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Bio - Eric L. Mihelbergel is an intermediate/advanced windsurfer, kiteboarder, and foiler living in the Great Lakes Region of New York State who enjoys sharing about windsports and fitness. |
Know Your Sail
By Eric Mihelbergel (Written March 2019)
Have you ever put a different sail on your board and wondered why it was more bouncy compared to the last sail you used on the same board? Or have you ever tried a new sail and gotten more spin-out even though it was the same sail size, same board and same fin. None of this is a big deal if you are using rental equipment at a resort because you can just return the gear and try something else. But if you are buying new gear you don't want to find these problems after your purchase. Much of this can be avoided if we understand a little more about sails and sail design. This discussion will be geared toward wave sails, however, we will do some comparing between wave sails and freeride sails to help in our understanding and selection of sails. As with many pieces of gear there are trade-offs in performance. For example, if you want more speed you will generally sacrifice some maneuverability, and vice versa. We will explore some trade-offs between wave sails and freeride sails.
Here are items we will consider in this discussion of sails. Remember these items when you purchase a sail:
1. Luff Curve
2. Twist
3. Center of Effort
4. Clew Grommet Position
5. Mast
6. Battens
7. Batten Rotation
8. Panel/Seam Shaping
9. Foot of Sail
10. Aspect Ratio
1. Luff Curve
2. Twist
3. Center of Effort
4. Clew Grommet Position
5. Mast
6. Battens
7. Batten Rotation
8. Panel/Seam Shaping
9. Foot of Sail
10. Aspect Ratio
Luff Curve: The luff curve influences many things. Less luff curve will make for a flatter sail shape and the sail will have more twist. This situation does not require much downhaul. More luff curve makes for a fuller sail shape with less twist, and this requires considerably more downhaul. When the curve of the luff is greater than the curve of the mast this is called excess luff curve. More excess luff curve creates a deeper pocket in the sail compared to a sail with less excess luff curve. The battens will rotate hard around the mast if the sail has a lot of excess luff curve. We will discuss batten rotation more below. Please note that "luff curve" and "excess luff curve" are two different things.
Twist: It is very important to have twist in the top of the sail. The wind at the top of the sail is stronger than the wind at the bottom of the sail because the wind experiences less friction with the water surface near the top of the sail. Therefore, we need a different angle of attack at the top of the sail so that the sail does not stall with the stronger wind at the top. If the sail has proper twist then it will be at the correct angle of attack to get the proper amount of power out of the wind without the wind detaching from the sail (stall). In addition, twist in the top of the sail will allow big gusts of wind to roll off the sail without jerking the rider around too much.
Center of Effort: Wave sails are designed with a higher center of effort so that the sail pulls the rider over the board in order to allow the rider more maneuverability and to keep body weight over the fin so that smaller fins can be used with less spinout (click here for an article Spinout: Your Fin or Your Sail?). Freeride sails are designed with a lower center of effort so that the rider can get their body out over the edge of the board to hold down larger sails that provide more speed.
Clew Grommet Position: One of the ways to affect location of the center of effort on a sail is by placement of the clew grommet. A higher clew grommet helps put the center of effort higher in the sail while a lower clew grommet puts the center of effor lower. The clew grommet position can also affect twist, with a lower position giving more twist and a higher position giving less twist.
Mast: The mast is one of the most important components of your entire kit. You MUST use a mast that the sail designer intended. Masts have very different amounts of stiffness and flex. The mast must bend in the correct places to allow the sail to assume the proper shape. If you use a mast with a different stiffness and/or bend curve than the manufacturer intended your sail will NOT work the way it was designed, and you will not get proper performance from your sail (click here for an article The Importance of Your MAST).
Battens: The sail shape is affected by the taper of the battens. The battens can be thicker or narrower at one end than the other thereby providing different stiffness at each end. This difference in stiffness from one end of a batten to the other affects the shape of the sail. The batten pocket is also very important. If the pocket is attached directly to one side of the sail then the sail will not be entirely symmetrical. One side of the sail will perform slightly different than the other. The solution is for the manufacturer to sew two panels together, where the battens belong, and sew the battens directly into that seam between the panels so that the batten is NOT on one side of the sail. Another component is the number of battens. Generally, the more battens a sail has the more shape and thus power the sail will have and the more control it will have at the high end of the wind range. The fewer battens a sail has the lighter and more maneuverable the sail will be, and it will depower easier for wave riding as it loses shape.
