Why Sails Twist

This article originally appeared on windwing.com and is now available via the Internet Archive

Since much material is often subsequently lost to public view, we are placing a copy here for future use.

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TECH TALK

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D2T (Decoupled Truncated Tip)

Windwing's exclusive D2T technology
is a significant development in sail design which maximizes aerodynamic
efficiency. D2T provides better performance, improved range and easier handling.

Truncated Tip
A 'truncated Tip' is 'squared-off' such that the curved apex of the tip is replaced by a straight line or 'flattened' curve. This shortens the luff considerably without a loss of area and allows the use of shorter masts in larger sails for improved mast compatibility. Added benefits are a lower center of effort and lower center of gravity for significantly improved handling.

De-Coupled Head
An internal headcap suspension system 'de-couples' (separates) the downhaul tension from the sail's head, upper luff and mast sleeve. This allows unencumbered aeroelastic 'twist' in the upper sections of the sail for improve aerodynamic performance and range.

<A NAME="anchorsailtwist"></A>

Why Sails Need Twist

There are three reasons for twist in a sail. In descending order of magnitude and importance they are:

I - Increase the wind range
II - Compensate for the wind 'gradient'
III - Delay circulation-induced tip stall.

Following are brief technical discussions of these items relative to sail twist.

Reason I - Increased Wind Range

The aerodynamic force generated by a sail is proportional to the square of the wind velocity. The force is also directly proportional to the force coefficient (determined by shape and sheeting angle), the air's density and the sail's square area. The following formula defines these relationships:

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Allowing the head of the sail to 'twist off' in response to increased aerodynamic loading at higher wind velocities 'smooths' this non-linear response. The 'sheeting angle' and resulting force in the upper sections of the sail are thereby reduced allowing sailors to maintain control when sailing 'overpowered' or
in gusty conditions. The following plot demonstrates this effect.

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The total aerodynamic force
may be split into two components - a lift component which is perpendicular to the flow and a drag component which is in the same direction
of the flow. These components are both proportional to the 'sheeting' angle. As the sheeting angle increases, lift increases to a maximum which is reached at the critical 'stall' angle. Above the stall angle, a rapid and significant loss of lift results. The following plot illustrates the effect of 'sheeting' angle on the lift coefficient for a camberless sail. (A 'cambered' sail will have a non-zero positive coefficient of lift at zero sheeting angle due to its 'pre-inflated' mechanically induced
shape.)

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The relationship between the lift and drag components is
a measure of sail efficiency. At zero sheeting angle, the sail still has a drag component due to the frontal area (shape) and 'wetted' surface (overall area.) As the sheeting angle and lift increase, the drag also increases. This additional drag is known as 'induced' drag. The following plot
illustrates a typical L / D relationship:

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Reason II - Wind Gradient
or 'Shear'

The wind increases logarithmically with altitude above the water. A compensatory 'twist' in the sail maintains an optimum spanwise angle of attack which improves efficiency and performance. Following are plots of the wind as a function
of height above the water.

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Windspeed vs. Height plots compliments of W. L. Kleb

Reason III - Circulation-Induced Tip Stall

The discontinuity at the head (sail and then nothing) causes 3-dimensional circulation flow at the tip. A component of this flow is in a direction which serves to increase the angle of attack. At high (near critical) angles of attack, the circulation flow is increased resulting in tip stall. In aircraft, this phenomena is reduced by 'washout' (twist), a more stall resistant foil section, a winglet, or a combination of all three. In a sail, twist serves to decrease the angle of attack at
the head thereby reducing tip stall.and improving efficiency and performance.

D2T Provides

Better Performance

Properly aligned flow and reduced turbulence eliminate tip stall and lower drag for increased power, higher speed and
improved pointing.

Improved Range

The mast sleeve, head and leech are de-coupled from the downhaul tension allowing enhanced twist for a lower center of
effort and improved wind range.

Easier Handling

Luff lengths are reduced allowing a shorter, more resilient mast. The sail and mast have a lower center of gravity, lower
center of effort and less weight for improved handling.

