The almanac gives high and low water. The rest you work out: the depth at any hour, the water over a sill, the room under a bridge, the stream on each leg.
1 · Springs, neaps, tables
Reading the day's tide
How do I work out the time and height of high water at a secondary port?
Apply the secondary port differences, with their signs, to the standard port's HW: interpolate the time difference by the standard port's HW time, and the height difference by today's height between neap and spring levels.
On a semi-diurnal coast high waters come about 12 h 25 min apart, so tomorrow's tide is some 50 minutes later. Springs, the biggest ranges, come a day or two after new and full moon; neaps follow the quarters, a week later. Springs grow near the equinoxes and with the moon at perigee.
Range = HW − LW of the same tide. Compare it with the mean spring and neap ranges: streams follow the range, not the height of HW. Add the zone to times in UT.
A secondary port borrows a standard port's table. Interpolate the time difference by the standard port's HW time between the tabulated times, and the height difference by today's height between neap and spring levels. Apply both with their signs.
Secondary port
Standard port HW 0200 · 5.2 m
Time diff −0010 at 0000, −0040 at 0600
0200 is 2/6 along: −0020 → HW 0140
Height: MHWN 4.6 −0.2 · MHWS 5.8 −0.6
5.2 is halfway: −0.4 → HW 4.8 m
LW the same way, then the range
2 · Chart datum and depth
How much water is really there
How do I work out the actual depth of water from the chart?
Add the height of tide to the charted depth: both are measured from chart datum, about LAT. Over an underlined drying height, the water is the height of tide minus the drying height.
Soundings and tide heights share one zero, chart datum, about lowest astronomical tide (LAT). Depth = charted depth + height of tide, so you seldom find less than charted, except on mean-sea-level charts (much of the Med). A minus height dips below datum, and an old chart reduced to MLWS over-reads at big springs. An underlined figure is a drying height: water over it = tide − drying height.
MHWS, MHWN, MLWN and MLWS are the mean spring and neap levels. At anchor, depth at low water = depth now − the fall still to come; the next low is lower as springs build. Allow for the sounder's offset and half the swell height in the troughs. Dry out on firm, level ground.
It falls 3.4 m more: 2.6 m at low water, 0.7 m under the keel.
3 · Twelfths and curves
The height between high and low
How do I use the rule of twelfths to find the height of tide?
Hour by hour from low or high water, the level moves 1, 2, 3, 3, 2, 1 twelfths of the range: the rule of twelfths. Add the rise to LW on the flood; take the fall from HW on the ebb.
Where the tide runs evenly over six hours, the rule of twelfths fills the gap: hour by hour the level moves 1, 2, 3, 3, 2, 1 twelfths of the range, fastest in the third and fourth. Add the rise to LW on the flood, take the fall from HW on the ebb; half an hour takes half that hour's share.
Hours from LW or HW
Twelfths gone
1
1/12
2
3/12
3
6/12
4
9/12
5
11/12
A stand, a double high water or an uneven rise and fall breaks the rule. Use the port's tidal curve instead: height = LW + factor × range. Read the spring or neap curve that matches today's range, or interpolate.
4 · Sills, bars, bridges
Water below, air above
How much tide do I need to get over a marina sill?
The sill's drying height + your draft + a margin. A sill drying 1.8 m, with a 2.0 m draft and 0.5 m to spare, needs a height of tide of 4.3 m; then find when the tide reaches it.
Work out the height of tide you need. Over a sill that dries: drying height + draft + margin. Over a bar: draft + margin − charted depth. Then find when the tide reaches it. On the ebb go early: at half tide the level can drop a metre in an hour.
Overhead clearances are charted above HAT, on older charts above MHWS: room now = clearance + that level − height of tide. A surge, or a high water above MHWS, eats into it. Measure to the top of the aerial.
Bridge: 14.0 m above HAT
HAT 5.6 m · tide now 2.6 m
Room now: 14.0 + 5.6 − 2.6 = 17.0 m
Mast with aerial: 15.8 m
To spare: 1.2 m
5 · Diamonds and the atlas
The stream for this hour
How do I read a tidal diamond on the chart?
