Pointing Ability, Long Keel vs Dagger Boards etc

Discussion in 'Sailboats' started by LSaupe, Nov 2, 2010.

  1. LSaupe
    Joined: Nov 2010
    Posts: 3
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Galway, NY

    LSaupe New Member

    Greetings All:

    Hopefully you can help sort this out (I am quite green to this stuff and am trying to sort out the physics).

    I often see professed that increasing keel performance (decreasing drag for a given side load) can increase pointing ability. Often this takes the form of improved daggerboards, leeboards or similar.

    How is "pointing ability" defined? Is this how close you can get to the true wind, the appearant wind, or maybe just VMG?

    What seems odd to me is that; let's say your sail can get within 35 degrees of the appearant wind and no higher. If you redesign your keel such that your drag is now lower (for the same given side force); my impression is that your forward speed is now higher and... now also your effective wind is stronger and has moved forward toward the bow. The new forward position of the appearant wind should force the sailor to bear off a bit to keep the sails active (say back to 35 degrees). In this case the boat does not point as good as with a higher drag keel relative to true wind, but may remain the same to appareant wind??? Maybe I am missing something here, or maybe my definition is wrong. VMG is most likely higher owing to the higher speed, even though the tack to tack angle is, or might be, higher. If this is the case, is this where the improvement to windward ultimately is (VMG only)?

    Any help here greatly appreciated. Does decreasing drag actually get you closer to the true wind or just a better VMG?

    Best Regards,

    Larry S.
     
  2. Stumble
    Joined: Oct 2008
    Posts: 1,896
    Likes: 71, Points: 48, Legacy Rep: 739
    Location: New Orleans

    Stumble Senior Member

    LSaupe,

    A more efficient keel can work in a number of ways to increase pointing ability.

    1) More righting moment can keep the keel closer to vertical which increases the ability of the keel to keep the boat from slipping through the water. Remembering that pointing ability is NOT the angle the bow points towards the wind, but instead is the angle the bow points PLUS the number of degrees the boat slips sideways through the water.

    2) A more efficient shape can create more lift through the water, thus again reducing the slippage of the boat.

    Reducing drag works in a number of ways as well.

    1) By reducing drag the speed through the water is increased. This increased speed translates into more water passing by the keel, increasing its efficiency (See 1 & 2 above).



    The downward spiral you described is a reality, in that at some point the cut of the sails limits the maximum angle to the wind they can be set at (This is called the alpha angle BTW). However a skilled sailmaker can and should design sails specific to the alpha of the boat in question.

    The clearest example of this is with spinakers, so for demonstration purposes think of a Code 0. Which is a very flat cut upwind reaching asymetrical sail. The luff is shortened compared to a downwind sail, and the chord (the maximum depth) is reduced. Further there will be no segment of the sail projecting to windward from the luff. Now if we take a Code 2A (a downwind running spinnaker again for an asymetrical) we find a longer luff, deeper chord, and material projecting to windward from the strait line drawn down the luff. Two radically different sails, identifiable visually, but both are asymetrical reaching spinnakers.

    For upwind the theory is the same. The sailmaker would create a sail with a shorter luff (this allows for a tighter forestay to reduce leeward sag), and the chord would be reduced as well. In fact many racing sailboats already have this distinction in their sails. The first being a light/medium #1 (typically here a 155%), and the second is a medium/heavy (again a 155%). Since the increase in windspeed has the same effect of having a more efficiant keel.
     
  3. RHough
    Joined: Nov 2005
    Posts: 1,792
    Likes: 61, Points: 0, Legacy Rep: 793
    Location: BC Summers / Nayarit Winters

    RHough Retro Dude

    The angle to the true wind that a boat will sail is a function of total lift/drag of the vessel. Reducing drag allows the boat to sail closer to the wind. In your example, if a boat requires 35 degrees of apparent wind to sail, reducing the drag of the keel might allow it to sail at 30 degrees apparent wind. You would not need to bear off to 35 degrees apparent since you don't need as much drive from the sails.

    So yes, reduced drag allows you to sail closer to the true wind.
     
  4. RHough
    Joined: Nov 2005
    Posts: 1,792
    Likes: 61, Points: 0, Legacy Rep: 793
    Location: BC Summers / Nayarit Winters

    RHough Retro Dude

    The lift from the equals the lateral force from the sails. You can't have more of one than the other without turning the boat. All you can do is reduce the drag at the amount of lift needed to balance the forces from the sails. This results in a reduction of leeway angle.
     
  5. Stumble
    Joined: Oct 2008
    Posts: 1,896
    Likes: 71, Points: 48, Legacy Rep: 739
    Location: New Orleans

    Stumble Senior Member

    Thats not really true RHough. You can have more lateral force from the sails than lift from the keel, what you cannot have (to my knowledge) is more lift from the keel than lateral force from the keel. For instance a sailboat beem to the wind with the sails sheeted in hard will slip sideways (more lateral force from the sails than lift from the keel). This technique comes in handy sometimes in docking under sail.

