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Scientific Definition Of Sailing Terms

Discussion in 'Sail Trim with Don Guillette' started by Don Guillette, May 8, 2017. Add this thread to a FAQ

  1. David in Sandusky

    David in Sandusky

    Joined Nov 8, 2007
    913 posts, 48 likes
    Hunter 27_75-84
    US Sandusky Harbor Marina, Lake Erie
    Circulation is not a force, but a description of an air flow that must be added to the two linear flows on the top and bottom of the foil/ sail to completely solve the flow equations. The principle physical impact of the circulation is the generation of vortices that are shed from the end/head of the wing/sail where they contribute to drag. High aspect ratio wings/sails (long/high and skinny) are more efficient because they reduce the relative influence of the tip vortices to the rest of the wing/sail.

    The physical equations for the terms are:
    Force = Mass times Acceleration;
    Work = Force times Distance;
    Energy is the ability to do Work; (the wind has the energy to do work on our boat, applying a force (lift) that moves us over a distance against the resistance of the water.)
    Power = Energy/Work per unit of Time.
     


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  2. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    6,977 posts, 1,049 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    I'm sorry guys, but I'm having a hard time understanding how these 10,000 foot views of lift theory and textbook definitions of equations help answer the OP's practical questions regarding what it means when a sail 'powers up'.

    Convert some of that into personal practical advice and your getting somewhere. ;-)
     


  3. Kings Gambit

    Kings Gambit

    Joined Jul 27, 2011
    2,208 posts, 272 likes
    Bavaria 38E
    US Ventura Harbor
    An airplane wing does not have to be moving with thrust to experience lift as long as air is passing over and under it. It's just that the lift is not great enough to actually lift the plane's wings. In fact, even if air (or water) is moving along the ground and then flows over an isolated mound, there will be low pressure above the mound, etc., relative to the surrounding ground, as long as that flow is laminar. The Lugworm, a type of burrowing marine worm, uses this Bernoulli effect to passively force water through a U-shaped burrow. At the upstream end the burrow hole is flush with the bottom; at the downstream end the hole sits atop a raised mound. As water flows over the mound, low pressure above it "draws" water through the burrow from the upstream hole.

    As air flows (laminar) over the surface of sail cloth of certain shape and draft, the low pressure created on the outboard (leeward) side "draws" the boat into that low pressure area. The boat moves b/c the resistance floating on water is much less than anything on the ground. Nevertheless, if the airplane wing were oriented vertically instead of horizontally and of the same shape and air blew over it in laminar flow, the plane if not too heavy could be "drawn" into the low pressure area as well, etc. In any event, the boat is so "powered" by sail shape and wind velocity.

    A sail seems shaped very much like an airplane wing. Its draft is deep near its leading edge into the wind, and then tapers toward the trailing edge, producing the same effects.
     


    Last edited: May 8, 2017
  4. Don Guillette

    Don Guillette

    Joined May 17, 2004
    1,858 posts, 32 likes
    Other Catalina 30
    US Tucson, AZ
    Thanks for all the replies -- it was very informative. Some of the posts were a bit too technical for my taste. I'm like Scott T-bird - some explanations put me to sleep. I've used the sailboat - plane analogy a time or two but can't remember the context I used it with. I only throw it out in one sentence. Generally, it was a pilot asking me a sailing question and relating it to his pilot training. Years ago, at the Long Beach Boat Show, I saw an experimental boat with an actual plane wing for a sail.

    Here's an example of how I explain aspects of sail trim. In 1949 there was a movie called "Battleground". It depicted the 101 Airborne Division at Bastogne. In one scene, all the troops are being pressed into the fight and a soldier is explaining the rifle. He says "this is a M1 Garand, semi automatic, high velocity, gas operated, 30 cal clip feed rifle". The other soldier says "look, you're not selling it to me, you're showing me how to fire it!!". That's one way of describing how something works. Here's how I would have explained it even though I know how the Garand function (US Air Force vet, small arms instructor) -- "this is the clip and here's how you insert it, line up the front/rear sights on a target and squeeze the trigger". Some folks would prefer the more technical answer but that's just not me.

    One last point: I really don't know scientifically why a sail boat does what it does. I can't explain things like the lift/drag curve and a bunch of other stuff but when I'm standing on the boat I can "feel" what the boat is doing -- it talks to me.
     


