Secondary ports to exam standard
Scope: RYA Coastal Skipper/Yachtmaster Offshore theory: tidal knowledge. Builds on: Day Skipper Theory L2.1–L2.2.
The layout of secondary port data
Almanacs give secondary port differences against a named standard port, in a block like this (exercise data):
| HW times | LW times | MHWS | MHWN | MLWN | MLWS | |||
|---|---|---|---|---|---|---|---|---|
| Standard port | 0000, 1200 | 0600, 1800 | 0500, 1700 | 1100, 2300 | 4.7 | 3.8 | 1.9 | 0.8 |
| Secondary port differences | −0010 | +0020 | −0005 | +0015 | −0.3 | −0.2 | −0.1 | 0.0 |
The differences only apply exactly when the standard port tide falls at those times or heights. Otherwise you interpolate.
Worked example (exercise data)
Standard port today: HW 0336 UT 4.4 m, LW 0948 UT 1.2 m.
Times
| Working | Result | |
|---|---|---|
| HW position | 0336 is 216 of 360 minutes between 0000 and 0600 = 0.6 | |
| HW difference | −10 + 0.6 × (20 − (−10)) = −10 + 18 | +0008 |
| Secondary HW | 0336 + 8 | 0344 UT |
| LW position | 0948 is 288 of 360 minutes between 0500 and 1100 = 0.8 | |
| LW difference | −5 + 0.8 × 20 | +0011 |
| Secondary LW | 0948 + 11 | 0959 UT |
Heights
| Working | Result | |
|---|---|---|
| HW position | (4.4 − 3.8) ÷ (4.7 − 3.8) = 0.67 of the way to MHWS | |
| HW difference | −0.2 + 0.67 × (−0.1) = −0.27 | |
| Secondary HW | 4.4 − 0.27 | 4.1 m |
| LW position | (1.2 − 0.8) ÷ (1.9 − 0.8) = 0.36 of the way to MLWN | |
| LW difference | 0.0 + 0.36 × (−0.1) = −0.04 | |
| Secondary LW | 1.2 − 0.04 | 1.2 m |
Secondary port range: 4.1 − 1.2 = 2.9 m.
Using the result
Put the secondary port HW time and heights on the standard port curve and work as usual. Then answer the real question.
Can a 2.2 m-draught yacht cross a bar drying 0.6 m with 1.0 m under the keel, and until when?
- Height needed: 0.6 + 2.2 + 1.0 = 3.8 m.
- Factor: (3.8 − 1.2) ÷ 2.9 = 0.90, on the falling side after HW.
- From the curve, read the time for factor 0.90 after HW.
- Rule of twelfths check: the first hour after HW falls 1/12 × 2.9 = 0.24 m, to 3.86 m. The remaining 0.06 m at the second hour's rate (2/12 × 2.9 = 0.48 m per hour) takes about 7 minutes. Latest time about HW + 1 h 07 = 0451 UT.
CAUTION — Twelfths are only a check This tide runs 6 h 15 min from HW to LW, not six hours, and real curves are not symmetrical. The examiner expects the curve; use twelfths only to catch a gross error.
TIP — Label everything Write "UT" beside every time. Add the hour for BST once, at the end: 0451 UT is 0551 BST.
The yacht angle
Bars, marina sills and shallow harbour entrances around the UK, Brittany and the Channel Islands limit when a yacht of 2–3 m draught can move. The mate who can produce a clean secondary port calculation before breakfast gets listened to.