Secondary ports: time and height differences
1The lesson in plain words
A secondary port has no daily predictions of its own. You take a standard port's HW and LW and apply interpolated time and height differences.
Standard and secondary
A standard port has full daily predictions of high water (HW) and low water (LW). A secondary port has only a table of differences from a named standard port. That port is chosen because its tide has a similar shape, so it is not always the nearest. The table has four parts: HW times, LW times, HW heights and LW heights. In both worked examples, seasonal corrections are zero and all times use the tables’ printed zone.
Time differences
The table gives a difference for two standard-port HW times, such as 0000 and 0600, each repeating 12 hours later, and the same for LW. When today's standard HW falls between them, interpolate: work out how far it lies between the two times as a fraction, and take the same fraction between the two differences. A difference of −0110 means 1 h 10 min earlier, not 110 minutes.
Height differences
Heights work the same way. HW differences are given at MHWS and MHWN (mean high water springs and neaps), LW differences at MLWN (mean low water neaps) and MLWS (mean low water springs). Find where today's standard-port height lies between the two mean levels and take the same fraction between their differences. On a tide bigger than mean springs, extend the same straight line with care. Round the answers to 0.1 m and the nearest minute.
Worked HW example
Standard HW is 0900, 4.6 m. HW time differences are −0040 at 0600 and −0020 at 1200, repeating 12 h later. 0900 lies halfway, so the difference is −0030: 0830. Height differences are −0.4 m at MHWS 5.0 m and −0.2 m at MHWN 4.0 m. The height fraction is (4.6 − 4.0) ÷ (5.0 − 4.0) = 0.6. The correction is −0.32 m: HW height = 4.28 m, rounded to 4.3 m.
Worked LW example
Standard LW is 1530, 1.0 m. LW time differences are −0015 at 0100 and −0035 at 0700, repeating 12 h later. 1530 is 2.5/6 from 1300 to 1900, so the difference rounds to −0023: 1507. Height differences are −0.1 m at MLWN 1.8 m and 0.0 m at MLWS 0.6 m. Height fraction = (1.0 − 0.6) ÷ (1.8 − 0.6) = 1/3. Correction = −0.033 m: LW height = 0.967 m, rounded to 1.0 m.
Seasonal changes and time
Mean sea level varies slightly through the year. The Admiralty standard-port predictions already include this seasonal change, but the differences do not. For best accuracy, remove the standard port's seasonal change, apply the differences, then add the secondary port's; around the UK it is rarely more than 0.1 m. Calculate in the printed time zone. Convert to local civil time at the end, adding summer time only if the published times exclude it.
2Key data
- Differences from
- A standard port with a similar tide, named in the table
- Time differences
- Interpolate by today's standard-port HW or LW time
- Height differences
- Interpolate between MHWS/MHWN or MLWN/MLWS
- Seasonal change
- Remove the standard port's, then add the secondary port's
- Worked answer
- HW 0830 4.3 m, LW 1507 1.0 m
- Spanish terms
- Puerto patrón, puerto secundario, diferencias de hora y altura
3Key points
- 1
A secondary port is worked from a standard port with a similar tide, not always the nearest.
- 2
Interpolate time differences by today's HW or LW time at the standard port.
- 3
Interpolate height differences between the mean spring and neap levels.
- 4
Use the printed time zone; convert to local time once, only if needed.
4Frequent mistakes
Wrong: Use the nearest tabulated difference without interpolating.
Right: Interpolate. A standard HW halfway between the two tabulated times takes the difference halfway between theirs.
Wrong: Read −0110 as 110 minutes earlier.
Right: Differences are hours and minutes: −0110 is 1 h 10 min earlier.
Wrong: Apply the HW height difference to the LW height as well.
Right: HW and LW have their own height differences, interpolated against their own mean levels.
5Practice case & exam keys
All times are UTC; seasonal corrections are zero. Standard HW is 0300, 5.0 m. HW differences are −0020 at 0000 and −0040 at 0600; at MHWS 5.0 m the height difference is −0.4 m. You need HW of at least 4.7 m before 0300. Does this tide meet both limits?
Show the answer
0300 lies halfway between 0000 and 0600, so the time difference is −0030. Secondary HW is 0230, meeting your time limit. The standard height equals MHWS, so apply −0.4 m: HW is 4.6 m above chart datum. This fails your 4.7 m height requirement, so the tide does not meet both limits. These answers remain in UTC.
Exam keys
- Secondary port = standard port HW/LW + interpolated time and height differences.
- Time differences are interpolated by time, height differences by the height between mean spring and neap levels.
- −0110 means 1 h 10 min earlier.
- Seasonal changes: remove the standard port's, add the secondary port's.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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