Unless noted otherwise, color coded speed scales are in meters/second. Multiply by 2 (1.94) to get knots.

Friday, January 25, 2013

Friday Jan 25 00z currents + Note on the forecasts

Here are currents along with boat track to date (have to look carefully for the track off Dakar). We are still a long way from major current influences... according to this RTOFS data.  Keep in mind that the 0h forecast is as much an assimilated picture of known knowledge as it is a forecast, but because there are no sensors in this near region it does remain a model prediction.

I have just received a very good question about the current data, namely: do we trust the current forecast data yet?

The answer has to be: very sorry to say... but NO.

This is all wishful thinking!!

Many tests of this model for the Gulf Stream region have been excellent, but i think they also tweak the model to work well there.  So we are in NEW territory, which frankly to me is  one of the exciting parts of this voyage.  

At present we are not yet automatically collecting the knotmeter speed from the vessel, so we do not have automated current computations yet.

To figure the current while rowing, we need S from knotmeter, H from heading sensor, and COG and SOG from the GPS.  The set and drift of the current is then just the vector that connects those two vectors.  If those two are the same, then the current is 0.

We always have cog and sog. We can figure that simply from the timed position plots, but if we ever lose either the S or the H, then we have to change tactics.

We lose the convenience of reading the currents live from a spread sheet, and have to ask the crew to stop periodically at a watch change and just drift for a few minutes to report back to us the cog and sog.  When drifting (without leeway) the cog and sog are the set and drift.

We need wind data during that measurement, however, so we can estimate any leeway that might be in place.

Thus without S and H if we see them heading anywhere near a strong eddy we will ask for the tests.

also, of course, if they discover they are going nowhere in seemingly good conditions, we can most likely took at the current prediction to see what eddies might be in the neighborhood and from the shape of these  we can most likely direct them to the best way out of it.  Or if we see prominent favorable current on the horizon, we can do the best to direct them toward it.

For now, though, we just wait till things settle down and all instrumentation is working, and watch to see how things evolve.

--david





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