A vang is the tackle or strut that runs from the base of the mast to the underside of the boom, a metre or two aft of the gooseneck, and pulls the boom down. In Britain it is called the kicking strap or simply the kicker; in North America the boom vang. Its job is to control mainsail twist by holding the boom down when the mainsheet cannot, that is, whenever the boom is eased out beyond the end of the traveller on a reach or a run. A beginner learns that tightening the vang stops the boom skying and the top of the mainsail flogging off the wind. An experienced sailor knows the vang also does a good deal of work upwind on many boats, bending the lower mast and flattening the mainsail, and that a rigid vang doubles as a topping lift that holds the boom up when the sail is down.
How a vang works
Upwind, the mainsheet pulls the boom down as well as in, because the sheet runs nearly vertically from the boom to the traveller. Leech tension and twist are set by the sheet and the sail’s angle by the traveller. Bear away to a reach and ease the sheet, and the boom swings outboard until the sheet is pulling nearly horizontally. It no longer holds the boom down, the boom lifts, the leech opens, the top of the mainsail twists off to leeward and the upper half of the sail stops driving. The vang takes over the vertical job. Because it is attached close to the mast, it acts on the boom as a lever with poor mechanical advantage, so it needs a powerful purchase, typically eight to one or more on a cruiser and a hydraulic or high-ratio cascade on a racing boat.
- Rope vang: a simple block-and-tackle purchase, light and cheap, found on dinghies and smaller keelboats. It only pulls down; a separate topping lift holds the boom up.
- Rigid vang: a telescoping strut with an internal gas spring or coil spring that pushes the boom up, combined with a tackle that pulls it down. It replaces the topping lift, supports the boom when the sail is dropped or reefed, and is the standard fitting on modern cruising yachts.
Why the vang matters for sail shape
Twist is the difference in angle of attack between the bottom and the top of a sail. A little twist is desirable because the wind at the masthead is stronger and freer than at deck level, but too much twist off the wind means the head of the mainsail is luffing while the foot is stalled. The vang sets that twist on a reach and a run. Tighten it until the top leech telltale streams most of the time and the whole sail is working. In heavy air downwind, a firm vang keeps the leech from pumping and helps prevent the boom rising during a gybe and the sail wrapping around the spreaders. Ease it in a gust on a tight reach and the top of the sail depowers instantly, which is the quickest way to stop a broach short of dumping the sheet.
Upwind, on a fractional rig with a bendy mast, hard vang pulls the boom down and forward, bending the lower mast, flattening the lower mainsail and opening the leech. This is vang sheeting, and it lets the trimmer ease the mainsheet in gusts to depower without the leech falling completely open. On a masthead rig with a stiff spar the upwind effect is much smaller, and the mainsheet does most of the work.
Common mistakes
Forgetting to ease the vang before sheeting in to go upwind is the classic error, leaving the leech hooked and the boat overheeled and slow. Leaving it slack on a reach lets the mainsail twist away and wastes power. Setting it very hard downwind in a big breeze without a preventer invites a broken boom when the boat rolls and dips the boom end. And on a rigid vang, running the boom hard down with the mainsail lowered strains the spring; ease the tackle when the sail is stowed.
Frequently Asked Questions about Vang
Is a vang the same as a kicking strap?
Yes. Kicking strap and kicker are the British terms, vang or boom vang the American ones. They describe the same fitting.
How tight should the vang be downwind?
Tight enough that the top leech telltale flies and the top batten is roughly parallel to the boom. In heavy air firm it up further to stop the leech pumping, but be ready to ease it fast if the boat starts to roll or round up.
The vang is a small piece of gear with a big influence on how the mainsail flies once the boom leaves the centreline. Treat it as a live control rather than a set-and-forget fitting, ease it before you harden up, and use it to shape the top of the sail, and you will find the boat faster on a reach, steadier on a run and far less inclined to broach.