What you need
- The day's high and low water times and heights for the standard port, from the tide tables.
- The port's tidal curve, which shows how the tide rises and falls between low and high water.
- The mean spring and neap ranges, printed with the curve.
It's a summer's day, so BST is in force. The tide tables for Standard Harbour, in UT, give:
| Time (UT) | Height | |
|---|---|---|
| Low water | 0418 | 1.0m |
| High water | 1042 | 4.6m |
Step 1: the range, and springs or neaps
Range = high water height − low water height = 4.6 − 1.0 = 3.6m. The curve says the mean spring range is 4.1m and the mean neap range 2.0m, so today is much nearer springs. Read mostly from the spring curve, interpolating by eye if the two curves differ much at the time you need.
Step 2: height at a given time
Question: what is the height of tide at 1310 BST?
- Convert to UT, because the tables are in UT: 1310 BST = 1210 UT.
- Work out the time relative to high water: 1210 − 1042 = HW +1h28, so about HW +1½ on the curve's time scale.
- Go up from HW +1½ to the curve and across to the factor scale. Say the curve gives a factor of 0.85.
- Height = low water + factor × range = 1.0 + 0.85 × 3.6 = 1.0 + 3.06 = 4.1m.
On the printed curve you can do the last step graphically instead: draw a line from the low water height on the bottom scale to the high water height on the top scale, then go across from the curve to that line and read the height off. It's the same sum.
Step 3: time for a given height
Question: on the morning flood, when does the height first reach 3.0m?
- Factor = (required height − low water) ÷ range = (3.0 − 1.0) ÷ 3.6 = 0.56.
- Go across from 0.56 to the rising side of the curve and down to the time scale. Say it gives HW −2h20.
- 1042 − 2h20 = 0822 UT, which is 0922 BST.
Step 4: depth and clearance under the keel
Charted depths and drying heights are measured from chart datum, so:
- Over a charted depth: depth = charted depth + height of tide.
- Over a drying height (underlined on the chart): depth = height of tide − drying height.
Question: a bar in the entrance dries 1.2m. You draw 1.8m and want 0.5m under the keel. When can you cross?
Height of tide needed = 1.2 + 1.8 + 0.5 = 3.5m. Factor = (3.5 − 1.0) ÷ 3.6 = 0.69. Say the curve reaches 0.69 at HW −1h50 on the flood and drops below it at HW +2h30 on the ebb. That's 0852 to 1312 UT, so you can cross between 0952 and 1412 BST.
Checking with the rule of twelfths
The rule of twelfths assumes the tide rises or falls 1, 2, 3, 3, 2 and 1 twelfths of its range in each of the six hours between low and high water. Here one twelfth is 3.6 ÷ 12 = 0.3m.
At HW +1h28 the tide has fallen one twelfth in the first hour (0.3m) and about half of the second hour's two twelfths (0.3m): about 0.6m. So the height is roughly 4.6 − 0.6 = 4.0m, close to the curve's 4.1m.
It's a useful sanity check, but it only works where the tide is roughly symmetrical and about six hours each way. In places with distorted tides, such as the Solent with its double high waters, it can be badly wrong. Use the port's own curve whenever you have it.
Common mistakes
- Mixing UT and BST. Do the whole calculation in UT and convert once, at the end.
- Reading the wrong side of the curve: rising for the flood, falling for the ebb.
- Using the spring curve on a neap day, or not interpolating when the range is in between.
- Adding the tide to a drying height instead of subtracting the drying height from the tide.
- Forgetting the clearance you want under the keel.
Next step: ports without their own tables. Our secondary port worked example adds the time and height differences on top of everything here.