Height of tide and depth of water
It tells you how much water there is under the boat at a given time, and when there is enough to pass or enter.
Types of exercise
- Height of tide at a given timeYou have the times and heights of LW and HW; you want the height at a given time.
- When can you cross (time for a height)It is the reverse of height at a time: you know the water you need and look for when it is there.
- Secondary portThe almanac gives tides for standard ports only; for the others it gives differences.
- Tidal stream at a diamondThe diamond table gives, hour by hour, the set and two rates: springs and neaps.
How it works
Start from the day's table of the standard port, which gives time and height of high water (HW) and low water (LW). You need three numbers: the duration (time from LW to HW, about six hours but worked out), the range (HW − LW) and the tidal hour (duration ÷ 6). Example, Colville: LW 09:25 at 0.8 m, HW 15:31 at 4.4 m. Duration 6 h 06 min = 366 min; range 4.4 − 0.8 = 3.6 m; tidal hour 366 ÷ 6 = 61 min.
Height at a given time, with the rule of twelfths: in the six tidal hours the level moves by 1, 2, 3, 3, 2, 1 twelfths of the range, so the running totals are 1, 3, 6, 9, 11, 12. Height at 12:31: since LW 3 h 06 min = 186 min = 3.05 tidal hours have passed. At 3 hours it has risen 6/12; in the 3 minutes after, a little more: about 0.51 of the range. Height = 0.8 + 0.51 × 3.6 = 2.6 m.
Time for a given height: reverse the calculation. When does the tide rise to 3.0 m? You need 3.0 − 0.8 = 2.2 m out of 3.6, that is 2.2 ÷ 3.6 × 12 = 7.3 twelfths. The running totals say 6 at the end of the third hour and 9 at the end of the fourth: you are in the fourth hour, (7.3 − 6) ÷ 3 = 0.44 of the way through. That is 3.44 tidal hours × 61 min = 210 min = 3 h 30 min after LW. Time: 09:25 + 3:30 = 12:55.
Depth of water = charted depth + height of tide, because the chart measures from Chart Datum. With 2.4 m charted and tide 2.6 m: 5.0 m. If the seabed dries, the chart gives a drying height and you subtract: tide − drying height. A rock drying 0.5 m with tide 2.6 m: 2.1 m of water. The under keel clearance is depth of water − draught: with a draught of 1.8 m, 5.0 − 1.8 = 3.2 m.
How much tide do you need to cross a shallow spot? Water needed = draught + safety clearance: 1.8 + 0.5 = 2.3 m. Height of tide required = water needed − charted depth: with 0.5 m charted, 2.3 − 0.5 = 1.8 m. Rise from 0.8 to 1.8 m: 1.0 out of 3.6 = 3.3 twelfths, 0.11 into the third hour: 2.11 hours × 61 = 129 min, at 11:34. With the tidal curve it comes out near 11:38: a few minutes' difference is normal. If the spot dries: required = needed + drying height.
A secondary port has no tables: take the time and height of the standard port and apply the almanac differences. If the standard port time falls between two columns of the block, interpolate, never extrapolate. Example, Jackson Bay from Colville: HW 15:31 lies between 13:00 (−65 min) and 19:00 (−5 min). 151 min out of 360 have passed since 13:00: −65 + 151 ÷ 360 × 60 = −40 min, so HW at Jackson Bay is at 14:51. Heights are treated the same way, between MHWN and MHWS: 4.4 m + 0.3 m = 4.7 m. At the end, if needed, correct for daylight saving.
Common mistakes
- Getting the running twelfths wrong: the third hour adds 3/12 and takes you to 6/12, not 3/12. Write 1, 3, 6, 9, 11, 12 before you start.
- Using twelfths where the tide is irregular (for example the double high water in the Solent) or always assuming 6 hours: the duration must be worked out for the day.
- Adding the height of tide to a drying height: for a drying seabed you subtract, and a negative result means the spot is out of the water.
- Extrapolating instead of interpolating the secondary port differences, or forgetting the time zone and daylight saving: the tables are in standard time, the boat lives on local time.
- Trusting the figure to the centimetre: the rule is approximate and strong wind or pressure changes the real height (about 1 cm per hPa). Always leave a safety margin.
Exercises from the course book
9 exercises from the school's material, each solved step by step with the reason and the formula.
- Exercise book 16.8Height of tide at a given time, using the curvea) At Victoria, on Thursday 3 January at 1600 UT: what is the height of tide? b) At Colville, on Saturday 23 November at 1710 (time zone −0100): what is the height of tide? Use the port's tidal curve and say whether it is rising or falling.
