Making the leech stand up

[tonyquoll]

When sailing to windwards, some guys seem to be able to point higher and go faster than everyone else.

I first learnt that if I was pointing low and going slow, it was because the sail was over-full and stalling. I needed to pull the outhaul and front lowers more to flatten the sail.

I heard the latest cut of the Redhead radial sail has a tighter leech, to help it stand up, which helps pointing. This possibly means the upper trailing edge of the sail is pulled in tighter, so an average the sail is closer hauled and operating more efficiently.

In this photo, Alex (middle) and Bryan (blue sail 2nd from right) have the leech and mast almost parallel, and are pointing higher than everyone else, who have the leech twisting out. Photo taken during Lauderdale Nationals by Rochelle (from shore 24x optical zoom).

Alex seems to have the traveller a long way out; between the footstaps and leeward hull, with the mainsheet cranked in very tight. Meanwhile mine (blue sail 2nd from left) is near the centre. It seems incredible the Alex is pointing higher. The trick seems to be holding the mainsheet tighter, and luffing; pointing up to when overpowered instead of easing the main.

At the recent NSW State Titles I tried this in the stronger winds, and managed to point higher and faster than some bigger guys in strong winds, although I was still a long way off the pace of the "A-Grade" guys.

Any comments?

[tonyquoll]

Related to "making the leech stand up", is what we call "the loose rig". By having the rear lowers very loose, it allows the mast to bend to leewards. This seems to help get the leech and mast parallel.

At Napier NZ, Ian at left has the loose rig, with mast and leech parallel. Scott and Hayden have flatter sails, traveler closer in and point higher, but with poorer velocity made good to windward.

One measure of the loose rig to compare the difference in rear upper and rear lower stay lengths, which pulled straight down the mast. A loose rig has rear lowers 165 - 180mm longer. The figures are not directly comparable between boats, as mast rake and height the hounds are installed at varies the figure.

An alternative measure is to pinch rear lowers and rear uppers together, when the boat is rigged up and the lower forestay is loose. If these can be pinched together about 800mm above the deck, then you have a loose rig.

The rig is too loose if the mast starts flipping backwards and forwards on windy broad reach; a very worrying effect. It can be controlled by grabbing the windward rear lower and pulling it aft, but I've found shortening the rear lowers 5 -10 mm (depending how loose they were), eliminated the problem.

[tom]

Great discussion Tony. I think for me the key here is getting the sail to work efficiently at a large angle of attack (traveller out). When the traveller is eased, the boat tends not to get blown sideways, and rather than point higher, i think the likes of Bryan just get less leeway. To get the sail to work at a large angle of attack you need to have the leech tight, the easiest way i've found to assist this is to have the mast raked back so that you are just clear of being block-bound on the mainsheet with max. downhaul and mainsheet tension. Also the uppers need to be tighter than you would have them for light conditions.

[tonyquoll]

Thanks Tom. The idea that it provides greater forwards component of drive seems true. But in the photo at top, we are looking almost directly along Alex and Bryan's hulls, while all other boats are angled to port. They really are pointing higher!

A mast rake measurement is done by hanging the halyard down from the top of mast, and measuring distance to front beam and rear beam. The extra distance in rear beam measurement is a guide to rake. Bryan informed me that his is 70mm, while Bruce says 90mm is optimum. Others have agreed 100mm is maximum.

I was running 140mm, and experimented at 100mm, then 60mm while at the Nationals. I found more rake did help with speed and pointing upwind in strong conditions, but at the expense of speed on reaches and runs. At the NSW States I set it at 90mm, plus had looser uppers to allow the rig to slop forwards for offwind speed.

There may be two aspects to mast rake that help upwind;

- moving the centre of drive aft increases effort needed by rudders to keep the boat going straight, therefore generating lift to windwards as they correct weather helm. The hulls may also help with this.

- a raked mast generates upwinds lift (like a sailboard holds the sailor's weight), which would help reduce drag and improve speed.

[tom]

It's hard to argue with a picture! I always thought that increasing upper rig tension and mast rake helped in windy conditions as it meant the forces from the mainsheet were put to greater effect in bending the mast as it pulled on the leech. This helps to flatten the sail while still maintaining good leech control. When the mast is more upright you can get the same leech tension, but the mast doesn't bend as much and the sail is fuller (good for reaches).

I wasn't at Tassie unfortunately, but in the many times i've sailed against Bryan and watched him dissappear into the distance i've not really noticed that he points any higher than the rest of the fleet as shown in that picture. Scary thought if he has worked out a better upwind technique!

[tonyquoll]

Comment received by email from Ian Marcovitch (Redhead Sails):

Hi Tony

Note in the photo that 2 redheads are pointing higher and 2 are pointing lower . Alex's sail is new and Brians is two years old both the same cut. Yours is 5 years old and the same cut. I used a sail of Steve Halliday’s exactly the same age as yours that had done more work yet after winning two regattas with it could not notice any performance difference between it and any of the newer sails i have been using lately. Upper leech tension aids offwind performance and in conditions shown in the photo the last thing needed is upper leech tension. Lower leech tension is what is required for pointing ability and the ability to maintain that tension by technique--steering and sheet tension- or sheer weight on the gunwale- (which allows more sheet tension). Brian is heavier than Alex , but Alex has grown up sailing lasers and dinghys and so has really good steering skills. What you are looking at are the two sailors who are really good sailors, and if they want to good sailors can sail high or sail low.

Now yes there is a subtle increase in upper leech tightness between your sail and Alex's sail. This has been done to address the need for the upper leech to be a little more curved in section so that when the sail is twisted off the leech is not so straight in section and perhaps a little extra power can be gleaned from it in certain moderate upwind and down wind conditions. The same can be achieved with gentle use of the leech line . It is not an absolute difference in the sails but a matter of altering the point at which the downhaul is used . The newer sails require more down haul to flatten the section shape of the upper leech.

I hope this explains a few things

Regards, Ian.

[tonyquoll]

Thanks Ian; excellent and honest advice which demonstrates why I advocate Redhead Sails.

I keep trying to strategise my way around the lack of "sheer weight on the gunwale", and from this discussion I can see some improvements can come from better mainsheet tension and steering.

[knobblyoldjimbo]

here's a tuppence worth from a Mari sailer, if you look at other cats, eg the A their square head mains fall off upwind, I've always assumed that this is because of engineering but Frank Bethwaites books show quite a big difference in wind direction between the top and bottom.

I've set up my new main (Eastwind) so the downhaul (the only real shape control) is independant of the gooseneck, this way I can heave it tight and the upper leech falls off and the mast bends which, in high wind is exactly what is needed. What I don't have at the moment is the knack of bearing away to point higher, something I have trouble understanding and also implementing but its what the hotshots do.

look also at the Moths, uphill it looks like they're on a reach (which I guess they are) trading speed for pointing and getting the pointing as a bonus.

another thing I read (on Sailing Anarchy so must be right!) but apparently most of the top A sailers don't use their main cleat.

[tonyquoll]

Mate, you've really got me thinking now. I was going to argue that Bethwaite only show's wind shear in very light breezes, and above 6 knots he says no change in wind speed with height. After checking the book (Bethwaite F. (1996) High Performance Sailing, Adland Coles Nautical), I've seen this whole chapter on sails that I haven't read yet... that book blows my mind and I have to recover before going to the next bit; taken only a year to get through the section on wind!

My experience is cross-cut sails are flattened with downhaul, while the radial Redheads shown above on PTs are shaped with outhaul and lower forestays, plus downhaul to remove the creases, and the right amount of vang for smooth flow off the lee of the mast onto the sail. Yep PTs are complex.

The foiling Moths seem to keep the leech pretty tight too. They would have to get up enough speed to lift onto the foils, then with reduced drag & heeled to windwards, they could point up a bit. Maybe analogous to cats pinching up on a reach to get a hull flying, then able to bear down with the apparent wind. However, as noted above, the guys with the tight leech both go faster AND point higher!

The PT is very responsive, so no probs steering over waves or luffing in gusts. Haven't sailed an A, but they seem to go in fairly straight lines. Wouldn't you'd have to work the main in and out to stop them tipping over?

Maybe I'll figure out the dynamics of how it all works after reading more Bethwaite! Cheers

[tonyquoll]

Looking at the A-class; square-head main not twisting off that much I'd reckon. If they ease the main to de-power it would though.

It seems a bit confusing to talk of leech tension, as separate from sail twist. I'm tending to conclude that reducing sail twist helps keep power up; luffing the leading edge rather than twisting off the top. Cant explain why yet.

On the NS14, instead of a traveller they just crank the vang on (operates separate to mast rotation spanner), so when the main is eased the twist is reduced.

[tom]

I think this is why mast rake has such an importance upwind.. Not much to do with the drag of the overall rig, but more that the leech is more perpendicular to the forces from the end of the boom. (taken to the extreme if you can get the tip of the mast directly over the rear beam all the mainsheet forces will go through the leech.) I also get a headache when i try to seperate leech tension and twist. I know from working with a sailmaker on my current sail that there is twist built into the sail, and for a given leech tension, the top of the sail will twist as a gust hits. This can also be altered by upper rig tension, so it's a matter of trial and error to see what settings give the most drive. There is a parameter for twist in the sail design software, Maybe Ian could tell you how that works? I have to say i have a bit of an idea, but not enough to try to explain in writing. My sailmaker is very forthcoming with information, but i'm a bit too dumb to comprehend him most of the time unfortunately. All i can do is get out on the water (which is hardly ever), and try things and see what happens!

[tonyquoll]

In the book "High Performance Sailing", Bethwaite explains that allowing the sail to twist decreases efficiency. Drag is caused by turbulence from the top of the sail (or wing or foil); hence the most efficient shape is long, narrow with maximum effective shape and minimum tip turbulence.

This principle is demonstrated by the efficiency of glider winds, or the high-aspect ratio centreboards, or even the very tall rigs used on the America's Cup winning trimaran.

When the mainsheet is held in tight, and the leech held vertical, it maximises the usable sail height. If the leading edge of the sail is luffing while the leech is flowing, this effectively creates a tall, narrow, low-angle-of-attach sail; which is very efficient.

The opposite effect is generated by allowing the sail to twist: that effectively creates a short, wide sail with minimum height and a lot of tip turbulence; less power and more drag.

I'm hoping to present this information more concisely in article, for the next APT magazine. Latest issue now available at:

http://aptca.papertigercatamaran.org/index.php?option=com_content&view=article&id=60&Itemid=53

[tom]

And yet interestingly in ice and land yachts, where the speed is many times greater than a PT, they use extreme twist ?

Gliders aren't the best example, as they can cope with plenty of lift from the tips of the wing as there is a wing on the other side to balance it out.

I'd like to sail with a rig that generates as much as possible power as low down in the rig as possible with minimal drag.

If you look at the wing on Macquarie Innovations, it is quite stumpy relative to the beam of the craft.

Challenge with the PT is to retain fullness required to get desired power in the bottom 1/3 of the sail, then make the top 2/3 of the sail have minimum drag. It is pretty much universally accepted that downhaul tension induces twist, and i know that all the top skippers use PLENTY of downhaul in conditions around 20knots. (hence broken sail hook shackles). I believe getting the mast to bend the corresponding amount to leeward reduces drag associated with this twist.

I'm very much looking forward to seeing your article in the next issue of APT Tony.. Always interesting to discuss different aspects of the PT.

[tonyquoll]

Good argument Tom,

The points you raise are very helpful, as it helps me understand what range of information relates to this issue. You also help me confirm my beliefs.

Here's a simple approach: look at the leech ribbons; steer off the bottom, pull the mainsheet in until the top one flows. If this results in pointing too high and going slowly, drop the traveller.

Bethwaite's book also mentioned ice boats; which have such little drag their apparent wind ranges from 7 degrees upwind, to just 10 degrees on a beam reach! Why would they want heir sail to twist out further than those angles? A quick scan of some images shows sails twisted in extreme winds, to de-power the sail. As they are tightly sheeted & close hauled most of the time, they haven't installed travellers or vangs to cope with the extreme conditions.

Macquarie Innovations say "While there would be a loss in efficiency due to the greater "end loss" from the low aspect rig, this loss was easily compensated for by the reduction in overturning moment." from: http://www.macquarie.com.au/mgl/au/speedsailing/design

In the photos of PTs above, you can see the top sailors at this year's Nationals and last years Internationals; all with quite flat sails and little twist. The Kiwis have the outhaul cranked so hard, the bottom of the sail has no curve at all.

The power sail you describe is fine on reaches; not upwind. I've found a full sail will not point as high & be slower.

Downhaul tension induces rearwards mast bend. On cross-cut sails, the downhaul is most important for flattening the sail. On radial sails we use the lower forestays to control mast bend, rather than the downhaul. A few comment that "it doesn't do anything"; while I would say it still pulls the drive of the sail forwards and removes creases. In 20 knots, the outhaul, forestays and vang are all cranked on a fair bit, so corresponding downhaul tension keeps the sail in shape. Broken shackles happen when people dip them in water after welding, which hardens the metal and makes it brittle. Ideally they'd be slow cooled in a furnace, the anneal the metal and make it flexible and strong instead.

[tom]

I wasn't really disagreeing with what you are saying Tony, and i think the area you have identified which is being able to get an efficient rig in wind over 15knots upwind is the key to success in the PT.

As you will have noticed, under 15knots and the fleet is waaaaay more compressed as more people are able to find an efficient set-up for the lighter winds.

I still think a full sail will be quicker upwind if you had the bulk to keep the hulls flat!

Point taken about the shackles breaking due to poor bonding technique.. I've also seen mast head hooks ripped out of masts, etc... When i was able to sail near the front of the fleet i watched the likes of Ben Deed pull his downhaul on and it was VERY tight (and that was with a radial sail). Not sure about the Redhead's.