A Custom-fitted Clew Strap
Velcro, buckle, or hitch? PS compares the pros and cons of each approach.
After settling on the material, one of the most basic mainsail design questions is whether to have an attached foot or loose-foot. A sail with an attached foot, secured to the boom with a bolt rope or sail slugs, has a small advantage in area, while a loose footed sail is easier to adjust (flat for windward work and smooth seas, fuller for reaching and rough seas), slightly cheaper to fabricate, and much easier to take off the boom for storage. Both are used on both high performance and cruising boats. Most new mainsails are loose footed.
A common misconception, or at least exaggeration, is that an attached foot provides better support for the sail. In fact, the sheet load will always be carried in a radial pattern originating from the clew, with most of the load carried by the clew ring. In a loose footed sail, the clew patch is slightly enlarged to compensate. For most boats, switching to a loose footed sail is as simple as attaching the clew shackle to the existing final slider (reinforced slugs are made for this purpose) or car. Unfortunately, the car or slugs are often undersized or prone to jamming without help from an attached foot. The simplest solution is a clew strap.
Simple Velcro wraps first appeared on racing dinghies such as Lasers, where they provided a lightweight solution that adjusted smoothly, allowed for fast sail installation and removal, and eliminated complicated and heavy track systems. They became standard equipment on larger race boats as well, including Farr 40s and TP 52s.
Sailmakers donít always include this simple device with a new sail, since they donít know what hardware you have or what your preferences may be. When we rigged our new main on our test boat, the existing sail slugs didnít make us happy, so we lashed the clew to the boom with a sail tie, wrapping it twice and securing with several half hitches. In truth, that simple webbing strap would have lasted the life of the sail, but as dedicated tinkerers, we soon replaced it with an adjustable strap with a buckles, which has functioned well for five years. This wasnít our first loose footed sail, but it was the first weíd installed on a boat that was not factory equipped for this.
The original sail, with attached foot and sail slugs, often stuck in lighter winds, requiring pushing against the center of the sail foot to get the clew to slide in and the sail to increase in fullness. Flattening the sail required using a winch, even with no wind pressure, because the cloth had to be forcibly stretched.
With a clew strap, the clew moves nearly as easily as a traveler car system. Flattening the sail requires only a firm pull and no purchase, and a winch is needed if there is wind in the sail. When the outhaul is eased, the clew automatically slides forward in all but the lightest airs. Obviously, there can be no hardware on the sides or bottom of the boom within the sliding range.
Lubricate the strap and the underside of the boom with a dry lubricant such as McLube Sailkote. If you use a buckle, consider covering it with tubular webbing to avoid scratching the mast. A lashing also slides better when it is covered with tubular webbing. With Velcro straps, the fuzzy loop side should bear on the boom, since it slides better. Plain webbing straps also slide better if you sew or stick self-adhesive loop Velcro to the bearing surface.
With end-boom sheeting the added stress on the boom is minimalónothing compared to vang loads. However, with mid-boom sheeting, the bending load on the boom will increase slightly, and attaching the clew to a rigid† track may better distribute the load. Consult a rigger or the owner of a sister ship to determine what works best.
If the clew strap is too tight (actually difficult to accomplish) it can be difficult to slide. If the clew strap is too loose, particularly if the clew is within four inches of the end of the boom, the outhaul will carry much of the sheeting load and will become difficult to adjust.
Spectra webbing slides more easily on the boom, but if the loop part of the Velcro is against the boom, this is irrelevant. There is no functional improvement in strength or stretch. Keep it tight.
The strength of Velcro clew straps rely on having enough wraps so that the shear load on the wrap is less than the shear strength of the Velcro, which Practical Sailor testing has shown is 6 to 12 pounds per inch of 1-inch-wide strap of new material. UV rays, use, and dirt or fibers in the contact area can reduce this by half.
