Saturday, August 6, 2011

Kanaha Wind Sensors

Warning: this entire post is kind of pointless. If you're not a geek, I suggest you don't bother reading it :)
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During the past 5 weeks here in Maui, we have often wondered about the Kanaha wind sensor on iWindsurf/Windalert. Sometimes, it seemed to understand the wind by 5-10 miles; sometimes, it seemed accurate; and every now and then, the wind sensor readings seemed to high.

Yesterday was a day where the sensor readings were both too low and too high within a 3-hour period. This is what happened on the water:

  • 3:20 pm - 3:40 pm: First few runs on a 4.5 m sail, 77 l board combo. Very windy, I had to sail partially sheeted in quite often, which means gusts were above 40 mph.
  • 3:50 pm - 4:30 pm: Next set of runs - more comfortable, but still fully powered, at first, then the wind dropped. Towards the end of the 40 minutes, I did not plane the whole time anymore (nor was anyone else, except perhaps the racers on slalom gear and big sails).
  • 4:30 pm - 5:10 pm: The wind had dropped a lot, and almost everyone came off the water. Around 5, the wind picked up again a bit.
  • 5:10 pm - 5:45 pm: I switched to  bigger board (93 l FSW) with a big (30 cm) fin. I needed both the bigger board and some push from the waves to get planing. Towards the end, the wind dropped a little, so planing consistently became harder and harder.
Here's the Kanaha wind sensor readings for the day:
A few things here are very wrong:
  1. The drop shown is just 5 mph, and even less in the gusts.
  2. The graph shows that the wind supposedly picked up around 5 pm, and then was stronger than before.
That's not what happened on the water. The drop was at least 10 mph, and the wind after 5 pm was significantly weaker than earlier during the day, especially in the gusts. This is exactly what the (free) airport wind sensor shows:

The averages look a bit too low on the airport sensor, but the gusts look about right (perhaps a few miles low). What's easy to see on the airport sensor graph is that the wind direction shifted a bit, from ENE to NE. The Kanaha sensor is located "on the outer breakwater of Kahului Hbr", more than 2 miles from the Upper's launch spot. That's about half way to where the West Maui mountains start. Easterly winds would run straight into the mountains, so we'd expect a windward wind shadow. As the winds turn more northerly, the mountains redirect the wind (so that sailors in Kihei get the strong north winds). It seems that this can generate winds that are significantly stronger in the harbor than a couple of miles upwind at Uppers. 

Even sailing straight out at Uppers, there's quite a bit of local variation in the wind. Of course, there's less wind close to the shore, where the trees block the wind a bit. But at some days, there's also a very noticeable drop in wind strength when you sail out between half a mile and a mile; other days, the wind seems to keep going up the further out you go. Keeps life interesting...

So, as many locals have known for a long time, the free airport sensor appears to more accurate than the Kanaha sensor - just make sure to add a few miles to the averages, or go by the gusts. The one drawback it has is that the readings are updated just once an hour. If you'd rather use the Kanaha sensor, keep in mind that it probably will be low for ENE directions, and may be high for NE directions. But then, who uses the sensors here, anyway? In the summer, the question usually is whether to rig 4.2 or 4.7, with 5+ m sails reserved for the "low wind" days. Since I don't own a 4.2 here, the question for me is between 4.5 and 5.0; I've used the 5.3 and 5.7 sails here only 8 of the 32 days we've sailed. To pick between these to sails is usually easy enough, and any "mistakes" can easily be fixed with a bit of efficiency or high-wind technique. Well, I warned you that the post is pointless, did I not?

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