Weather helm is a phenomenon where the force on the sails wants to turn the yacht up and into the wind. This is caused by having the Center of Effort (CE) of a sail aft of the Center of Lateral Resistance (CLR) of the underwater profile. Balancing the sails is simply the act of playing around with the CE to get it to be directly in line with the CLR.
If the CE is ahead of the CLR, the boat will have lee helm and will want to turn downwind. If the CE is aft of the CLR, the boat will have weather helm and will want to turn upwind. Only when the CE and CLR are directly over each other will the sailboat be balanced and sail straight, not venturing upwind or downwind.
Balancing the sails is not the same as trimming the sails for maximum performance. A lot of time, you will need to have your sail working at less than peak performance to properly balance out the whole setup. This is easy when you are sailing on a beam reach, where all the forces acting on the yacht are coming from the side, but what about on a run?
To be on a run, you need to ease your mainsail all the way out so that it can act as a large drag to the apparent wind and be pulled along, likewise pulling your yacht along with it. Here is where it gets tricky.
As you ease your sail out, the CE of the sail will not only move forward, which will help give you lee helm and pull your yacht downwind, but it will also move the CE laterally and outboard. This long and very powerful lever arm, also known as your boom, will act on the mast and try to turn your yacht opposite of the direction you have eased the sail. In this case, with the boom eased to port, the force on the sail will try to turn the yacht to starboard. This may feel and act like weather helm, but it is not caused by the aft movement of the CE, instead it is caused by the leverage of the mainsail held out by the boom.
The further outboard the clew of the sail is, the longer the lever arm is that is acting on the yacht, and more leverage the sail will have to turn the yacht in the opposite direction.
Your options here are simple, either you can move the clew further in or not ease the sail as far. Moving the clew inboard is effective at reducing this torquing. This can be achieved by either reefing the sail or simply not easing the clew past the beam of the boat. Obviously, these options seem counterintuitive as the apparent wind while on a run is less than the true wind; more sail would seem logical!
Instead of reefing, and dealing with the boom on a run, an alternative is to raise the trysail.
The trysail has no boom, so the foot will curl more easily when the sail is eased. As you ease the sail, the clew will tend to move directly forward from the sheeting point, and as such will stay inboard of the beam of the yacht.
The sail is also smaller but when eased will fill up and provide a great amount of drive downwind. Since the sail will curl up, the majority of the sail will be right along the mast, in the middle of the rig, further keeping the CE close to the midline of the yacht.
Being a small sail, it also won’t block the wind from reaching your headsail. This will increase the headsails effectiveness which will then aid in keeping the bow of the ship pointed downwind.
Having a trysail up on a wonderful day may not seem intuitive, but it really does help take out the weather helm while sailing downwind. This will balance your sailplan and let your yacht sail more balanced towards your downwind destination.