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  5. Meriachee

    Meriachee

    Joined Aug 1, 2011
    1,755 posts, 266 likes
    Catalina 270
    CA Edmonton, Ab Wabamun - on the orange ball
    Looking at the graphs that show all the nice airflow over a sail in a given circumstance reveals a pretty picture for most of us. From a technical perspective, it usually ends with the notion of moving the car forward or aft. But that's a problematic discussion for some too.
     


  6. Kings Gambit

    Kings Gambit

    Joined Jul 27, 2011
    2,208 posts, 272 likes
    Bavaria 38E
    US Ventura Harbor
    In the "old days" I doubt soldiers firing artillery were using calculations of Newtonian physics to determine the elevation of the piece, shell velocity, and distance to hit their targets. They just kept firing until they found the correct formula empirically. There's just a common sense aspect of sailing that works for most of us :wink:.
     


  7. jon hansen

    jon hansen

    Joined May 25, 2012
    442 posts, 111 likes
    john alden caravelle 42
    us toledo ohio-----sturgeon bay wis sturgeon bay, wis
  8. jon hansen

    jon hansen

    Joined May 25, 2012
    442 posts, 111 likes
    john alden caravelle 42
    us toledo ohio-----sturgeon bay wis sturgeon bay, wis
    [QUOTE="Don Guillette,

    One last point: I really don't know scientifically why a sail boat does what it does. I can't explain things like the lift/drag curve and a bunch of other stuff but when I'm standing on the boat I can "feel" what the boat is doing -- it talks to me.[/QUOTE]

    ................ , what?
     


  9. Daveinet

    Daveinet

    Joined Sep 20, 2014
    661 posts, 58 likes
    Rob Legg RL24
    US Chain O'Lakes
    Not really. The problem is that you are ignoring the time component that is used to determine wind speed. Wind speed is measured in miles/time. Power is force*the rate force is applied quantified in distance/time (MPH). Wind speed is the determining factor of how much energy you can extract from the wind. Power is the correct scientific term for describing the winds affect against the sails.
    This is not quite right. The definition of energy is (power*time)=energy. You have time in your equation twice, once listed as a part of energy and once as time. The formula is "1 watt * 1 second = 1 Joule". Joule is the unit measure of energy. Since power is force * rate at which the force is applied, it must be applied for a given amount of time to accomplish work or energy. A hydraulic press can apply force for an indefinite amount of time, but unless there is motion, which takes time, no work or energy is done.

    So why not mess everything up here, because I like to use the concept torque in sailing. Yes, I understand torque is rotational, but if you are thinking in terms of engines, the concepts translate fairly well. So here you go:
    If one thinks of engines in a car, there are two factors that define the engine's output. RPM and Torque. For a given RPM, the engine puts out a given amount of torque. Multiplied together defines HorsePower. We can quantify the wind as HP. In an engine, for a given amount of HP, we convert that to motion. The transmission will either multiply torque, which lowers the RPM, resulting in more force, but slower speed. If you don't need as much force, you can up shift, which gives you more speed, but not a much force. So now when thinking of a sail, the wind is your engine, and sail draft is your transmission. If the wind is slow, there is very little power, so we downshift into a lower "gear" by increasing draft. We loose speed, but we gain force to keep the boat moving. When the wind is fast, the "engine" contains more power, so we can reduce the draft or shift to a higher gear to gain speed. This analogy works up to the point where the wind contains more power than can be translated into speed, due to the limitations of the hull. At that point, you just are making the sail inefficient as you are no longer converting the power into the motion of the boat. To me it helps understanding what to do with draft if I think of it like a car transmission.
     


    Last edited: May 14, 2017
    Scott T-Bird likes this.
  10. Scott T-Bird

    Scott T-Bird

    Joined Oct 26, 2008
    2,911 posts, 191 likes
    Starwind 27
    US Allamuchy Barnegat, NJ
    Nice translation! It works for me.
     


