Secondary port tidal calculations

Tide tables give full predictions only for standard ports. For the many smaller harbours in between, called secondary ports, you get a table of differences: how much earlier or later, and how much higher or lower, the tide is than at the standard port.

The differences change with the time of day and between springs and neaps, so you usually have to interpolate between two columns. That’s where most of the marks are lost.

What you need

HW or LW time and height at the standard port
The secondary port’s time and height differences
The standard port’s mean heights (MHWS, MHWN, MLWN, MLWS)

The method, step by step

Time difference: find the two tabulated standard-port times your HW (or LW) falls between. Work out how far it is from one to the other, and take the same share of the way between their differences.
Apply the time difference, keeping its sign: a minus difference makes the secondary port earlier.
Height difference: compare the standard-port height with its spring and neap heights (MHWS and MHWN for high water, MLWN and MLWS for low water). Take the same share of the way between the spring and neap differences.
Apply the height difference, again keeping its sign.

If the standard-port height is outside the spring and neap heights, carry on at the same rate (extrapolate) rather than using the nearer column.

Tide tables are usually in UT. If you’re working in British Summer Time, convert at the end, and check the convention your course uses.

Worked example

Question 222-222P · fictional practice data

HW at Tamsin Bay	1420 5.9 m
Secondary port	Hollin Sound

Using the differences table, find the time and height of high water at Hollin Sound. Times are in the same time zone as the tables.

Tidal differences (fictional)
Times	HW	HW	LW	LW
Tamsin Bay	0400	1000	0400	1000
	1600	2200	1600	2200
Hollin Sound	+0005	+0030	+0020	+0005

Heights (m)	MHWS	MHWN	MLWN	MLWS
Tamsin Bay	7.1	5.6	2.6	1.0
Hollin Sound	+0.4	+0.3	+0.2	+0.1

Time difference +0030 + 0.72 × (+0005 − (+0030)) = +00121420 is 0.72 of the way from 1000 to 1600 (6h).
HW time at Hollin Sound 1420 + 12 min = 1432
Height difference +0.30 + 0.20 × (+0.40 − (+0.30)) = +0.32 m5.9 m is 0.20 of the way from the neap height 5.6 m to the spring height 7.1 m.
HW height at Hollin Sound 5.9 + 0.32 = 6.22 ≈ 6.2 m

Answer: Time difference for HW +0012 · HW at Hollin Sound 1432 · Height difference for HW +0.32 m · HW height at Hollin Sound 6.2 m

Common mistakes

These are the slips TideTutor catches in this exact question, in the words it uses to point you back on track:

1420 is between the two tabulated times, so interpolate between their differences rather than using the nearer one.
Check the direction of your interpolation: the closer the time is to a column, the closer the difference is to that column.
Apply the difference with its sign: a plus difference makes it later.
That's high water at the standard port. Apply the time difference.
That's the spring difference. 5.9 m isn't a spring height, so interpolate between the spring and neap differences.
Apply the height difference with its sign.

Questions people ask

What is a secondary port?

A port without full tidal predictions of its own. Its tides are found by applying time and height differences to a nearby standard port.

Do I interpolate time differences or use the nearest?

Interpolate. If HW is two-thirds of the way between the two tabulated times, take two-thirds of the way between their differences.

Which heights do I interpolate between?

For high water, between the MHWS and MHWN differences; for low water, between MLWN and MLWS, using the standard port’s height on the day.

Practise until it’s easy

TideTutor gives you endless fresh secondary ports questions like this one, with three-step hints, feedback on the classic mistakes and the full worked solution. Two questions in every topic are free.

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All ports, charts and tidal data in these guides are fictional, for practice only. Never use them for navigation.