Secondary ports

Standard ports and secondary ports

In Lessons 24 and 25 you worked from the Port Kerran tide table, which gives the time and height of every high and low water, day by day. A port with its own full daily predictions is called a standard port. There are only so many of those: printing a full table for every harbour, bay and anchorage would fill a library.

Most other places are secondary ports. Their tides are found by comparing them with a nearby standard port whose tide behaves in a similar way. The almanac (or the Admiralty Tide Tables) prints a short block of differences for each one: how much later or earlier high and low water come, and how much higher or lower they are, than at the standard port. You take the standard port’s figures for the day and correct them.

On the practice chart, Kerran Roads is the anchorage on the north side of the island, about 5.6 M from Port Kerran on the other side of it. Its tides are referred to Port Kerran. Like everything on the Isle of Kerran, its data are fictional, made up for practice.

Secondary port = standard port + differences. Work out the standard port first, then correct it.

Reading the differences block

Here is the Kerran Roads entry, set out the way an almanac prints a secondary port:

Read it from the top:

• The Port Kerran row of times lists the standard-port times of high water (0000 and 0600) and low water (0100 and 0700) that the differences were worked out for. The second row (1200, 1800 and so on) is the same times twelve hours on: tides repeat roughly twice a day, so a high water at 1800 takes the same difference as one at 0600. These times are in UT, like the table.
• The Port Kerran heights are its mean levels: MHWS 5.2, MHWN 4.0, MLWN 1.6 and MLWS 0.4 m. They tell you what a typical spring or neap tide looks like there.
• The Kerran Roads row gives the differences to add. A time difference of +0010 means high water comes 10 minutes later than at Port Kerran; −0005 means low water comes 5 minutes earlier. A height difference of −0.3 means a spring high water is 0.3 m lower than at Port Kerran.

Why do the differences change? At a port with regular twice-daily tides, high water at springs comes at about the same time of day each fortnight (at Port Kerran, within a couple of hours of midnight and midday UT), and high water at neaps about six hours later. So the 0000 and 0600 columns roughly stand for springs and neaps, and the tide reaches Kerran Roads a little sooner or later depending on which it is. Heights work the same way: the difference at springs is not the same as at neaps.

Time differences: interpolate, in UT

A real tide rarely falls exactly on a printed time, so you interpolate: find the two printed times either side of the standard-port time and work out how far between them it lies. The difference is the same fraction of the way between the two printed differences.

You can do the same thing on a graph, as many navigators do: put the printed times along the bottom and the differences up the side, join the points with a straight line, and read off the difference above your time. The explorer below does exactly that.

Low water works the same way with the LW columns. On Sat 5 Jun, LW Port Kerran is at 1007 UT, between 0700 (+0015) and 1300 (−0005). It is 187 of 360 minutes along (0.52), and the difference falls by 20 minutes across the gap: +15 − 0.52 × 20 = +4.6, so +5 min. LW Kerran Roads is 1012 UT, 1112 BST.

Height differences: between neaps and springs

The height difference depends on how big the tide is. Compare the standard port’s height for the day with its mean levels: a high water is compared with MHWN and MHWS, a low water with MLWS and MLWN. Then interpolate between the matching differences, just as you did for time.

The secondary port form

With two times, two heights, two sets of differences and a time-zone change, it is easy to lose track. Training centres and the Admiralty use a printed secondary port form: a grid with columns for HW and LW times and heights and a row for each step. Here is the Sat 5 Jun example from the table to the finished answer:

Official forms also have rows for small extra corrections, such as seasonal changes in mean sea level; they are not needed for Kerran Roads. Now try one yourself.

Heights in between: using the curve

The form gives you high and low water at Kerran Roads. For the height at any other time, use the Port Kerran tidal curve from Lesson 25, but with the Kerran Roads figures:

1. Count the time from HW Kerran Roads, not HW Port Kerran.
2. Draw the range line from the Kerran Roads HW height (top scale) to its LW height (bottom scale).
3. Compare the Kerran Roads range with the mean ranges (springs 4.8 m, neaps 2.4 m) to choose between the curves.

Putting it to work: anchoring in Kerran Roads

Suppose you arrive in Kerran Roads at 0813 BST on Sat 5 Jun and plan to anchor on the 6.5 m sounding just south of the anchor symbol. As Lesson 8 showed, the depth you actually have is the charted depth plus the height of tide:

With a draught of 1.8 m there is plenty of water under the keel at low water. The depth at high water matters too: it decides how much chain or warp you need to let out, so work out both before you drop the anchor. If you stay overnight, check the next low water as well, since it may be lower.