Above all else, you must have the bridle set up so that it is center lined at all times. Having it set even a few centimeters off, in one direction or another, will seriously jeopardize your ability to sail fast upwind. Before you hoist your sails, pull the bridle forward along the centerboard. Make sure, when pulled taught, that the block on the bridle is perfectly centered in the boat.
The purpose of the bridle is to enable sailors to obtain the ideal mainsail shape, while keeping the boom in the center of the boat, as you sail upwind. As you pull in the mainsail, you not only move the boom towards the center of the boat, but you also pull downward on the boom. The further down you pull the boom, the more tension and power you put into the leech of the sail. Thus, the higher your bridle is, the earlier the blocks all come together, the less you pull down on the boom, and the less tension you are able to apply to the leech. In other words, a higher bridle will yield a center lined boom with less leech tension (more twist) on the main, and a lower traveler will yield a center lined boom with more leech tension (less twist) on the main.
In light air, you want the ability to center line the boom, keeping the flow off of the jib from interfering with the mainsail. At the same time, you want to avoid putting tension on the leech that will stall the main and slow you down. Thus, you want to raise your bridle to the point where, when the boom is center lined, your leech tell tail is streaming back 50% of the time and disappearing 50% of the time. Leave about two inches of trim between the blocks on the bridle and boom so that you can add a bit more leech tension in puffs around the course.
Tuning the bridle in medium air is dependent on how you define medium air. In a Club 420, you have the ability to trapeze, which considerably increases the wind ranges that you are able to sail in. For the purposes of this article, medium air is defined as any wind speed, other than light air speeds, where you do not have to dump the main, or you seldom have to dump the main, in order to keep the boat flat. This range can cover anywhere from 8-15+ knots, depending on the size of the sailors in the boat.
The point is, as the wind increases, up to the point where you start to get overpowered, you will want to increase the power and pointing ability in your boat. This is accomplished by trimming the main in harder, which will apply more tension to the leech of the main, the bend in the mast, and the luff of the jib. If your bridle is set too high, you will end up block to block too early, and won’t have the ability to get the desired amount of leech tension when your boom is center lined.
Ultimately, the same concept applies in both medium and light air. Your goal is to get the boom as close to center lined as possible, while applying the desired amount of tension on the leech of the main (50/50 rule on the top batten tell tale). Always leave a couple of extra inches, just in case you need a touch more trim in puffs. There is the widest range of adjustments to the bridal in this wind range.
As you start to get to the point where you are overpowered, you will want to start raising the bridle again. As you start getting overpowered, you will need to apply more vang, and dump the main to keep the boat flat. The more you dump the main, the more it will affect the slot (the gap between the main and jib). While easing is necessary, you want to ease while having as small an impact as possible on the flow over the jib. Thus, by raising the bridle, when the sail is trimmed to centerline, you will not have the ability to apply much tension to the leech. In other words, your sail will reach the centerline with more twist in it, spilling excess breeze out of it, allowing you to depower more with the boom in the ideal position. With the right amount of vang on, the boom will just go outboard from there when you ease.
How you use the bridle will depend significantly on your weight range, and the conditions. The important thing to understand is that you ideally want your boom in the center of the boat at all times, when sailing upwind. Depending on where you set the bridle, when the boom is center lined, the main will have more or less tension on the leach. The more you want the leech to twist off (i.e. windy or light winds), the higher you will need your bridle. Likewise, the more you want to take the twist out of your sail (i.e. medium winds), the lower you will want to set your bridle.
See you on the water,