white Oman trimaran in Lorient harbour 2-A foiling windsurf 3-An A-Cat Exploder All of them seem to share common features: 1-A sharp leading edge 2-A maximum thickness which seems a little aft 3-A Concave shape on both sides of the last 35% of the wing section leading to very low exit angle. Main Difference: The bigger the boat the larger the camber and thickness (for structural reasons I guess). We still have to wait to see what kind of foils the new AC boat will carry, in the meantime that a few infos to chew for the addicts. Cheers
  10. Erwankerauzen

    Moth decksweeper and lowered mast stumps

    PhilS, The 5.70 m luff is an assumption regarding a new rig trade-off regardless of the rule box. But your remark prompts me to crunch the figures again with a 5.185 max luff lenght, in order to compare apples with apples. So with a full decksweeping foot sail with a 5.185 luff lenght and a 8.25 m^2 we have: Oswald coefficient =2 So we should have the following results: Aspect Ratio = 6.52 2D Lift Coefficient = 0.91 (for a 0.70 3D lift coef as above-mentionned) Induced drag coef = 0.0404% Induced drag = 20.33 Newtons (apparent wind @ 10 m/s) So even in this case, the decrease for induced drag remains significant relative to the classic rig: 20 Newtons vs 46 Newtons. Wish you a nice week Cheers Erwan
  11. Erwankerauzen

    Moth decksweeper and lowered mast stumps

    Thanks Phil, Doug, Connor & Inebriated for taking time to post. I was travelling last week, and I am not confortable writing from a smartphone, that is why I waited to be back home to post. "Poor reasonning" Is almost a compliment as it was more or less intuitive, and based on former A-Cat induced drag calculations. As most of you are engineers, I will try to post a few calculations to illustrate the intuition: Of course it is purely theorical and the ability to tack or gybe in 1 second as can be seen on Youtube, is not considered . Classic Moth Rig Theorical DeckSweeper Rig Sail Area 8.25 m^2 8.25 m^2 Luff lenght 5.185 m 5.70 m Gap(% luff lenght) 10% 0% Oswald Coef 1.19 2 Geometric AR 3.26 3.94 Effective AR 3.88 7.87 For 10m/s Apparent wind velocity and a 350 Newtons actual sail lift, we should have Cl(3d) 0.70 0.70 Cl(2d) equivalent 1.06 0.79 Cxi (Induced drag coef) 0.092% 0.0253% Induced Drag 46.3 Newtons 12.73 Newtons Detla= 33.5 Newtons Main assumption : both rigs have the same height for their center of effort. The 33.5 Newtons Induced Drag difference is interesting to consider relative to the total driving force created by these rigs. With Apparent wind angle around 20° relative to the boat: The driving force is around F= 350 Newtons * Sin (20°) = 119 Newtons. Of course it is a bit meaningless as it is not possible to have both: a full decksweeping sail and the possibility to tack or gybe under the footsail. If the crew moves his weight aft, in order to go behind the leech for tack or gybe, I dont believe it is possible according to the relative weight of the crew vs total boat displacement. But, because the performance "carrot" if even larger than for A-Cat, I feel confident a smart Aussie or Kiwi will bring a breakthrough sooner or later. Best regards Erwan
  12. Erwankerauzen

    Moth decksweeper and lowered mast stumps

    Thanks Teknologica, In other words an actual breakthrough cannot happen in the Moth Class due to the rule box. So I feel confident a smart guy will create a new rig/platform which will use the standard Moth foils and will outperform the Moth at cheaper price, but of course, nothing will be compliant with the Moth rule box. It will be another serie. Happy week end Erwan
  13. Erwankerauzen

    Moth decksweeper and lowered mast stumps

    Thanks Mika, Nice Moth champion interviews :
  14. Erwankerauzen

    Moth decksweeper and lowered mast stumps

    Thank you Tecknologica for taking time to answer my candid post. Regarding the loss of power, it should be interesting to guess if it is a consequence of a lower rig or a less powerfull sail ? Decksweeping sails for A-Cat, not only have a lower induced drag, but everything else equal they are likely to deliver more hp, as the bottom is more efficient. And if you have an elliptical lift distribution (with an elliptical area distribution like today's A-Cat sails) your sail CoE is at 42% height when trimmed for max power. Of course, the above statements work well for decksweeping sails with 0 gap. Decreasing gap is better, but as some gap still remains, improvements are probably less obvious and could be offset by other effects. This development class is amazing and I feel confident it will remain so for a long time. Thanks again for posting Tecknologica and fair winds. Erwan