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Go Back   SailNet Community > Skills and Seamanship > Learning to Sail
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  #1  
Old 12-02-2006
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Leach line aerodynamics

I was reading zfactor's wire cable question and the responses regarding leach lines, and I got to thinking about what the leach line is really doing, and where and when would be the best time to have it deployed.

Obviously, all of the following discussion refers to upwind work.

From my understanding of fluid dynamics*, I know that inducing a curve at the trailing edge of an airfoil can increase lift. The leach line accomplishes this by shortening the roach length and warping the trailing edge of the sail, right? That increases lift, good, but it also increases drag, bad. That's why airplanes only deploy their flaps on takeoff and landing, when extra lift is required, and drag can be tolerated and countered with increased throttle. Flaps are low speed, low angle of attack equipment. So it seems to me that, theoretically, I ought to tighten the leach line as I come out of a tack and harden up on my new heading, but that once I've climbed onto the wind and achieve a steady course, the leach line should be slacked.

The other thing I know from fluids is that the amount of lift available from any given airfoil is deterimined by the width of that airfoil, not its length. That's why masts are tall, and airplane wings are wide, not long. We do curve the tips of wings, especially these days after what they've learned about trailing vortices and crowded flight lanes, but technically there is no aerodynamic advantage to being able to curve the wingtip. Again, drag is the issue. So it seems to me that having a line in the foot of the sail, as suggested by Giulietta, wouldn't provide any advantage. This is reinforced by the observation that I use an outhaul on the foot of my main to flatten the sail, rather than to curve it.

I'm really just an eager novice of a sailor, and I'm certainly no sailmaker, so I'd be grateful if one of you knowledgeable folk would enlighten me as to what is considered optimal use of a leach line. Any racers out there with ideas?

(I should probably just dig out my copy of Colgate or something, right? Anyone know of a good source for technical discussion on the subject?)

*BSc Mechanical Engineering w/ a minor concentration in aerodynamics. But that was ten years ago, and I've been doing piping systems since then. I'm struggling to remember my theorems without cracking a book, so feel free to correct me if I'm all wet. And as for practical sailing application, what the heck do I know?
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Old 12-02-2006
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morganmike,

Slow down in your conclusions, ok? And yes, you may need to read or research a little more.

Racing sails, are allways fabricated including all the triming possibilities in mind. The leech and foot lines are one more of those possibilities, because sometimes that makes a difference, and it is better to have all the possible tools than not having them.

(I have used the leechline, when cruising with my wife and small baby, only to stop the leech of the genoa from vibrating and annoying us, was is fine triming?? no... but made the ride smoother!!)

Here is an example of a well known sailmaker :

http://na.northsails.com/One_Design/...tailedSail.htm

attention: Foot lines only used on Genoas, not mainsail. (but I have seen a boat with them, too!!)

The outhaul can and is used to stretch the sail to meet particular sailing conditions, such as in "stronger winds", for example, (and no crew on the rail!!) but also used to give more camber (curvature) to the sail in lighter winds, (more lift) it is a very good tool to shape the sail, and should be used together with the Boomjack, the backstay, the cunningham (also have one)and the main track car, to make sure that the wind entering the sail, does "its" work and exits the wing, sorry sail, as fast as possible to make it smooth for new wind coming in. The leech line (and I used the flap name just to explain it in an easier way, because english is not my first language!! ) is just another tool.

Note that wind velocity and angle 30' above the water are different from the ones at water level, thus the need for twist (I hope the right word) - backstay, boomjack and traveler.

On my boat, my outhaul can travel up to 30', because that is just one more tool I have should I need it.

This normaly gives you more camber along the vertical axis, but the leech line "closes" (I need help with the english here) the the trailing edge. of course common sense must be used because it can overshoot your needs, just like the traveleres, the boom jack, the backstay etc.

As far as using it on the genoa, it is doing excatly what you were saying by curved wing tips!!! It is there to camber the lower end of the sail, (horizontaly) and making sure that the wind coming thru the lower end of the sail is nor washed down verticaly but rather pushed towards the trailing edge of the sail. I don't sail with it closed at all times, but it is there should I need it.

