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Main to the centerline?

Discussion in 'Sail Trim with Don Guillette' started by Jackdaw, May 22, 2018. Add this thread to a FAQ

  1. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    8,656 posts, 2,029 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    Most sailors know that using your traveler to move the boom up to the centerline as you adjust for twist provides the best upwind performance. Under perfect conditions, slightly up from this is OK, but not too far.

    Now comes reports that the fast boats in the white-hot TP52 fleet are dragging their booms WAY to windward. It would be funny if they were not winning. But they ARE. What's up? Its it for everyone? Discuss.

    cannonball-haked-768x486.jpg
     


  2. victorhoisington

    victorhoisington

    Joined Jan 22, 2008
    169 posts, 24 likes
    Islander Freeport, 41 Ketch
    US Longmont, CO
    SO a couple of questions. If I'm not mistaken, don't these boats have movable keels that at least pivot side to side? and aren't they also semi planning or even foiling hulls. The speeds that these boats sail at are considerably faster than most of our cruiser/racer boats so my thought is that the apparent wind has moved considerably forward meaning they are able to "over--trim" the mainsail when close hauled. But, I'm not a racer and I'm constantly learning when it comes to sail trim so, it will be interesting to hear from everyone else.
     


  3. justsomeguy

    justsomeguy

    Joined Feb 20, 2011
    6,181 posts, 740 likes
    Island Packet 35
    US Tucson, AZ/San Carlos, MX
    High-aspect ratio mainsails?
     


  4. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    8,656 posts, 2,029 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    mainsail spec and design totally factors into this.
     


  5. Rick D

    Rick D

    Joined Jun 14, 2008
    6,511 posts, 151 likes
    Hunter Legend 40.5
    US Long Beach, Shoreline Marina, CA
    Just thinking out loud... could it be because the jib is so far in to center that the main has to be "over trimmed" to compensate for the wind off it? It seems like the jib clew is about a foot or less to the mast. Otherwise, I have no clue. I seem to remember that they (to my eye) have more twist than I would expect, from the overhead photos I recall seeing. This one photo seems to show the main trimmed above the center. Edit to add: I would think you would want to add pressure to induce heeling to reduce wetted area especially in lighter conditions. TP 52.jpg
     


    Last edited: May 22, 2018
  6. Joe

    Joe

    Joined Jun 1, 2004
    6,384 posts, 300 likes
    Catalina 27
    US Mission Bay, San Diego
    I like this comment..... it makes sense that the high speeds, narrow jib sheeting angles, super wide beam, canting keels, planning hulls, forward rudders and dagger boards, etc. all lead to a dynamic none of us have any experience with. It's an evolving paradigm....
     


  7. BobbyFunn

    BobbyFunn

    Joined Apr 16, 2017
    229 posts, 111 likes
    Hunter 170
    US Tampa
    1. Sails rotate the hull. 100% of time at all heel.
    2. Higher sail = more rotation with more torque.
    3. More torque = faster boat (if heat isn't wasted in heel. Heel leads to drag and misdirected torque, and drag leads to suffering.
    4. This design wins with heavy bulb and longer keels.
    5. To capture the higher lift the loft/draft appears to be much higher than the casual sails. The jib is crazy fat at midsection.
    6. With draft at a higher location and the mainsail having large roach there isnt as much leverage for controlling the position of the leach.

    The leverage required to control the draft is out of whack. I tried an extreme version of this with an rc sail where the mainsail was reefed upwards. You could move the boom as much as you wanted, but the angle of attack and draft barely moved.

    Just my thoughts.
     


  8. Brian D

    Brian D Moderator

    Joined Feb 17, 2006
    3,891 posts, 581 likes
    Lancer 27PS
    US MCB Camp Pendleton, Ca KF6BL
    Looks nice at the WL, but I sure would love to see what the main is doing aloft. Bet it has some wicked twist.
     


  9. weinie

    weinie

    Joined Sep 6, 2010
    1,146 posts, 178 likes
    Jeanneau 349
    US port washington, ny
    saw a comment over at SA that centerline is just one of many possible possible positions.
    I agree on that there is nothing inherently special about the centerline.
    Only the AWA has any consequence.
     


  10. Will Gilmore

    Will Gilmore

    Joined Oct 19, 2017
    2,395 posts, 976 likes
    O'Day 19
    US Littleton, NH
    I'm going too bet not.
    Looking closely at those pictures, as limited as the view is, and in conjunction with what I've learned on some of jackdaw's previous threads, I notice the jib foot is right down on the deck. There was an analysis site that pointed out how much drive was lost by raising the foot above the deck. It was ridiculous. Look at how tight everything is. The jib in the second photo is like a solid wing. The other factor is how the main serves to improve the lift of the jib. Most of the driving force is generated by the jib.
    Just guessing here, but by hauling the main windward, the back, leeward side of the main develops a stronger low pressure. The pressure variant across the jib is increased, this speeds the wind across the jib to help fill that low. The result is the jib's efficiency and thus its driving force is increased. The main contributes nothing towards forward movement, but it improves the air the jib is in.
    just a thought.

