AC 36 Foiling Monohulls

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

  1. Doug Lord
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    Doug Lord Flight Ready

    Whups!
     
  2. AlexanderSahlin
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    AlexanderSahlin Junior Member

    Interesting video. Obviously, they have a very light rig. But watch the motorboat that first approaches the bow of the AC-75, and then seems to tow the bow into the wind to enhance righting. I don't have the resolution on my screen to tell for sure that there is a towline, but the motion of the boats indicates that. I will be more impressed when they can demonstrate righting without assistance. Are they allowing this kind of assistance during the AC-races? As they have chosen a boat that capsizes as soon as the speed and also the hydrodynamic lift drops, so it can no longer stabilize the boat, such capsizes can be expected to occur quite frequently.
     
  3. HJS
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    HJS Member

    Alexander
    At 0.27 in the video, the black towline is clearly visible.
    JS
     
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  4. AlexanderSahlin
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    AlexanderSahlin Junior Member

    Now even I could see the towline, and they were not just towing the bow of the AC-75 into the wind, -they attached the towline near the shroud-attachment, to actively right the AC-75 with the motorboat, like you can do with any catamaran, that has some buoyancy in the mast!
     
  5. Konstanty
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    Konstanty Junior Member

    With normal flying canting keel and heavy bulb they will should can get up without any motorboat and towline.
     
    Last edited: Jan 1, 2020
  6. AlexanderSahlin
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    AlexanderSahlin Junior Member

    Correct, e.g. all new Open-60 monohulls have to demonstrate that they can right the boat from fully inverted with their ordinary keel without any outside assistance.
    If the AC-75 needs assistance for righting the boat after a capsize, I cannot see any reason to have ballast in the foils. The movement of CG to windward you get by lifting the windward foil a little higher than the leeward foil is quite marginal. To get stability when towing back the AC-75 they can take help from a 40-ft RIB on each side to support them.
     
  7. OzFred
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    OzFred Senior Member

    The rule requires 300 kg of buoyancy in the top 4 m of the sail.

    The boats are designed to be self righting, ETNZ haven’t explained why they didn’t attempt it. There may have been a malfunction of the FCS, but after righting they continued training so that seems unlikely.


    About 1.25 tonne placed 3 m off the windward rail provides significant RM. Removing ballast from the foils would make the boats unstable to the point of being impossible to keep upright unless foiling.
     
  8. AlexanderSahlin
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    AlexanderSahlin Junior Member

    Good that they have buoyancy in the top of the sail.
    I have heard the argument that you get righting moment from the weight of the windward foil a lot of times. But have a look from a forward view of an AC-75 in normal foiling configuration: As I can estimate from the pictures available, the windward foil's CG is at an angle +15° from a horizontal plane through the pivot-axes of the foils, while the leeward foil's CG is at an angle -35° from the horizontal. The moment arm from a pivot axis to a foil's CG is approx. 4 m. The righting moment you get from this is: RM=4m x 1.25 tonne x (cos 15° - cos 35°) = 0.73 tonne x m, which is about the same as if one of the bigger crew-members walks from the leeward to the windward side of the boat. I still claim that this is marginal for such a big boat. OK, you have some buoyancy in the submerged leeward foil, but that would also apply for a foiler without ballast.
    The foil's total CG is a little lower than the pivot axes also, OK. In the same model as above this will be: 4m x (sin35°-sin15°)/2 =0.63m, or about the same as if you attach the ballast directly on the bottom of the boat. I don't have the actual hullshape of the boat, but there is an effect that can have more influence, and that is that you increase the inertia of the floating plane of the boat when you increase the weight, because the waterline becomes wider when the boat floats deeper, so if they need that stability they can as well put some rocks (or some extra tonnes of hydraulic gear, if that looks more hightech than granite) on the bottom of the boat.
     
  9. Dolfiman
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    Dolfiman Senior Member

    Luna Rossa on sea state offshore Cagliari : uneasy to have a stable platform, the advantage to be over the waves seems lost in the disadvantage to be in a permanent compensation of a high unstability by concept.
     
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  10. AlexanderSahlin
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    AlexanderSahlin Junior Member

    Yes, a multihull would have been a more stable platform. The canted T-foils (but without ballast) is a good development. A configuration like William Sunnucks' Vampire, or the trimaran-like foiler I suggested on page 19 or 20 in this thread, would have been easier to handle. And would also keep the speed better when you somtimes dip a hull. Maybe the idea behind the narrow canoe attached on the bottom of Luna Rossa is to have something with a narrow waterline when you make a light touch-down with the hull? Anyway, impressing that they can control the boat so well under those conditions, The crew is certainly very skilled.
     