    But since there is always a difference in the two forces, making a more efficiant keel can help to reduce the slippage of the boat. This is one of the reasons why ice boats and sail carts (boats on wheels) are much more efficiant than sailboats. They come as close as possible to a perfectlt efficiant keel shape.
     
  6. LSaupe
    Joined: Nov 2010
    Posts: 3
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Galway, NY

    LSaupe New Member

    Thanks for the replies.

    If a boat is slipping sideways at a constant speed; then are not the force of the keel (and rest of the hull) equal and opposite to that of the sail? If there is no acceleration, then no unbalanced forces? F = MA

    Wether it is moving forward or sideways (or the vector sum) everything should be in balance if there is no acceleration?
     
  7. RHough
    Joined: Nov 2005
    Posts: 1,792
    Likes: 61, Points: 0, Legacy Rep: 793
    Location: BC Summers / Nayarit Winters

    RHough Retro Dude

    If the lateral force from the sails is greater than the lateral force from the keel the boat bears away (turns) until the forces are equal or the boat has a dead downwind course. The docking situation you describe is a case where the boat's heading can be 90 degrees to the course. On any course under sail other than dead down wind the lateral forces are equal or the boat is turning.

    LSaupe is correct, in steady state motion the forces must balance (no acceleration).

    Aero forces of lift and drag act relative to apparent wind. Hydro forces act relative to course sailed.
     

    Attached Files:

  8. Stumble
    Joined: Oct 2008
    Posts: 1,896
    Likes: 71, Points: 48, Legacy Rep: 739
    Location: New Orleans

    Stumble Senior Member

    HA,

    and this is why I am a lawyer not an engineer. I always forget that ∆V is a necessary component of force as opposed to simply V.

    Either way it doesn't change my thoughts on the effect of better keel design, simply the process by which they work.



    Interestingly enough I just ran across an article written by Steve Dashew suggesting that putting a wing between the main and the cabin roof increased the pointing ability of Beowolf by 5-7 degrees...
     
  9. gggGuest
    Joined: Feb 2005
    Posts: 831
    Likes: 24, Points: 18, Legacy Rep: 76
    Location: UK

    gggGuest ...

    t really isn't as simple as that...

    Over simplification... As the guy said as speed increases the apparent wind comes back and all else being equal you point lower and go faster.

    Any boat can be set up to point so high that forward progress is minimal although this is masked on a lot of boats because the jib leads are set up at around an optimal point.

    The fastest upwind progress comes at a compromise point between pointing too high, which means you go too slowly to make any progress at all, and too low, in which case you zig zag across the ocean and make no ground upwind. You feel this most in the very high performance boats (and ice boats must be even more extreme) where there is virtually no difference in the sheeting position between a beam reach and the optimum beating angle. You can scream into a mark on a high speed reach and just point the boat upwind at the desired angle and wait for the boat to slow down so the sails fill properly again without touching the sheets.

    Yes the lower the drag the better the vmg (speed made good) upwind available to you, but using lowered drag to physically point higher may well be a losing game. This is especially true in boats that plane well upwind.
     
  10. RHough
    Joined: Nov 2005
    Posts: 1,792
    Likes: 61, Points: 0, Legacy Rep: 793
    Location: BC Summers / Nayarit Winters

    RHough Retro Dude

    Sorry, it is that simple. But it would be easy to prove wrong.

    If you can resolve the forces for any sailing machine at the best (closest to the true wind) sailing angle, then reduce drag and show that the vessel will point lower, I'll buy the beer. :)

    If the apparent wind angle stays the same, reducing drag forces the vessel to accelerate until drag = drive again or the vessel must sail closer to the wind until drive is reduced to the new lower drag value.

    The key relationship is drive to drag. If drag is low (ice boats & land yachts) the apparent wind angle is also low. Once the vessel is sailing faster than the wind the range of AWA becomes smaller.

    In theory, any AWA greater than zero produces drive. The vessel will accelerate until drive = drag. Thus the AWA needed to move forward is solely a function of drag. As drag nears zero the required AWA also nears zero.

    To recap:
    Reduce drag and you must sail faster at the same AWA or sail closer to the wind at the same speed.
     
  11. Gary Baigent
    Joined: Jul 2005
    Posts: 3,001
    Likes: 118, Points: 63, Legacy Rep: 509
    Location: auckland nz

    Gary Baigent Senior Member

    LSaupe, the comparison between long keel and dagger(s) is such that there is no comparison at all. If you're thinking of changing to vertical or angled dagger foil(s), then do so. Randy is dead right: reduce drag, improve underwater appendage(s) shape ... and you will sail faster and higher with an improvement all round. It is very basic and very simple.
     