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C520 Triaxial Aramid X-Ply

Windwing's tear and UV resistant C520 X-Ply is the

most technically sophisticated sailcloth available. Warp-oriented Technora aramid fibers are superimposed on a bi-directional polyester grid and laminated within a low stretch monofilm sandwich for ultimate strength, low weight

and superior performance.

Dual Grommet Clew

Standard on all sails, dual grommets allows high or

low rigging positions for optimum boom geometry. Utilizing both grommets also provides a highly stable intermediate position for fixed or variable

outhaul systems.

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Leech Stabilizers

Reduce turbulence and drag at the leech for improved power and speed.

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'T' Cam Batten Stabilizer

Windwing's exclusive 'T' Cam batten Stabilizer provides a secure direct

compression link between the mast and batten for ultimate leading-edge stability. Due to its relatively small size, the 'T' cam also fits inside a tight fitting RAF sleeve providing a 'Cam Option' for user-friendly freeride sails.

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CS</A>

ITB

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Low drag, screwdriver adjustable Internal Trim

Batten fittings provide convenient and precise batten tuning for

ultimate performance.

6:1 Stainless Steel Downhaul Tack Fitting

The 6:1 downhaul tack fitting provides a secure low-friction connection for proper rigging and ultimate strength.

<A HREF="http://web.archive.org/web/20000312154125/http://www.windwing.com/downhaulfitting.html" TARGET="View Frame">Rigging the 6:1 Fitting

</A><A HREF="http://web.archive.org/web/20000312154125/http://www.windwing.com/cat.html" TARGET="View Frame">CATALYST</A> / <A HREF="http://web.archive.org/web/20000312154125/http://www.windwing.com/wave.html" TARGET="View Frame">WAVE</A> / <A HREF="http://web.archive.org/web/20000312154125/http://www.windwing.com/air.html" TARGET= "View Frame">AIR</A> / <A HREF="http://web.archive.org/web/20000312154125/http://www.windwing.com/syn.html" TARGET="View Frame">SYNTHESIS</A> / <A HREF="http://web.archive.org/web/20000312154125/http://www.windwing.com/race.html" TARGET="View Frame">RACE CS</A>

<A HREF="http://web.archive.org/web/20000312154125/http://www.windwing.com/masts.html" TARGET="_self">WINDWING MASTS</A>

&

CONSTRUCTION DETAILS

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	1) Heavy duty tip plug system with stainless steel buckle.
	2) Head, tack and clew patches on both sides.
	3) Leech stabilizers reduce turbulence and drag for improved power and speed.
	4) High quality custom tapered RBS epoxy carbon battens.
	5) Low stretch laminated leech ply.
	6) Glued and laminated batten pockets on both sides.
	7) Superior grade optical quality monofilm.
	8) Extended clew patch protects against wear and abrasion from the boom.
	9) Extra wide edge webbing from tack to clew.
	<A NAME="anchorssdwnhlfttng"></A>10) <A HREF="http://web.archive.org/web/20000312154125/http://www.windwing.com/downhaulfitting.html" TARGET="View Frame">6:1 stainless steel tack fitting.</A>
	11) Exclusive 'T' cam batten stabilizers.
	12) Glued and laminated seven-layer seams.
	13) Abrasion resistant full length tapered mast sleeve.
	14) Luff plys on both sides.

Construction Quality vs. Weight

The subject of weight regarding Windwing sails is common because they are

traditionally among the strongest, most durable sails made and this durability can only be gained by using extra reinforcements, thicker materials and higher quality battens and fittings. Windwing continues this tradition because we firmly believe a quality sail should last through many seasons of repeated

hard use without a reduction in performance.

Sail Weight vs. Rig Weight

At Windwing, we feel a slightly heavier sail is a small and reasonable sacrifice

if the additional weight provides enhanced performance, durability and value. This philosophy is substantiated by years of positive performance reviews and the fact that the sail is only a portion of the total rig weight. A difference of a pound in the sail's weight can mean many extra months of

use but only results in about a 5% change in the overall rig as follows:

Mast	=	5 pounds (430 30% carbon mast)
Boom	=	6 pounds (Chinook 4'6" Body)
Base	=	1 pound (Windsurfing Hawaii 12")
Sail	=	7 pounds
Rig	=	19 Pounds
1 Pound	=	5.3% of the total rig weight.

