By the time you arrive at the lake your choices of how to tune your boat are limited to about five actions. Most other possibilities are resolved during construction of the boat and cannot be changed easily. Such things as fin placement, bulb placement, balance point, radio board placement are already set. These should have been covered in the design of the boat. The five major points left to consider in tuning for conditions at the lake are:
1. Position of mast on the deck.
2. Flatness of the sails.
3. Rake of mast.
4. Control of twist to control heeling and weather helm.
5. Use of a storm rig.
These points will be discussed below but first a few definitions:
Mast rack - the multiple position fitting that holds the base of the mast.
Weather helm - the tendency of the boat to turn up into the wind by itself.
Lee helm - the tendency of the boat to turn downwind by itself.
Twist - when the top 1/4 to 1/3 of the sail falls off to leeward (downwind) at an angle to the foot of the sail.
Boom vang - the fitting going from the main sail boom to the mast. It keeps the foot of the sail from lifting.
CE - Center of Effort is the point from which the total effort of the sails can be considered as acting.
CLR - Center of Lateral Resistance is the center point of resistance of the boat's underwater area.
1. Mast position: If the design was done properly there is basically only a minor adjustment to be made by moving the mast. If the boat is properly balanced it should have a very minimal weather helm or no helm at all. In big sailboats a slight weather helm is a safety factor. In any case you certainly have adjustments to make based on wind speed changes. In very near dead winds an even helm will keep your boat moving in the direction you want much more readily than slight weather or lee helm. In 0 to 1 mph winds a slight helm (either lee or weather) can cause the boat to turn off by itself since you need water moving past the rudder to control the boat. With even helm any thrust is more readily converted into forward movement, not turning movement. As the wind picks up you find that weather helm creeps in rapidly with increasing wind speed because of the heeling of the boat. When this happens the first thing you do is flatten the sails followed by moving the mast forward on your hopefully multiple position mast rack. This is true even if you started with even helm. Because of the heeling and other dynamic factors, weather helm naturally increases as wind speed increases. Moving the mast forward tends to counteract this effect by moving the CE forward towards the CLR. Usually no more than about 1/2-inch should be required. Beyond this other factors come into play, such as increasing twist which will be discussed in step 4. Another factor that comes into consideration is that a slight weather helm can improve pointing ability when on a beat.
2. The second most basic tuning adjustment is the flatness or fullness of the sails (both main and jib). In light air you want as much drive as possible from you sails, which are the engine of the boat. To get the drive you put bag in the sails. The more bag, the more drive. Light air needs all the help it can get. As the winds pick up you need less drive because you start to get all the wind you can handle. This changes up to where you have very nearly flat sails in high winds simply because otherwise the boat is so overpowered that it cannot do anything but lay flat in the water and wait for you to learn what I am trying to tell you. Another factor that comes into play in extremely light winds is that the wind does not have enough energy to get around the bag in the sails so you need to go to flatter sails, matching those of high winds. The actual amount of bag for the differing wind conditions can be generally stated but is to some extent unique to each boat. This is why you need to practice with different amounts of bag in similar wind conditions. This also assumes that you have built your boat with all the proper adjustable outhauls, downhauls and uphauls, etc. If you are not prepared to put in this practice, be prepared to always be in the middle of the pack, at best.
3. A third tuning factor that can be used for fine-tuning is the rake of the mast. Normally boats are designed to have either a small rake back (to the stern) or to be straight up. Having less backward rake will decrease weather helm (the same as moving the mast forward on the mast step). This is a change that you want to be very careful with. Normally, you want to make sure that you do not have forward rake of the mast unless absolutely necessary to control weather helm.
4. Another controlling factor that comes into play in very high winds is to increase the twist of the mainsail. Increased twist lets the upper part of the sail fall off to leeward, thus slipping wind out of the main to maintain the forward motion necessary to win races, instead of losing it to heeling. Twist is controlled primarily by the boom vang. Loosening it will let the main sail achieves greater twist.
5. Finally, if the class boat you are sailing allow is it, plan on having a storm rig for your boat. This is a complete rig with reduced sail area. For heavy winds, this should be the primary tuning factor of choice, combined with necessary variation of the other four factors.
If you put all of these to use properly, you will find that you are finishing ahead of many people that formerly were sailing to the dock while you were still out on the course racing. John Emery is a good example of someone who is pursuing all this in a logical and planned manner with his self-developed tuning sheet. All of you have noticed that he is the one who is on the end of the dock changing his tuning setup, sometimes after every race. Maybe in the next newsletter he will write up what he has been doing.