Regulate power with mast bend

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 flatterwith 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:
  1. Pulling the backstay shortens the distance between the top of the mast and the back of the boat.  This reduces tension on the leech,  so if you want to keep the same amount of twist you have to pull harder on the mainsheet.  The opposite is true when you ease the backstay – the leech gets tighter so you have to ease mainsheet.
  2. As the tip of the mast moves aft,  the middle of the mast arcs forward.  The entire spar acts like a lever with its fulcrum at the ‘hounds’ (the point where the forestay and shrouds attach to the mast).  
  3. When the middle of the mast levers forward,  it pulls cloth from the middle of the sail.  This makes the main flatter overall (3a) and sucks the fullness out of the forward part of the sail,  which moves the position of maximum draft farther aft (3b).
  4. If the mast bends enough,  it flattens the main past its designed sail shape.  This results in the appearance of “overbend” or “inversion” wrinkles running from the  middle of the mast toward the clew.
  5. Because it pulls the top of the mast aft,  the backstay also tensions the headstay. This reduces jib luff sag,  making the jib flatter and more draft-aft overall.

Other Tips and Notes

  • When you pull harder on the backstay,  the  mast bends,  the sail flattens and the position of maximum draft in the sail moves aft toward the leech (solid draft stripes). 

    When you ease the backstay,  the mast gets straighter ,  the sail shape becomes fuller and the draft moves forward toward the mast (dotted stripes).
    Watch for overbend wrinkles. The backstay bends the mast and helps make the mainsail flatter when you are getting overpowered. However, it is very easy to pull the backstay too hard. One clear sign that you are near maximum backstay tension is the appearance of overbend wrinkles running from the clew toward the middle of the mast. If you have more than a hint of these wrinkles, ease the backstay slightly (or tension the checkstays if you have them) to straighten the mast and put a little more shape into the sail.

  • Ease backstay downwind. When you’re sailing on a run, it’s not fast to pull the top of the mast aft. Ease the backstay until the mast is vertical in the boat or actually raked forward (but in windy conditions don’t ease the backstay so much that the mast has reverse bend). On an overpowered reach, it’s OK to keep the backstay on to depower the sailplan.

  • Adjust backstay, adjust mainsheet. When you tension the backstay, it pulls the top of the mast aft. If you don’t touch the mainsheet, the leech of the sail will twist off to leeward because the top of the mast is closer to the aft end of the boom. Therefore, whenever you adjust the backstay you should make a corresponding change in mainsheet. If you tighten the backstay,  tighten the sheet. If you ease the backstay, ease the sheet or the leech will get too tight (because the tip of the mast moves farther away from the end of the boom).

  • Use the backstay to change gears. When conditions change, don’t forget the backstay. As you get a puff, pull on more backstay to flatten the main and reduce headstay sag (which flattens the jib). In a lull, ease the backstay to add power to the overall sailplan.

  • Don’t get hung up. On some boats, the backstay often gets caught on the mainsail leech or the top batten during tacks or jibes. You may be able to prevent this by using some sail repair tape (with lube spray) to smooth over the spot that catches. It also may help to keep the backstay slightly taut (rather than loose), or to ease the main out quickly (and then re-trim it) just after completing a tack.