Sailboat tacking explanation

Discussion in 'Sailing Talk' started by Dani, Nov 13, 2011.

  1. Dani

    Dani New Member

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    dear all
    i'am a newbie here, not even sure if the subject was already discussed or is of interest. tried to find a simple explanation to tacking, google brought up either oversimplified explanations, historical notes, and technical "how to" information, or learned articles crammed with high math. my interest is an elementary explanations based on high school knowledge, namely newton's law of action and reaction forces, and vector decomposition of forces (rule of the parallelogram). see my recently devised explanation at http://www.ee.bgu.ac.il/~censor/sail-boat-tacking.pdf. is anybody aware of other explanations of this kind posted on the internet? is the explanation sound? thanks for responses-- dani
     
  2. mindgasm

    mindgasm Member

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    Dani,

    What aspect of tacking are you seeking to have explained?

    At the simplest definition level, tacking is changing "boards", by moving the bow through the wind. In contrast, gybing is changing "boards" by moving the stern through the wind.

    If you are seeking information as to why boats must tack upwind, you will want to look into Bernoulli's Principle. While it is based in fluid dynamics, the core principles are fairly rudimentary. In essence, once the angle of attack decreases too far, the pressure difference between the inside and outside of the foil (sail) is not sufficient to overcome the other forces, specifically wind and friction from the water.

    If you are looking into how moving the tiller/rudder causes the boat to change course, including tacking, look at Bernoulli's Principle with regard to the changing angle of attack across all three foils, especially the rudder.

    Hopefully, that helps. If you have more info, feel free to post it.
     
  3. Dani

    Dani New Member

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    dear mindgasm

    thanks for response. i am aware of the principles you mention, this was not the point. sorry if i was not clear enough. in my explanation of sailing upwind, http://www.ee.bgu.ac.il/~censor/sail-boat-tacking.pdf, it is emphasized that only flat surfaces (sail and keel) are assumed. only vector decomposition and newton's action and reaction are needed. within this premise, a concise analysis is presented, on the level every highschool student can understand.

    i look for references (internet, articles, books) that use simple mechanics. could not find. thanks again,

    dani
    ...
     
  4. mindgasm

    mindgasm Member

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    Okay, I think I understand a bit better now. You are actually looking at the process of sailing upwind, as opposed to the action of tacking. Within that constraint, you are looking for resources that explain the physics behind upwind sailing, at a fairly elementary level. Is that correct?

    Unfortunately, I think you are going to have a difficult time finding that. The forward and lateral forces (the parallelogram) are derived from the fact that the keel and sail are not flat surfaces. Any discussion of of upwind dynamics that neglects to address the "lift effect" in both air and water from the foil shape, will be either oversimplified, or just wrong.

    That said, I don't think any of the principles involved extend beyond high school level, though calculation of the values very well might. If you are looking for the definitions and principles involved, most of them derive from Newton's Second Law. If you are looking for a simplified method to calculate the forces involved, it becomes a bit harder. Most of the forces are directly related to the curved nature of the foils, and will require a basic understanding of Calculus.

    Reducing the foils to flat surfaces not only overlooks the basic reason why sailing upwind is possible, I am pretty the math just won't work.

    But, maybe I am still missing something.
     
  5. Dani

    Dani New Member

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    thanks mindgasm

    yes, i am looking for a simplified model. the way i see it, flat surfaces might be less efficient, but sailing upwind does not exclusively depend on the sail curvature, i believe. i know enough of the mathematical physics involved to follow a theoretical argument, but i have not found yet a satisfactory detailed discussion. i am not sure further details might interest many readers, so we can continue this privately if you wish (<censor@ee.bgu.ac.il>).

    dani
    ...
     
  6. mindgasm

    mindgasm Member

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    Without a curved surface to act as a foil, no lift is generated. The deflected flow would yield a lateral force, and a negative forward forward force. Supposing the same forces were generated by the keel, with the lateral force opposing that of the sail, the lateral forces would cancel out, leaving only the combined negative forward force.

    In short, you sail backwards.

    The ability to sail upwind is the result of the lift generated by flow over a curve surface (sail), with the lateral forces canceled by those created in an opposite direction by flow over a different curved surface (keel). The resulting non-canceled vectors combined for forward motion.

    Efficient sailing upwind is derived from combining multiple foils in series (jib and main), creating an altered angle of attack, as well as a greater pressure differential between the upper and lower flows.

    I will see if I can't put something together that explains it visually.
     

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