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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.
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Spin-Out: Your Fin or Your Sail?
(Category: Wave Sailing)
By Eric Mihelbergel (Written February 2019)
When it comes to windsurfing in waves, speed is not of much interest to me. Some people like to blast, but I really like to play. Because of this, smaller sails are of great interest to me. Smaller sails are more playful, lighter, more maneuverable, less tiring to use than their larger counter parts, and they are not in the way as much when you are on a good wave and just want to experience the board. Several years ago I started thinking that since a smaller fin has less drag than a larger fin, maybe a smaller sail could be used if a smaller fin was paired with it. Less drag? I found this to be somewhat true as it does allow for a slightly smaller sail to be used. We can only decrease the size of our fin to a certain point, however, before the sail side-force and the moment of inertia caused by the board width and sailor weight will produce stall on the fin. Then we get spin-out! After playing around with this last season, I decided to refresh my memory from the physics degree I earned over 20 years ago and re-learn a little bit about foils (fins). I wanted to explore drag, lift, and particularly stall (spin-out).
A fin is a foil. It produces lift under the same principles as an airplane wing, except it is vertical instead of horizontal and used in the more dense fluid of water rather than air. The size of a fin is generally stated in terms of length by the windsurfing community and manufacturers, but length alone is a terrible way to compare fins. Area of a fin is a better measure to compare fins, however, it's still not enough. We also must consider the width of the fin at the top and the bottom (chord), the maximum thickness of the fin and the taper away from that thickest part that determines the shape of the foil, the profile shape (outline) of the fin, the comparison of fin length and area (aspect ratio), the rake angle of the fin (vertical or swept back), and the stiffness, flex, and twist of the fin. All of these things define a fins overall character. Rather than comparing fins by length, we should compare them by overall character.
Without the fin our windsurfing board would mostly drift sideways from the side force of the sail. The board would provide a little bit of opposing side force to resist the sideways drift, but not enough to effectively thrust the board forward. So we need to select a fin with the proper overall character to match the sail and board we are using. Most windsurfers know that we need a big enough fin to resist the side force of the sail. If the sail is too big for the fin we are going to get spin-out (stall). While area of the fin can be used to compare fin size, there are many other factors besides area that affect lift and spin-out.
Here are other factors that will affect lift and spin-out:
1. The rake of your fin: A fin with a lot of rake, like a weed fin, will produce less lift, so you may get spin-out more easily. A fin of the same area that is straighter up-and-down will generate more lift.
2. Your speed: The faster you go the more lift your fin will produce thus reducing the amount of spin-out you might get.
3. The shape or profile of the fin: Different fin shapes produce more of less lift thus affecting spin-out.
4. The geometry of the foil (cross section of fin): The shape and thickness of the foil affects the performance of the fin.
5. Aspect ratio: If we divide the square of our fin length by the area of the fin we get aspect ratio. High aspect ratio basically means long and narrow while low aspect ratio means short and fat. Generally, high aspect ratio will produce less drag while low aspect ratio will produce a more maneuverable fin.
6. Angle of attack: This is the angle between your fin (board) and the direction you are traveling. Yes, you read that right. Our boards are not actually traveling in the direction they are pointed. There is a very slight sideways travel which creates an angle of attack for our fin to generate lift from.
7. The water density: Salt water is a little more dense than fresh water which is why things float better. The denser salt water also gives your fin a little more lift and a little less spin-out. This concept of fluid density also explains why the lift from a small fin can equal the lift from a much larger sail. Water is much more dense than air, and so the smaller fin can resist the force of the larger sail because more lift is generated from the denser water.
8. Moment of inertia: When you pressure the heel of your rear foot with your body weight there is a large amount of leverage (moment of inertia) that is exerted against the fin. The fin must resist this leverage. When this leverage against the fin is greater than the lift that the fin can produce the whole board will spin-out. The wider your board, the greater leverage you will apply to the fin when pressuring your heels thereby increasing spin-out.
9. Air under the board: If the front of your board is too high relative to the back, or if your board is bouncing a lot, you can force air under the board. This air can travel onto the fin and interrupt the flow of water over the fin thus reducing lift and causing spin-out. Make sure you have the proper sail and mast to match your board and water conditions to prevent bouncing (click here for an article Know Your Sail)
10. Imperfections on the fin: Chips, scratches, and roughness on the surface of your fin can affect the performance and cause spin-out. Even one small strand of a weed stuck to the fin can interrupt the flow of water against the fin and cause spin-out.
