Basic Mainsail Trim for Racers, Part Two - SailNet Community

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Old 12-02-2001
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Basic Mainsail Trim for Racers, Part Two

This article was originally published on SailNet in May 2000. 


Once you've gotten the boat up to speed, particularly in light air, fine-tuning the mainsail controls for max flow attachment will help you maintain optimum pointing ability and speed upwind.
In Part One of this series, I described the trepidation I initially felt when asked to trim the mainsail aboard a new 40-footer in a regatta some time ago. After the first awkward moments of realizing that I was in over my head, I slowly started to get the hang of it and managed to get progressively more proficient with each race. Once you've got a boat up to speed, there are certain nuances regarding mainsail trim that you should be aware of to help you maintain optimal performance. Take a look at the following three scenarios and then go give them a try on the water.

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.)


Taming the mainsail in big breeze is the best way to assist the person on the helm and keep the boat performing optimally.
Maintaining Control  Sometimes the breeze builds. That's a fact. When it gets to a certain velocity, the forces acting on a mainsail may be so strong that the boat becomes overpowered. Air pressure on the main affects the balance of the boat. A perfectly balanced boat will drive upwind with almost no feel on the helm. As pressure increases on the mainsail, the boat wants to head into the wind. To counteract this tendency, which we call weather helm, the driver has to turn the rudder against the force. But too much angle on the rudder will stall the water flow and significantly slow the boat. It's not unlike having too much shape along the leech of the mainsail.

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.


Once you begin the downwind leg, it's still critical for the mainsail trimmer to pay attention and adjust the trim to suit the fluctuations in wind strength and angle.
Downwind Dynamics    Think of the mainsail downwind as a big wind blocker. The more shape the sail has, the more area that is exposed and the faster the boat will go. The simplest way to trim downwind is to ease the mainsail out until it starts to luff, and then sheet it in just until it stops luffing. Easing the outhaul will expose more area, so ease this control until the foot of the sail becomes round without wrinkles. Blow off the cunningham and the backstay, and set the vang so that the top batten looks parallel to the boom. This is all pretty simple, and fairly easy to remember, but you'd be surprised at the number of racing crew who make these adjustments by rote and never look at the sail to see the effect of the changes they're making. You may also want to let a half inch of mainsail halyard tension off to round out the luff.

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."
Each of the three preceding scenarios requires slightly different trimming techniques. However, when you're out there racing in the real world, you'll find that it is never that clear cut. You might encounter any combination of the conditions described, forcing you to rethink the set-up of your mainsail. For example, you might need to power-up right after the start, then as your breeze gets clear you find that you need to depower for a puff. Or you could be going along at your best velocity with the sails flattened slightly to maintain flow attachment and then the boat sails into a hole forcing you to power-up again.

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.


Suggested Reading:

Basic Mainsail Trim for Performance, Part One by Pete Colby 

Mainsail Controls for Performance, Part One by Dan Dickison

Mainsail Controls for Performance, Part Two by Dan Dickison

Buying Guide: Backstay Adjusters

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