  11. rgranger

    rgranger

    Joined Jan 19, 2010
    4,217 posts, 422 likes
    Hunter 26
    US Smith Mountain Lake
    How about foot-pounds per square hectare... :poke:
     


  12. Cayennita

    Cayennita

    Joined Nov 28, 2009
    493 posts, 15 likes
    Catalina 30
    US St. Croix
    In 5th grade, were asked to bring a couple of business cards to perform an experiment on lift.
    We bent the cards in s slight curve and holding them apart about 1/2' apart in front of my mouth. I blew between them and they moved towards each other demonstrating "lift". From there we started building various paper planes again demonstrating lift.
     


  13. Calif. Ted

    Calif. Ted

    Joined Jun 8, 2004
    1,947 posts, 53 likes
    Catalina 320
    US Dana Point
    Yeah, you can feel it if a boat is "laboring" or starting to power up or "running free" and fully powered, Jackdaw's first post about covered everything I've ever been able to explain to others.
     


  14. normofthenorth

    normofthenorth

    Joined Nov 13, 2015
    45 posts, 2 likes
    Hunter 290
    Ca Toronto Toronto Ontario
    Jackdaw, for me - admittedly much more of a dinghy sailor than a keelboater - there's a different first dividing line. It's between wind so light that it won't follow the full airfoil shape of a high-draft or "full" sail, and "normal sailing wind" - everything above that line but below wind that is overpowering.
    In that "normal" range, I'm ALWAYS looking to maximize the power my sails squeeze out of the wind, because that's how the boat will swim through the water the fastest. (Of course, sailing upwind is about maximizing vmg to windward, not exactly power, but you'll forgive the slight overgeneralization.)
    In both the underpowered and the overpowered regime, I want flat sails, though for different reasons. In a drifter, flat sails may keep the light breeze attached to more of the sail, generating more lift and power - physics power, = energy per unit time. In strong overpowering winds, I want flat sails because they generate LESS lift & power and I've got too much.
    In between, I always want full sails on a reach to maximize lift, power, and speed. (I sail and race a planing dinghy - Albacore - so hull speed is not the speed limit that it is on most keelboats.)
    And BTW, the post that says "no twist in big winds" needs to be modified for some overpowering winds at least in some boats. At least some of the time, letting a flat mainsail twist will luff/depower the top of the sail, which has the most leverage to heel the boat too much. There are different tricks on different boats to bend the mast while opening the leech, but they don't work well if the sail is cut so full high up that it won't flatten with max mast bend. A full top of the main won't luff effectively, it will just replace lift with drag, and both can create excessive heeling. (Of course reefing turns overpowering wind into normal wind!)
     


    Last edited: Jul 10, 2017
  15. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    6,977 posts, 1,049 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    I agree with all of that!
     


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  16. normofthenorth

    normofthenorth

    Joined Nov 13, 2015
    45 posts, 2 likes
    Hunter 290
    Ca Toronto Toronto Ontario
    Actually two UNbent (flat) business cards will also draw together when a breeze blows between them. Higher velocity does produce lower pressure. And Bernoulli's complicated equations are also correct. But using them to explain to non-phDs how sailboats sail or how airplanes fly leads to constant multiple errors - like "proving" that airplane wings have to be asymmetrical (Just Plain Wrong!) or that airplanes can't fly upside down (ditto!).
    In fact, UNDER the water, our boats all rely on generating lift from rudders and keels or centerboards or dagger boards - ALL of which have SYMMETRICAL foil shapes, but generate lots of lift very effectively, just like symmetrical airfoils on modern airplanes!
    Fortunately, Newton's equal and opposite reaction law explains ALL of it, even in very simple versions! So my advice is to forget Bernoulli in any non-phD explanation and concentrate on Newton. An air plane's lift comes from deflecting air (over both sides of the wing) DOWN. A sail generates lift (and) force and energy by bending the wind from its initial direction to a direction that is more AFT. If that direction is also less to leeward, as usual, the deflection also causes heel, after the keel/board uses ITS lift to resist letting the boat slide sideways to leeward. The rudder steers the back of the boat around by deflecting the passing water in the opposite direction. The forces, energy, power, math, and physics all work, and they mostly work at High School level, UNlike Bernoulli's explanation!
     


