erdb

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  1. erdb

    Boats and foils comparison

    Hey Basiliscus, really enjoying your posts. On the NYYC thread I posted this diagram below. On several videos it's quite apparent that the AC75s sail in a bow-to-the-lee rotated manner. I was wondering what the purpose of this may be since they must be getting higher hull aero drag sailing a bit sideways. I thought it may be to get more righting moment from the rudder if that exerts downforce or opening up the slot between the main and the jib. Or it's maybe just necessary to burn off excess AOA on the main foil at high speed that needs to be there when lifting off at low speed. What do you think?
  2. erdb

    Team NYYC

    I think crabbing is the wrong term. It implies that it directly affects VMG. It doesn't. VMG is determined by the foils and the rigs. You could in theory stick a hull between them pointing in any direction you wanted. For some reason they sail with the bow slightly turned off the wind relative to the direction of movement, and it seems they do it downwind, too. Why? I'm not sure but would love to find out. That video above is pretty conclusive I think. This is from around 1:00.
  3. erdb

    Team NYYC

    I don't think the foil arm produces any significant lift in either direction. Its job is to hold the foils and it's faired to minimize drag.
  4. erdb

    Team NYYC

    Great post and I agree, it's hundreds of little compromises here and there. I just thought generating more righting moment with the rudder could be one of the factors. That, and opening up the gap between the jib and the main were the only two things I could think of that may explain the "crabbing" that we clearly see in the videos. Reducing the foil AOA at high speed could be another. One thing is for sure, they pay a price in hull aero drag, so there must be a reason why they do it.
  5. erdb

    Team NYYC

    I guess the boats are designed to sail this way, so the foils are already angled accordingly, plus you can fine-tune with the flaps. Rudder is turned relative to the hull as if the boat was turning off the wind, but it goes straight, because the rudder is parallel with the direction of movement.
  6. erdb

    Team NYYC

    That's a different kind of rotation, more like twisting. But, the cats did some crab walking, too, which may have been for the same reasons - increase the lateral distance (across the direction of movement) between main foil and windward rudder that provided additional righting moment.
  7. erdb

    Team NYYC

    You don't need more lift for this, and rotating the hull doesn't affect VMG directly either. Once the hull is in the air, VMG depends mainly on the efficiency or lift/drag ratios of the foils and sails. The rotation of the hull only affects hull aero and the geometry of how the foil vs rudder are positioned and how the sails are set relative to the hull. It doesn't have a direct effect on which direction the boat is moving. It's really just rotating the hull relative to the direction of movement.
  8. erdb

    Team NYYC

    I guess it's determined by the angle and lift of the main foil and then the rudder angle is adjusted accordingly.
  9. erdb

    Team NYYC

    I think the differences in hull design/keel size may indicate how much they want to rotate the boat across the direction of movement (some call it crabbing). I always though that they do that to increase the righting moment from the rudder foil by increasing the lateral distance between the rudder and the main foils. Maybe it also helps opening the gap between the jib and the main. Of course the downside is that now you have more aero drag on the hull. They'll have to find the balance between minimizing drag and maximizing righting moment. The differences in the keel size and roundness may indicate that AM plans to rotate the boat across the most with the smallest keel, INEOS the least with the big, sharp-edged keel and LR (and NZ B1) somewhere in the middle.
  10. erdb

    Boats and foils comparison

    I always thought most of what they say is BS, and their explanation of AM's jump is wrong, too. On the video it's clear that it's the rudder that sinks first, and only then lifts the main foil out of the water. Plus, how could you increase righting moment by increasing the lift of the foil if the boat is already completely out of the water? It's the weight of the boat that gives the righting moment, the foil is only the fulcrum point. I think what happens is that they heel over, Dean tries to fall off the wind to correct (you can see the rudder angle when it lifts out), but that tiny piece of submerged rudder can't do it, so they have to let the main out. At that moment, driving force pointing forward is gone, plus drag from the apparent wind suddenly points aft on the rig, which pushes the rudder foil deep into the water. That produces that rooster tail. Now the bow points to the sky, AOA and lift suddenly increase on the main foil and off they go.
  11. erdb

    Emirates Team New Zealand.

    Crab-walking and platform rotation are the same thing. It's the efficiency of the rig and the foils that determine how close they can sail to the wind. You could put the hull between the rig and the foils at any angle you want (ignoring class rules). What's interesting is that they chose to sail somewhat sideways into the wind, which for sure must increase aero drag. One reason I can think of why it's worth it is that by going somewhat sideways, you increase the lateral distance between the main foil and the rudder. If the rudder is used to create righting moment, the larger distance allows more righting moment / less rudder foil drag. Or maybe it's for changing the relative positions of the forestay and the mast.
  12. The mast track is kind of inside on my mast, but maybe I could drill two holes on the two sides and thread the dyneema in - around the track - then out on the other side. That way the holes would be on the aft, compression side of the mast. I'll think about that.
  13. That's an interesting idea. I'm not sure I can achieve ferrule-like smoothness, but I guess I could try on some aluminum test pieces. I'll definitely contact Sparcraft.
  14. I'd bring it down about 4.5 ft (mast is 31ft long). I think it's be more stable to have the sail area lower.