By Airwaves Writer Mike Ingham
I just finished up coaching in Howth Ireland for the 2013 J24 Words, sadly having to sit out while my collar bone fuses back together. As I watched the practice race I paid particular attention to downwind tactics, and could not help but observe how well the top teams managed lanes. Last article, we spoke of some rules of thumb for downwind such as heading for pressure in light wind and taking the long gybe in heavy. To execute whatever strategy we choose, we need to understand apparent wind and how to manipulate it effectively to our advantage.
Apparent wind is the vector of the true wind and our boats motion. When we sail, the apparent wind is forward of the true wind because our motion pulls it that way. The faster we go the farther forward the apparent wind is of the true wind. To illustrate by an extreme example, America’s Cup catamarans go 1.8 times the speed of the wind downwind, so their apparent wind is almost as far forward as when they sail upwind.
Typical boat downwind sailing angles has its apparent wind forward of the true wind.
Fortunately, knowing the direction of the apparent wind is simple: we just use our mast head fly. I don’t like to use shroud, backstay or any other indicators below the mast head fly because they are influenced by the rigging.
For every condition (wind and waves), there is an optimum angle for each type of boat to get downwind fastest. We call this Velocity Made Good (VMG) mode.
-Sail a little higher than VMG and we go faster forward, but not as deep. As we do so, our apparent wind shifts forward.
-Sail lower than VMG and we go slower, but deeper, shifting our apparent wind aft
-It is not just that it changes because of the boat turns (as if you swung on an anchor), but the effect is amplified because you go faster or slower depending if you head up or bear off.
–When changing angles (either up or down) from optimum VMG, there is a slight loss of VMG so it better be worth it!
Apparent Wind Shifts forward when head up
Apparent wind shifts aft when bear off
Let’s discuss how we can move this apparent wind around to our advantage downwind:
1. If our mast head fly points to a boat less than 10 boatlenghts back, we are in dirt
a. To clear it we have 3 options, bear off, head up, or gybe until our apparent wind is free
2. It is surprising how small a change in course clears the lane
a. The mast head fly only needs to be slightly away from boat behind
b. < span style="font-size: 12pt; font-family: Calibri, sans-serif;">Small changes in course can quickly free up your air because your apparent wind changes as well as your boat positioning
The red boat on the right has its mast head fly is pointed in between the two blue boats, the air is clear.
Point apparent wind at puffs
1. Puffs come from where our apparent wind is
a. NOT true wind
2. Like clearing air, to line up with a puff, we can gybe, head up, or bear off
3. And once in the puff we can constantly re-adjust our angle (and thus apparent wind) to stay in it as long as possible.
a. It is surprising how a little heading change can change the apparent wind considerably because of the change in speed
The Green boat is smartly heading up to change their apparent wind so that the mast head fly points at a puff. Blue is missing the opportunity.
Red gibes changing the apparent wind by 90deg to line up mast head fly with puff
Calling lanes and puffs
1. Have someone on the boat in charge of apparent wind and puffs.
2. We like having someone constantly pointing:
a. The direction of our apparent wind
b. At any potential dirty air boats
c. And at puffs
3. And make changes to our apparent wind
a. By pointing it becomes crazy obvious if our apparent wind is lined up with dirt or not lined up with a puff
b. And we can change our apparent wind by changing heading to clear air or to a puff
Our tactician has specific easy things he says depending on what he wants to do. For example:
1. If he wants us to sail at optimum speed, he says “VMG mode”
2. If he wants us to go up he will say something like “up 5”
a. Meaning he wants us to sail 5 degrees higher than we are now
3. For bearing off, he says something like “down 3”
4. And I listen for urgency in his voice to decide
a. To turn at an optimum time (like “down 5” in a wave)
b. Or if I need to change course immediately to take advantage of a small window of opportunity
c. I don’t look at the compass, I just keep changing course until he is happy
Through all this, we need to sail fast. In “VMG mode” we have the spin trimmer constantly giving feedback on how high or low he wants to go for optimum speed / depth. He is telling me to bear off when he feels good pressure and to head up when he feels the wind is soft. He is essentially keeping us in the VMG mode. When we go out of the VMG mode, the tactician has already said “up 5” and I say “trim to me” so he knows not to change gears and trim to my angle instead of having me steer to his optimum.
These moves can be surprisingly powerful, but since there is always a VMG loss, they need be weighed against the gains. In the case of clearing air, if we think we will lose a boatlength because our masthead fly pointing at another boat, but do get free you will lose 2 boatlengths in VMG, then it is not worth it. Same goes for chasing puffs, if we will only make a small gain then it is not worth it. We need to know our boats well so we can make those tradeoffs. And we need to still sail as fast as possible when high or low VMG mode. But to oversimplify; if we keep track of our mast head fly, it will point us to puffs, and keep our air clear.