  17. sea_casper

    sea_casper

    Joined Jun 28, 2005
    423 posts, 4 likes
    Hunter H33 2004
    US Mumford Cove,CT & Block Island
    Scientifically it's complicated, empirically you can observe and "feel" the results, without understanding the science. For example when adding "twist" to lessen heel, has any one noticed that the lift vector for the twisted top of the sail is pointing forward, adding to forward speed, as it reduces the lift vector causing heeling. The America's Cup Boats use true airfoils, and can't twist and their engineers noticed the loss in forward thrust available from twist. Myself with an unbattened, loose footed furling mainsail, have found a dearth of "expert" advice on sailing optimally, e.g., slackening the halyard has little effect, the outhaul is the primary draft control, to flatten I must also furl a tad, but I do have infinite reefing control when necessary. Like I said the science is complicated, educated "feel" is everything.
     


  18. Meriachee

    Meriachee

    Joined Aug 1, 2011
    1,755 posts, 266 likes
    Catalina 270
    CA Edmonton, Ab Wabamun - on the orange ball
    Thankfully enough twist in the main will eventually produce enough downdraft to rip the explanations from your hands, but not enough to displace the beer.
     


  19. normofthenorth

    normofthenorth

    Joined Nov 13, 2015
    45 posts, 2 likes
    Hunter 290
    Ca Toronto Toronto Ontario
    If you're sailing in strong winds, where the wind shear (velocity difference) between the top and bottom of the sail is usually very small (unlike low winds), if adding "twist" to lessen heel adds to forward speed, I think it proves that your sail is trimmed/sheeted too tight, or you're footing excessively (sailing too low) if going close-hauled. If the sail was trimmed optimally before you got overpowered and let the top twist off to de-power, then the top should be luffing, trimmed looser than optimal, afterwards. The sail will then generate LESS forward force and power, not more. OTOH, if excessive heeling was slowing the boat down, it may still go faster with the extra twist, same as an overpowered boat that reefs.
     


  20. normofthenorth

    normofthenorth

    Joined Nov 13, 2015
    45 posts, 2 likes
    Hunter 290
    Ca Toronto Toronto Ontario
    Getting back to one of the letter writer's original points, the analogy between how sails generate lift and how airplane wings generate lift -- and ESPECIALLY between how (modern high-aspect) fin keels and spade rudders generate lift and how airplane wings generate lift -- is extremely apt and tightly related. In fact, I think a light-weight version of a modern high-aspect spade rudder would probably make a near-ideal airplane wing for a powered plane or a glider. I also make and fly kites, and many kites generate their lift with fabric airfoils that the kite people call "sails", and with good reason. Again, the physics explanations -- Newtonian or Bernoulli (but PLEASE not High School Bernoulli!) -- are virtually identical in all cases. Yes, there are differences between water and air, and yes the speed regime also makes a difference, but these differences are quite trivial compared to the similarities, IMO.
    And there's another very close analogy that many of us see every day: Modern (ordinary horizontal-axis high-aspect) wind turbines use airfoils (to generate lift out of moving air) that closely resemble the foils sections many of us use on our keels, boards, and rudders. What's amazing about those foils is how amazingly efficient they are with their almost toothpick-like slimness. When a typical modern wind turbine is stopped, the area of the blades' described "disk" (circle) that is actually filled with blades is on the order of 5%. But I think I've read that the % of air passing through that whole "disk" that transfers energy to one of the blades is on the order of 95%! And they don't spin fast like engine propellers, either! The difference between the rotating/lifting blades and the stalled/stationary blades is vitally important in storms. If the wind velocity rises above the turbine's design range, the operator/computer applies the brakes, stopping the blades from rotating. As long as they're stopped, only about 5% of the power of the wind actually pushes the blades. But if the brakes fail and the blades start turning, that % rises to ~95%. Most catastrophic wind turbine failures happen when the brakes fail in a storm, then the wind on the 95% disk knocks the tower down!
    Similarly, when my Albacore dinghy is stationary at the dock with full centerboard extended, a child could push the boat sideways (admittedly slowly). But when the boat is sailing at hull speed close-hauled in a breeze, that same centerboard -- now slicing through the water with high efficiency and generating lift as if it were a full keel or bigger -- is resisting a sideways force of (I'm guessing) 100s of pounds and sideways power of ~10 HP. Same "blade", but very different effect at speed vs. stopped/stalled.
     



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