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Keeping the pressure on
Efficient downwind sailing relies on constant apparent wind pressure and angle. The more stable you are, the less wind you'll need to maintain your average speed. Downwind, more than at any other point of sail, the wind can seem very unstable. This is because its strength and angle are linked to the boat's speed, which itself can fluctuate by as much as a factor of two.
To maintain a constant pressure and apparent wind angle, remember the temperance adage: "The best is the enemy of the good". It's easy to fall into a sinusoidal cycle: you luff up to find the right angle, but a little too much, then you shoot down as the trimmer tries to adapt to wind variations... By overcorrecting the angle of sail, helmsman and trimmer quickly lose their coordination, giving the impression of running after each other. What's more, a lot of extra ground is covered on the way.
Descent angle depends on wind strength
It's also important to remember that the optimum angle changes with wind speed. Experienced sailors, especially racers, know the best angles of descent for various conditions: when it's best to luff (lulls) and when it pays to bear away (reefs).
Finding your way downwind in light winds
Downwind in light to moderate winds, you can't go straight to the goal, or you'll never see the spinnaker inflate.
So you'll luff up until there's pressure in the spinnaker sheet. This will be your reference sailing angle. A glance at the windvane to memorize the apparent wind, another at the compass for the course, and for the more equipped, the electronic windvane will give them the real wind at which the spinnaker is inflating.
The helmsman will endeavor to maintain a certain stability of course and apparent wind angle despite the inevitable zones of calms and reefs that he will have to cross.
Finding your way downwind in medium
Downwind, waves come into the equation as soon as the medium wind picks up, causing the boat to surf from time to time. It's tempting to luff up to accelerate and surf while downwind. Remember, a stable course and apparent wind are the keys to downwind sailing.
It's often wiser to adopt a fairly steep course in line with the swell. You won't be sailing close to the plan, but you'll get small accelerations with each wave. These little pushes maintain the boat's speed and, consequently, the apparent wind speed. This is not a slow, downwind ride, but a compromise offering a more stable course and more constant pressure.
In a regatta, making good use of the swell will enable you to sail lower and faster than the others. And, above all, it prevents you from luffing up in front of a close competitor.
Finding your way downwind in a breeze
If the boat doesn't glide and is content with good surfing, you'll need to take advantage of the wind and waves to go downwind. When the sailboat is on top of the wave and starts to accelerate, the helmsman pulls down as the boat accelerates and the spinnaker seems to unravel. With his eye riveted on the spinnaker tear, he maintains the same apparent wind angle. Then, as the boat begins to decelerate, it's time to reef to maintain the apparent wind angle.
If the boat is fast and catches the wave in front with each surf, it's probably a good idea to luff a little to avoid tiring the rig with each deceleration. By luffing, surfs can last longer, and if the boat planes well, speed will increase VMG. In certain conditions, with very fast sailboats, this behaviour is more akin to piloting, where you look for a way through the waves to avoid crashing into holes or bumps.
