When racing, I always use my the adjustable backstay on my Catalina 22 Sport to adjust rake and bend.
^ This! Spoken by someone who know the boat well.
Moving on to discuss a the mechanics of backstay tension, tightning a sagging forestay, and bending the mast:
It's important to be able to visualize what happens when you tighten the back stay on most masthead rigs. There are two distinct phases:
Phase 1: Tighten the backstay enough to remove sag from the forestay. This reduces draft in the headsail and depowers it. After this the masthead can't move significantly aft any further because the forestay prevents it.
Phase 2: As you continue to tighten the backstay, you will compress the mast along the vertical axis, which will cause it to bend and depower the mainsail.
All masts are flexible to a greater or lesser degree, mast head rigs included. If you look at the geometry of a rig, you will see immediately that the backstays exerts compressive force on the mast as well as pulling it backwards.
When you tighten initially tighten the backstay up to the point the forestay has very little sag, you are depowering the headsail by reducing the draft.
If you continue to tighten the back stay past that point as much as possible, you are compressing the mast to bend it in the middle, which pulls the luff forward, reducing the depth of the draft out of the middle of the mainsail, which in turn depowers it.
TUNING
On a Catalina 22, first I'd set the stick straight and in the middle and no rake, with backstay off, everything else hand tight. Then I'd tune the forward lower shrouds and upper shrouds to the same tension, around 8-10% (?) of breaking strength. Next, I'd set the aft lowers visibly slack. With the backstay off, I'd check that the forestay is loose enough to deflect it by hand somewhere between 6-12" at 5' off the dock (I'm not very tall!) Next, I'd adjust the length of the aft lowers to limit how much bend I could induce with the backstay on hard (Hard is defined as up to 25% of breaking strength). I'd set the length of the aft lowers to limit bending to 50% to 100% of the mast width from front to back when the backstay was on hard. Probably 50% would be enough for the average cruising mainsail on a C22. That's a pretty good dock tune for performance cruising.
Then I'd take it out sailing in 10-12 knots of breeze with a genoa and fine tune it.
Then I'd use the backstay the way EditorC22NSA recommends above
STATIC TUNE for PREBEND
On most masthead rigs, there is usually only a very little bit of prebend tuned into the rig, if any. The stiffer the mast, the less likely we are to put prebend in a masthead rig. Static prebend, if there is any, is tuned into the Catalina mastrig rig by shortening and tightening the forward lower shrouds whilst lengthening and loosening the aft lower shrouds.
On the typical Catalina masthead rig, the forward lower and aft lower shrouds prevent mast inversion and pumping, allow the use of a lighter, less stiff mast.
I probably wouldn't tune any significant prebend into a Catalina masthead rig unless I were going to sail all the time in high winds and lumpy water. If I sailed in constant high winds and lots of waves and chop, I'd tune in an inch or less of prebend.
There are two more reasons you might want to put some prebend in the mast. One would be if the mainsail draft was too deep due to age.
The second reason to have prebend is that the sail maker built the luff with a curve in it to match a mast that had some prebend. (If there is too much luff curve compared to the bend in the mast, you will see a "knuckle" at the front just behind the luff, because A "knuckle" is tight arc running vertically, within a foot of the mast.)
I'd inform my sailmaker if there's any mast bend if I were buying a new mainsail.
DYNAMIC BEND
Dynamic bend is induced by tensioning the backstay, which primarily causes compression of the mast, which causes the mast to bend. Dynamic bend occurs only when additional backstay tension is applied after the mast is raked aft enough to tigthen the forestay. The mast head will move down a bit. It will also move aft a very tiny bit. For every inch of downward movement, the middle of the mast will bend forward about 3 inches.
In order for the middle of the mast to bend forward, the aft lower must be loose enough to permit the middle of the mast to bend. As the mast bends forward, the aft lower will tighten, limiting the amount of mast bend you can induce. Simultaneosly, the forward lower will get looser.
Judy
Edited several times to remove ambiguity.
