Have a look at this GPS track from one of my windsurfing sessions last summer:
This is how almost everyone windsurfs at Kalmus (and many other spots): back and force, back and force, and then some more back and force. If we plot the speeds against the angle to the wind in a "polar plot", here's what we get:
The filled are shows the maximum speed at a given angle to the wind; the inner blue line shows the average speed (ignoring speeds below 5 knots). The plot includes speeds while turning, which confuses the picture a bit. Here's what it looks like if we ignore all turns, and look only at speed when going in a straight line:
In GPS Speedreader, we can also look at how often we surfed at any given angle to the wind by selecting "Frequency" in the pulldown menu at the bottom:
This tells a pretty boring story: almost always on a beam reach in one direction, and pinching upwind just a little bit when going the other direction. The highest upwind angle is just around 30 degrees (60 degrees relative to the wind).
For comparison, let's look at the tracks from a windfoil session:
The tracks are spread out a little more, which is easier to see in the polar plot:
The maximum upwind angles are about 15 degrees higher than with a fin. While speeds drop quickly when going upwind on the fin, the speed drops on the foil are much smaller. This was a session in about 11 to 13 knots of wind, gusting to 15 knots, with a 6.5 m sail, a Slingshot Infinity 84 foil, and a Fanatic Stingray 140. The i84 foil is known to not go upwind as well as the smaller Infinity 76. I was reasonably well powered, but not really fully powered; a bit more wind would have allowed a few more degrees against the wind. But even with this setup and in the lighter wind, going upwind was totally effortless - much easier than with a fin.
Freeride foils are known to be easy to use and can be tons of run, but they are slower than race foils, and do not go upwind nearly as well. I don't have a race foil, but after reading on the Seabreeze forum about someone who had just gotten a race setup, I downloaded the track from ka72.com, and had a closer look. Here's the polar plot:
That's quite a different story! The speeds are comparable to my windsurf session, but both upwind and downwind angles are a lot better. Where I had a hard time pointing 30 degrees into the wind, the race foil reached almost 60 degrees, while barely slowing down relative to the beam reach speed. That enabled the foiler to take a nice excursion around some big sand banks at his spot:
At least for me, this would have been impossible to do with a fin - maybe on a longboard with a huge daggerboard, but even that would have required a lot more tacks.
The "frequency" polar plot for the race foil also looks very different from the one with the fin:
The distribution is much wider than in the fin plot further up, especially on the left side (starboard) - meaning he did not go upwind or downwind a couple of times, but rather most of the time. More options, more fun! Maybe I need to get one of these race foils...