Batten Rotation: Battens will rotate harder around the mast when there is more excess luff curve. More excess luff curve gives a deep pocket in the sail when powered and also allows the sail to depower when on the wave. This is good for off-shore and side-shore conditions on steep clean waves. However, the downside is that the sail will have less power in light wind (when the battens are rotated around mast) and more power in gusts (when the battens are pulled away from mast), which is the opposite of what we want when we are not on a wave. It makes the sail twitchy with the draft moving backward and forward, and it gives us power at the exact wrong time. As an alternative to using only excess luff curve for sail shape, a sail maker can use less excess luff curve and more panel/seam shaping and outhaul to get shape, which would be much better in on-shore and side-on conditions where more consistent power is needed on the mushy waves. A combination of excess luff curve and panel/seam shaping can be used to find a balance.
Panel/Seam Shaping: In order to solve the problem of hard batten rotation around the mast, and loss of low end power, a manufacturer can build less excess luff curve into the sail. This, however, reduces much of the 'foil' shape that gives the sail lift. So to compensate for the lost shape and lift the manufacturer will add shape to the sail by building the shape into the panels through seams. Imagine that you are going to sew two pieces of fabric together. One piece has a straight edge and the other piece has a curved edge. Now sew the pieces together keeping the edges parallel. In order to keep the straight edge and the curved edge parallel while sewing them together you must slightly bend the curved edge fabric as you sew. The result is two pieces of fabric sewn together that now have an overall curve in the resultant single piece of fabric. This is called panel shaping in sail making. These seams are often hidden under the battens so you may not see these seams. Sails made with this type of shaping generally require more outhaul to pull the battens away from the mast a bit.
Foot of Sail: The foot of the sail is that part near the bottom. Most windsurfers know that a wave sail has less fabric in the foot of the sail so that the sail does not hit the wave when performing maneuvers on the wave, while freeride sails have more fabric in the foot so the rider can get maximum power for speed. Another reason for adjusting the amount of fabric in the foot of the sail is to control the center of effort in the sail. If we reduce fabric in the foot of the sail then it moves the center of effort higher, which is what we want for wave sails. The higher center of effort pulls the rider over the center of the board to increase maneuverability and to keep leverage off the fin thus allowing a smaller fin to be used. The smaller fin has less drag which allows a slighly smaller sailer sail to be used. In wave riding it is desirable to keep things as small as possible, without sacrificing much power, in order to remain maneuverable. (Click here for an article Riding Smaller Wave Sails).
Aspect Ratio: This is a comparison of the height of your sail to the sails area. Is your sail tall and narrow, or is it short and fat? A tall and narrow sail (high aspect ratio) generally has less drag for the amount of lift it provides. Think of a glider plane with those efficient long narrow wings. A short and fat sail (low aspect ratio) generally is much more maneuverable. Think of the short fat wings of a maneuverable fighter jet or space shuttle. But there are differences between windsurfing sails and airplane wings. For example, the leverage of a 30 foot tall windsurfing sail would pull the rider right off the leeward side of the board while an airplane could handle the leverage of a long wing. In addition, a 30 foot tall sail is gonna flex like an al dente noodle, and it would be very difficult to lay down a sail this tall on a frontside bottom turn. Forward loops would also be a real bitch with a 30 foot sail. At the other extreme, a sail with an excessively long boom would not be possible to sheet-in with all that leverage against the rider, nor would it be very easy to duck jibe. So a balance must be found in aspect ratio when designing a sail. To make things more complicated, the aspect ratio is going to change depending on how much downhaul and outhaul you put in your sail. Personally, I don't think aspect ratio is very useful in comparing windsurfing sails because there are so many other things going on with the sail, but it's worth mentioning here so we have a fuller understanding of sails.
Here is a brief comparison of wave sails and freeride sails:
Wave Sails Freeride Sail
Less Luff Curve More Luff Curve
Higher Center of Effort Lower Center of Effort
Less Foot Area More Foot Area
Higher Clew Grommet Position Lower Clew Grommet Position
Fewer Battens More Battens
Wave Sails Freeride Sail
Less Luff Curve More Luff Curve
Higher Center of Effort Lower Center of Effort
Less Foot Area More Foot Area
Higher Clew Grommet Position Lower Clew Grommet Position
Fewer Battens More Battens
Now, let's tackle a few trouble shooting areas:
My board is more bouncy with the new sail that I tried: This is likely to happen if you put a wave sail on a freeride board. Wave sails are designed to lift the nose of the board slightly to make the kit more maneuverable, while freeride sails are designed to pin the nose of the board down to go fast. A freeride sail has a lower center of effort than a wave sail which will pin the nose of the board down more. Use a freeride sail on a freeride board.
