Absorpt? Do you mean stalling?

Yes. If the sail is stalled, the air flows from the leech to the middle of the sail, hence sucks or absorbs the telltale to the leeward side of the sail. Normally the sail stalls from the leech to the mast, so the leech telltale is the most sensitive for avoiding a stalled sail.

Cheers,

Klaus

I was confused by your choice of words. "Der leech telltale sindt absorpt" will go into our sailing dictionary now (my excuses for "radbrechen" your language)
I am glad an aerodynamist have the same view on leech telltales. We had a hard discussion with Rick White amongst others some time ago who advocated dropping leech telltales and going by the telltales at max draft or thereabout. Simple is fast, but that was a bit too simple I think.
Would you care to guesstimate how large the difference could be between sailing with a streaming telltale at max draft and the section aft of that stalling, vs. sailing with telltales at max draft(or in front of max draft) and the leech streaming?

Not too bad. In strict German it would be "Die Achterlieksverklicker sind absorbiert." I am sure that normal german people wouldn't understand it.

It depends a bit on the boat and point of sail.
First some numbers:
alpha, cL, L/D, % sep
10, 1.23, 65, 100
12, 1.33, 50, 90
15, 1.38, 28, 60
18, 1.33, 14, 40
The first number is the angle between wind and sail (angle of attack). The second number is the lift (cL) of the sail, the third number the lift to drag ratio and the last one is about the point where the flow stalls (0% is the mast, 100% the leech).
While beating with fast boats cL and L/D are important, but with stronger wind cL is less important. The higher L/D the better the vmg. For only a bit more cL you pay with a lot of drag and you will be slow. On all other courses, cL is relevant and you can forget about L/D.
Going upwind, leech tell tales are very useful. Going downwind without spi, tell tales at 60% at the leeward side of the sail are useful. With spi, the main is tight anyway and you control the heel with the traveller (or main).
Tell tales at 30% on the windward side are useful to find the right angle of attack.
The numbers above are from a aircraft airfoil, they will be different to a sail, mainly in terms of drag, and they include only what we call profile drag. But overall effect will be the same.

Cheers,

Klaus

"Achterlieksverklicker".. So what do you call a telltale on the jib? "Vorsegelverklicker"?

Thanks for the numbers. Stalling the flow over any foil (like a sail) is pretty bad! So there you have the numbers Pete. Undersheet and you loose a bit, oversheet (stall the flow over the sail, or or not helming straight) and you are parked compared to those trimming it right.

Of course, undersheeting a lot is not fast either..

Danke.

Gina, I've seen your hands after you've crewed for Matt. I can assure everyone here she is NOT just "along for the ride"!

And she's got guns guys, and not afraid to use them!

Okay, what do you did you do when the big headers or lifts came along?

I think I just yelled stuff like: "Go faster!" and "Do you see the mark?" Just stuff like that. I'll find out what Matt does and get to you. And Tim, you are so very good for my ego. Thank you.

"Go faster!"- universal crewspeak!

I spend a lot of time yelling at my skipper too; "Go Faster" or "What the Fark are you DOING??!!" or "Tack NOW you Jackass!"

But that's only when I'm sailing Uni. ;^) Seriously, I talk to my self alot out there. Keeps me focused.

Pete, according to Matt, sometimes you can see the lifts and headers and anticipate. But usually, there is a trend. Gusts in one part of the course and almost always a lift or almost always a header. You just have to get into it and see what happens. Ex. There is a big lift just before "A" mark or whatever the case might be. Oh, and he always asks his crew what is going on on the course and she always replies with "everyone else is going faster and higher". It always seems to work for me. He plays great "catch up".

Klaus,

Thanks for the information. On the downwind I also trim the main that the leech tell tales are flowing and I have the feeling it adds a lot of lift (so alows me to sail deeper).
To adjust the main I always leave the traveller centered and ease the main. Anyone that does the opposite and is this better/worse for drag?

I dont know what it is worth on the F16, but in the T we always eased the traveller about 10-15 cm before trimming the main. Then, if you were daring with the alu mast, you trimmed the mainsheet in gusts/lulls. Playing the traveller was the safe method if you could not afford a new mast.


