Here is some interesting information about Vangs/Boomkickers.
Boomkickers are the simple, affordable solution to supporting the boom and eliminating the chafe, hang-ups and adjustments of a topping lift.
Sailboat performance is improved with less weight and windage aloft. In light air, lifting the boom opens the leech for a faster sail shape and more speed. Reefing is safer and easier with better boom control.
Boomkickers use two coated fiberglass spring rods to provide the force necessary to support the boom and mainsail. The rods are clamped into hardcoat anodized aluminum end pieces and attached to universal mast and boom fittings. Fitting are included along with machine screw fasteners and the proper drill and tap. The mast fitting has a standard round slide which, used in the luff groove, avoids drilling into the mast. It also allows adjustments later if needed. Flat slides in varying widths are available from the factory. The slides can often be inserted at the sail opening above the gooseneck and slid down reassembling the fitting in position. If slides are not usable, the fitting can be attached directly to the mast using the same drill and tap provided for the boom. The custom extruded boom fitting is countoured to fit both round and flat bottom booms.
The two sketches below show the basic set up of the Boomkicker models 750, 1000 and 1500. When installed, the Boomkicker is straight with the boom positioned above the usual sailing height. As the mainsheet or vang is tensioned, the Boomkicker flexes smoothly from straight to flexed. The force stays relatively constant as the Boomkicker continues to flex, which helps keep the vang efficient. Each standard model has two sets of spring rods of different diameter and force so you can choose the set that best matches your boom weight and preference for support.
The more forceful the rods, the more support for the boom but the harder it is to pull on the vang. In general only about 10lbs should be needed to pull the end of the boom down by hand. The larger diameter or more forceful set is initially installed. If the boat is at the smaller range of the model, switching to the lighter rods before installation is recommended. At their initial length, the rods provide as much as 6 feet of travel at the end of the boom depending on the set up. If needed the spring rods are easily shortened up to 10" with a hacksaw to fit above the vang, with some loss of range.
Reproduced courtesy Mauri Pro Sailing
You can read more about standing rigging set up in my ebook 'Voyage of the Little Ship 'Tere Moana' on my website www.sailboat2adventure.com