how to display local currents?

[seandepagnier]

Maybe this is obvious but I don't find a way to display current currents.

What I mean by this, is using gps speed over ground, and comparing it to water speed sensor and compass.

It would help to have leeway or water speed vector, but still interesting and useful with just waterspeed.

For this to work the compass must be accurate, but how to display the current over ground?

[nuku34]

I am sure there are expensive systems that would compare gps heading and speed as a vector with compass and speed through the water as a vector and display the difference. But that would not automatically tell you speed and direction of any current that is affecting your progress.



Be aware that the difference between the vectors might be due not only to current but also to leeway especially if you are on a yacht. Some yachts make a lot of leeway! Leeway and current may be in different directions, so it gets pretty complex!



That said, you can set a waypoint on your chart plotter (or navigation software such as opencpn) and then look at the cross track error over time. Cross track error is the distance you are away, at a right angle, from a straight line between your waypoint and your original starting point. Its a basic function of any electronic charting system with gps input.



So, if you have sailed for an hour and you are now 0.5nm to the right (or left) from your original track to the waypoint, you can assume that something (current and maybe leeway) is pushing you to the right or left off course. Even if you are motoring in a flat calm, cross track error is still only the cumulative effect of the speed and direction of some current. That doesn't prove that the direction of current is at 90 degrees to your course. A stronger current at a 45 degree angle to your course could have the same effect.



If you are on a sailboat and on any course but dead downwind, cross track error will be a combination of leeway and current, (generally the more wind the more leeway), and the direction of current will probably be different from the direction of leeway. So the effect of both will be a sum of their vectors.

Unfortunately there's no easy (automatic) way to separate the 2 influences. If you had some idea of how much leeway your particular boat makes in various conditions with no current, then you could do a manual correction to the cross track error.



Sorry for the complexity of my answer, but you are really talking about a complex situation.
Good luck!

[goboatingnow]

Originally Posted by seandepagnier

Maybe this is obvious but I don't find a way to display current currents.



What I mean by this, is using gps speed over ground, and comparing it to water speed sensor and compass.



It would help to have leeway or water speed vector, but still interesting and useful with just waterspeed.



For this to work the compass must be accurate, but how to display the current over ground?

Measuring the current in what context , sea river , lake, under sail , motor .

In lakes etc. You can use the difference between STW and SOG to indicate current as a scalar

Determining the vector is basically very difficult as you have to remove wind leeway and other influences

[seandepagnier]

Originally Posted by nuku34

I am sure there are expensive systems that would compare gps heading and speed as a vector with compass and speed through the water as a vector and display the difference. But that would not automatically tell you speed and direction of any current that is affecting your progress.

Unless your waterspeed sensor measures leeway as well. This is all really just vector scaling and subtraction, no need for an "expensive system" It should be part of OpenCPN and I am halfway surprised it isn't or unsure if I overlooked where it is or which plugin provides it. At least dashboard could.

Now, the accuracy of the current prediction would be related to the accuracy of the waterspeed and compass sensors. The good thing is at least with my 3d printed water speed sensors I implement auto-calibration from gps when you send it gps (you are meant to disable output of gps to the sensor unless you know there are no currents) and the compass also auto-calibrates. Not sure how much leeway will throw it off, I have not done testing just yet, but I can imagine this will affect calibration.

Be aware that the difference between the vectors might be due not only to current but also to leeway especially if you are on a yacht. Some yachts make a lot of leeway! Leeway and current may be in different directions, so it gets pretty complex!

I am still pondering how to measure leeway. I have seen some electromagnetic transducers and also ultrasonic that can do it, but question the accuracy unless carefully calibrated over many conditions because of how the hull affects water flow. It is bad enough with paddle wheel needing calibration at every speed and heel (not to mention sea state) especially depending on its location.

So not really sure how to measure leeway, perhaps an underwater "vane" could work?? For example point ahead normally, but pointing behind if the boat is dragging anchor. I will look into trying this. It will of course be difficult/impossible to get perfectly accurate readings especially in different sea states but it probably would give a useful indication and with enough data it could be calibrated to be more accurate as well.

The interesting thing is in 2017 the LWY sentence was added to nmea0183 to represent leeway! Still waiting on sentence for 3d wind...

If you are on a sailboat and on any course but dead downwind, cross track error will be a combination of leeway and current, (generally the more wind the more leeway), and the Good luck!

Actually.. it also depends on the angle relative to the current. It is only leeway and current if you sail exactly 90 degrees to the current.

So really any measurement you make from "cross track error" is a product of:
1) current speed
2) current direction
3) leeway
4) water speed
5) bearing (direction boat is actually facing)

So that is 2 measurements from gps (ground speed and cross track error) with 5 unknowns.

With compass and water speed (including leeway) there are 2 measurements and 2 unknowns (only currents) which can be calculated and displayed.

It may be possible to build a leeway table, but really difficult as changing sail trim (or sails) or dagger board setting as well as just standing on a different part of the boat would all change it. So better to measure as much as possible in real time.

[goboatingnow]

I cant see a reliable consistent way of measuring leeway.

Without leeway calculations , you can’t resolve a current vector.

I’m not sure why any would would want accurate current measurements. SOG versus STW is enough to estimate currents to an accuracy sufficient for navigation decisions.

[seandepagnier]

Originally Posted by goboatingnow

I cant see a reliable consistent way of measuring leeway.

Without leeway calculations , you can’t resolve a current vector.

I’m not sure why any would would want accurate current measurements. SOG versus STW is enough to estimate currents to an accuracy sufficient for navigation decisions.

Sure. So first to display estimated currents either assuming no leeway or predicting leeway from a table.

I don't see why you don't think leeway can be measured with a sensor. The same way a wind vane measures wind direction, a water vane at the bottom of the keel could measure leeway.

[seandepagnier]

I found the tactics plugin it can display the currents and everything else I was looking for.

[rgleason]

This discussion is dealt with and explained quite well in tactics both the plugin and the documentation.
There are also definitions and good vector charts.