Weight of '99 Windwing Sails

Of particular interest regarding the weight of '99 Windwings is the institution

of truncated tips which significantly lowers the center of effort and the center of gravity providing a surprisingly noticable feeling of lightness and easy handling. Following are recent sail tests by Ken Winner, technical

editor for American Windsurfer magazine which point this effect out.

'99 Windwing Sail Reviews

Windwing Designs '99 Synthesis 6.7 (Windsport Magazine) Ranked 1st in Power, 1st in Stability.
The new '99 Windwing Synthesis is designed to combine the race winning performance of the Windwing Race CS with the easy handling of a narrower mast sleeve no-cam sail. The sail features a decoupled Truncated Tip (D2T) and multiple removable camber inducers so you can rig the sail as a 3, 2, 1, or no-cam sail. The custom tapered RBS epoxy/carbon battens are among the highest quality in the business.

We spent most of our time sailing this sail in the no-cam mode and this sail is the most slalom-oriented of the no-cams in the test. The extra batten results in outstanding stability and overall wind range. The sail features a fairly full profile combined with a fairly long boom length for good acceleration when coming out of the gybes. In the no-cam mode the sail's rotation is smooth and tacking is easy, even in overpowered conditions when cambered sails tend to knife back and forth a little. In its cambered mode we found the rotation to be easy and effortless, with instant acceleration once the sail rotates. The overall handling is more like a powerful compact race sail than a maneuver oriented no-cam sail, making it ideally suited for flat water race and chase action with your friends and local club racing. The narrow mast sleeve makes uphauling and waterstarting considerably easier than most compact race sails.

Summary: If you're not sure how many cams you want, look no further than this sail. Sailors looking for a powerful, fast, flat water blasting and bump and jump sail with smooth rotation in the turns will appreciate this sail's overall performance. The extra batten makes the sail feel a little heavy for first time shortboarders but intermediates and advanced sailors will enjoy this sail's combination of speed, power and stability.

Windwing Designs Catalyst (American Windsurfer)
The Catalyst is an excellent sail for serious, radical wave sailors. Its high-cut foot enables riders to perform radical aerials and on-the-wave transitions without getting in the way of the sailor's legs or the board, and without dragging in the water. It also has Windwing's striking new head design, a feature that we feel works quite well to extend the sail's range. You have to use plenty of downhaul, but the reward definitely follows. As is usual for Windwing sails, the Catalyst is built to last, which is to say it's best for medium- to heavy-weight riders who need the durability and can deal with the extra weight.

Windwing Designs '99 Interface Wave (American Windsurfer)
"The Interface Wave is a conundrum. It's arguably the most durable sail on the market, and it's clearly one of the heaviest for its size, yet one of our lightest testers, Renata Fuzetti, commented that the Interface Wave felt light. In fact, no one seemed bothered by this sail's weight, which we think proves that the feeling of weight in a sail comes as much from the way a sail is designed and tuned as from it's actual physical weight. In the case of this sail, the design must dominate perceptions, because everyone who tried it, regardless of weight and skill level, enjoyed it."

Windwing Designs '99 Interface Air (American Windsurfer)
"Billed as a light-weight, highly versatile sail, the Interface Air is much like the Windwing Interface Wave except that it has a stepped monofilm body and a cam option. This is one of the keys to its versatility, as the freeride sailor can decide to go with a little more speed and stability by installing the cam, while the wave sailor can go for better handling by leaving the cam aside. We agree that this is a versatile sail, but the scale doesn't agree that it's light. No matter, no one seemed to mind as the Air's excellent power, speed and range seemed to trump all other concerns."

