The most common tide calculations such as the exact interpolation method have already been discussed in a previous lesson about tidal calculations. In this online lesson we will go a few steps further.

## Calculate time of high or low water in a secondary port

- Find the time of HW or LW of the Standard port. You can see from the arrow if you have to browse forward or backwards to find the standard port. This means browsing to Standard Port -> and this means browsing back to Standard Port <-
- Determine the time difference using the times / differences table.
- Add the time difference to the time in the Standard Port.

For example: In the almanac you find this table at the Secondary Port Scheveningen (Primary Port is Vlissingen):

**Times**

High Water Low Water

0300 0900 0400 1000

1500 2100 1600 2200

**Differences SCHEVENINGEN**

+0105 +0100 +0220 +0245

- If the time of high water in the Standard Port Vlissingen occurs at 3:00 am or at 3:00 pm, it is 1:05 am later high water in Scheveningen. So at 04:05 and 16:05.
- If the time of high water in the standard port of Vlissingen occurs at 09:00 or at 21:00, it will be high water in Scheveningen at 01:00. 10:00 or 22:00.
- If the time of high water in the Standard Port Vlissingen takes place at 06:00 or at 18:00, it will be 01:02:30 later high water in Scheveningen. 06:00 is exactly between 03:00 and 09:00 and therefore interpolated between 01.05 and 01.00.

This interpolation is sometimes difficult and is best done by drawing a graph with the timeline on the x axis and the correction time on the y axis. Note that the correction time can also be negative, in which case you put the correction times on the negative Y axis. Keep in mind that this is actually very similar to linear interpolation as 1/7 rule is used to calculate height of tide between spring and neap tide. In this case we are actually talking about a 1/6 rule, because there are always 6 hours between the values we use.

## Calculate height of tide at high or low water in a secondary port

- Look up the height of tide in the Standard Port.
- Determine the difference in the Secondary Port using the table below that can be found at the Secondary Port in the Nautical Almanac.

**Heights (meters)**

MHWS MHWN MHLN MLWS

4.7 3.8 0.8 0.2

- 2.6 -2.1 -0.6 0.0

- Height of tide in the Secondary Port = Height of tide in the Standard Port + Difference:

a. If the height of tide in the Standard Port at high water is 4.7 meters, then the height of tide in the Secondary Port is 2.6 meters lower, ie 2.1.

b. If the height of tide in the Standard Port at high water is 3.8 meters, then the height of tide in the Secondary Port is 2.1 meters lower, ie 1.7.

c. If the height of tide is between 4.7 and 3.8, you have to interpolate (or if it is outside, for example 4.9, extrapolate). Draw a graph similar to the previous example, with the time on the x-axis and the difference on the y-axis.

## Case 2: Secondary Port

At 10.50 a ship runs aground while it is in a secondary port. The question is: When will the ship come loose again? The date falls exactly between spring tide and neaps tide. The tide table of the corresponding Standard port states the following:

Time m

08.30 4.1

15.01 1.0

21.16 4.4

The secondary port states the following:

Times | Heights | (meters) | |||||

High | Water | Low | Water | MHWS | MHWN | MLWN | MLWS |

0100 | 0700 | 0100 | 0700 | 4,8 | 3,9 | 1,4 | 0,5 |

1300 | 1900 | 1300 | 1900 | ||||

Differ. | |||||||

-0105 | -0005 | -0035 | -0035 | +0,4 | +0,2 | +0,2 | +0,2 |

So we solve this by answering these two questions:

- What was the height of tide when the ship run aground?
- When will that height of tide occure again, so that the ship starts floating again?

We will of course solve these two questions by applying the graphical exact interpolation method twice, as explained in the previous Coastal Navigation course. However, in order to draw the tide graph, we must first calculate the height of tide and time of HW and LW in the secondary port, by applying the corrections to the standard port data. We do this in one of the following ways:

### Approach 1: Estimate / draw

To make a good estimate, it is best to make a drawing on a square of paper, as below.### Approach 2: Use the Secondary Port Tidal Prediction form

### Approach 3: Calculate

Calculate the height of tide when the ship run araound.

HW Time

- 0005 - (1,5 hr / 6 hr) x 60 min = - 0020

LW Time

- 0035 + (2 hr / 6 hr) x 30 min = - 0025

HW Heights

+0,2 + 2/9 x 0,2 = + 0,24

LW Heights

1 + 0,2 = + 1,2

Calculate the differences to determine when the same height of tide occures again so that the ship starts floating again.

HW Times

-0105 - 2,25/6 x 60 = - 27,5

HW Heights

0,2 + 5/9 x 0,2 = 0,3

## Computation of rates

This method assumes that the rate in the whole of NW Europe varies with the range in Dover. That in itself makes sense, because if the range in Dover is large, then there will be a strong tidal rate too. Moreover, if the range in Dover is large, the range in the rest of Europe will also be large. The computation of rates is a more accurate way than interpolation if you only know the tidal rates at spring tide and neap tide. You can find that rate in the nautical chart or from the HP33. Note: 1020 means 2.0 knots at spring tide and 1.0 knots at neap tide.

1. Draw a vertical line from the spring tide flow rate to the red horizontal Spring tide line.

2. Draw a vertical line from the neap tide rate to the blue horizontal neap tide line.

3. Draw a diagonal line between the intersections of the vertical lines and the horizontal lines of spring and neap tide.

4. Calculate the average range for Dover for the given day.

5. Draw a horizontal line from the mean range in Dover on the vertical axis to the diagonal line

6. Draw a vertical line to read the rate from one of the horizontal axes.

Note that the horizontal line of the day in question will almost always be below the red spring tide line and above the blue neaps tide line, unless it is an extreme situation.