Large luff pocket sails with sharp leading edge

I twist my jib up to 45'. This reduces jib power but increases the man's power.

With very light and fluky wind I twist the main up to 45' so that it can get some power in very shifting winds.

When over powered, I twist off 70' of main and jib as a fisherman's reef.

I only twist a few degrees for wind sheer.


How much will mast bend affect your twist?

 

I was quite surprised how hard it is to find actual research/measurements on wind gradients. Even Wikipedia makes a few claims and just mentions "according to a source....." Does anyone have a link to some actual measured wind gradients? Preferably in different conditions and locations?

 

Wind gradient is inconsistent!

Hoist a few flags up the flagpole you selected as a mast. Watch them flap around in the breeze for an hour. You will have all the empirical data you need.

Very little Twist is ever sewn into a sail. I achieve 0' thru 70+' with spar manipulation.

I perceive that you are looking to achieve a 70% increase in performance with a double skin sail. With an additional point zero zero one percent increase with Twist.

However your selected flagpole mast will rob 60% or more.

Changing your mast selection will be a greater increase in performance than anything you do to the sail.

 

I asked about wind gradient not because of twist. I know it is inconsistent. I ask because I would like to know how much difference, on average, mast height makes. Especially in light winds gradient is significant. How much performance is lost by having two shorter masts instead of one mast. Everything is a tradeoff so I would like to know what I am sacrificing by having sails reach just that little less high. And how much extra sail area will make up for the loss in height? But I still haven't found much real measured gradient data.

 

You haven't found data because the gradient is too little to be important.
500 ft elevation difference absolutely.
Fifty not so much
Fractions of a percentage of wind speed

 

Thanks for taking the time to answer Blueknarr.

Now my whole understanding of sail twist is out of the porthole.... I almost asked another ignorant question but found some answers already. Mainly here Wind Shear - Fact or Fiction? - BLUR https://www.blur.se/2009/02/09/wind-sheer-fact-or-fiction/

So I hope I understand the issue now. Before, I thought wind shear was caused by the interplay of wind gradients and boat speed creating different apparent wind directions along the height of the mast. I have seen this explanation all over the place. But now it seems that plays only a small part in the apparent wind shear. Most of the apparent wind shear is actual wind shear that can be observed when measuring from a stationary point.

For my situation it means: My sails can be on two lower masts (biplane) instead of one larger one and a small increase in sail area will easily compensate for missing the slightly higher wind speed up higher. And if I go for soft wing sails (or any sails for that matter) I have to make sure I can control camber and twist.

 

Now you got it.
You don't need expensive computer systems to adjust your sails for shear.
Yarn tell-tales have been doing a great job of showing exactly how air is moving across the sail in real time for quite a number of decades.

 

If speed is not important, why bother with a large luff pocket sail anyway?

The first boat I ever owned was at a club that had been using big luff pockets in the 1960s, on International Moths. I got my own first big luff pocket sail in about 1986. They are marginally faster; enough to make a difference when racing but not enough to really notice unless you have a comparable craft well sailed and alongside you, in my experience.

Also I'm not an aerodynamics guru at all, some of the guys who are have mentioned that much of the time the most efficient way to make a rig is by reducing complexity and using the reduced weight to increase span. Increasing span dramatically reduces induced drag.

 

I've seen that chart before, and if I recall correctly it had zero evidence to back the claims it contains. It's also basically complete and utter BS, as practical experience shows. If you get a standard (Type A) rig and replace it with a rotating wingmast, the "max boat speed to wind speed" does NOT rise by 15% as claimed. For example, in Australia there's a popular dinghy that's similar in length to a Laser, much lighter, and carrying a larger sail on a wingmast. It's actually rated slower than a Laser, which is arguable. The point is that even when sailed by a sailmaker who is a world champ and Olympic medallist in wingmasted cats, the boat is only faster in a breeze and actually slower in light stuff.

Similarly, the Finn dinghy went from a type A-style rig to a type B one, and it certainly did not suddenly increase its boatspeed/windspeed ratio by 15%. If it had, the Finn would now be the world's fastest hiking dinghy, almost as fast as an International Canoe. I think it may have increased the speed by a fraction under 1%, as far as I can see.

The claim that a rotating luff pocket is faster per se than a wingmast is laughable. There's a reason A Class cats, F18 cats, F16s and other classes that could use pocket luffs use wings instead. According to the figure, if I fitted a wider luff pocket on my Laser, with windsurfer style camber inducers to lock the shape in, my Laser would then beat the 49ers and International 14s around the course. It would also beat our old Formula 16 style cat in singlehanded form, complete with its 32' tall wingmast. It is utterly ludicrous that anyone could concieve coming up with an comparison that indicates that a modified Laser could beat a 49er around the track.

Sure, there's a note that the chart is "rough", but in fact it's not rough - it's completely off into pixieland. The people who make that sort of claims are just utterly ignoring what actually happens on the water, and cannot be believed in any way.

 

The interesting thing about the Omer wingsail is that they have, like so many involved with similar sails, utterly failed to provide any evidence to back up their performance claims. Omer came here himself years ago and promised to provide some comparative data. I think that was 11 years ago, and we're still waiting.

If the Omer rig was as fast as Omer claimed, then the little Seascape 18 - a ballasted cabin mono - would have been faster than a Tornado or F18. It was a laughable claim. The Seascape guys went as far as buying an Omer rig and testing it. They dumped the rig. They said publicly here or on SA that it did not perform as claimed.

 

For reasons of budget, availability and simplicity I am going for unstayed free standing masts. Also I like the fact there are no shrouds. Less parts, less maintenance less windage. I am aware of the drawbacks. Slightly more weight, not the most suitable for a multihull because no automatic depowering through heeling and maybe less sail shape control.

One of the biggest drawbacks though is the large diameter tapered round mast. The smaller oval section of a stayed mast has a lot less windage and isn't blocking the sail so much. So in this case I think a luff pocket or double sail wins a lot of performance and is almost essential. I realize this is a challenge to implement correctly but this forum is a great help. Some myths I had in my mind are cleared up so I have a better chance of succeeding. I will test my designs with scale models to see how they behave so if I don't find a solution I feel confident in I will definitely reconsider my options.

 

I noticed that the Omer sail seems to have a rather big gap where the double sail meets at the trailing edge. That may create a significant amount of drag.

 

Looking at the chart.

How much of of the AC's speed is due to hard wings vs foils vs professional crew?

Would they still use 70 foot hard winged filers if the regattas were held in a full wind spectrum of 1-35 knot winds?

Qmaran

Rig style one of a fixed mast and single skinned sail is by far the most common.

It is the cheapest and most versatile. Maximum bang for the buck.
Reality is that the other rigs rarely produce any extra drive. The often add complexity and complications.
The exception is sleeve luff on dinghies and windsurfers. Simply slide the mast into the sail. No halyard or tracks or standing rigging.