- Exercise book 16.9Depth of water at a port and at low watera) On Saturday 21 September, at Victoria, at 1340 DST you are moored in 6.0 m of water. How much water will there be at the next low water? b) At Colville, on Tuesday 19 November at 0800 (time zone −0100), the charted depth of the marina is 3.1 m: how much water is there?
- Bob Exam 1When the boat floats offOn 8 October at 0800 the boat (draught 1.3 m) is aground on mud at Endal Bay, at a spot that dries between 1.4 m and 1.3 m. At low water at 0611 the height was 0.4 m and at high water at 1244 it will be 4.3 m. At what time does she float off?
- Manuale Yachtmaster p.12Height of tide at 0820 at PlymouthOn Tuesday 16 April at Plymouth high water is at 0422 UT (5.3 m) and low water at 1047 UT (0.6 m). It is springs. What is the height of tide at 0820 BST?
- Manuale Yachtmaster p.13From what time you can enter the marinaOn the evening of 21 July you want to enter Victoria Marina, where the sill dries 4.4 m. Your boat draws 1.8 m and you want 0.5 m of clearance. High water is at 2248 BST (8.1 m) and the previous low water at 1652 (2.6 m). From what time can you enter?
- Manuale Yachtmaster pp.14-17Secondary port: Braye from St HelierAt St Helier high water is at 0445 UT (9.2 m) and low water at 1050 UT (3.2 m). Find the time and height of high water at Braye and the height of low water. Braye differences for high water: standard 0300/1500 +0050, standard 0900/2100 +0040; for heights: MHWS 11.0 m −4.8; MHWN 8.1 m −3.4; MLWN 4.0 m −1.5; MLWS 1.4 m −0.5.
- Manuale Day Skipper p.22Water over a charted depth and over a drying bottomAt 1316 the height of tide is 4.6 m. On the chart you find a charted depth of 5.0 m and, a little further on, a bottom that dries 0.5 m. How much water is there over each at 1316? And at low water at 1957, when the height will be 1.8 m?
- Exercise book 8.8Clearance over the shoal under high pressureYou want to enter a small bay. The survey dates from 1908, your boat draws 1.6 m and at the entrance the chart gives soundings of 5.5 m and 2.2 m. For ten days a 25-knot onshore wind has blown and the barometer is at 1026 mb (standard pressure 1013 mb). How much clearance do you have under the keel over the 2.2 m sounding, with the tide at chart datum, before and after the effect of pressure?
- Seaman's Notes p.18How many hours before high waterHigh water at Victoria is at 1125 UT. Which row of the tidal stream table (hours relative to HW) does 0810 UT belong to?
Esercizi d'esame del corso
Gli esercizi del libro, con la curva di marea del porto: come li trovi all'esame.
- Altezza di marea a ColvilleStai pianificando l'ingresso a Colville, lunedì 9 dicembre. Usa la curva di marea per trovare l'altezza di marea a ore date e la profondità d'acqua nella marina. Gli orari del tabellone sono nel fuso di Colville (UT −0100).
- Finestra di ingresso a ColvilleLa tua barca pesca 1,8 m e vuoi almeno 0,5 m sotto la chiglia. Il punto più basso sulla via d'ingresso a Colville ha una profondità di carta di 0,5 m. Trova la finestra di tempo in cui puoi entrare il 9 dicembre.
- Incagliati: a che ora si riflottaA Colville il 9 dicembre la tua barca (pescaggio 1,8 m) si è incagliata, con la marea in calo, su un punto con profondità di carta 0,3 m. Calcola quando tornerà a galleggiare.
- Jackson Bay: porto secondario e ancoraggioVuoi dare fondo a Jackson Bay, porto secondario di Colville, lunedì 9 dicembre alle 16:30. Ricava le maree di Jackson Bay dal tabellone di Colville e dalle differenze del porto secondario, poi la profondità d'acqua che serve dove getti l'ancora.
- Corrente al diamante B e rotta da governareSulla RYA Training Chart 3 pianifichi una tratta di 6 M vicino al diamante B, tre ore dopo l'alta marea di Victoria. Ricava velocità e direzione della corrente dal diamante, poi la rotta da governare per fare una traccia sul fondo di 355° veri a 7 nodi.
- Rotta da governare, passo per passoDa A alle 21:05 vuoi raggiungere B, a 6 M di distanza su una traccia di 186° veri. La barca fa 9 nodi. La corrente (diamante K, HW+3) è 033° veri a 2,3 nodi. La variazione è 7°W. Trova la rotta da governare, vera e magnetica, la velocità sul fondo e l'ora di arrivo.