By David Dellenbaugh. Republished by permission - Speed and Smarts
[Note that on the Cavalier 28, being a masthead rig, you will not see any appreciable mast bend unless the baby-stay is also tensioned]
The backstay is a powerful tool that gives you control over mast bend. The harder you pull on the backstay, the more you bend the mast. As the mast becomes more curved, the mainsail gets flatter, the position of maximum draft moves aft and the main leech twists more. Therefore, one rule of thumb is that whenever you adjust the backstay you almost always have to change other controls as well. For example, you may need more cunningham to keep the draft forward and more sheet to maintain the desired twist.
Since bending the mast reduces the power in the mainsail, the backstay is used primarily when you have enough wind that you need flatter sails to point higher or to start depowering. It is very seldom that you would want any backstay tension in lighter air when you are looking for power (except you might want to take the slack out of the backstay to keep the mast tip from bouncing around in waves).
Once your crew is hiking and the boat is powered up and going fast, then you can think about adding backstay. Of course, the amount you need depends on wave state as well as wind velocity. In flat water you might start pulling the backstay quite a bit earlier than in chop (when you need more power).
When it gets really windy, then you need to pull the backstay very hard. This flattens the main, adds twist and thereby depowers the sail plan so you can sail the boat flatter with less windward helm.
However, if you bend the mast too much, you will flatten the sail beyond its designed sail shape. In that case you will likely see ugly overbend wrinkles and the leech will fall off to leeward, which is bad for speed and pointing. So normally pull the backstay only until you see a hint of overbend wrinkles.
If the backstay affected only the mainsail, then life would be easy, but that is not the case. When you pull the backstay, you also tighten the headstay, which flattens the jib and moves its draft aft. The trick is finding the backstay setting that optimizes the shape of both sails at the same time, and sometimes this means you must compromise.
In light air, for example, the mainsail likes enough mast bend to match its designed luff curve. But if you get this by pulling the backstay you will depower the jib too much. The solution is to ‘pre-bend’ the mast (with tuning) so you can keep a loose backstay and powerful jib.
When you pull (or ease) the backstay, it affects the mast and sails in many ways: