Ding, Ding, Ding!!

[Fly4rfun]

I was reading some of the info, and explanation for us novices in the Jib sheet section, when the bell went off. we have a airfoil !! its called a sail. this i understand as I flew as a Private pilot for many years and hours. now I might catch on to the whys and what for's of this. not just to do "this " because I was told that's what was done.

[GreenLake]

Glad you are enjoying reading the Basic Concepts and Techniques thread and making the connection. The main difference are the wind speeds involved. So think Wright Brothers more than jet plane, and you are in the correct aerodynamic equivalent (same Reynolds number).

There are some very nice papers out there that explain the flow around the "wing" (main) and "slat" (jib) of our dual airfoil system. Arvel Gentry has written a number of popular articles on the aerodynamics of sailing and they are still available online (just a search away). One of these days, I'll get inspired again and add a diagram to summarize the main findings, but in the meantime, why not go to the source?

[Fly4rfun]

GL,

I get lost with the math, can't do it, kept me from getting more advanced deg in Nursing.

[GreenLake]

I don't recall a lot of math in Arvel's articles. They were written for sailors.

[tomodda]

Gary:

Yes, you have it exactly right, the Sails are low-speed airfoils. Also worth considering, the centerboard acts as a "hydrofoil." Since water is so much thicker than air, the CB's area is much smaller than the sails, but the forces (sails vs CB) actually cancel each other out! Which brings us to what I call the "Watermelon Seed Theory of Sailing".... I wrote it up somewhere else on this forum, but it goes like this - If you add up all the force vectors on the sails (lift, drag, etc) you wind up with one big force propelling you forward and to the lee, in a diagonal. If you add up all the forces on the underbody (It's not just the CB, you also have to account for the hull itself and the rudder), you wind up with a force propelling you forward and to windward, also in a diagonal. So you have two diagonal vectors acting on your boat, your boat is shaped like a watermelon seed and gets "spit out the middle!" Now, if you've ever spit a seed out from your mouth, remember how you could control the direction of the seed by twisting your lips this way or that, maybe tensioning your lower lip more? Well same deal with sailing, you change the vector and force of your aero- and hydro- dynamic surfaces to change the direction of the boat. The most obvious being the rudder, pull on that and you are changing your underwater shape and therefore the hydrodynamic vectors. But sail trim is the same, all those controls change the aerodynamic vector. Sailing is all about constant subtle inputs into your controls surfaces (aero- and hydro-) to keep that "watermelon seed" balanced right on the knifes-edge, going in the direction you want it to, at the fastest speed possible for the wind you have.

Or you can just read Avrel Gentry's articles. I have all of them on disk.. @GreenLake, how can i post and link them here? Warning - Gentry doesn't use too much insane math, but he does have a very controversial theory of lift - he shows that lift comes from a circular "flow' around the airfoil, rather than pressure differentials, Venturi effect, etc. In practice, it doesn't make much difference to how we sail, with the possible exception of how we manipulate the "slot" between the jib and the main. IFRC, he advocates a way more open slot than most of us use. I don't have steady enough winds around here to really try it out, running measured miles and comparing, etc. But it's fun theory Overall, his articles are well worth reading, and I've set up "Gentry Tufts" on my jib (a set of 3 tell-tales right behind the luff, about halfway up). Quick illustration - and mind you, I was just ghosting along and in the middle of a tack so don't lecture me about sail set.. the mainsail is very unhappy and the rudder is too hard over:

Gentry Tufts

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Anyway, digression to a digression Sailing has a lot in common with flying, except think of it going upwind/downwind instead of up and down. And one wing (sail) is a lot bigger than the other (CB). Other than that, it's exactly like an airplane.. well, maybe glider anyhow!

Best,

Tom

[Fly4rfun]

Tom,

Np lecture here, i barely know how to fold a sail, let alone set it. !
thanks for the info. Enjoyed reading the redoing of your boat. My mast also came with the stabilizing stays. One was broke so removed the other one. ill consider replacing them once i get some water hours under my belt.
Garry

[GreenLake]

If you mean the diamond stays and jumper struts: considered opinion has that they are not needed.

When it comes to different aerodynamic theories, what distinguishes one from another is how easy they make it to predict the effect of sail trim.

In other ways you expect them to be ultimately equivalent. For example, we know that Newton's law of conservation of momentum holds. Therefore if wind (and water) impart a momentum to the boat, some air and water will have to be imparted an equivalent opposing momentum (get deflected) to make up the balance. Knowing that, however, won't let you predict how that momentum transfer happens around an air foil.

So you need something that gives a better description. Many people cite Bernoulli's law. It's the same thing: it's not that Bernoulli's law is not applicable, it is just that it's difficult to predict from it how best to trim your sails. (In order to attempt to do so, you have to set some boundary conditions, that is, make assumptions what happens at the trailing edge of a sail, and I find Gentry's case that finds the prevailing assumptions erroneous rather persuasive).

