The mainsail on most boats is effectively the power plant—the figurative engine that drives your boat under sail. If you understand how the various controls that affect the mainsail work, you'll be a step ahead of the game when it comes to optimizing performance. Two controls are integral to the rig that supports the mainsail—the backstay and the vang. By adjusting them (along with the other controls discussed in Part One of this series), you can affect the shape of the sail to produce the most productive form for the given conditions. Here's how.
Backstay Not every boat is commissioned with an adjustable backstay; in fact, some boats don't even have backstays. However, most rigs that have backstays can be adapted by adding components that will make it possible to adjust the tension on this important mast support. And changing the tension of the backstay will alter the shape of the mainsail and the headsail. In general, if you tighten the backstay you bend the mast, which tends to flatten both the leading edge of the headsail and the upper portions of the mainsail by creating additional luff tension. The amount of change varies on masthead rigs (those with their headstay attached at the top of the mast) and fractional rigs (those where the headstay is attached at some point down from the top of the mast). Depending upon the bend characteristics of the mast, a boat with a masthead rig will usually have a smaller range of adjustment, and will consequently experience a less-pronounced change in sail shape.
Let's say that you're sailing along in flat seas and 10 knots of breeze. Due to the conditions, you'll probably want a moderate amount of tension on your backstay. If the wind increases by five knots and looks like it will stay there, experienced sailors respond by altering their sail shape. Usually they start by getting a little more outhaul tension and then begin increasing the backstay tension to flatten the mainsail somewhat and keep the boat from becoming overpowered. As I mentioned above, tensioning the backstay will also affect the headsail by tensioning the forestay and thereby tightening the luff of the jib or genoa. When you do this, have a look at the jib or genoa trim to make sure you still have proper tension on the sheet and the lead is still in the right place. If you put the backstay on hard, chances are you'll need to make a slight adjustment to the sheet tension and lead.
If you're sailing along in a fairly strong breeze with the backstay tensioned pretty hard and the wind gets significantly lighter, releasing the tension on the backstay will help you to power-up the mainsail and the headsail by straightening the mast, which will generally add more fullness to both sails. The mainsail will become deeper because there will be less tension on the luff and the headsail will become fuller because you've essentially just added some sag to the headstay by moving the tip of the mast forward. Powering-up, as this is known, is also a good technique to have handy if you have to sail through some steep chop or powerboat wake.
Now the general rule of thumb with adjustable backstays is that you want some backstay tension on when sailing upwind—unless the wind is extremely light—and you want it off downwind. There are various kinds of backstay adjustment systems, from cascading purchase systems with line and blocks to mechanically operated adjusters, to hydraulic cylinders. It's important to choose a system that can operate under the loads that pertain to the rig on your boat. Most grand-prix racing boats, for instance, generally rely upon hydraulic systems due to the enormous loads their rigs can generate, while smaller daysailing boats like Catalina 22s or Melges 24s utilize purchase systems. Boom Vang
Definitely the redheaded stepchild of mainsail adjustments, the boom vang is often overlooked and almost always last on the list of adjustments that sailors think about. But that shouldn't be the case because the boom vang can be a critical tool.
Essentially what a boom vang does is control the upper leech of the mainsail—the tighter the vang, the tighter the leech. Of course racing sailors often use some vang tension in light air while sailing upwind to ensure that the boom doesn't bounce, but this control really comes into play in heavy air. Upwind it can help depower the sail plan by flattening the upper leech. Downwind, you simply use the boom vang to ensure that the upper leech of the mainsail is working for you. Until you really have a good feel for what the proper boom vang tension is, it's best to sight up the sail from below the boom. Too much tension on the vang and the top batten in the mainsail will be hooked to windward of the boom; too little and it will flop to leeward relative to the boom's angle. With just enough vang tension, the top batten of the mainsail will be oriented parallel to the boom.
If the breeze gets really strong, on some boats it's productive to employ the vang-sheeting technique upwind. This essentially involves setting the vang tension on hard and dumping the mainsheet or traveler in the puffs as you drive along on the jib. What the vang does in this situation is that it keeps the boom from rising when the mainsheet is eased, thus allowing the sail to stay flat and depowered. If you're sailing downwind in these conditions, it's prudent to have someone standing by to release the vang just before the boat gets overpowered and spins out of control, particularly when sailing on a tight reaching angle with the spinnaker up. On board many keelboats in these conditions the vang is regarded as the throttle, and the person trimming it needs to be in constant communication with the helmsman to keep the boat on its feet.
On some boats—particularly racing dinghies—the boom vang can be attached to the deck just aft of the mast or to the mast step itself. If this is the case, you'll notice that a lot of boom vang tension will bend the mast down low. Just be aware of this as you trim your mainsail so that you can make the appropriate adjustments elsewhere.
Like backstays, boom vangs range from line-purchase systems, to mechanical devices, to hydraulic cylinders. There are various factors that will help you determine what system is right for your boat, but certainly having a system that can withstand the loads is a principal consideration. Once again, on-the-water experience using this control in various situations will expedite your education regarding the proper use of the vang, so get out there and give it a try.
Mainsail Controls for Performance by Dan Dickison
Basic Mainsail Trim for Racers, Part One by Pete Colby
Basic Mainsail Trim for Racers, Part Two by Pete Colby
SailNet Store Section: Vangs