(I often look at the foot of my son's Optimist, that is attached to the boom, and see that as a very very good example of that "horizontal" camber in the lower end of the sail. can you look at an optimist sail??)


The leech and footlines (footlines are used almost on every racing sail) are just more tools, and I like them. Sailing is a constant learning curve, and I like the "gizmos" ehehehe

The fact that I include them in my cruising sails, is just another tool to fine trim the sails. My boat was built for racing, but because we sail all year round, and do some cruising, (inside it is 100% cruiser, 3 king size cabins, 2 heads, etc.), however, even cruising, I can't stop..... tell tales, cars, cunningham, I monitor them all the time... Its my way of amusing myself on a boat (which is my objective), other people have wine and listen to Pavaroti.

Hope I helped and excuse my lack of technical words.

PS I am an Aeronautical Engineer, also, so perfectly understood your questions. Planes don't use their speed brakes all the time, but they are there!!

While you are at it look at aspect ratios and downwash!! Just a tip!!:

the photo bellow shows the Horizontal camber that the foot line can help provide. A picture is worth 1000 words.

Also note that my genoa works inside the rails. And the rail at the fron is open so we can bring it out when needed.
Attached Images
File Type: jpg 1.jpg (58.6 KB, 135 views)

Last edited by Giulietta; 12-02-2006 at 12:03 PM.
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Old 12-02-2006
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There's a good explanation of the purpose and use of leech lines on page 106 of Wallace Ross' "Sail Power." The short answer is that they're needed to support the leech of the genoa, which, unlike the mainsail, doesn't have battens to support it. When the leech of the genoa starts to flutter, you should tension the leech line just enough to stop the fluttering, but no more. The purpose of a leech line is not primarily to shape the leech of the genoa, so as to increase its power. It's purpose is to prevent the unsupported leech of the genoa from collapsing, which, if it happens, will allow the wind to spill out of the sail and thus to decrease the power of the sail. In other words, using a leech line won't make the boat any faster or increase the power of her sails, but it will prevent the loss of power due to an unsupported leech.
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Sailormon6,

mine do have battens. But yes, I can't explain because the words fail me.

Mine however, and excuse my "butting in" do alter the shape of the sail, too. believe me.
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G, thanks for the informative reply. It's when to use the tools that is exactly what I'm trying to figure out. I have used the leach line on my genoa exactly as you have described, to stop fluttering, but my sails are relatively old and I'm never sure if what I'm doing is really supposed to optimize performance or just compensates for tired cloth. I guess (because of condition) my sails have enough camber that I haven't given much thought to trying to achieve more. I'm usually trying to flatten them. I'm sure that's where knowing what the design shape of my sail is supposed to be would be helpful.

With regard to the foot line, there is still an open question in my mind as to which controls should be used for upwind as opposed to downwind work? I have understood from reading that adding curvature is more desirable when going downwind.

Re: twist, ok, I can see that, but as far as curving the wingtip goes, how much can curving the bottom of a genoa really effect flow along the sail? Isn't the velocity profile such that flow feeds into the sail from ahead and that some of the flow direction is vertical - UP, which wouldn't be impeded so much by a low pressure zone at the foot of the sail as by a high pressure zone at the flow exit (i.e. the 'wingtip', or top of the sail)?

Anyway, thanks for the picture, I'm sure I couldn't camber the foot of my main like that, as it's on a track. I'm always interested in seeing alternate designs for things.

Anyone ever sailed on a day foggy enough to observe airflow over the sail? I'd like to see that.