    - Will (Dragonfly)
     


  11. Davidasailor26

    Davidasailor26

    Joined May 17, 2004
    1,105 posts, 172 likes
    Beneteau Oceanis 37 LE
    US Havre de Grace
    Agree that AWA is the only thing that dictates how the main needs to be trimmed to work as an airfoil. The thing that I'm surprised by is that as the main goes above centerline the low pressure on the Lee side isn't essentially pulling the boat backwards. I guess if the draft is well positioned and the top is twisted off there's still forward drive, or at least not too much drag to overcome the advantages.
     


  12. weinie

    weinie

    Joined Sep 6, 2010
    1,146 posts, 178 likes
    Jeanneau 349
    US port washington, ny
    It's not pulling the boat backwards because the boat is pointing exceptionally higher.
     


  13. Rick D

    Rick D

    Joined Jun 14, 2008
    6,511 posts, 151 likes
    Hunter Legend 40.5
    US Long Beach, Shoreline Marina, CA
    OK, Jackdaw.... and the answer is.............
     


  14. BobbyFunn

    BobbyFunn

    Joined Apr 16, 2017
    229 posts, 111 likes
    Hunter 170
    US Tampa
    Square top and big roach. There's no good way to control that floppy top section without pulling the lowest part way over.
     


    DaveJ and Rick D like this.
  15. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    8,656 posts, 2,029 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    Yes but there is more to it than that. The TP52s have been flying squaretops from day #1, by world class sailors. But with the boom near centerline.
     


  16. Daveinet

    Daveinet

    Joined Sep 20, 2014
    844 posts, 137 likes
    Rob Legg RL24
    US Chain O'Lakes
    For a moment, lets pretend the sail is just a flat wall and the wind is hitting the at 90 degrees. Will the boat move forward? Depends on the angle of the keel. If the keel trailing edge of the keel is to windward, the boat will move forward as it is pushed sideways.
    So while we seem to think in terms of draft and lift. There is a component of just a sideways force against the keel that moves the boat forward. If the sail is slightly cupped, it may increase the sideways force component against the keel as it tends to trap the wind.
     


  17. shemandr

    shemandr

    Joined Jan 1, 2006
    3,369 posts, 492 likes
    Marblehead Skiff 14'
    US Greenport, NY
    I've been looking at that picture off and on since this thread was posted. It makes me realize I know nothing about TP52's. I'm surprised at how close the sheeting angle is for the jib. So like others I assumed that has something to do with it. But I guess all of them are like that and only some are hauling the main up to where most of us would think it would stall. The other idea I have is that the hull is so wide that maybe it's necessary to reduce wetted surface and reduce drag. Its crew is sitting forward I would think to get that transom out of the water. This was the case with Commanche and Wild Oats. In light air Wild Oats with less wetted surface had the advantage and in higher winds Commanche with its broad flat aft underbody could plane faster. I notice that Phoenix is more narrow and its boom is in a more normal position. It's crew is sitting in the rear. That's all I've got.
     


  18. walt

    walt

    Joined Jun 1, 2007
    3,228 posts, 368 likes
    Macgregor 26S Hobie TI, Capri Coronado 15
    US Denver, Colorado
    My speculation.. (since I am guessing)

    Sails still have to operate at a positive angle of attack to generate forward thrust. The pictures shown are just the very bottom of the sail. If you google TP52 sailboats and look at more pictures, you can see that even through the boom may be very close to or even over the centerline, the mid to top section still has some twist and does operate with with a positive angle of attack. I think this is what Bobbyfun already mentioned.

    From the math standpoint, another interesting thing happens when sailboats get fast. The equation for lift force has a "coefficient of lift" that is proportional to angle of attack over some angle range. The lift force is also proportional to velocity squared. This means that if you kept the lift force constant and doubled the speed, the required angle of attack for the same lift force would be one fourth what was needed for half the speed. You cant keep lift force constant to go faster as you need higher lift plus the lift vector has less forward component as you go faster because of apparent wind angle change but a there is still that relationship of the necessary angle of attack being reduced by higher speed (or higher speed squared).
     


    Last edited: May 25, 2018
  19. Will Gilmore

    Will Gilmore

    Joined Oct 19, 2017
    2,395 posts, 976 likes
    O'Day 19
    US Littleton, NH
    I have not seen pictures of these boats with much in the way of top twist. Despite the square tops, they seem to stay pretty flat. There is some twist, but it doesn't look like enough to account for, to me. Another advantage that pulling windward of center could be a better balance to the helm. With most, if not all the lift/drive force being generated by the headsail, there has to be a tendency for lee helm when the wind moves that far forward. This would mean the rudder is working to stall out forward motion, so balancing that by dragging the main in that close would allow the helm to stay straighter and improve movement through the water.
    I still think the slot generates better lift for the whole rig with the headsail nearly, but not completely, closing that up and the leach of the main moved leeward.

    - Will (Dragonfly)
     


  20. DrJudyB

    DrJudyB

    Joined Jun 25, 2004
    178 posts, 108 likes
    Corsair F24 Mk1
    003 US San Francisco Bay, CA


    Will Gilmore likes this.

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