Old DataMarine instrument …does it speak NMEA0183?

Davidasailor26 · Jul 30, 2023

Unlikely that you can do anything to get the data from the Datamarine. That transducer is probably just a dumb hash effect sensor or something like that. All of the “thinking” is probably done inside the display to translate the pulses from the paddle wheel into a speed.

kappykaplan · Jul 30, 2023

I get STW from my Datamarine instrument into my Raymarine CP for TWS.

jviss · Jul 30, 2023

Manuals for S110KL here:
Index of /boat/manuals/Datamarine/

kappykaplan · Jul 30, 2023

Tally Ho said:
How do you get the data from the DM device to your RM CP? That is exactly what I am trying to do…do you utilize NMEA0183 or some other protocol?

I've got a converter of some sort installed in the pod for the wind instrument. I'll try to remember to check next time I'm at the boat.

jviss · Jul 31, 2023

The DM doesn't have an NMEA183 output. The only thing I can imagine is a microcontroller connected to the remote counter output, calculating STW periodically, and transmitting it to an NMEA183 port. Cool Arduino project. <- @Davidasailor26

[The remote counter output on the S100KL puts out a pulse every 0.1 mile (I'm assuming nautical mile). A simple program to time the duration between pulses and scale would give 'instantaneous' knots, then just format and transmit the NMEA183 sentence. I'm pretty sure you can get an off-the-shelf Arduino with the necessary interfaces built in, as NMEA183 uses RS-232 and/or RS-422 to drive, i.e., for talkers. Either interface will work with typical opto-isolated listeners.]

LloydB · Jul 31, 2023

Required STW doesn't make sense because if you were adrift in a 4 knot current your speed through water would be 0 but the true wind could be 8 knots off depending upon which way it was blowing and the direction you are drifting. I think of true wind as being the speed and direction of the wind over the surface of the earth.

jssailem · Jul 31, 2023

You might try to examine the Raymarine ITC5. The unit fires the transducer, takes the transducer signal and will put it out onto SeatalkNG

kappykaplan · Jul 31, 2023

jssailem said:
Raymarine ITC5

I'll be at my boat on Fri and will determine what I have + photos.

jviss · Aug 1, 2023

LloydB said:
Required STW doesn't make sense because if you were adrift in a 4 knot current your speed through water would be 0 but the true wind could be 8 knots off depending upon which way it was blowing and the direction you are drifting. I think of true wind as being the speed and direction of the wind over the surface of the earth.

It is true that it doesn't make complete sense, but that seems to be the way most "consumer" systems do it these days. A common definition is that:

"The True Wind is actually the sum of two other wind vectors:

The Ground Wind: is the wind that is measured relative to a fixed point on the ground (a weather station in the harbour, the instruments of a boat at anchor etc).
The Tide or Current Wind: is the wind created by the motion of the water relative to the land."

CURRENTS & TACTICS II – THE WIND TRIANGLE

I think it's just that the current wind is notoriously difficult to measure or otherwise determine in a way that integrates into instruments, or at least it seems so. Given that most currents people encounter are relatively slow, with the exception of rivers and the Gulf Stream, as a first level of approximation STW works. At least that's my analysis, FWIW.

Davidasailor26 · Aug 1, 2023

Using STW is the "proper" way for measuring TWS from a performance perspective. STW accounts for currents which is what you need to do if you want to track how you're trimmed and if you're hitting your performance targets.

Imagine (to use a simple example) your polars say that going dead downwind in 2 knots of wind you should go 1.5 knots. Now imagine you're pointed down river with a 2 knot current, and the ground wind speed is also 2 knots in the same direction as the current. Since there is no wind over the surface of the water the boat will just drift in the 2 knot current, showing STW of 0, SOG of 2, and apparent wind of 0. If you're using SOG for the TWS calculation your instruments will say that since you're going 2 knots with 0 apparent wind the TWS must be 2. So you look at your polars for 2 knots downwind and assume you should be going 1.5 knots. Then you look at your STW of 0 and SOG of 2 and think neither of those match the polars, so something must be wrong and in need of tuning.

That's a contrived example and you could probably figure out on the water what's happening and how to compensate, but it demonstrates the type of confusion you can cause using SOG/GWS for performance monitoring. If you use STW/TWS you're always getting good performance measurements for the boat.

jviss · Aug 1, 2023

Davidasailor26 said:
it demonstrates the type of confusion you can cause using SOG/GWS for performance monitoring. If you use STW/TWS you're always getting good performance measurements for the boat.

Did you mean SOG/GWS, or perhaps SOG/TWS? If so, what's GWS?