However, successive wraps increase the shear strength by a combination of the friction and compression-loading of the Velcro by overlying wraps. With two wraps, the shear strength is multiplied by over five times, and with three wraps the shear strength is increased by over eight times.
Finally, the load is reduced by half when the webbing folds back around the ring (not all clew straps do this). Thus, three wraps of 1-inch webbing (24 inches each) should hold about (8 pounds/inch) x (20 inches) 0.5 x 6 x 2 legs x 8 multiplier = 7,680 pounds. For comparison, two wraps of 1-inch nylon webbing closed by a strong buckle or soft shackle (test boat) would have a breaking strength of about 10,000 pounds.
As an example, the maximum sheet load (measured when it is time to reef) on our test boat is about 825 pounds.† A three-wrap Velcro clew strap gives us a safety factor of about 900 percent when the strap is new, and 400 percent when it is old and dirty. Shackles and blocks would typically be rated for about 1,500 pounds working load and a breaking strength of about 7,500 pounds. Longer straps and more turns increase the strength potential.
You donít need a strain gauge to determine the amount of load on your clew. Harkenís online calculator (www.harken.com/MainsheetLoading/) closely matched our tested numbers.
A clew strap can also be used to hold a reefed (or unreefed) mainsail tack tight to the mast in the absence of a properly positioned tack hook. Webbing straps can replace bails for booms and vangs as well, though in both cases you need to prevent it from sliding. A line with loops, running the length of the boom, can work. In these latter cases, sewn loops are more usual.
The Laser-style clew strap, used successfully on boats up to 24 feet, has a second Velcro flap, which provides added grip and secures the tail.
To install, thread the tails through the clew until the ring rests on the clew. Orienting the short strap away from the boom,† wrap the longer strap around the boom, through the ring, and back down under the boom. Then lock the tail of the strap in place by pressing the shorter strap over the tail. This online video can help: www.youtube.com/watch?v=IV2CJvuUwnU
A conventional lashing is perhaps the simplest approach. Simply thread light line or webbing through the tack and under the boom until the combined strength of the lashing exceeds the mainsheet load† by about 10 times (safety factor including wear).
For example, a double wrap of 3/16-inch Dyneema or webbing is enough for sails up to 500 square feet. A simple attachment method is to splice an eye in each end and connect the eyes with a soft shackle. Unfortunately, the length is only adjustable by re-tying, and removal requires unthreading.
Alternatively, bend the tail around a single spliced loop and secure with a string of half hitches, as we did on the test boat. This makes it secure and not difficult to adjust or untie. To keep the Dyneema lines together, thread them through a 10-inch length of 2-inch tubular nylon webbing and locate this against the underside of the boom.
Bottom line: This is the simplest approach, but not the most convenient.
Rings and Buckles
Numerous approaches using a buckle and rings are possible. The main advantage here is that you avoid using UV-vulnerable Velcro.† The metal bits are covered with webbing to avoid scratching the boom. We have used this system on our test boat for the past five years. The only downside is that it takes slightly longer to set and remove than Velcro straps.
Bottom line: These are very durable, but youíll need to sew your own, or hire a sailmaker to make a custom strap for you.
Installing a Velcro strap is lightning fast. The loop becomes very stiff when the Velcro layers grab together under load, and this combined with the low-friction characteristics of the loop portion of Velcro, makes it easy to adjust the outhaul. For larger boats, your sailmaker should have the design experience to size something appropriate. Cruiser and racer durability experience has been very good.
Bottom line: Properly sized, the Best Choice for boats up to 40 feet.
The concern with a Velcro strap is its strength over time, but this should be observable; when the tail begins to peel easily, itís time to replace. For racers who frequently replace sails, Velcro straps should last as long as the sail.
Cruising sailors may prefer the UV resistant reliability of a lashing or buckle system. Either way, unless the clew is equipped with a low-friction traveler car system, a clew strap will make adjustment easier, and, more importantly, it will encourage you to get the most out of your new loose-footed mainsail.