# Relative velocity America’s Cup style

By Marcus Wilson 19/09/2013

Well, I have to get in an Auld Mug post before the end of the event. Today's cancellation gives me an extra day to do it.

Watching the coverage of the boats for the first time was very interesting for me. First of all, it was alarming the speeds that they got to. This is far from the sailing that I've done, in inefficient dinghies that might hit 5 knots on a broad reach in a good breeze (and that's scary stuff). And second, not unrelated to the first, is the position of the 'sail' ('wing' I think is a much more accurate description of what the main sponsors stick their logo over), particularly on the downwind legs. I'm used to letting the sail out when heading downwind. Watch these things and the sail position is barely changed from what it is on the beat (upwind leg). And not a spinnaker in sight.

It's to do with the relative wind. There might be 20 knots of wind, coming from a direction somewhat towards your stern, but that is not what is going to be felt by those on the boat if it's shifting at 35 knots. What's important is the wind relative to the boat. In maths terms, that's the velocity of the wind, minus the velocity of the boat. It's a vector calculation – both wind and boat speed have a direction as well as a size. The effect is that the wind as experienced on board moves towards the bow. And if you're doing 35 knots, it will move a long way towards the bow. (On the rough diagram below, that's the blue dotted line.)  That changes your sail setting quite considerably (and renders a spinnaker entirely pointless – it would just be a large brake).

It also means that the wind shadow (the dirty air) is in a place you might not immediately expect. The boat most upwind, despite being closest to the wind, may actually be in the shadow of the downwind boat. All rather tricky at these speeds.