Experience with Airmar DX900+?

[pjShap]

Those are all corrections for known and measured causes of measurement deviation, like the mast rotation and mast head movement. None of it corrects for a sensor that reads 11 knots when the actual apparent wind is 10 knots.

[Dockhead]

Originally Posted by pjShap

Those are all corrections for known and measured causes of measurement deviation, like the mast rotation and mast head movement. None of it corrects for a sensor that reads 11 knots when the actual apparent wind is 10 knots.

Of course. But so what? You're missing the point. There are many factors which distort the direct wind measurement. Back calculating apparent wind after all those corrections is one way to deal with that. Again, whether it's the right way to deal with that or not I don't know. But it is a thing.

[s/v Jedi]

Originally Posted by Dockhead

Of course. But so what? You're missing the point. There are many factors which distort the direct wind measurement. Back calculating apparent wind after all those corrections is one way to deal with that. Again, whether it's the right way to deal with that or not I don't know. But it is a thing.

What pjShap means is that the back calculation is fully dependent on the accuracy of the wind instrument used. If that instrument has a 5% error for windspeed, the back calculation, at best, will have more than 5% error. Same for wind angle.

[Dockhead]

Originally Posted by s/v Jedi

What pjShap means is that the back calculation is fully dependent on the accuracy of the wind instrument used. If that instrument has a 5% error for windspeed, the back calculation, at best, will have more than 5% error. Same for wind angle.

Well, this is obvious. I understood him the first time. But does not detract from the point that other errors can be corrected using the back calculation methods.

Whether it's really worthwhile or not I don't know. SailMon and at least for some time B&G really advocated back calculation of apparent wind. Ockham are vehemently against it. It's an interesting question.

Ockham argues that you don't want to correct out wind shear -- that you can't deal with that issue by normalizing AWA to deck level or whatever. The wind shear still exists. I find that argument to be quite persuasive. Wind shear is a WHOLE other problem which can't be dealt with by calibration or correction. Top racing programs deal with wind shear by having a second deck level wind transducer. Then wind shear can be measured directly.

In any case, I'm not going to be doing this on my boat (or on my friend's boat I'm managing the electronics refit for). We are trying to choose between B&G H5000 and Sailmon computers, and we will try to get the best possible corrections to the apparent wind data for heel angle, upwash, downwind acceleration, and masthead motion which we can do directly to apparent wind, but we will leave off the rest. YMMV.

So I'm not arguing in favor of back-calculating apparent wind. I'm just refuting the incorrect statement which was made, that it's pointless or doesn't exist.

[s/v Jedi]

Originally Posted by Dockhead

Well, this is obvious. I understood him the first time. But does not detract from the point that other errors can be corrected using the back calculation methods.

Whether it's really worthwhile or not I don't know. SailMon and at least for some time B&G really advocated back calculation of apparent wind. Ockham are vehemently against it. It's an interesting question.

Ockham argues that you don't want to correct out wind shear -- that you can't deal with that issue by normalizing AWA to deck level or whatever. The wind shear still exists. I find that argument to be quite persuasive. Wind shear is a WHOLE other problem which can't be dealt with by calibration or correction. Top racing programs deal with wind shear by having a second deck level wind transducer. Then wind shear can be measured directly.

In any case, I'm not going to be doing this on my boat (or on my friend's boat I'm managing the electronics refit for). We are trying to choose between B&G H5000 and Sailmon computers, and we will try to get the best possible corrections to the apparent wind data for heel angle, upwash, downwind acceleration, and masthead motion which we can do directly to apparent wind, but we will leave off the rest. YMMV.

So I'm not arguing in favor of back-calculating apparent wind. I'm just refuting the incorrect statement which was made, that it's pointless or doesn't exist.

Agreed. Assuming the sails move with the mast, this movement is a factor for apparent wind and thus should not be corrected for in case the data is used for sail trim.

Anyway, top importance imo is that the boat’s polars are used and a percentage of performance vs theoretical performance. Even when that absolute number is not very accurate, sail trim improvements are immediately obvious. Getting the absolute number correct is 2nd importance because even after optimum sail trim, overall performance can still be below optimal, for example when a different sail combination is used.

[ixnax]

Originally Posted by pjShap

I've been using the Airmar DX900+ for about 3 years now. I found calibration, last tried when I first installed the unit 3 years ago, impossible. There's a phone app that connects to the sensor's processor box via Bluetooth. The app has an auto-calibrate option, which I presume uses the phone's GPS as the calibrated speed. After the auto-calibration I saw seemingly random and totally ridiculous readings, like 1500 knots. I'm really not that fast. However, I set it back to default values, and the readings seem reasonably accurate ever since. I say reasonably accurate because I have not made the effort to test in smooth water with zero current, or run back and forth on a measured distance, etc. I have a steel boat, and after the initial purchase, when Airmar had insisted this was a great transducer for a steel boat, tech support told me they no longer recommend it for steel hulls. I rarely clean the sensor, and it continues to work. In Maine this summer, with colder water and less growth, it appeared reasonably accurate for two months without cleaning. In warmer waters it benefits from cleaning every few weeks.

Basically that was my experience, except I had it in for just 5-6 months before I hauled; it seemed about right. Prepping for bottom paint the yard guys accidentally sanded down the sensor heads. I put in a spare DST800 and planned to try the 900 again when I found super calm conditions to calibrate it but Covid, etc. etc. Maybe this year.