Current Speed Graphs

[Kelkara]

For US waters I have both Navionics and CMap chips in my plotter ... recently I was looking at some of the current data and I noticed a bit of a discrepancy...

Navionics has current speed continuously increasing/decreasing through slack-water, but CMap has in inflection. This has implications for trying to decide when your window of opportunity is for crossing a rapid.

This example demonstrates it well, although it happens at all current stations ... notice that, although both agree o the timing of slack-water, on CMap a 1kt current is predicted at 12:32, but on Navionics it is predicted at 11:33 ... a whole hour earlier! Obviously this example is only a 2kt maximum current which my boat can handle, but there are currents around in some passes that I don't want to mess with.

So which model is more correct? is there generally an inflection in the speed graph at slack water or not?

[StuM]

The also have 15-20 minutes difference in Ebb/Flood on that particular day.

The only way to tell which is most correct on that day would be to actually measure it. And that won't tell you which one will be more accurate on another day.

As for the inflection, I would guess that the CMap version is a more accurate representation of the real world. Invert the lower lobes of the Navionics version and consider what that says is happening at slack water.

[Kelkara]

Originally Posted by StuM

The also have 15-20 minutes difference in Ebb/Flood on that particular day.

eek I didn't notice that ... usually they at least agree on the times of peak/slack tide ... that would explain why the time difference was quite so large, I assumed it was the unusually steep gradient on the navionics graph causing that.

... looking up in my printed current tables it looks like Navionics is closest there ... I guess the old paper tables aren't obsolete after all.

As for the inflection, I would guess that the CMap version is a more accurate representation of the real world. Invert the lower lobes of the Navionics version and consider what that says is happening at slack water.

It seems to be saying that there is a maximum acceleration of water at slack water, and zero acceleration at peak flow, cycling twice daily ... which kinda makes sense ... so now you've made me think more about it ... I'm inclined to believe the Navionics.