Digression: When the carbon masts was put on the T, the traveller was eased less but the mainsheet could safely be eased more than with the alu stick.

Not quite I believe Drag is still relevant in my experience, which is purely practical and founded on just my own meager experience. The classic example was back in 2002 when the Gran Segel spis from Sweden made their debut at the worlds in Marthas vineyard. These sails had less draft than the current best spis from other sailmakers (but were also made with more panels=more stable design). The boats with the spis from Sweden were much faster than the others. I think this was becouse of less drag (and the ability to build more virtual wind). Together with Marcus we did a Tornado spi with more draft back in 2006 (right Marcus?). The spi had loads of power, but it was very slow. Drag was too high I think as it did not accelerate much in gusts (but was outright scary to sail with).
I also read about the L/D dont matter theory in one of Marchajs books, and now you say the same. Klaus, could you explain just why this is for us simpletons?

Hi Gilo,

I does the opposite, because I don't know, what my mast can support. But for boats where the gennaker doesn't end at the top of the mast, I guess that trimming the main with a fixed traveller is more logic (the top of the sail needs to be more open, hence more twist, because it isn't in the 'downwash' of the gennaker. 'Downwash' is the flow affected and behind of the lifting surface. On aircraft it goes down, hence the origin of the name. On a sail it is a sidewash).
It is certainly better for drag, but we want maximise speed and not minimise drag... and keep my boat alive.

Cheers,

Klaus

Hi Rolf,

I am not less 'simpleton' than others. I sail since 2000 on week-ends in the summertime.

On my Dart 18, the gennaker was too full as well. To understand we have to have a consebe clear about lift, cL and draft (or camber) of the sail. More draft doesn't automatically means more cL or lift.
Lift is a force, cL is a lift coefficient and the relation between the two is lift ~ cL x v², where v is the apparent wind speed.
The luff of a sail without battens collapses in case of small entry angles. A full sail needs greater entry angle than a flat sail. Hence the smallest possible angle of apparent wind depends on the draft of the gennaker. It is rather geometry than L/D.
You can create lift with cL or speed, and due the power of the square, speed has the longer lever.
Take a flat sail and point higher so the apparent wind is stronger, and you have more lift with less cL than the full sail.
Now the lift accelerates the boat. The lift increases even more, but instaed of sheeting out, you bear away and keep the apparent wind speed more or less constant, but sail more deep and fast than the full sail.
So it is the entry angle and not drag, which requires to have a flat gennaker.

Cheers,

Klaus

and right now 1:0 for Germany:England (U21 championship, soccer)

Ok, you are saying that the advantage of the spis with less draft was the sharper entry angle and thus being able to build more virtual wind. Not much to do with L/D at all but "simple" geometry. Mmmm, I'll have to re-evaluate Marchaj Thanks!

Enjoy the game!

On the H16 we used only the leach tells to set the sheet tension. With the big head sails, we started putting on tells there as well, but quickly found they told us nothing. The top ones especially as the way the sail is designed to operate, they always flowed due to the natural twist induced with downhaul and rotation. It may be different without the wing mast, but I just use the stock tells now just off the luff as that shows me in much better detail if I have the correct rotation and sheet tension.

Off the wind with crew I can fine tune the sheet tension with changing wind pressure, but to keep heeling in control or to reach we play the traveler.

We have found we like the flatter spins as the point of sail is safer to keep the apparent wind forward. I do not think (wthin a reasonable range of cuts) they are faster around the course though. You have to sail to the cut you have. What is slow is they guy trying to run deep (or high) to match his buddy who has a differnt cut or different condition sail.

Sorry for resurrecting an oldish thread.



So are you qualified for the next worlds yet? Norway is probably not going to qualify for this event


To the point, here is a nice article about Ben Halls wing: http://www.sailingscuttlebutt.com/news/09/0810/

What I found relevant to our discussion was this:


So flatter was faster even with a solid wing. I am so fond of the "less drag" thinking based on my experiences that I want to say that this is why they increased the speed with the new setting, but of course geometry is still geometry.