Working a secondary port
A secondary port is predicted by applying tabulated time and height differences to its associated standard port. The differences for high and low water times, and for the heights at springs and neaps, are listed in the almanac; the navigator interpolates them for the actual range on the day, applies them to the standard port's predictions, and then works the height of tide as normal. The method looks fiddly but is just careful arithmetic, and it is essential because the standard ports are few while the harbours a skipper visits are many.
Finding the height of tide at a secondary port
- Look up the day's high and low water times and heights at the standard port.
- Take the secondary-port time and height differences from the almanac and interpolate them for the day's range.
- Apply the differences to the standard-port times and heights to get the secondary-port predictions.
- Work the height of tide at the required time using the tidal curve or the Rule of Twelfths.
WARNING — Interpolate for the day's range Secondary-port differences are given for springs and neaps and must be interpolated for the actual range on the day. Using the spring figure on a neap day, or vice versa, throws the answer out and can put the boat aground.
TIP — Answer the two real questions Tidal work exists to answer two practical questions: what depth is there at a given time, and at what time will I have the depth I need? Frame every calculation as one of those and it stays purposeful.