AC 36 Foiling Monohulls

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

  1. MikeGBR
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    MikeGBR Junior Member

    Hi, I've just started looking at the AC75 rule, for my own interest, and been reading the back posts on this forum. I have also been looking at the videos of the Ineos and American Magic prototypes sailing. One thing that surprises me is that there appears to be no attempt to get the hull and rig to heel to windward. Would anyone like to comment?
     
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  2. Doug Lord
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    Doug Lord Flight Ready

    Welcome to the forum, Mike! Best I can tell is that with the NZAC 75 foil configuration the gain with Veal heel would be minimal to non-existent as opposed to the 20-30% gain in RM with a Moth and the other benefits below.
    The benefits of Veal Heel on a Moth are:
    • it increases the righting arm between the foil lifting
    force and the hull and helm weight
    • it produces a side force component from the lifting foils
    that augments the side force generated by the struts to
    reduce leeway. This is critical if strut area (and drag as
    we have seen) is minimized by flying high
    • it produces a lift component from the sail force that
    reduces the lifting foil loading
    • it shifts the helmsman closer to the water decreasing the
    impact of his wind shadow on the sail and putting him
    in a slower moving portion of the wind boundary layer
    --
    Most of these would not be possible with an NZAC 75
    ============================================

    Veal Heel dl 2012 N.jpg
     

    Attached Files:

    Last edited: Apr 5, 2019
  3. MikeGBR
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    MikeGBR Junior Member

    Hi Doug, Thank you for your welcome and immediate reply. Thanks too for the very interesting paper on the International Moths. The described benefits of 'Veal Heel' are exactly as I would envisage. Please excuse my ignorance but why do you think that the similar gains could not be achieved with the NZAC 75 foil configuration?
     
  4. OzFred
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    OzFred Senior Member

    The benefits of windward heel for a Moth can be achieved in an AC75 without heeling. An AC75 can:
    1. Cant the arm to gain maximum leverage
    2. Possibly further maximise leverage by creating more lift on the outer section of the main foil than the inner section by using independent flaps
    3. Anhedral on the main foil means that at the right cant angle, the outer portion is providing more of its lift vertically and the inner portion more of its lift horizontally (to windward), so more righting moment without heeling the boat
    4. There is likely little aerodynamic benefit to them healing to windward as it presents a larger silhouetted to the wind (though that can be designed around I guess)
    As for the benefits of the rig heeled to windward, a number of Moths have canting rigs that heel to leeward (i.e. stand it upright) because they think it's better to have more power from being upright than lift from heeling. All very marginal though. Others think the complexity of a canting rig isn't worth the hassle, but it highlights that the lift from the heeled rig isn't that much of a benefit given the loss of efficiency over an upright rig.

    Some offshore multihulls have canting rigs to stand the rig upright when they are heeled to leeward. They don't seem to think it's advantageous to heel the rig any further to windward than that.

    Windward heel on any boat other than a Moth is very difficult to sustain. Skiffs do it from time to time, but only a few degrees and only in fairly steady conditions. Moths are somewhat unique in that they really can't be sailed to windward without windward heel. It's much easier to sail the boat upwind that way than to try to keep it flat. Downwind is a different matter, they are sailed much flatter.
     
  5. Doug Lord
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    Doug Lord Flight Ready

    ===================
    Mike, the biggest benefit of Veal Heel on a Moth is the substantial extra RM and that would not be possible on the NZAC 75 configuration due to heel. But the NZAC 75 already develops substantial RM due to the lee foils' extension to leeward so heeling the boat to weather would do nothing to increase that other than moving the CG of the mast to windward but that would be really tricky and would probably be better accomplished by canting the mast to weather-if thats legal and desirable(from a CG standpoint). Veal Heel is only possible on some-but not all-foilers. It is not possible on a non-foiling boat.
    ---Note-of the two proto's currently sailing only one has anhedral and I'm not at all convinced that independent moving of the flaps would have any great benefit-I could be wrong. At this point, though, I tend to think a straight foil will be better all things considered.

    American Magic-straight foils:
    AC 75 straight foil amer mjc proto (1).jpg

    INEOS-UK-anhedral foils. Note different degrees of anhedral port and stb:
    AC 75 Proto anhedral.jpg
     
    Last edited: Apr 6, 2019
  6. MikeGBR
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    MikeGBR Junior Member

    Thank you Fred and Doug for you further comments and insight. Very interesting. I had envisaged that the combination of moving the C of G to windward and generating vertical lift from the sail plan would enable the loading on the foils to be reduced, allowing the use of smaller, thinner foils with less drag. I don't know if it is a relevant comparison but, if I understand correctly, the Vestas Sailrocket gained efficiency by cancelling out the healing moment by inclining the sail plan to windward. I also note that VSR2 was using supercavitating foil sections, which would seem to make sense for a NZAC 75 operating at high speeds. I did a quick Google search which actually brought up a YouTube clip in which Prof Stefano Brizzolara proposes some possible foil sections. Further food for thought!
     
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  7. Doug Lord
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    Doug Lord Flight Ready

  8. OzFred
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    OzFred Senior Member

    VESTAS Sailrocket is the only boat I know of that incline the rig to windward. There are a small number that cant the rig upright and which could therefore incline the rig to windward if they felt it beneficial. Given that Moths cant the rig away from windward, I take it that it only makes sense to incline to windward in a very limited set of circumstances.

    Sailrocket is an extreme sailboat that only has one point of sail.

    That term is a Doug Lord invention that is not used by anyone else. The name of the class is "AC75".
     
  9. Doug Lord
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    Doug Lord Flight Ready

    Here's a shot of the Ultim Macif with the rig canted a bit to windward:

    [​IMG]

    BMW US1 with rig canted to windward:
    BMW USA1 with canted mast.JPG
     
    Last edited: Apr 6, 2019
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  10. OzFred
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    OzFred Senior Member

    Macif typically sails with leeward heel, so it's canted to be nominally upright. It likely oscillates between slightly to windward and slightly to leeward.

    Just because the boat can cant the rig to windward doesn't make it a good idea. The BMW Oracle Racing trimarans were designed to be sailed with only the leeward float in the water, so quite a bit of leeward heel. Likely they are in a transition, or testing, or whatever. You can make whatever you want from a single still image.

    Here's one lifting two hulls in 12kn of breeze (and making over 20kn to windward).
     
  11. Doug Lord
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    Doug Lord Flight Ready

    A little shaky but great video-thanks.
     
  12. CT 249
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    CT 249 Senior Member

    You don't get vertical lift without giving up horizontal driving force, which is why so many windsurfers try so hard to keep their rigs vertical instead of raked to windward.
     
  13. OzFred
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    OzFred Senior Member

    Here's Banque Populaire V with rig canted upright from 2009 (skip to about 1:00, the links don't seem to support starting at a particular time):



    Here's Benoit Marie and his canting rig Moth during the EFG MothEuroCup Act 5 at Lake Wittensee in 2014:

    [​IMG]
    Full article: EFG Moth EuroCup Act 5 at Wassersport Club am Wittensee https://www.sail-world.com/news/178744/EFG-MothEuroCup-Act-5-at-Wittensee

    And here Paul Larson (of Sailrocket fame) talks of canting the rig on a MOD 70 to keep the mast upright (skip to 0:40):
     
  14. Doug Lord
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    Doug Lord Flight Ready


  15. MikeGBR
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    MikeGBR Junior Member

    Thank you for your further comments, pictures and videos.
     
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