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Airmar leeway measuring log/depth sensor


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Another from the upcoming DAME awards. A log sensor that measures leeway. IIRC, one of the boats in the Volvo before last (?Puma?) had one, but it was a big pile of cash item designed for ROVs.

 

post-419-0-80901900-1476827886.jpg

 

http://www.metstrade.com/mets/innovation/dame-nominees/?filterId=%7BA9D07E0A-31E3-4F18-9E98-9F511A52CFD8%7D

 

DX900+ MultiLog
Airmar Technology
Industry’s first Bluetooth®-enabled, multifunctional sensor Available outputs include: Dual axis, electromagnetic speed sensor, integrated water depth, fast response temperature sensor, and three-axis accelerometer all with no moving parts. In addition, the sensor measures and outputs leeway.

 

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Not sure how many instruments would read it, but should be capable of software updates.

 

130578 - Vessel Speed Components Category: General & or Mandatory This PGN provides a single transmission that accurately describes the speed of a vessel by component vectors.
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  • 1 month later...

Asked for more info on the DX900+, but the folks at Airmar EMEA did not know anything else than that delivery will not start until 2017 and that the price is still open. As the list price of Airmar’s ST900 (that measures boat speed + water temperature) already is 733e it is likely that DX900+ will be well beyond 1000e.

According to the brochure the DX900+ measures the water flow rate both in the longitudinal and transverse directions, water temperature, depth, heel and trim. From the above parameters, the system then calculates Leeway both in knots and as an angle.

 

The ultrasonic based sensor has no moving parts, is reportedly insensitive to foiling and the flow measurement accuracy is according to the spec very good, but the question is still how well the outputs correspond to the boat's real STW and Leeway. My own view is that the DX900+ has the same basic problem as traditional rotating speedlog sensors have that is, they must be calibrated against a known boat speed and then this sensor will also be heeling dependent as the curvature of the hull in the direction of the water flow changes. And my guess is that the same effect also affects the Leeway measurement. But the question is how much?

 

Another comment regarding one of the graphs in Airmar's preliminary brochure - There are spikes in the Leeway output during tacking. In my opinion these have nothing to do with Leeway, but indicate an increase of the transverse water flow due to the turn. The length of the spikes depends thus not only on the angular velocity, but also on the location of the transducer, i.e. how far ahead of the keel the sensor is installed. So if you just want to know the Leeway these spikes should actually be removed. Note that the old rough Pendrick formula for calculating Leeway does not have the same problem.

 

So the challenge here is how to calibrate the Leeway measurement. If the sensor is installed far enough in front of the keel perhaps the yacht could be moored perpendicular to the flow in a river where the flow is measured with a reference instrument? And if so perhaps also the heeling dependence of the transverse water speed measurement could be clarified at the same time?

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At METS I was given a price 1150 €, if I remember correctly. I don't know, if that includes VAT. It's an electromagnetic sensor, not ultrasonic. It should be possible to measure leeway with an ultrasonic sensor as well.

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So the challenge here is how to calibrate the Leeway measurement. If the sensor is installed far enough in front of the keel perhaps the yacht could be moored perpendicular to the flow in a river where the flow is measured with a reference instrument? And if so perhaps also the heeling dependence of the transverse water speed measurement could be clarified at the same time?

 

Wonder sometimes if having a motorboat alongside for calibration runs might be useful. Assuming the mobo has an accurately calibrated log, you could store COG and SOG and heading and speed. Use the mobo data to calculate any tide/current and then reverse out the required calibration for leeway.

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Assuming leeway is same on each tack, (though likely it's not unless your rig is well tuned) then you can do the solution of COG & HDG by sailing at mid tide in a reasonably eddy free region and making multiple (5+) tacks after a brief period of full BSP speed on each board. Set and rate will be constants, as will leeway. Autopilot wind mode on well calibrated instruments is a help.

 

A direct vector measurement as this promises will be an interesting adjunct.

 

Question is whether the local transverse flow is representative of the whole hull?

 

Siting the sensor will be a bit more significant.

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Good observation Joakim, did not pay attention to this. Took for granted that the DX900+ was based on the same speed measurement technology as in the ST900 (which is ultrasonic).

This is actually an interesting detail - Why is Airmar using different flow sensing technologies in these two sensors? My guess is that this is because of the depth measurement, which is missing from the ST900+. It is also interesting to notice that Airmar has specified the accuracy for the speed measurements in DX900+, but not for the ST900, which could indicate challenges with the ultrasonic speed measurement.

In regard to calibration - The best would be if one could find a simple and trustworthy way of calibrating the Leeway. Collecting data from a series of tests in slack water is one way to go, but it means that only those that put the effort of logging this and doing the math will perform the calibration. And due to the Bernoulli effect the amount of transverse flow will depend on where the sensor is installed so the calibration will always be specific to the location. The question is how big the error is if no adjustment of the calibration is made? Less than 10%? If so this measurement is probably as such better than Pendrick's old Leeway formula, which by-the-way also must be calibrated as the Leeway correlation constant is boat specific.