Windwing Designs '99 Synthesis 6.0 & 8.3 (American Windsurfer)
195-pound tester Glenn Fuller loved both of the Sythesis models that we tested. He's the sort of windsurfer who likes a solidly built sail and doesn't mind if it's a bit on the heavy side, as long as it has durability and performance. Durability is a Windwing hallmark quality and the performance of this year's Sythesis is better than ever. We like the versatility conferred on the sail by the three removeable cams, a feature that's good for buyers who aren't sure whether they like camless sails and who need some time on the water to figure it out. We also like the decoupled truncated tip (D2T in Windwind parlance). Like the Neil Pryde Shear Tip, D2T seems to make the sail work well even when it's not rigged just right and gives it a light, responsive feel. In cambered mode we found the Sythesis to be stable and fast, whether equipped with cams or not. Without cams, however, it was a little easier to pump; with cams it was more stable and a tad rangier. One detail that strikes us as noteworthy on the Synthesis is the functionality of the screw-type batten tensioners (for non-cambered battens). They not only can be adjusted with a standard flat screwdriver but also follow the "righty-tighty convention we're all used to.

Windwing Catalyst 5.0 (Windsurfing Magazine)
Low-End Power-Oriented Sailor: The Catalyst is modern in design and capable in a wide range of conditions. With minimal outhaul, the sail keeps shape along the entire luff, from foot to tip. It has low-end punch but seems softer in feel than an all-monofilm sail. It's clearly much more durable, too.
High-End Control-Oriented Sailor: It's great on the high end as long as you use enough downhaul to take shape out of the head. It's stable, fast and maneuver-oriented, yet still has a forgiving and slightly softer feel. The weight alone will favor heavier riders. Lightweights and less powerful sailors won't be able to take full advantage of what this sail has to offer.
Onshore Sailor: Keep this sail in breeze and you'll have one heck of a fun time sailing in onshore conditions. It'll be easier making windward progress by staying on the wave face and riding backside. It's fast enough for lofting big airs when sailing out and super-maneuverable on the wave coming in.
Down-the-Line Sailor: Being a maneuverable sail, going down the line is effortless. In addition, the Catalyst is well built and ready for any task at hand including wipeouts!
Comments: The Windwing Catalyst is built as an aggressive sail for the aggressive sailor. It's high-end-oriented and built to last. Heavyweights and more powerful sailors will enjoy what this sail has to offer. The less experienced sailor may find this sail more difficult to rig initially because of the interior and lowered head cap system. After that, the battens rotate smoothly and the sail is bomb proof.

Windwing Interface Wave 4.9 (Windsurfing Magazine)
Low-End Power-Oriented Sailor: The Interface Wave is a high-wind and high-end-oriented wave sail built to withstand even the toughest of conditions. It's modern in shape with a proportionally shorter luff and larger squared-off head. At 9 pounds, it may seem heavy; however, when you're sailing, the sail is well balanced and well behaved. It's difficult to feel the weight once you're on the water.
High-End Control-Oriented Sailor: In a breeze, the Interface is truly a high-end-oriented sail built for performance and durability. It feels stable but not stiff and tight. It has a softer and more forgiving feel than an all-monofilm sail. It's somewhat tricky to rig it perfectly at first for particular conditions. It may take some added time on the beach to tune it properly for maximum performance.
Onshore Sailor: In a solid breeze, this sail has maneuverability and is excellent for slashy backside hits and on/off power sailing. The transition from loading and unloading power between maneuvers is easy, and the sail is quite responsive. In lighter winds and more puffy conditions, we felt a little more low end may be appropriate.
Down-the-Line Sailor: On a wave, it's stable and actually light in feel through bottom turns and transitions. The swing weight -- short luff and low center of effort -- makes this sail handle and maneuver surprisingly well. Lighter-weight riders will certainly have an advantage in a variety of conditions. You heavyweights will need more and consistent wind.
Comments: The Interface Wave is very modern in design. It has a short luff and a large square head. It will rig with or without shape along the entire luff (up high), depending on the amount of downhaul. It has fair range but seems to favor lighter riders. Heavyweights can enjoy all that this sail has to offer as long as it's windy.

