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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
    887 posts, 41 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
    6,507 posts, 799 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
    2,009 posts, 219 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
    1,839 posts, 24 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
    1,507 posts, 187 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
    2,009 posts, 219 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
    221 posts, 49 likes
    john alden caravelle 42
    us toledo ohio-----sturgeon bay wis sturgeon bay, wis
  8. jon hansen

    jon hansen

    Joined May 25, 2012
    221 posts, 49 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
    593 posts, 38 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
    2,779 posts, 155 likes
    Starwind 27
    US Allamuchy Barnegat, NJ
    Nice translation! It works for me.
     


  11. rgranger

    rgranger

    Joined Jan 19, 2010
    4,129 posts, 391 likes
    Hunter 26
    US Smith Mountain Lake
    How about foot-pounds per square hectare... :poke:
     



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