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Had a good chat at METs with one of the engineers

 

They're really happy with the Bluetooth under testing ... should of asked in the BT is in the unit or the optional n183 to n2k box. Power draw and voltage requirements seem reasonable. It also outputs the readings from the on board IMU so that leave calibration for pitch, roll and surge open. He seemed pretty happy that cross flow wasn't a massive issue.

 

Have heard it said since that it's reading speed sufficiently outside the boundary layer but didn't have time to go back and ask.

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Angular errors will null out in calibration.

Doesn't that assume leeway is the same on both tacks? What if it's not? Asymmetric hull, keel, rudder, rig...?

Angular error of installation nulls out when motoring straight ahead. 30 sec constant speed & HDg. Leeway probably will be different, until boat tuned.

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  • 8 months later...

There are prototypes in use, results seem good. The device fits in a standard skin fitting, same as the DST, I believe. The technical story around interfaces has been a nightmare. Documentation that's available is lousy, ranging from misleading to wrong. The marketing story is even worse. It's really disappointing.

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  • 8 months later...

I have one on order to be installed with B&G electronics on a J/112E. My understanding at this point is that the speed/temp/depth info will make it to the mast displays, but the leeway data will only show up on the Airmar app running on the iPad. Like the idea of no paddlewheel, interested to see if it truly displays speed as a negative number while moving backwards!! With any luck, I'll have some real-world feedback for you by the end of June!

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Ordered mine a few months back. Bluetooth version with depth.   Had to return it as measurements were all over the place.  There was some speculation it was because of the carbon hull but that did not make too much sense to me

There we’re some posts in the topic on Panbo.com

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On 5/23/2018 at 4:48 PM, 11235 said:

Like the idea of no paddlewheel, interested to see if it truly displays speed as a negative number while moving backwards!! 

I don't have this one, but the 4500 (speed only). It senses motion by counting the bubbles as they go by (to over simply a bit) so it can't tell forward from backward. It is sensitive to motion down to tenths of a knot, when a paddlewheel would be still, so in a tidal berth you can travel quite a distance overnight (on the log) if you keep it on. 

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  • 2 months later...
  • 2 months later...
On 5/23/2018 at 6:48 PM, 11235 said:

I have one on order to be installed with B&G electronics on a J/112E. My understanding at this point is that the speed/temp/depth info will make it to the mast displays, but the leeway data will only show up on the Airmar app running on the iPad. Like the idea of no paddlewheel, interested to see if it truly displays speed as a negative number while moving backwards!! With any luck, I'll have some real-world feedback for you by the end of June!

Any feedback? Do you run Expedition by chance?

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  • 10 months later...

I installed a DX900+ this June.

At first I was having a lot of trouble, as I am integrating (new boat to me) new electronics, including a new Zeus3 with some annoying 0183 stuff like wind.  Initially the speed and depth worked great, they a connector on the DX900+ came loose and I lost data, when I hooked it up again the speed was off by a couple knots, which I had a hell of a time correcting. Turns out by ignoring it, and just working on other things, now everything seems to be working fine, including Temp, which has always said 32  degrees. at least, after it was saying 106 degrees. When I look at Network, sources, data, it shows all the stuff I thought you could only see on a bluetooth connection.   On the B&G, there doesn't seem to be, at this time anyways, any screen to show the pitch, roll, and leeway.

I do think the depth is a little more accurate than previous ultrasonic transducers, at least when in weed infested areas. I have not noticed a - showing up when in reverse.

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  • 11 months later...
On 9/19/2019 at 2:36 PM, coltrek said:

I installed a DX900+ this June.

At first I was having a lot of trouble, as I am integrating (new boat to me) new electronics, including a new Zeus3 with some annoying 0183 stuff like wind.  Initially the speed and depth worked great, they a connector on the DX900+ came loose and I lost data, when I hooked it up again the speed was off by a couple knots, which I had a hell of a time correcting. Turns out by ignoring it, and just working on other things, now everything seems to be working fine, including Temp, which has always said 32  degrees. at least, after it was saying 106 degrees. When I look at Network, sources, data, it shows all the stuff I thought you could only see on a bluetooth connection.   On the B&G, there doesn't seem to be, at this time anyways, any screen to show the pitch, roll, and leeway.

I do think the depth is a little more accurate than previous ultrasonic transducers, at least when in weed infested areas. I have not noticed a - showing up when in reverse.

On the Z3, I don't imagine the leeway shown is from the DX900+, am I wrong?  Did you compare leeway on Z3 vs Bluetooth from DX900+

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