Downhaul tension

[jimwilkins]

I have noticed a repeating comment almost every where there is windsurfing: Down haul tension. I had heard somebody qoute that most sails do not have enough downhaul tension on them when rigged. So, say the specs on the luff sleev says Base:32 cm for a 5.5 on a 430 mast. is the supposed to be take as a min setting.

Most have told me that the top two battens should be loose and flapping, the air is drawn up the sail and of the twist. More twist is more low end power to get you going but alot of air escapes due to the less shape to send the air back across the sail... wtf??? In a wing the more surface across equals more lift, the case where air needs two foil off the tip, the wing reacts by flexing near the tip.

[spennie]

"...in a wing..." is a great analogy, because a windsurf sail IS a wing, and NOT a parachute. Air "escaping" out the twist has a near-zero effect on your sail's performance, since it's NOT a parachute, it’s an airfoil. There are a lot of people who think like you do, because it feels like the sail is pulling more when you rig like that, but that’s just bad handling you’re feeling, not more power. There’s an older sailor here in L.A. who does that, and he’s usually the first one off the water since he’s fighting his sail all the time and it tires him out.
Sails get their power down low, in the boom area, and the upper part is just drag reduction. I’m sure you’ve seen those little winglets at the ends of modern jet wings, that’s what the top of your sail is doing, it’s keeping the high-pressure air on the upwind, or sailor side of the sail from chasing around to the lee side and spoiling that nice low-pressure zone that’s pulling you across the water. You’ve heard people tell you to “close the gap” at the bottom of the sail, between the bottom of the sail and the board, well, that’s how you prevent that same thing from happening at the bottom. You can easily feel this happening when you sail if you experiment with it.
Twist at the top allows a sail to bend open during strong gusts, which changes the shape of the foil. It’s built into the sail by the sail designer, who knows a lot more about this than you do, sorry. YOUR SAIL'S TOTAL PERFORMANCE DEPENDS ON IT BEING DOWNHAULED PROPERLY!!! The overall shape of the foil is created by the bend in the mast, and the bend in the mast is determined by downhaul tension. Not downhauling properly will change the overall shape of the foil and lessen the tension in the rig, causing your sail to perform poorly. Try to get as close as you can with the mast characteristics as well, since every sail is designed around a certain type of mast, so the closer you can get the better.
Rig as close as you can to the sail’s specifications, then do SMALL adjustments (1 cm. at a time, or a pinky width) to see what kind of effect it has. Too little will make your sail jerk when a gust hits it, and it will want to wander around, never feeling comfortable & smooth. Too much will eventually kill the power and give it a “dead” feeling. A good sail properly rigged will power up smoothly, giving what I call that magical feeling, when all of a sudden you start accelerating, but you don’t feel it in the boom at all, like magic.
Your friend’s advice to have the top two battens floppy sounds pretty good, but you really should check with a tape measure at least once, until you figure out the best settings for each sail in your quiver.

Spennie the Wind Junkie
Southern California Regional Teamrider, Sailworks
www.WindJunkie.net

[swchandler]

Spennie's comments are spot on. However, I did want to add a couple of thoughts regarding a properly rigged sail. It's important to note that the appearance on the beach after rigging doesn't necessarily reflect the same shape characteristics of the dynamically loaded sail while out sailing. I've noted that a lot of the floppy looseness in the leech smooths and evens out once out in the water and the true shape of the foil is realized. Also, depending on the type of sail being used (race, wave, freestyle, etc.) the looseness in the leech can vary considerably. For instance, the leech of many top of the line race sails are often quite loose all the way down to the boom.

[Jrobb]

An easy way to tell if your sail is properly downhauled is while on the water, when a gust hits it you accelerate or are able to accelerate. If you just feel the sail power up and you don't speed up...the sail feels heavy and dead or top heavy when powered up, you need more downhaul.

If the sail creases just above the boom when a gust hits or you hit chop (sail appears to want to fold in half just above the boom), you don't have enough.

If you are struggling to plane, the leech flutters in a gust or the rig feels twitchy, you probably have too much. If the leech flutters in a gust and can plane just fine and it feels twitchy, get a smaller sail out.

I usually use the printed spec as my start point or min setting. Then go 2-4cm up from there. This works on my KA sails. Other manufacturers may be closer or different in their specs.

J

[isobars]

[LeeD]

Always like to compare with mother nature's examples rather than man made devices....
Like to look at eagles, hawks, condors, vultures, and other gliding and flying birds (perequine falcons) for my examples..
All have lots of tip twist, widely spaced outer feathers, and flowing even twist from stem to tip, maybe like a windsurfing rig should have.
Hummingbirds and bees are more closer to old style windsurf sails, with less twist, but operate at really high RPM's, variable, to control needed lift.
I suspect birds do need a certain amount of lift, as well as forward drive.
We, as windsurfers, need lots of forward drive (the reason we are planing windsurfers), and some lift to hold up our bodies...

[spennie]

Isobars refers to sails having "An inch or two at the top"? Nothing I've seen in the past 18 years! I was referring to 99.999% of sails out there, trying not to confuse anyone with rare variations or specialty sails.
Ditto on the "floppy all the way down to the boom" comment, the only time I've seen that is when Formula racers are trying to use 11.6s in 25mph wind, or someone is just plain crazy.
Your sail should be shaped like an airplane wing at the boom, and loose at the top, down 2 or 3 panels. We could all discuss thousands of variations on this till the end of time, but I'd rather be sailing!

[isobars]

[cbknap]

All of you sail engineers are missing a key point....the manufacturer's luff length measure has to do with setting the mast base, not with how much downhaul the sail needs. The two are related but quite different.

North was one of the first sail manufacturers to put small circles on the sail corresponding with minimum, medium and maximum downhaul. After setting the mast base and boom, simply downhaul until the wrinkle (triangle shaped area of slack tension) touches the dot corresponding to the wind conditions. Sailworks, Maui Sails and Ezzy have followed North on this idea.

Experts may choose to ignore the manufacturer's specs and use, for example, more downhaul, or less extension, but a novice can rig to the marked specs and be 95 percent sure of excellent performance.

Despite all this, the concept eludes some people. Just this Sunday, a friend asked me why his cams kept coming off. Before I got within ten feet of his sail I spotted the problem....his downhaul wrinkle was 6" shy of the MIN dot!

If your sail is older, doesn't have downhaul marks, or just isn't working, ask someone who seems to be sailing effortlessly--preferably someone on that same sail brand.

chris

PS: Spennie's comments on what your sail should feel like are spot on.

[U2U2U2]

Some sails are rigged flater than others , SuperFreaks "can" be sailed with a floppy leach but aren't designed for best performance that way.

Gaastra's Remedy have moderate downhaul.

I agree to start with the manufacture's spec and change a little DH & outhaul at a time. ( Big surprise there) The spec can vary quite a bit when using different masts and even a different mast extension can some variation, like maybe 4CM.

Lots of the sail websites have rigging videos,with full support, several come to mind that I can call and talk straight away.

Lots of great advise above.

I don't think you can generalize on how its supposted to look, without a specific sail and mast. I don't see quite so much looseness with smaller sails, using a small mast which is softer, but maybe its my type sail.