Windwing Interface Air 5.5 (Windsurfing Magazine)
Low-End Power-Oriented Sailor: The Interface Air has the capability to be rigged in various ways. In lighter conditions, you can set it up with a tighter leech and fuller draft through the luff of the sail. It has the option of a camber to help hold its shape when coasting through the holes and light spots. At 9 pounds it feels heavy until you get it on the water. The design makes the sail feel light and lively in your grip.
High-End Control-Oriented Sailor: With plenty of downhaul and subtle outhaul, this sail feels stable and balanced. The Camber option can add stability both at high speeds or when you get attacked by a savage puff. The twist characteristics allow for smooth acceleration, so you won't get jerked over the handlebars. Although itís stable and rigid, the sail is still forgiving.
Speed-Oriented Sailor: It ís well balanced between fast and maneuverable. The outline favors bump and wave sailing, but it's far from slow. Like many of the other sails, it's fast on beam reaches and downwind but suffers slightly upwind compared to more slalom-oriented sails. Sailing with the camber option does increase the overall speed, but that's probably not the reason to buy this sail.
Comments: The Interface Air is probably the most versatile sail in Windwing's high-wind line. It can handle waves like a true wave sail and bump-and-jump conditions easily, and with the optional camber, it has excellent stability. It's very modern in design, with the larger head and short luff length. It has the capability to face even the toughest challenges by all levels of sailors.

Windwing Synthesis 6.7 (Windsurfing Magazine)
This sail can go either way -- rigged with cambers or without. We rigged with the cambers and needed outhaul tension to keep the cambers on the mast. We felt that a sail this size loses too much stability sailing without the cambers. Outhaul and downhaul are easy to adjust. The battens tension easily and are clean for minimal drag.
Early-Planing-Oriented Sailor: The Synthesis is a power sail and planes early, but it could have been slightly better if the sail didn't weigh 11 pounds. At this weight, it feels heavy on the beach yet surprisingly light on the water. The design features keep the swing weight to a minimum. While sailing, it feels somewhat light and lively. It can carry shape through the head of the sail.
Control-Oriented Sailor: When sailing with the cambers in place, the sail holds shape and has decent stability. It can be downhauled to flatten the head and hold the draft down low in the sail where itís more controllable. Sailing without the cambers, the sail loses some of it's stability. The construction is durable and makes the sail feel forgiving during unexpected gusts.
Speed-Oriented Sailor: The Synthesis is more user-friendly than it is fast. Heavier and more powerful sailors will push it to its limits, but lighter riders may get overpowered. At high-speeds and in strong winds, the draft feels back on the boom and favors big guys. In comfortable conditions, it's fast on a beam reach and off the breeze. If you slack the outhaul, it can go deep well.
Comments: The Windwing Synthesis is sail built for various conditions with great potential at any angle of attack. You can sail with or without the cambers in the sail. The cambers maintain stability and shape in the sail. We preferred sailing with the cambers. It's an extremely well-built sail and will last several seasons under normal conditions. It's modern in design having a large head and short luff length.

Windwing Race CS 8.3 (Windsurfing Magazine)
Early-Planing-Oriented Sailor: The 8.3 CS Race is capable of being rigged with shape along the entire luff through the head of the sail. The head is modern in design and can hold shape. The leech still has a tendency to twist and open in lighter wind ranges. It powers up early and feels slippery at speed. It doesn't take much to get going, but the overall weight did hurt slightly in the lighter conditions. We feel it could even plane sooner with less weight.
The Ranger: It works well in any conditions -- super-light winds to fully powered race conditions. On the extreme high side, our lighter sailors were too overpowered and thought it was backhand heavy. Our heavier testers enjoyed the power, however.
Speed-Oriented Sailor: It's a race sail that works in any conditions. It might be easier to downsize for the added control that you can translate into speed. On the extreme high end, the sail felt punchy and slightly backhand heavy. Bigger and more powerful riders can take advantage of this sail's potential, while smaller riders may want to downsize sails.
Comments: Windwing sails are built strong and the quality is excellent. It may be slightly heavy on the scale, but tester Grant Kowalchickís comment says it all:"It sails really light and is super-easy to waterstart." The cambers rotate very smoothly and the acceleration is mellow and forgiving, not jerky. This is a quality race sail that will last several seasons. You'll save money every time you ride it.