Quote:
In other words, using a leech line won't make the boat any faster or increase the power of her sails, but it will prevent the loss of power due to an unsupported leech.
Well, every action =/opposite, right? Preventing power loss = effective increase in power. Either way, good point.
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A leech line does, of course, actually alter the shape of the sail slightly, but that isn't why we use it. We use it to keep the air flowing smoothly off the trailing edge of the sail. If the leech line is too loose, the leech (trailing edge) of the sail will flutter, creating turbulence. If the leech line is too taut, the leech will be cupped, which will also create turbulence. The way to adjust it is to ease the tension on the leech line until the leech of the sail starts to flutter, and then increase the tension just enough to make it stop. There is a point within the range of adjustment of the leech line where the flow of air off the leech is optimized. That's what you are looking for, not any particular shape.
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Quote:
With regard to the foot line, there is still an open question in my mind as to which controls should be used for upwind as opposed to downwind work? I have understood from reading that adding curvature is more desirable when going downwind.
When sailing off the wind, the objective is to maximize sail area and increase the amount of drag, or resistance, created by the sails. When sailing downwind, drag is good, because the force created by drag is going in the same direction as the boat. By easing the outhaul, cunningham, and similar controls, you increase the effective area of the sails by eliminating the curls caused by tension along the foot and luff of the sails. Increasing the curvature of the sails is also good, because it increases the resistance of the sail, but too much curvature results in a decrease in the effective sail area, and that's bad. So, the idea is to find the middle ground. With those general principles in mind, I think you could theoretically use the foot line to cup the foot of the sail a little and increase resistance downwind, as long as you don't do it so much as to decrease sail area. I have to admit, though, that I can't remember actually using a foot line that way more than once or twice in over 25 years, because it's a pretty subtle adjustment, and I think there are usually more productive things to think about during a race.

Coincidentally, I saw a lot of good racing sailors on the Chesapeake last year running downwind without adjusting the tension curls out of their sails. I suspect the reason was because they thought it was too much bother for too little gain, but, when you ease the controls, the increase in power is enough that you can feel it. You don't need to look at the instruments to know that you're going faster.
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Old 12-03-2006
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Interesting, thanks. Seems like I'm doing it right without really knowing it, since the times I've used the leach line it has been as you have both described, to stop the sail from fluttering. I'll definitely be playing around with it once the boat is back in the water.
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Old 12-20-2006
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Leach lines are PRIMARILY to keep that section of the sail 'stable' (not flopping or fluttering). They are definitely found on mainsails as well as foresails.
The leech of a sail should be FLAT (exit) with respect to the direction of air flow, as increasing camber in this section will result in air flow separation on the lee side, increase drag; with the aim that on the leeches the flow on both sides should be laminar, and the velocities of airstreams on the windward and leeward be EQUAL as they exit from the sail.
Sails are "thin foil sections" and DO NOT behave anything like how your high school science teacher explained them. Aerodynamics is simply NOT a science that can be intuitively rationalized into 'simple terms' (and your high school teacher was entirely ... WRONG).

For a good 'sailors perspective' on how sails 'really work' (and If you have an indepth 'fluids' background), go to http://www.arvelgentry.com/ and go through all the 'how do sails really work, etc' and the 'magazine articles' section. These are the 'seminal' articles of the aerodynamics of sails ... that *everyone else* uses as their reference basis.

So, to adjust the leech lines on a sail: relax the line until the leech edge starts to become unstable or 'just begin to flutter', then very slightly tension the line to stop the fluttering and make it fast into its jamcleat ... thats it!
;-)
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Old 12-20-2006
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The leech line keeps the leech from shaking and vibrating. That's it. Why? A shaking leech shakes the sail, and that spoils the shape and spills wind. But most important, shaking ages the sail rapidly by breaking down fibers and resin. The two worst things for sails are luffing and sun. That's why good racing sailors don't hoist their sails until just a few minutes before the prep gun. The jib and mainsail leech lines (some call them leech cords) should be JUST taut enough to keep the leech from chattering. A little cup in the leech won't hurt your speed, but a little shaking will hurt your pocket book. The best sails have leech lines that can be adjusted easily even when reefs are tied in, say by leading the mainsail leech line up through a block on the headboard and then down the luff to a small cleat near the gooseneck.

My sources are The Annapolis Book of Seamanship and many races and seminars with sailmakers.

Last edited by johnsail; 12-21-2006 at 09:23 AM.
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