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|Fifty-Ninety Rule (50-90 Rule)
This is a method of estimating current speeds in between slack and peak flow that was developed at Starpath School of Navigation in the 80s.
It proposes that you can divide the time period into 3 equal intervals and then approximate that the current at the end of the first interval is 50% of the peak and then at the end of the second interval the current speed is 90% of the peak.
Since this section of the current cycle is on average 3 hours long, you can approximate this approximation by just assuming that one hour after slack the current is 50% of how strong it will get at peak and then two hours after slack it will already be 90% of the peak.
Or using this rule, recognize that the current will be within 90% of its peak value for about 1 hour either side of the peak.
In our book called Navigation Workbook 1210 Tr, there is an exercise that shows how well this rule works, but even beyond that good agreement we rarely know the actual currents well enough to try for a more rigorous mathematical estimate.