It is, indeed, confusing. But then, the polars can't always be correct. For equal and opposite wind of the ground and current wind, the TWS will always be zero, regardless of the magnitude of the wind. So, You might be going 2, 3, 4 kt., with zero TWS, and the polars can't explain that. Isn't that so?

Davidasailor26 · Aug 1, 2023

jviss said:
Did you mean SOG/GWS, or perhaps SOG/TWS? If so, what's GWS?

It is, indeed, confusing. But then, the polars can't always be correct. For equal and opposite wind of the ground and current wind, the TWS will always be zero, regardless of the magnitude of the wind. So, You might be going 2, 3, 4 kt., with zero TWS, and the polars can't explain that. Isn't that so?

GWS is Ground Wind Speed - the actual speed of the wind over the unmoving ground, as opposed to TWS as the speed of the wind over the surface of the water.

The polars will always be correct (assuming you sail to your polars) if you’re measuring boat speed through the water and true wind speed relative to the surface of the water. If you have zero TWS that means you have no wind over the surface of the water, and your boat will not be moving (relative to the surface of the water). The boat will be moving with the current (speed over ground), but from the standpoint of measuring boat performance you don’t care about that - it’s not actionable for tuning or trimming the boat.

jviss · Aug 1, 2023

Davidasailor26 said:
GWS is Ground Wind Speed - the actual speed of the wind over the unmoving ground, as opposed to TWS as the speed of the wind over the surface of the water.

The polars will always be correct (assuming you sail to your polars) if you’re measuring boat speed through the water and true wind speed relative to the surface of the water. If you have zero TWS that means you have no wind over the surface of the water, and your boat will not be moving (relative to the surface of the water). The boat will be moving with the current (speed over ground), but from the standpoint of measuring boat performance you don’t care about that - it’s not actionable for tuning or trimming the boat.

Ah, I see. It depends on how you define TWS. The definition I'm familiar with isn't the same as yours. You're saying wind speed relative to the water's surface. I subscribe to the definition that TWS is the vector sum of wind over the ground and current wind (I quoted this in a previous reply):

"The True Wind is actually the sum of two other wind vectors:

The Ground Wind: is the wind that is measured relative to a fixed point on the ground (a weather station in the harbour, the instruments of a boat at anchor etc).
The Tide or Current Wind: is the wind created by the motion of the water relative to the land."

CURRENTS & TACTICS II – THE WIND TRIANGLE

Davidasailor26 · Aug 1, 2023

jviss said:
Ah, I see. It depends on how you define TWS. The definition I'm familiar with isn't the same as yours. You're saying wind speed relative to the water's surface. I subscribe to the definition that TWS is the vector sum of wind over the ground and current wind (I quoted this in a previous reply):

"The True Wind is actually the sum of two other wind vectors:

The Ground Wind: is the wind that is measured relative to a fixed point on the ground (a weather station in the harbour, the instruments of a boat at anchor etc).

The Tide or Current Wind: is the wind created by the motion of the water relative to the land."

CURRENTS & TACTICS II – THE WIND TRIANGLE

I think that is my definition of TWS. The first bullet is what I’ve seen called GWS. When you add in the current you’re measuring wind speed relative to the surface of the water.

jviss · Aug 1, 2023

Davidasailor26 said:
I think that is my definition of TWS. The first bullet is what I’ve seen called GWS. When you add in the current you’re measuring wind speed relative to the surface of the water.

Hmmm, I'll have to think about that.

jviss · Aug 2, 2023

jviss said:
The DM doesn't have an NMEA183 output. The only thing I can imagine is a microcontroller connected to the remote counter output, calculating STW periodically, and transmitting it to an NMEA183 port. Cool Arduino project. <- @Davidasailor26

[The remote counter output on the S100KL puts out a pulse every 0.1 mile (I'm assuming nautical mile). A simple program to time the duration between pulses and scale would give 'instantaneous' knots, then just format and transmit the NMEA183 sentence. I'm pretty sure you can get an off-the-shelf Arduino with the necessary interfaces built in, as NMEA183 uses RS-232 and/or RS-422 to drive, i.e., for talkers. Either interface will work with typical opto-isolated listeners.]

Updating my post on an Arduino adapter for this. The 0.1 mile pulse is going to result in a very slow update rate, like once per minute at 6 knots, once every 6 minutes at 1 knot, and so forth. Not very useful. I was thinking, with the schematic of the S100KL it's probably possible to tap the pulses from the transducer, which should be much, much more frequent! This would require an "invasive" adapter, but I'm sure it's possible.

Old DataMarine instrument …does it speak NMEA0183?

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SBO Weather and Forecasting Forum Jim & John

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