He uses a different set of boundary conditions, the Runge-Kutta ones, and that's when the model starts to have the "counter circulation" around the sails. Which isn't "real", but captures the fact that the air on the windward side of the sails flows more slowly than on the leeward side. Physicists love to change perspective, so they might look at the situation as if they were moving with the air (like flying in a balloon overhead). Then the wind is stationary and the sail moves. To make the air move faster on one side than on the other then looks like a counter circulation to someone looking down like that, because to them the wind doesn't exist (it seems stationary).

Anyway, once you construct a model like that, it begins to explain a lot about the effect of sail trim. Which is what you want. But you can be sure that at any point, it also obeys all those other laws of physics and doesn't contradict them.

Some people may like to just focus on the conclusions, including what the airflow along the sail does look like, and how telltales will help you monitor it and improve your sail trim.

Speaking of sail-trim: I don't see any telltales at the end of your main, Tom. You do want to monitor the flow at the leech, which usually means a streamer at each batten pocket. The ones you have only tell you what the flow is like on the first third of the sail.

Now, sail trim is affected by disturbances, whether from waves, or from less than attentive steering. Some trim settings are more forgiving than others (they are said to have a wider "groove"). While they may not be as efficient, nominally, they may be so in practice, because if you don't manage to stay in the groove you pay a penalty, so to speak. Even at higher levels of racing you will not find the uniformity of trim you might expect, and my amateur conclusion is that this is based on what works optimally for the combination boat and helmsperson. Just a thought.

[tomodda]

GL:

Thank you for the perspective on different aerodynamic theories... you're right, it's not either/or, both viewpoints are valid, any theory is just a way of describing natural phenomena. Kind of like the 5 blind men describing an elephant.

In the meantime, I DO have leech telltales on my main, 3 inches above each batten pocket:

Leech telltales

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You can see the top two pretty well, but the bottom two have wrapped back around the sail. Note also that the main is luffing - a big bubble all along the mast and my luff telltales pointing off forward or just hanging down. Remember, I was in mid-tack, still sheeting the jib in - look at the position of the clew. and I've left my bungee "auto-pilot" in charge of steering.

For Gary, this is just to point out that you have to consider the whole sail-plan, from the forestay all the way back to the main leech as one big wing, keep the air flowing smoothly across all of it. The most important telltales are the jib luff, to see the air coming onto the sailplan, and the main leech, to see it coming off. Also, notice that the top 1/3 of my mainsail, above the "DS" logo" has good flow, the rest of the main doesn't. This because it's in "clean air", not affected by the jib - the top leech telltale is streaming nicely, the next one down is at an angle, the 3rd one is flipped behind the sail, and the bottom one you can barely see against the treeline, hanging pretty much dead. Once I'm done my tack and have the jib set correctly, one hopes that the air flow reattaches to the main and all those telltales straighten out, flowing straight back again. But at the moment of this photo, only the top part of the main and the front-most part of the jib are driving me, the rest of the sailplan is just along for the ride. And you'll note that I'm barely moving, no wake, just a few ripples.

Note that I was sailing in VERY light wind, maybe 2mph. Winds are usually weaker and more turbulent near the surface (lower), especially in light conditions. So even after I get the sailplan settled down, I'll probably still be getting most of my drive from the top of the mainsail. Moral of the story - watch your entire sailplan, but depending on conditions some parts are more important than others. When I'm sailing, I'm constantly watching my sails and the wind - on the water, on trees, and flagpoles, and on my windex, and on nearby boats, even the flag on my transom gives me some wind info. Other things - running into other boats, the shoreline, etc - are also important, but occupy maybe 10% of my "scan." Of course, a good chunk of my "scan" also is looking for another beer in the cooler

[GreenLake]

Tom. In light airs like that, if you have a topping lift, you might allow it to take some of the weight of the boom - so that your main can twist (wind direction changes with height).

Otherwise we are in agreement on this. (I'll have to re-read Gentry on the triple tufts near the jib luff).

[Fly4rfun]

GL,Tom
thanks for the explanation

[tomodda]

Gl: You're absolutely right re: topping lift. Yes, I have one and I use it that way. To be honest, for these photos, I was just "messing about." My son was with me and I asked him to take some photos of me sailing, since I don't have any. So I dropped him off at the dock, fell off downwind and made a few lazy tacks back up to the dock. If I had been "actually sailing," I would have trimmed better (I hope!).

[GreenLake]

Tom, I totally know how this goes with taking photos near the dock.



Either they don't show the sails, the sails aren't trimmed, or they are reefed, or the cannons are out and block the view.

[tomodda]

I'm glad I'm not the only one with red topsides. And I love your cannons, what a great way to get kids out on the water.

As for the triple Gentry tufts, they're for fine tuning when you're in the "groove." Gentry writes that by the time your regular tufts start reacting to a header, it's already too late, you're out of the groove. The triple tufts are set forward of the usual telltales so you can use them while IN the groove. Why three? Well, it has to do with a separation bubble which forms at the leading edge of the sail, but I'd have to re-read Gentry myself to understand the details.