The most important difference between inland and coastal sailing is the tide, which can affect almost every part of your sailing. It can dictate where and when you get afloat, where you can go and how long it will take you to get there. When related to wind direction, it can even dictate how rough or uncomfortable the passage will be.

The practice of navigation is complicated by the presence of tidal effects and ocean currents. These effects, which may be favorable or unfavorable, tend to deflect the yacht from its charted course and reduce or increase its speed. A comparison of dead-reckoning positions and fixes reveals the extent of such effects and often helps the navigator to predict and adjust for future influences.

Tides are caused by the gravitational attraction of the Earth, Moon and Sun, together with the rotation of the Earth. The Sun has only about a third of the influence of the Moon on the tides. When Moon and Sun pull together the very high and low tides called Spring Tides are produced. When they work against each other the smaller Neap Tides result.

Times and heights of high and low water at Standard Ports are found in Admiralty Tide Tables and nautical almanacs, but both of these publications are far more comprehensive than the typical sailor requires. Local Tide Tables available from yacht chandlers, sailing clubs or the Harbor master provide the simple information required for any single place.

Because the water level is constantly changing due to the tide, a standard reference point is needed for charts. The point taken for a Chart Datum approximates to the lowest expected level of the tide, known as the Lowest Astronomical Tide or LAT.

As you can see from the diagram, the heights of high and low water and most of the other points are referred to this datum. The difference in height between each high water and the next low water (or vice versa) is known as the Range of that tide.

If you have proper tide tables or an almanac, there is a simple graphical way of working out intermediate tidal heights and times. If not, the rule of twelfths provides a rough guide.

Based on the assumption that the tide follows a symmetrical flow pattern during a 12-hour cycle between successive high waters, the rule nevertheless provides enough accuracy for most sailors.

During the 1st hour after HW, the range is 1/12 of the total
During the 2nd hour after HW, the range is 2/12 of the total
During the 3rd hour after HW, the range is 3/12 of the total
During the 4th hour after HW, the range is 3/12 of the total
During the 5th hour after HW, the range is 2/12 of the total
During the 6th hour after HW, the range is 1/12 of the total.

To use the rule, you only need to know the time and height of either high or low water together with the range for that tide.

Working Example
HW time is 1400; HW height is 8m
Range is 6m
What is height of tide at 1800?
Range divided by 12 = 1/2m
1800 is 4 hours after HW i.e. 9/12 of the range
9/12 of 6m is 4.5m i.e. tide will have fallen by 4.5m
8m - 4.5m = 3.5m
Height of tide at 1800 is 3.5m

Finally, to get the depth of water in any spot at a certain time, you need only add the charted depth to that figure for height of tide. Thus in the previous example, if the question had asked for the depth of water at 1800 in a spot whose charted depth was 3m, the answer would have been 6.5m.

To find the rates of tidal streams, either consult the chart Tidal Diamonds or use special Tidal Stream Atlases and Pilot Books which include diagrams showing the tidal streams for a given area, all related to times before or after High Water at the nearest appropriate port.