Tides at a diurnal port

What you do on the bridge

1. A Hon Dau height
2. A secondary port height

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 the difference between a semidiurnal, a mixed and a diurnal tide?Often asked

   A good answer covers:

   • Semidiurnal: two high and two low waters a day, of much the same height, as round the UK
   • Diurnal: one high and one low water a day, as in parts of the Gulf of Mexico and South East Asia
   • Mixed: two tides a day of very different heights, or changing between semidiurnal and diurnal through the month
   • At a diurnal port the range follows the moon's declination: largest at tropic tides, smallest at equatorial tides

   Show me on the bridgeThe tide tables for Hon Dau: one high and one low water each day.

2. How does working a tide at a diurnal port differ from a semidiurnal one?Often asked

   A good answer covers:

   • The rise and the fall each take about twelve hours, so the factor curve runs about twelve hours either side of HW
   • Use the curve published for that port, not a standard six-hour one
   • The tables give one HW and one LW a day; check which of them is the tide you want, as it may fall on the next day
   • Secondary port differences are applied the same way, by interpolation
3. What does a diurnal tide mean for planning a passage over a bar or a shallow patch?

   A good answer covers:

   • There is only one high water a day to cross on, so missing it can mean waiting a day
   • The stream may run one way for many hours, and turn only once a day
   • At equatorial tides the range is small, so there may be little gain from waiting
   • Work the tidal window carefully and allow for squat, swell and a margin

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

   Not every port has two tides a day. Hon Dau, off Hai Phong in Vietnam, has one high water and one low water a day: a diurnal tide.

   These are practice tables with our own figures. Look at the high and low waters first, then the factor curve. It runs about twelve hours either side of high water, not six.

2. A Hon Dau height

   In Tidal calculation choose Hon Dau, and find the height of tide four hours before high water. Read the factor off the Hon Dau curve.

3. A secondary port height

   Now Hon Gai. Apply the time and height differences to Hon Dau's high and low water, then use the same long curve.