This article was originally published on SailNet in May 2000.
Walking the Fine Line Once a boat gets up to speed, its apparent-wind velocity will increase and move forward, reducing the angle of attack of the mainsail. As water flow accelerates over the keel and the rudder, these appendages produce more lift and this allows the boat to point higher, which further decreases the angle of attack. So, to maintain the optimal angle for performance, you'll need to sheet the mainsail in more. And since you probably started with the sail shape set for power, you'll need to make some adjustments there too.
Faster-moving airflow has difficulty staying attached to a curve. Think of driving a car. You can do 100 mph in a straight line with no problem, but try turning a corner at that speed and you'll probably lose control and separate from the road. Similarly, the more pronounced the curve of the mainsail, the harder it is for the airflow to turn. So once the boat is up to speed, and you have sheeted in to increase the angle of attack, you'll need to flatten the overall shape to avoid stalling. Start by pulling on the outhaul so the bottom of the sail is flat, which reduces drag. Then pull on some backstay (if you have an adjustable backstay—some stock boats don't). Backstay tension will bend the mast tip aft, causing the middle of the mast to poke forward and consequently pull the sail flat.
When you get the boat up to speed, fine-tuning the mainsail controls for maximum flow attachment will help you maintain the optimum height and speed. The boat and sails will alert you when you are at this point. As you accelerate, and the breeze moves forward, the telltales on the inside of the jib will begin popping up or breaking, and the front of the main will begin to luff. This is your signal to pull in the mainsheet. If the boat starts to heel too much, there may be too much shape or power in the sail, so ease the traveler a bit to help reduce helm. Then pull on a little backstay tension. Always look up when you do this and you will see the overall shape of the sail get flatter. You may also need to pull a little more sheet on to compensate for the top of the main becoming too open. And consider the cunningham adjustment. As you pull the backstay on, the draft moves aft. You can use the cunningham to tighten the luff and pull the draft back forward where you want it. Remember, your backstay adjustment will ikely have an impact on the headsail (whether you're working with a fractional or a masthead rig) by changing the the headstay tension, so the person trimming that sail needs to be informed of the changes. (We'll deal with the interaction between the two sails in a future article.)
In heavy air, your job is to depower the sail effectively to help control the amount of weather helm the driver experiences. The desired mainsail shape in heavy air is flat with a twist in the upper leech to spill excessive pressure out of the sail. Of course you don't want to overdo it, so depower incrementally. As the breeze comes on, steadily flatten the sail. First pull on some outhaul, then pull on a little backstay, and then pull the mainsheet to find the correct tension for the upper leech. You may need to add some cunningham tension to pull the draft forward and flatten the leech. If the breeze is puffy, you'll want to play the traveler, dropping it down in response to the puffs (just as the boat starts to heel) and pulling it back up when the puff subsides.
At some point, you'll find yourself with the traveler all the way down, and the backstay on as tight as you can get it, yet the boat will still be overpowered. This is when you begin to vang-sheet. Vang-sheeting involves pulling the vang on as tight as you can get it to minimize the vertical movement of the boom before you begin playing the mainsheet. Ease the sheet in the puffs, which twists open the top of the sail, and then trim it in the lulls. This helps spill excess power off the upper leech while keeping the lower section working. The goal of depowering the mainsail is to spill off just enough power so the boat is not out of control, but not so much that you completely stall the sail—it's a lot like Homer Simpson's job at the nuclear power plant.
Bear in mind that the wind will continuously change in velocity and apparent angle and the boat will be steered at different angles, so you'll have to pay attention the whole time you are sailing downwind. Often, while racing downwind, I look around to call puffs for the driver and spinnaker trimmer. This is good information for them, but it takes my concentration away from trimming the main. Inevitably, the driver will change the direction of the boat or the breeze direction will change and I won't catch it. Then the mainsail starts to luff and sure enough someone will yell "Colby, Colby, watch your main trim!" So don't fall asleep when you turn downwind. Also, make sure that you take the time to organize your lines so that when you jibe, or turn upwind, you won't be the source of any foul-ups.
|"The key to successful mainsail trim on the racecourse is quickly recognizing what the boat needs and then trimming accordingly."|
Of course there is another key factor that I purposely haven't mentioned, which is how the mainsail interacts with the jib or genoa, but that's something we'll get into in a future article. For now, just remember that the key to successful mainsail trim on the racecourse is quickly recognizing what the boat needs and then trimming the main to achieve the desired results. Keep talking to the driver and keep looking around at the sails, your boat speed, and the competition. It's always a constant battle, but it's always educational and usually fun. Now go out there and get sailing. Good luck.
Buying Guide: Backstay Adjusters
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