Whether you lie afloat or camp ashore it is very handy to have an idea of how much water will be beneath your keel at any given time. To find your boat aground on the ebb, or to leave it too late to get back to your mooring, can be inconvenient, to say the least. A lead line and the 1:2:3 3:2:1 rule, plus a little brain work, will give a fairly accurate estimate of the future state of the tide. But brain work, however elementary, does not come easy after a long or exhausting passage. Mistakes are easily made, leading to a complete breakdown of the schedule for the following day.

The use of a see-at-a-glance diagram has its obvious advantages. Such is the ‘Tidal Clock’.

To its disadvantage is the fact that one is necessary for every port or harbour that you may visit. It is, however, a pleasant occupation to draw one for each of your favourite havens, in your new diary, during the time that you are laid up.

Various information is still needed, of course, to use with a Tidal Clock: the time of high water, a cast of the lead, and the datum from the chart concerning your anchorage or mooring. Also, to construct the clock, you need the duration of the flood, and the ebb, plus the average height of spring and neap tides.

First, draw a vertical line, graduated equally to represent the rise in feet of ordinary spring tides, let us say fourteen feet (Figure A). At half tide, seven feet, place the point of a pair of compasses, and using seven feet on the scale as the radius, draw a circle. Next draw a circle from the height of neap tides, say ten feet, then further circles from the intermediate heights between springs and neaps. (Figure B). The right hand semicircle will represent the ebb and the left hand semicircle the flood. Assuming that the flood lasts seven hours, and the ebb five hours, we now have to divide the left and right halves of the circle into seven and five segments respectively. This is most easily done with a protractor, e.g. 90÷7 and 9O÷5.The lines now radiating from the half-tide mark are numbered from the top 1 to 7 and 1 to 5 respectively. Horizontal lines are now drawn in at right angles to the tidal gauge from each foot of height. (Figure C).

The method of using the clock will readily be seen. To find the height of the tide at any given time, take the circle that cuts the height of high water for that day, and trace the horizontal line from the appropriate hour to the tidal gauge in the centre. (e.g. Height 12 ft., 2 hours after H.W., Height of the tide 9 foot. Figure C.) Too many horizontal lines tend to add a little confusion to the clock; they can be omitted however, and any straight edge placed across the clock will represent them.

As an example of the use of the clock, assume that coming over the bar 5 hours before high water on the early spring flood, you had to lift your plate to clear the bottom. The next day you will be leaving on the ebb. To clear the bar at a similar depth, you will be wise to be there before 3½ hours after high water, as a line drawn horizontally from 5 hours on the Flood side across to the Ebb side will show you.

Again, you may know that when there is 4’ 6” at the entrance on the ebb, your favourite anchorage is just accessible before it dries out. Putting a straight edge on the clock at 4’ 6” you will see that at neaps you have until just before 4 hours after high water, and at springs only approximately 3 hours after high water, to reach your anchorage.

It goes without saying that this is still only ‘guesstimation’, especially as the predicted tides, height and time, between springs and neaps (and neaps and springs) are not always accurate. Therefore see that your forecast is made in your favour, with plenty of leeway.

And Lord help me if it does not work out for you!