  11. sharpii2
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    sharpii2 Senior Member

    By the looks of this, these boats aren't even self-rescuing. But it is interesting to note that this particular one suffered so little damage from its capsize that it could continue sailing after it was righted. I doubt the catamarans could have done that. So, maybe these big foiling monos are the better deal safety-wise.

    I estimate their speeds at somewhere between the mid-thirties and the low-forties in knots. It seems the crash boats have little trouble keeping up. What does everyone else think?
     
  12. OzFred
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    OzFred Senior Member

    That last statement infers that an 80kg person sitting on the rail provides the same RM as 1.2 t of ballast on an arm extending 3 m further out.

    The pivot point when foiling is roughly the centre of the submerged foil, so:

    80 kg on the leeward rail provides about 3 m x 80 kg = 240 kgm
    80 kg on the windward rail provides about 8 m x 80 kg = 640 kgm, so an increase of 400 kgm
    1.2 t on the raised arm provides about 11.5 m x 1,200 kg = 13,800 kgm​

    In the docking position (fully lowered) the ballast from both foils is about 3.5 m below the centre of buoyancy, providing about 8,400 kgm.

    When in the lowered ("docking") position and sails down, the ballast in the foils provides enough stability for the crew to move around on the boat without fear of capsize. Once the sails are hoisted and the boat starts to make way, stability is mostly provided by balancing leeward foil lift and windward foil RM with sail capsize moment. Once foiling, stability is likely entirely from balancing the weight of the entire boat with the wind pressure on the sails.

    There may be some RM from downforce from the rudder, but I think that's irrelevant in regard to a discussion on RM from the foils.

    A very rough graphic is below. Precise weights, angles and distances are in the rule but I think this is good enough for discussion:
     

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  13. OzFred
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    OzFred Senior Member

    Better capsize safety comes from a much smaller beam, so smaller drop into the water from the high side, plus the sail and rig seems much more robust than a wing, so as well as providing stability when prone, there are no bits of rig flying around or tendency to turtle. The personal safety gear is also much more highly refined than at the start of the AC72 era, which is good to see.

    The port foil was in the fully raised position before the capsize and seems to have been lowered to the lower foiling position for righting. To right from capsize, I think both foils are supposed to go to the fully down position to provide maximum RM with the boat on its side. They might have tried a number of things but found that the surface tension plus water on the sail was too much for the ballast to overcome. Only ETNZ really knows and they aren't saying publicly.

    The voiceover on the YouTube clip is misleading, it says they "…had a bit of a high fly… then slowly rounded up in a high mode…and a very slow capsize". After the crash, the initial response was to bear away. The boat only starts to round up when it's heeling heavily to leeward and the rudder looses grip, at that point whatever the helm is doing becomes irrelevant. The initial bear away response fits with a skiff sailor's "keep the boat under the rig" instinct. Unfortunately a 20 m yacht with a tiny rudder just doesn't turn that fast in displacement mode.
     
  14. AlexanderSahlin
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    AlexanderSahlin Junior Member

    You increase the righting moment by adding weight on the boat, yes. Adding 2500 kg (the weight of the two foils) gives an increase in righting moment about the same as you estimated. And for a 7-ton boat that increase makes sense, yes. But the comparison I tried to make was to estimate how much you increase the righting moment by lifting the windward foil on an AC-75 compared with the righting moment for a foiler with the same overall weight and geometry, but without ballasted foils. And that increase is still about 730 kgm, which is about the same as if I walk from the leeward rail to the windward rail on an AC-75 (120 kg x 6 m =720 kgm).
    Concerning the discussion on how much the ballasted foils increase the stability in displacement mode, I thought we discussed the phase, when the boats were either accelerating to start foiling or had just landed on the water after foiling. And in that phase the foils are most likely in position for foiling. I thought the AC-75s were going to be supported by tender-boats while being towed back to the harbour after racing. It has passed some 100 years since the challenger had to sail to the America's cup on her own keel. (Imagine doing that on an AC-75 :)
     
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  15. Doug Lord
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    Doug Lord Flight Ready

    The "thing" running down the middle of Luna Rosa is a big mistake, I think, It will hurt light air takeoff and slow the boat in conditions where it makes frequent impact.
     
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