Been watching a few vids lately and it's easy to see the top of sails twisting off appropriately. But it also seems like the entire leech is doing absolutely nothing most of the time, except perhaps causing a bit of drag and adding weight.

Is there any logic to removing floppy leeches for certain types of sails?

Surf sails for example still find the extra grunt up high useful, but perhaps a speed sail does not need a floppy leech at all.

Just wonderin'.

Some of the newer race sails actually run a tighter leech in the mid 1/3 of the sail

On the wave sailing front - loose leech / flat sail does you no favours when its light and onshore - tight leech is good for getting first gear / extra low range planing power out of your sail. Bit of a Scarbs thing because you are up against so many elements trying to sail there

downwind current
3 directions of well / chop /back wash
Kites coming in from all directions
Kite lines in the break
flash in the pan ramps that require instant planing to take advantage of

Philip Koster runs some tight leech full sail set ups - he is the onshore master and i think it really gets him into a position to do all the stunts he does

Go somewhere like margs and the loose leech is your friend again because its windy and the wave pushes into the wind a lot of the time

To what degree do you suppose flex top masts contribute to the top of the leech twisting off perhaps too much?
Would a stiffer mast as in a taller same brand mast that would fit the same sail decrease the twisting off and help power up a sail in lower winds?

I hope your brain can take it Azymuth! Great comments following the article too!
http://www.windsomnia.com/2010/03/10/how-to-calculate-sail-twist/



"The answer is you design the sail with a lot of twist. The reason is because apparent wind decreases when you head off the wind and increases when you head up wind. So, by designing the sail with sail twist to match apparent wind twist at deep tacking angles, this ensures that when sailing at deep tacking angles, when apparent wind is low, and you need the most power from the sail, the entire the sail is optimally sheeted to produce the most power. Headed upwind, there is less wind twist, therefore the sail will be under sheeted at the top of the leach. This essentially depowers the top of the sail when headed up wind. Depowering the sail upwind is ideal because the apparent wind is so much stronger and you actually want less power from the sail. So therefore designing the sail with twist helps to provide more constant sail drive over a large tacking range."

I had assumed that the Naish chopper was an effort to design a wavesail that minimized the need for twist, though it seems to me that it would work best in quite even wind conditions when you can rig a bit smaller.

I thought this stuff on birds wings was pretty good too.

I am sure the change in wind speed relative to height above water is a contributing factor, but I don't think its the whole story.

I noticed the author of the http://www.windsomnia.com/2010/03/10/how-to-calculate-sail-twist/ has extrapolated data for a racing yacht, not saying he is wrong, just that it is an extrapolation.

There is quite a bit more to it, this is a good discussion if your interested:

en.wikipedia.org/wiki/Forces_on_sails#Influence_of_Aspect_ratio_and_Sail_Planform_on_Induced_Drag

The following paragraph from that link is a pretty good summary:

Influence on coefficients

Lift-induced drag is directly related to the narrowness of the extremities due to premature stall over the heavily loaded short chord profile. ... The more elongated the sail, the less lift-induced drag alters value of the lift coefficient.

The plan form, camber and twist will have an influence on the amount of drag created at the tip of the sail. With an optimum plan form, camber and twist giving the minimum drag for a given lift coefficient.

Interesting if you follow the theory on induced drag and span wise distribution of lift, It follows that foils with a very high lift coefficients (i.e. very cambered sails) will have a higher induced drag than a foil with a lower lift coefficient (i.e. flat sails). Which would suggest very cambered sails need more twist than flat sails. This might explain why your huge deep draft slalom sail, might need a bit more twist than your 4.0M...

Also a windsurfing has less camber at the tip, than at the boom, camber has the effect of increasing the lift coefficient but also increases the angle of attack at which the foil stalls, this would suggest you need to further decrease the angle of attack at the tip...

Of course you could just accept that for any given shape and size sail there is an optimum twist, and camber that gives minimum drag and just go sailing.

It would be interesting to hear from the sail designers on sea breeze if they use any calculations or follow any of this aerodynamic theory or simply use the tried and true of approach of real world testing?

LaoShi isn't that what I said but with no need for colour coding lol

Yep, yep. Got all the pretty pictures. Apparent wind is different at the top and bottom of the sail, and at different tacking angles (and speeds).

___________________________________________________________________
For anyone that doesn't get it:

A) Wind travels at different speeds the higher off the ground you go. This is the important bit. It goes faster as you go up. Lie on the ground one time; no wind. Fly a kite; heaps of wind up there.

B) If the wind is coming from the North at 15 knots, and I am traveling East at 15 knots, the apparent wind, where the wind is apparently coming from on my boat, is 15 knots from the North-East.

C) Because of both A) and B) the apparent wind will be at different angles at the top and bottom of the sail. Oh no! What is a sail designer to do?

Therefore: Sails have "twist" to provide the best trim angle for that part of the sail.

___________________________________________________________________

I am also led to believe the twist lowers the power in the sail moves most of the power to the lower half or so of the sail, making it easier to sail. Basic leverage.

I'm not denying this works, It does. Definitely!

But it appears that, for the top panel at least, there is often so much twist that the angle is matching the apparent wind; that is not producing any power at all. I certainly can't feel anything from the top panel, perhaps even the next down too, and looking up at it it just flops around there. Are the very top panels actually doing anything?

174 said...
The loose leech is *reducing* drag. Basically avoiding tip vortices you would get from a zero-twist sail. If you look at modern jumbo wings or F1 cars they have end plates to do the same thing.

Is it this oh great sail designers? Seems legit. Looking up...

Just like the winglets that are now common place on jet wing tips, as well as the theory outlined in Lao Shi's post I thought that the twist off of a sail releases the air cleaner creating much less drag. Less drag = faster (and also means less large womens shoe sizes being sold)

don't know... got rid of my floppy drive around 1999. Replaced them with CDrs and external hard drives.