  12. gggGuest
    Joined: Feb 2005
    Posts: 831
    Likes: 24, Points: 18, Legacy Rep: 76
    Location: UK

    gggGuest ...

    .....
     

    Attached Files:

  13. RHough
    Joined: Nov 2005
    Posts: 1,792
    Likes: 61, Points: 0, Legacy Rep: 793
    Location: BC Summers / Nayarit Winters

    RHough Retro Dude

    Are you going to explain what the triangles are trying show?
     
  14. tspeer
    Joined: Feb 2002
    Posts: 2,304
    Likes: 269, Points: 83, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    Nice sketch!

    Here are some mathematical relationships to go with it.

    Let:
    gamma = course sailed relative to the true wind (gamma = 0 -> straight upwind)
    Vb = boat speed through the water
    Vt = true wind speed
    Va = apparent wind speed
    beta = angle between apparent wind and course sailed, as shown in the diagram.

    Vb/Vt = sin(gamma - beta) / sin(beta)
    Va/Vt = sin(gamma) / sin(beta)
    Vb/Va = sin(gamma - beta) / sin(gamma)

    From these relationships, which are just the law of sines from trigonometry, you can see that the closer you can sail to the apparent wind, the faster you are going.

    What's interesting about the diagram is you can show geometrically that the following relationship is also true:

    beta = arctan(aero_drag/aero_lift) + arctan(hydro_drag/hydro_lift)

    So getting to small beta is all about achieving low drag/lift ratios. Or, as it's more commonly said, high lift/drag ratios. The relationship above is also why it's very convenient to resolve the total forces into lift and drag, rather than longitudinal drive and side forces in the yacht body axes. The lift and drag can often be calculated without having to know what the orientation of the hull actually is. For example, a gybing centerboard may have the same lift-drag relationship as a non-gybing board, but the orientation of the hull will be different. There are also different physical contributions to drag, so doing the force accounting in lift & drag instead of X and Y force is a lot cleaner.

    High performance craft do tend to become limited by angle rather than power. For example, a landyacht that can do 5 times the speed of the wind (a pretty typical performance) is sailing with an apparent wind angle of only 14 degrees. At any given point of sail (gamma), the apparent wind builds as the speed increases, so the power is there. But the apparent wind is clocking more forward, swinging the lift vector to the side, and eventually there isn't any forward drive component left after the tax of aerodynamic drag is paid.

    One thing I discovered that was a surprise to me was that best performance doesn't always happen when the aero lift/drag ratio is maximized. The reason is the lift on the hull isn't arbitrary. The hull lift is determined by what is necessary to oppose the side force from the rig. But much of the hydro drag of the hull will be there, regardless. So it's necessary for the rig to load up the keel if the hull is to achieve a good lift/drag ratio. Even if the rig and topsides are very efficient, so best aero L/D occurs below stall, it can still pay to go to maximum lift because that improves the hydrodynamic L/D more than it hurts the aero L/D.

    FWIW, the tires on a landyacht work very much like foils in water. There's a leeway angle that comes from the flexibility of the tires, and the leeway is proportional to the side force, just like with a foil. And there's a drag associated with the lift, too, that is similar to the induced drag of a foil. Even the leeway angles are comparable - on the order of 3 - 4 degrees. You can actually see and measure the leeway angle after the yacht has passed - the track from the front wheel will be closer to the track of the windward wheel than the track of the leeward wheel. It's quite noticeable.
     

  15. tspeer
    Joined: Feb 2002
    Posts: 2,304
    Likes: 269, Points: 83, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    Vmg is the best measure. It determines how quickly you get to the mark.

    Pointing more into the wind is the way to improve Vmg when you're sailing a boat that cannot appreciably increase its speed. Heavy keel boats are typically in this category, hence the value placed on pointing.

    You're absolutely right. The boat is going faster. It's not pointing as high. The drag is higher. But it has a higher Vmg. So it's going to weather faster by footing, rather than pointing. This is what you do in a high-performance boat. If you see a modern multihull sailing lower than the monohull fleet going to weather, it's not that the multihull can't point - if it has a deep daggerboard it can point with the best of them - rather it gets its best Vmg by footing off instead of pinching. Sailing faster but farther to get the mark first.

    If your boatspeed doesn't increase enough as you head down a bit, then footing won't work for you. You'll sail the extra distance, but you won't get the Vmg.

    The same thing happens in reverse with all boats. You can sail higher and slower to sail a shorter distance to the mark. But if the boat slows too much, Vmg suffers. Then you're pinching, and you need to back off a bit.

    You've got it all right. But different boats respond differently to changes in course. You have two opposing tendencies - sail faster or sail farther. Which one is better is a quantitative issue that depends on the details of the boat's design and the conditions. The best performance is right at the crossover point where the gains and losses are balanced. Finding that balance is where the art and skill of sailing lies.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.