J 109 Leeway or wind corrections anyone?

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Dan Corcoran
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Joined: Sun Jan 29, 2017 4:32 pm

J 109 Leeway or wind corrections anyone?

Post by Dan Corcoran »

Would anyone have corrections to offer when setting up instrument calibrations?

I am probably doing the wrong thing changing calibrations mid-season, but my Ockam system is in it's death throes, and I understand none of the wind calibrations are worth keeping as the B&G corrects for true wind and then back calculates apparent wind, and the blue signet boat speed sensor which replaces the red one, is more linear.
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Vento Solare
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Location: Newport, RI

Re: J 109 Leeway or wind corrections anyone?

Post by Vento Solare »

Dan - I believe your True Wind and Apparent Wind analysis is backwards. The Apparent Wind is sent without any additional sensor inputs. True Wind is vector math and uses either speed through the water & Heading or COG/SOG. More accurate systems also have the ability to correct for leeway and updraft.

Go for good sensor output first and calibrate before doing other calculations. I have my speed sensor mounted on the centerline to minimize tack-to-tack errors. I also have the Nexus high performancer compass with roll and pitch. The Nexus server has the ability to do higher level calibration. An alternative is to use an external program such as Expedition to take the raw sensor data and have it do the calibrations and corrections.
Dan Corcoran
Posts: 280
Joined: Sun Jan 29, 2017 4:32 pm

Re: J 109 Leeway or wind corrections anyone?

Post by Dan Corcoran »

Sounds crazy, but's it's different on B&G and some other instrument systems. What we consider AWA & AWS, they consider "measured data", they go through corrections to get to TWA & TWS, and then they use vector math to get AWA & AWS.

The basic flow is:
Raw data
Apply MHU Offset, Mast Rotation
Apply gyro corrections
Apply heading, speed
Apply shear & upwash
Obtain TWS, TWA, TWD
Derive AWA, AWS

Dan
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Vento Solare
Posts: 394
Joined: Wed Dec 31, 1969 8:00 pm
Location: Newport, RI

Re: J 109 Leeway or wind corrections anyone?

Post by Vento Solare »

Dan,

If that's the way B&G does it, the approach is wrong. I'm sure that B&G linearizes the sensors so that the angular and speed data is corrected for sensor errors. I know that Nexus does this. Measured wind would get this correction to yield apparent wind. Applying heading and speed for TWS and then backing them out to obtain AWS introduces errors if everything isn't precisely calibrated. Riddle me this - what do your wind instruments display if you disable the heading and speed? The answer is AWA. Turn off the heading and speed and see how accurate the AWA is. The best way to calibrate AWA is with no wind, drive under power and the wind angle should read zero, assuming there are no currents. The speed through the water should equal the apparent wind speed (assuming no currents). Once you have those reading correctly, you can factor in heading and speed through the water to get TWS. On a J/109 you have no mast rotation correction so unless that is zero, you have a problem. Other corrections for leeway and upwash are applied, but you need accurate pitch and heel sensors and either a table of data, or derive the data for the table using test runs. It is a long process.

If you run Expedition you can have it take the raw sensor data and do all the calibration and calculations, then send that data to the displays.
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