A tidal diamond points to a table giving, for each hour from HW at a reference port, the stream's true set and its spring and neap rates. Interpolate the rate by today's range; each row serves half an hour either side.
A tidal diamond points to a table: for each hour from HW at a reference port, the set (where the stream goes, true) and the rates at springs and neaps. A row covers half an hour either side of its hour; after HW+6 count back from the next HW. Interpolate between springs and neaps by today's range, extend the line beyond springs, and lay off each hour of a leg in turn.
Tidal diamond C
Hour · set · springs · neaps (kn)
HW+2 · 230° · 1.6 · 0.8
HW+3 · 240° · 2.2 · 1.0
HW+4 · 245° · 1.8 · 0.8
Range today 4.0 · sp 5.6 · np 2.4 m
Midway: HW+3 gives 1.6 kn
A tidal stream atlas has a page for each hour from HW at its own reference port; 10,22 by an arrow means 1.0 kn at neaps, 2.2 kn at springs. On the chart a feathered arrow is the flood, a plain one the ebb, with the spring rate.
6 · Streams and gates
Where it runs hard, when to go
When is the best time to round a headland with a tidal race?
At slack water, when the diamond's rate falls to nothing, or pass well outside, above all at night. Off a headland the stream makes a race with overfalls, and wind against tide steepens every sea, worst at springs.
Streams run hardest in deep water: mid-channel, in narrows, outside a bend, off headlands. Inshore they are weaker and turn earlier, and a bay may hold an eddy. Against a foul stream, work the edges.
Off a headland the stream makes a race with overfalls. Wind against tide steepens every sea, worst at springs. Round at slack water, when the diamond's rate falls to nothing, or pass well outside, above all at night. Cross a river bar late on the flood, never on the ebb against a swell.
A tidal gate opens only at a certain state of the stream. Time to leave = time at the gate − distance ÷ speed over the ground. Arrive as it opens; against a weak foul stream, pushing on may still beat waiting.
7 · Weather and the Med
When the tables are wrong
How does high atmospheric pressure change the height of tide?
Predictions assume about 1013 hPa: each hectopascal above lowers the sea about a centimetre, each one below raises it. An onshore gale piles water up, an offshore one drives it out.
Predictions assume about 1013 hPa. Each hectopascal above lowers the sea about a centimetre, each one below raises it. An onshore gale piles water up, an offshore one drives it out. These are estimates: add margin for them, never spend it.
Low water on the bar
Predicted LW 0.4 m · bar charted 1.6 m
The tables give 2.0 m
Barometer 1033 hPa: about −0.2 m
Offshore gale, two days: −0.3 m more
Plan for about 1.5 m
In most of the Mediterranean the range is a few decimetres, so weather moves the level more than the tide. The exceptions: about a metre in the northern Adriatic, more when a Sirocco blows up it, and up to two metres in the Gulf of Gabès. Narrows such as Messina have streams worth timing.
Practice questions
Questions from this topic, as the app asks them. Open one to see the answer.
Almanac — moon
Today: Friday
Full moon: Sunday
Last quarter: the following Sunday
You want to dry out against the harbour wall to scrub the hull, and you need the lowest low waters of the fortnight. Which days do you pick?
Monday–Tuesday, a day or two after full moon
Friday–Saturday, just before the full moon
The following Sunday, at the last quarter
Wednesday–Thursday, as the range builds
Show the answer
A. Monday–Tuesday, a day or two after full moon
Springs, with the lowest LWs, come a day or two after new and full moon; neaps follow the quarters.
The chart shows 1.8 m where you plan to anchor, and the title panel says soundings are reduced to LAT. With ordinary weather, how often will you find less water than that?
Hardly ever — LAT is the lowest predicted tide
At every spring low water
At every low water, twice a day
At every neap low water
Show the answer
A. Hardly ever — LAT is the lowest predicted tide
Chart datum is about LAT: predicted tides almost never fall below it; only high pressure or offshore winds take it lower.
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