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True or Theoretical Wind for a Trawler

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As i continue the search for a quality anemometer with NMEA2000 and no moving parts.... i find the ones that supply simple apparent wind are significantly cheaper than models that add theoretical wind (that subtracts out the movement of your boat).

For example with Airmar to get the latter, it appears to jump in price by about $300, which is significant if i don't really need it. In a trawler where we spend most of our time doing 8 to 10 kts, i'm wondering if I would use the extra feature. If i'm very slowly approaching a new marina deciding how to get to the slip, it seems likely apparent wind speed and direction would be fine for planning purposes.

The other time i'd use it; say i'm crossing lake Huron and a squall pops up and i just want to monitor how bad its getting.... For those that have one of these on a trawler, can you comment on whether the fancier model is really useful?

Ben Ellison
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Joined: 6 years ago
Posts: 172

Hi David, The Airmar is unusual (and cool), but you probably don't need a sensor that outputs "theoretical wind." In fact, you likely have something on your boat that can already compute true True Wind (aka Ground Wind) if it receives Apparent Wind speed and direction. Many NMEA 2000 instrument and multifunction displays can do it, and some autopilots. And in my view, that's what you really want. Besides the docking scenario you mention, keeping your eye on real world wind speed and direction despite whichever way you're going 10 knots can alert you to weather changes you might not otherwise sense.

Incidentally, if you get into it, you'll learn that in the marine world racing sailors passionately insist that True Wind is simply AWA/AWS corrected for boat speed which ignores set and drift. So some instruments use Ground Wind for true True Wind -- corrected for SOG/COG -- and others let you choose between STW and SOG for True. More discussion here:  

This post was modified 4 years ago by Ben Ellison

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Thanks Ben that makes sense. We’ve got Garmin 5200 series plotters and a GMI 10 and at least a couple GPS antennas on the net so I bet you’re right.