My board is getting more spinout with the new sail that I tried: This is likely to happen if you put a freeride sail on a wave board. It is the opposite of the above problem. A freeride sail has a lower center of effort which puts the rider out over the edge of the board more with greater pressure on the heels of the feet applying more pressure against the fin. The result is more spinout. Use a wave sail on your wave board and the higher center of effort will pull you more over the centerline of the board so that you are not leveraging against the fin. Avoid using a freeride sail on a wave board.
My wave sail does not have the same power as my friend who has the exact same wave sail model and size: Does your mast have the correct bend curve and stiffness? If your mast does not flex in the exact spots that the sail manufacturer intended (proper bend curve) then the sail will not have the correct foil shape, draft position, and twist. In addition, if your mast is too stiff or not stiff enough then the sail may jerk you around in gusts thereby disrupting your stance and result in power loss. Contact the manufacturer of the sail to learn what exact mast specs are necessary for your sail.
My wave sail does not give me enough power when riding frontside on the wave in onshore conditions: This is likely due to batten rotation and/or the number of battens. If your sail is designed with a lot of excess luff curve it will depower when riding waves frontside. A wave sail with less excess luff curve and more panel/seam shaping will retain its shape better and thus continue to deliver power. Additionally, you may need more battens. Five battens as opposed to three battens will allow the sail to keep its shape when frontside in onshore conditions so that you can maintain power.
My wave sail does not give me enough power when freeriding: Wave sails are not designed for freeriding, BUT....there are still things to address here. If your wave sail is designed with too little excess luff curve and too much panel shaping, then the battens may be derotated, which means an inefficient sail shape has been created. Power is lost as the sail does not maintain an efficient foil shape. Selecting a sail with a combination of panel/seam shaping and excess luff curve can be a nice balance to get a more versatile sail.
My wave sail jerks me around in the gusts: This could happen from several things. First, is your mast the correct stiffness? A stiff mast is going to jerk you around in the gusts because it does not have much flex. Second, does your sail have a lot of luff curve? If it has a lot of luff curve that means it requires more downhaul which bends the mast to its limits and makes the mast stiffer as it reaches that limit. Selecting a sail with less luff curve overall will give you more flex in the gusts. By checking with the sail manufacturer you should be able to learn what mast is best. Third, does your sail have a lot of excess luff curve. This will rotate the battens hard around the mast. In light wind you will have little power with the draft forward, while in strong gusts the power will increase as the draft moves back. This back and forth movement of the draft will jerk you around, and it will feel as though your harness lines are never in the right spot. Select a non-rotational power wave sail, with less excess luff curve, designed for onshore conditions rather than a rotational wave sail, with greater excess luff curve, designed more for offshore conditions. Fourth, have you downhauled the sail enough? If you have not put enough downhaul in the sail it will not have enough twist at the top which will make the sail jerky in gusts.
To conclude let's talk about choosing a good wave sail for use in Western New York on Lake Erie and Lake Ontario. We mostly have on-shore and side-on conditions here on the Great Lakes. That means our waves do not stand very tall, clean, and steep. We still need power in our sail when we are on the wave. When choosing a wave sail for these conditions we would want more battens as opposed to fewer battens to keep good power consistently in the sail. I like a 5 batten wave sail. We would also want a sail with less batten rotation so the sail does not depower on our small mushy waves and delivers more consistent power all the time. To accomplish this we should choose a sail with less excess luff curve and more panel shaping and outhaul. Different manufacturers have different design preferences. Some manufacturers are very committed to using lots of excess luff curve while others strongly oppose lots of excess luff curve. And, still, other manufacturers fall somewhere in the middle, sometimes offering a blended sail and sometimes offering two separate sail models. In addition, I personally prefer a sail with less luff curve in general so that the mast has a little more flex. You will need to check with the manufacturer to learn how they design their sails so that you can effectively choose a wave sail for use in Western New York.
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