Look up the tides at the standard port and refer to the secondary port difference table:
Avonmouth tides and Watchet differences, October 25th. At 1121 there was a HW of 9.4 metres at Avonmouth. Find the HW Height at Watchet:
Draw two lines at any angle you like; measure out the scales for difference values and height; connect the corresponding pair of values at the open end of the diagram; draw a parallel line and make your interpolation. (or extrapolation as in this case.) As a matter of interest I worked out the spring tide difference at Watchet one week later:
In each case the scale is set up to allow for the construction of the working parallel line. Neaps or Springs the Watchet low water difference value is always +0.1m so no interpolation required.
A satisfactory interpolation can quickly be achieved as follows: use the almanac to identify a ‘spring tide day’ within the week in which our day is located. There is a large range of tide on this day and it occurs within a day or two of the new moon or the full moon. The phases of the moon will be shown in the almanac. Count the number of days between spring tide day and the day in question. For each of these days, the HW difference value changes by a seventh of the total of the difference between springs and neaps. Thus work out the difference value on the day in question. Adjust the standard port high water height by this difference value to get the secondary port height.
The Watchet times:
Time difference graphs are always interpolations. The change in the difference value is directly proportional to the amount of time transferred between the columns in the table. In the example below it takes (14:00 - 08:00) = 6 hours for the difference value to change between -50 minutes and -35 minutes, a change of +15 minutes in total or of +15/6 minutes per hour as the clock advances from 08:00 to 14:00.
UKHO Easytide predicted:
The graphs work; the solemn procedure is a bit much to ask when out for a jolly cruise, in my honest opinion!
Secondary Port Worked Example