AC 36 Foiling Monohulls

Discussion in 'Sailboats' started by OzFred, Sep 13, 2017.

  1. CT 249
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    CT 249 Senior Member

    You said that JB Turner's remarks were ""absurd....uninformed and unfortunate...nonsense". Any reasonable and honest person with a grasp of the English language would know that it IS an insult to use terms.

    However, since you claim that such words are not insulting, we must be able to use the same terms to describe YOUR posts without you complaining that we are being insulting. After all, it's very simple - some of us will treat you in the same way you treat other people.
     
  2. Doug Lord
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  3. AlexanderSahlin
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    AlexanderSahlin Junior Member

    OK, Doug could have presented his thoughts about safety for sailing hydrofoils without commenting on how informed Mr Turner is. Posts on a forum like this shall be based on physics and design considerations. I hope to read more about Doug's development-work on sailing hydrofoils here in the future, not discussions about more or less experienced people.
     
  4. AlexanderSahlin
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    AlexanderSahlin Junior Member

    The paravane-sailer is different, yes. I just mentioned it as an example to tell that there have been more boats than Sailrocket with this kind of configuration, although I must admit that Sailrocket 2 was the most successful.
    As I tried to tell in my post, and also Tom Speer explained more carefully, canting the rig changes the heeling moment if the lifting foil is displaced laterally relative the mast-foot. For windsurfers canting the rig has no influence on the heeling moment, because the mast is stepped right above the lifting surface of the board. To maximise the forward thrust, windsurfers sail with very little windward cant of the rig.
    You can make a very simple model to estimate these effects if you consider a boat with the mast stepped laterally at CG and all the lifting foils at a position half the height of the sail's CE to leeward of CG. Also assume that the rig is canted only by adjusting the shrouds and that the weight of the rig is small relative the overall weight. Then you will find that a leeward cant of 30° gives the maximum forward thrust some 15 % higher than with a vertical rig. (More lateral displacement of the lifting foils will make the effect of leeward cant much larger.)
    However, the increased lift on both sail and foils will increase the drag, which makes the optimum cant of the rig vertical when foils and sail operate at their CL for maximum L/D. At marginal foiling conditions, where the foils and sail operate at higher CL than that for max L/D, some windward cant is most efficient to reduce CL a little, and at high speed where CL is lower than for max L/D some leeward-cant is most efficient, but probably never as much leeward-cant as 30°.
    The AC-75 is more complicated than the simple model above. -If they sail with fixed shrouds and pivot their foil-arms to trim the canting of the rig, the pivot-point of the foil-arm is somewhere between the foil and the CG. I hope some AC-team can report the results for rig-canting from their VPP-analysis on this forum :)
     
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  5. OzFred
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    OzFred Senior Member

    I really wish we'd forget about windsurfers/sailboards. Their windward heel is as much about the weight and height of the sailor and the length of their arms as anything else. I.e. the rig isn't canted independently.

    That would be pretty similar to an AC72, 75 and 50.

    You'll need to provide some more detail. I can't see how canting the rig 30° to leeward increases thrust. It reduces the profile exposed the wind, changes the angle of the sail to the wind so more is split off the tip and the reduced lift of the sail is angled downward, further reducing the horizontal component. Where does the 15% more thrust come from?

    The bottom line being that overall, vertical is best unless you cant the rig more or less continuously in response to gusts in the same way you might adjust a sail. Generations of sailors have come to the same conclusion through empirical means. :)

    Pretty sure AC75's can't cant their rig (i.e. it's prevented by the rules), so if they want windward heel it will be by heeling the whole boat. I think they might do that to promote foiling (i.e. prior to actually foiling in addition to pumping the main in marginal conditions) and when rounding the top mark and bearing away (as do other boats if they can).
     
  6. Doug Lord
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  7. AlexanderSahlin
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    AlexanderSahlin Junior Member

    OK, I shall try to explain how leeward cant can increase the forward thrust from the sail in more detail.
    Assume that the aerodynamic force in the sail is large enough to fly the central hull with only the leeward foil in the water.
    In the attached photo you see first a figure of the boat from forward. θ is the canting angle, CE is the height of the sail's centre of effort above the foil without any cant of the rig, while CE' is the heeling-moment arm. CS is the height of the mast-step above the foil. b/2 is the horizontal distance from the centre-line to the leeward foil. This is also the righting-moment arm. The windward foil is lifted out of water.
    Figure 2 shows the boat from above. FH' is the horizontal projection of the heeling force, FH. β is the apparent wind-angle. In this case it is most convenient to consider the horizontal projection of the forces. The horizontal projection of the heeling force, FH'=FS'cosβ, (where FS' is the horizontal projection of the aerodynamic lift of the sail), while the forward thrust, FT=FS'sinβ, or FT=FH'tanβ.
    In my example for b=CE, where I found that the forward thrust has a maximum at mg/√3tanβ for θ=30°or 1.15FT(θ=0°) , I assumed that the height of the mast-step above the hydrofoil was so small that it was compensated by the movement to leeward of the weight of the rig.
    Of course, the increased aerodynamic and hydrodynamic forces due to leeward cant can increase the drag. At marginal foiling conditions, where the sails and foils operate near their CLmax, leeward cant will make them stall, so the hull falls back in water. But at high speed, where sails and foils may operate near or even below their CL for minimum drag, the increase in forward thrust due to leeward cant can increase the speed a little.
    For the AC-75, where the hull has a lot of friction drag before foiling, the speed and also apparent wind will increase a lot as soon as they get the hull out of water. Therefore, you can expect that their foils and sails quite often will work well below CLmax, so leeward cant can be an advantage.

    Bild 2019-04-30 kl. 13.33.jpg
     
    Last edited: Apr 30, 2019
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  8. OzFred
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    OzFred Senior Member

    Another INEOS video that appears to show some foiling tacks, a couple more crashes and they're testing flat and anhedral foils.
     
  9. OzFred
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    OzFred Senior Member

    Very interesting and thank you for the detailed response. Please give me some time to think about this.
     
  10. Doug Lord
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  11. Doug Lord
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    Cool video:
     
  12. Doug Lord
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  13. schakel
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    schakel environmental project Msc

    The nosedive might become dangerous for the future AC 75':
    Ineos Nosedive.PNG
     
  14. OzFred
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    OzFred Senior Member

    I don't think the nose dives will be an issue per se as the crew should be prepared for them, e.g. they should be in their crew stations and braced during tacks and gybes. However, they don't bode well for close manoeuvring where, say, one boat attempts to tack into a leeward bow position and sticks it in right in front of the other boat, which might cause a collision with much more serious consequences for those on the leeward side of the windward boat. A sharp evasive swerve to windward will cause the stern to go to leeward, much like the Titanic trying to "port around" that iceberg, but faster.

    It might encourage Dean Barker to stop helming from the leeward wheel… ;-)
     
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  15. Doug Lord
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