Secondary ports made easy

“Let’s be honest, the easiest way to find the height of tide in one of the Solent’s many harbours is to use an app. But for those who prefer to take the more traditional approach, students on a training course or exam candidates, the good old tidal curve is needed.

If you’re visiting one of the Solent’s many anchorages or rivers then you’ll also need to do three secondary port calculations before you have the information needed to complete a tidal curve:

  1. the time of high or low water
  2. the height of high water
  3. the height of low water

Below, I explain my quick method to calculate secondary port info  … and there’s not a crocodile tooth in sight!”
Alex., Chief Instructor at HPSS

Secondary port grids

Below is a High Water Time Differences grid  that I’ve created to help interpolate the time difference for high water at the entrance to Beaulieu River (Stansore Point). The lowest charted depth when entering the river is 0.9m so an accurate tidal height calculation is critical.

Portsmouth is the Standard Port for Stansore Point so the insert is an extract from a tide table such as those found in Reeds Nautical Almanac. I’m using Sunday 9th August 2020 in these examples.

Step by step

  1. Mark the time of Portsmouth HW on Sunday 9th August (1510 UT) along the bottom axis of the High Water Time Differences grid.
  2. Draw a vertical line (red) upwards from this mark.
  3. Draw a horizontal line (blue) from the point where the vertical line and the zigzag line meet.
  4. Read the time difference from the vertical axis and round this up or down to the nearest whole minute. This is -19 minutes in the example.  Therefore the afternoon HW at the entrance to Beaulieu River on will occur at 1451 UT (1510 UT – 19 minutes).
  5. Follow the same process to find the low water time difference between Stansore Point and Portsmouth. This requires the Portsmouth low water time of either 0750 UT or 2009 UT and the Low Water Time Differences grid. The example uses the 0750 UT low water and shows that low water at Stansore Point will happen 10 minutes earlier than in Portsmouth, (0750 UT – 10 mins = 0740 UT).
  6. Use the High Water Height Differences grid and follow the same steps to calculate the difference for the height of high water. The height of HW in Portsmouth at 1510 UT is 4.3m. Mark this along the bottom axis then draw a vertical line upwards.
  7. Draw a horizontal line from where the vertical line meets the diagonal line and read the height difference from the vertical scale. In the example, the difference is – 0.6m. This means that the height of high water at Stansore Point will be 0.6m less than in Portsmouth. (4.3m – 0.6m = 3.7m)
  8. The final step is to find the Low Water Height Difference.  This one is easy as the differences grid tell us that regardless of the height of low water in Portsmouth it will be 0.3m less at Stansore Point (1.3m – 0.3m = 1.0m)

You now have the data needed to complete a tidal curve. Download copies of the tidal curves for Portsmouth and Southampton using the links below.


Download secondary port grids

A selection of pre-prepared secondary port grids in PDF format are available to download: