Jim, you’re in command of a brand new passenger liner, on a great circle passage from Bermuda to the English Channel Light Vessel. The distance is 2,800 nautical miles. Your cruising sea trial speed is registered as 22 knots, in dead calm weather and no current whatsoever. The general average Gulf Stream current along the route is estimated at 2 knots by Pilot Book and the Weather Routeing agency forecasts light winds all the way.
Once on Full Away of Sea Passage, the perfectly rigged patent log starts reading, the Doppler Log Water Tracking push button is depressed and the GPS Distance Run Over the Ground is activated, so that all instruments starts reading and recording the distances run through the water and over the ground at the exact same time.
At End of Sea Passage, all instruments arrived at the same time or after 117 hours and are compared. Since everything went according to the voyage plan, the difference in distance between the patent log and the Doppler log water tracking is nada zero zilch! But compared to the GPS, it seems that something went wrong. The recorded distance over the ground is effectively 2,800nm but the distance run through the water from the patent log and the Doppler log is only 2,567nm or a difference of minus 233nm against the GPS.
On your way back, in the exact same conditions and route, the GPS still shows 2,800nm. The difference between the patent log and the Doppler log water tracking distances are still nada zero zilch. But now, it took 140 hours to cross and the distances recorded by both water logs are 3,080nm or plus 280nm against the GPS.
Eastbound Log (2,567nm) + GPS Difference (233nm) = 2,800nm
Westbound Log (3,080nm) - GPS Difference (280nm) = 2,800nm
The least we can say is that these results are absolutely astonishing, aren’t-they!!! How can that be possible?
Now Jim, do the same exercise only westbound but in that existing chaotic current system, while crashing into a few gales low along the run and under an overcast sky all the way through and tell me how you are going to manage to reach Saint George’s Cut by magnetic compass and by patent log alone. Please be advised that there are many hundreds of shipwrecks all around Bermuda Island … might is well you provide the lookout with a good set of glasses!
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