Tidal calculations: UK standard and secondary ports

What you do on the bridge

1. A standard port height
2. A secondary port height
3. A secondary port time

The oral questions, and what a good answer covers

The key points are in our own words. In the course, these questions come once you have done the job.

1. What is chart datum, and what are heights on a UK chart measured from?Often asked

   A good answer covers:

   • Chart datum is about the level of Lowest Astronomical Tide on UK charts: the tide rarely falls below it
   • Charted depths are below chart datum, drying heights are above it (underlined on the chart)
   • Depth of water is the charted depth plus the height of tide
2. What causes springs and neaps, and how do you use the range on the curve?Often asked

   A good answer covers:

   • Springs come a day or two after new and full moon, when the sun and moon pull together; neaps near the quarters
   • Work out the day's range at the standard port and compare it with the spring and neap ranges
   • Use the spring curve, the neap curve, or interpolate between them, on the standard port diagram
   • Secondary port height differences are interpolated the same way, by the height of high or low water

   Show me on the bridgeOpen the tide tables at the chart table. Today is between springs and neaps.

3. What is the rule of twelfths, and when would you not trust it?

   A good answer covers:

   • A quick estimate: in the six hours of rise, 1, 2, 3, 3, 2 and 1 twelfths of the range come in each hour
   • Good enough for a check when the tide is regular and semidiurnal
   • Not where the curve is odd, such as the Solent with its double high water, or Portland and Weymouth
   • For a real clearance, use the tables and the curve for the port
4. How do you find the tidal stream at a place and time in UK waters?

   A good answer covers:

   • From the tidal diamonds on the chart: rate and direction each hour before and after HW at the reference port
   • Or from the tidal stream atlas for the area, again by hours from HW Dover or a local port
   • Interpolate between the spring and neap rates for the day's range

How the lesson runs

The tutor's narration from the lesson, step by step. On the bridge, each control lights up as it is named.

1. Brief

   Today we're at the chart table, working tides the way the UK tables lay them out. The standard port is Plymouth, with secondary ports along the Devon and Cornish coast.

   These are practice tables with our own figures, not the Admiralty Tide Tables. The method is exactly the same. Each form is checked step by step, and the check names the first step that is wrong.

2. A standard port height

   Open the tide tables. In Tidal calculation choose Plymouth, and find the height of tide at a time this afternoon. Today's range is between springs and neaps, so look at it before you read the curve.

3. A secondary port height

   Now a secondary port, say Dartmouth. Interpolate the time differences for the time of Plymouth high water, and the height differences for its height. That gives Dartmouth's own high and low water. Then the curve, as before.

4. A secondary port time

   Last, the other way round: the time the tide reaches a height you need at a secondary port. Work out the factor from the height, then read the time from high water off the curve.