11. Size of your sail: We already briefly discussed this in the paragraph above. A bigger sail produces more lift, and requires a fin of greater lift to resist the bigger sail (all other things being equal). So, use a smaller sail. You can learn to ride smaller sails. Here is a whole article on Riding Smaller Wave Sails.
1. The rake of your fin: A fin with a lot of rake, like a weed fin, will produce less lift, so you may get spin-out more easily. A fin of the same area that is straighter up-and-down will generate more lift.
2. Your speed: The faster you go the more lift your fin will produce thus reducing the amount of spin-out you might get.
3. The shape or profile of the fin: Different fin shapes produce more of less lift thus affecting spin-out.
4. The geometry of the foil (cross section of fin): The shape and thickness of the foil affects the performance of the fin.
5. Aspect ratio: If we divide the square of our fin length by the area of the fin we get aspect ratio. High aspect ratio basically means long and narrow while low aspect ratio means short and fat. Generally, high aspect ratio will produce less drag while low aspect ratio will produce a more maneuverable fin.
6. Angle of attack: This is the angle between your fin (board) and the direction you are traveling. Yes, you read that right. Our boards are not actually traveling in the direction they are pointed. There is a very slight sideways travel which creates an angle of attack for our fin to generate lift from.
7. The water density: Salt water is a little more dense than fresh water which is why things float better. The denser salt water also gives your fin a little more lift and a little less spin-out. This concept of fluid density also explains why the lift from a small fin can equal the lift from a much larger sail. Water is much more dense than air, and so the smaller fin can resist the force of the larger sail because more lift is generated from the denser water.
8. Moment of inertia: When you pressure the heel of your rear foot with your body weight there is a large amount of leverage (moment of inertia) that is exerted against the fin. The fin must resist this leverage. When this leverage against the fin is greater than the lift that the fin can produce the whole board will spin-out. The wider your board, the greater leverage you will apply to the fin when pressuring your heels thereby increasing spin-out.
9. Air under the board: If the front of your board is too high relative to the back, or if your board is bouncing a lot, you can force air under the board. This air can travel onto the fin and interrupt the flow of water over the fin thus reducing lift and causing spin-out. Make sure you have the proper sail and mast to match your board and water conditions to prevent bouncing (click here for an article Know Your Sail)
10. Imperfections on the fin: Chips, scratches, and roughness on the surface of your fin can affect the performance and cause spin-out. Even one small strand of a weed stuck to the fin can interrupt the flow of water against the fin and cause spin-out.
11. Size of your sail: We already briefly discussed this in the paragraph above. A bigger sail produces more lift, and requires a fin of greater lift to resist the bigger sail (all other things being equal). So, use a smaller sail. You can learn to ride smaller sails. Here is a whole article on Riding Smaller Wave Sails.
Of the eleven items above I want to focus on one item in particular, NUMBER 8, because it makes a HUGE difference in performance. You can easily improve your riding by making a few adjustments to avoid spin-out while riding small fins. The wider your board the further the edge of your board is from the fin. When you put weight on the heel of your rear foot at the edge of the board it can cause spin-out as we noted above. You could certainly put a big fin on the board to generate more lift and get rid of the spin-out, but we don't want big fins when we are wave sailing. We want to be maneuverable and have less drag. We want everything smaller. So how do you go small AND reduce spin-out at the same time? It's really pretty simple. Get your body weight over the center of the board, NOT on the edge of the board. How do you do this? First, make sure you have a modern "wave sail", not a freeride sail. Wave sails are designed with a center of effort that is higher up on the sail as you will notice from the higher clew grommet and less area in the foot of the sail. This higher center of effort allows the sail to pull your body more over the center of the board (click here for an article Know Your Sail). If you put the same size freeride sail on your wave board the freeride sail will shift your body weight out onto the edge of the board over your heels because the center of effort is much lower in a freeride sail. This will increase spin-out unless you put on a bigger fin. If you put on a big fin and a freeride sail, however, it would defeat the purpose of wave sailing. So use a wave sail. Next, use longer harness lines and lower booms. This will get your sail more upright so that you get maximum power from the smaller sail you are using. Drop your hips lower to compensate for the longer harness lines and lower boom. Your knees are forced to bend more with your hips now dropped, and bent knees can absorb the energy from bounce that would otherwise transfer to the fin and cause spin-out. It's a whole package deal. Today's compact wave boards have wider tails than traditional wave boards, so it is very important to keep the weight off of your heels to avoid spin-out. I also like a wave sail designed with less luff curve so that the mast stays more flexible. That way the force from a gust of wind does not transfer through the mast and down to the fin thus causing spin-out.
As you can see, both the fin and the sail play a major role when it comes to avoiding spin-out. Pay close attention to them and you will be much more efficient.
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