Board Speed vs. Wind Velocity

Contributed by Dr.Peter I Somlo
email: somlo@ieee.org

A theoretical solution of the numerous parametric equations which determine speed as a function of course and wind velocity is of great interest to the technical windsurfing enthusiast. Dr. Peter Somlo has developed such a solution utilizing the aeodynamic and fluid dynamic relatonships for lift and drag of the sail, fin and board over a range of courses to the true wind and at two wind velocities. Peter's solution is presented below for true winds of 12 and 15 knots and a sail size of 7.0 sqm.

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Method

The calculation was performed for the board-speed (vector) as follows:

1. Set up the equations for sail-lift, sail & board-drag as functions of air temperature, air pressure, sail size, wind-velocity and board-velocity. Here we must remember that sail-lift and sail-drag depend on the velocity of the apparent wind (which we don't know yet because it depends on the board-speed) and the board-drag depends on the board-speed (not known).

2. Set up an equation for the apparent wind, which is a vectorial subtraction of the wind vector minus the board-speed vector. (The faster the board, the more the apparent wind comes from the front.) To make the vectorial calculations simple, all vectors were represented as complex numbers (which have magnitudes and angles).

3. Assume a sheeting-in angle which will be used to modify the angle of the sail-lift.

4. Assume that the sail-lift is at right angles to the sail direction.

5. Break this sail-lift force into two components: in the direction of the board and right angles to it.

6. Chose a direction of board-movement (relative to the true wind direction).

7. Realising that when moving at a steady speed, the forward-force must equal the backward drag (otherwise the board would be speeding up or slowing down), solve the equation(s) for the board-velocity vector - for the condition of the net forward-force to be zero - yielding the board-velocity vector, and so all the forces will become known.

Notes:

a. Although the computation was carried out in SI units, speeds were converted to knots (from m/s) and forces were converted in results to kilogram-force (from N) - more familiar units.

b. An artificial function was devised to increase the hull-drag smoothly by a factor of 3 at the planing transition as the board-speed drops from 12 to 10 knots . (The choice of 11 knots was the result of a calculated hull-speed-limit of a commonly used board.) The factor of 3 came from estimating the reduction of the wetted surface when planing properly - or not.

c. The 'Forces' diagram shows the total sail-lift. The fin-lift is the component which is at right angles to the board, and the forward-force is the component in the forward direction. We see that most of the sail-lift is transferred to the fin (which is preventing the board going sideways), and the forward-force is a fraction of the sail lift only. For a 7m^2 sail, reaching in a 15 knot wind, the sail-lift is about 68 kgf (so you can hang most of your weight on the sail), but the force that makes you go forward is only about 11 kgf. The curves are not quite applicable for sailing downwind (square running at 180 degrees) because the transfer of lifting force to drag force only at square running is unknown, i.e. there is no sail-lift, only drag.

d. The Cartesian graph for board-speed shows two wind-speeds 12 and 15 knots, for a 7m^2 sail. Note the 'kinks' in the curves indicating the transition to planing above 12 knots.

e. The polar plot is the curve re-plotted for the 15 knots case, with laser-gun measurements by Ken Winner, technical editor of American Windsurfer magazine superimposed. It can be seen that the curves are very similar, differing mainly in scale (Ken suggested to reduce the drag coefficients.) However, at close-hauled sailing there may be another reason for the difference. If a competitor is asked to see how fast can he/she sail up-wind, they will bear off first to get on the plane, and then sail upwind. The computer program does not accomodate this behaviour. In the 10 to 12 knots regime the results are multi-valued, and one can chose which root to find: the planing one or the non-planing one. Below 11 knots the non-planing one was chosen.

For more info contact:
Dr.Peter I Somlo
Microwave Consultant | email: somlo@ieee.org
<A HREF="http://web.archive.org/web/20000312154125/http://www.zeta.org.au/~somlo/default.htm" TARGET="_blank">http://www.zeta.org.au/~somlo/default.htm</A>

Stay tuned for more in-depth tech info on these items and much more....

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Why Sails Twist

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Contents

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TECH TALK

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Windspeed vs. Height plots compliments of W. L. Kleb