remery said..
One of my other polar plots didn't make sense at all, it was as if the wind was coming from a different direction. I wonder if the difference is OAO or SPB files.

I'll have a look at the track, the wind direction is very predictable at that spot in summer.

It is very unlikely to be a difference in the file format. Both should have the same Course over Ground and location numbers

I think it's possible, proximity to shore changes wind direction a tad?

sailquik said..

decrepit said..
I think it's possible, proximity to shore changes wind direction a tad?

I think this is a well known thing Mike.The sailing instruction and training literature I have read, refer to this a lot. And it can be quite significant.

I think you will find the same thing explained in Meteorology texts as well. I remember a meteorologist friend explaining to me that the 'surface roughness' of land and sea dramatically affect wind angle to the isobar lines close to the surface. If I remember correctly, he said over a smoother surface like the ocean, the winds will blow almost parallel to the isobars, whereas over land they diverge much more in the direction of the pressure gradient.

It is used to significant advantage in sailing races. On an upwind course you're looking for the tack where gusts are "lifting". Otherwise called a header, the wind veer allows you to sail a course closer to the upwind mark. The opposing tack, gusts will "knock" you further off breeze and force you to take a course further from the direction of a mark. So much so that if the knock is severe enough, it is enough cause to expend the loss of speed and tack back onto the favoured direction.

the opposite is true for downwind courses. A knock will allow you to make a direction change further downwind, giving you a better angle towards the downwind mark. You notice the effect much more when on a craft like a skiff or windfoil that can travel faster than the wind, but not by much. In this case turning further off breeze is more a part of surviving the gust than anything else. I think us windsurfers travel a bit too quick off breeze to feel just how much it can play on direction of travel, that is, when you get hit by a knocking gust you could point downwind a bit further, but you don't have to. You don't get knocked flat by it if you don't.

I've watched Slowie sailing off breeze before plenty of times, I think to a degree he does actually just point lower than most though.

decrepit said..
I think it's possible, proximity to shore changes wind direction a tad?

Good point, I know when pinching upwind at that spot, it is easier as I move away from the shore. That's probably what gives me the feeling I'm going dead downwind.