This project is a companion to my previous projects, How to Add Numbers to a Sail and How to Reproduce a Class Insignia on a Sail. If you’re getting started in club racing or if you just want to get the best performance out of your sails for cruising, draft stripes can help. A draft stripe makes it easier to see how small adjustments in sail trim affect the shape of your sails and therefore, how air moves over them. A draft stripe can help you to optimize the amount of lift your sails produce in different wind conditions and become a better sailor. Becoming a better sailor means you make more efficient use of your time when cruising and have more fun. And if you race, better sailors sail faster.
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A draft stripe is a horizontal strip of dark colored sailcloth placed at one or more points along the luff on each side of a sail. Each stripe indicates the path of the air as it passes over the surface of the sail. Smaller sails typically have one draft stripe located at the midpoint of the luff or two draft stripes that divide the sail into thirds. Larger sails may have three or more equally spaced stripes depending on the length of the luff. Draft stripes can be added to headsails or mainsails. They are not used on flying sails like spinnakers and drifters.
Draft stripes allow you to quickly and easily see the airfoil shape of a sail, how deep the draft is (belly as a percentage of width) and where the maximum draft is located along the width of the sail. The optimal shape for a particular sail might be 14% draft at the foot increasing to 20% near the sail’s head with the apex of the draft located around 40% of the sail width (arc chord). These amounts depend on the specific sailboat design, sailcloth characteristics, sail cut, expected sailing conditions, purpose (cruising or racing), and so on. That’s where a sailmaker’s expertise comes in to design the best sail for your particular needs. The average skipper doesn’t need to be concerned so much about technicalities.
For example, in the picture below, I’ve drawn imaginary lines that show the actual draft (solid red), preferred draft (dashed red), and twist (green). From these lines, you can tell that the draft is deeper than it should be (old, blown-out sail), farther aft than it should be (also from being blown-out), and there’s a slight twist from the foot to the head (acceptable for the sailing conditions). One or more draft stripes would allow you to see these without using your imagination. In this example, the panel seams are roughly horizontal and can be used as built-in draft stripes. But seams that are not horizontal will be misleading so a draft stripe is more useful.
Draft stripes also indicate the amount of twist in the sail relative to its foot, which determines how much air is spilling out of the top of the sail and affects the amount of heel.
Don’t be a draft dodger
Draft stripes are a good aid when you:
- Are learning how each of the sail trim controls work
- Getting familiar with the performance of a different sailboat
- Have crew trimming the sails who are not familiar with your sailboat
If you sail often and long enough, you should develop an innate sense of how sail trim affects performance; to the point that you don’t need draft stripes—you just “know.” It’s like driving your car at a certain speed without looking at the speedometer. Your senses of sight (the speed of the water flowing by), touch (vibration, heel angle), and hearing (wind and water noise) tell you everything you need to know.
It’s easy to make draft stripes from 1″ wide self-adhesive polyester tape. Our friends at Sailrite offer it in red, blue, and black sold by the foot.
To add a draft stripe:
- Lay the sail on a large, clean, flat work surface.
- Determine how many stripes you want and their locations (for example, one stripe at 50% of luff length or two stripes at 33% and 66% each). If possible, space them apart from the sail’s other features (seams, battens, reef points, numbers, and insignia, if any) like shown below. Avoid placing draft stripes over seams that can cause wrinkles and can cause the stripes to come loose over time.
- Add the lengths of all draft stripes on both sides of the sail, round up to the nearest foot, and purchase that amount of draft tape.
- Use a pencil and straightedge to draw a light guideline at each stripe location. The line must be horizontal to the waterline. If you’re adding a stripe to a mainsail, mark the endpoints by measuring equal distances up from the foot of the sail, which should already be horizontal. If you’re adding a stripe to a headsail, the foot is usually not horizontal and the luff is not perpendicular. Tie or pin one end of a string at the halfway point of the luff. Hoist the sail until that point is waist high off the deck. Hang a bubble level on the string. Then pull the leech and the string taught until the string is horizontal and mark where the string intersects the leech. Apply the stripe tape straight between those two points.
- Cut the draft tape to length so that it fits between the luff tape/rope and leach line/seam and lays flat.
- Peel back two or three feet of the backing paper, align the tape edge with your guideline, and lightly press the tape to the sail without making any bubbles or creases. Take your time and reposition the tape until you get it right. Repeat the process to the end of the tape.
- When you have the draft tape where you want it, roll it firmly with a J roller if you have one. You can improvise one with a short length of dowel or other cylindrical object used like a rolling pin.
- Turn the sail over and apply a second piece of tape directly over the first.
You’re done! Now go sailing and practice using your draft stripes to optimize your sail’s shape in various conditions.
Below are the common sail controls and how each affects sail trim. The links take you to the corresponding projects on this site that show you how to make the controls yourself.
|Boom downhaul/Cunningham||Flatten luff, adjust draft position|
|Outhaul||Adjust draft depth|
|Vang||Adjust sail twist|
|Halyard||Flatten luff, adjust draft position|
|Sheet||Adjust draft depth|
|Lead blocks||Adjust foot/leech tension, sail twist|
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