Section 8

Tidal Waters

A tide is the periodic rise and fall in the height of the sea. It's caused by the gravitational attraction of the sun and moon and the centrifugal force of the earth's rotation. Tide causes the depth of coastal waters to increase and decrease, resulting in a high tide and a low tide. It also causes tidal current, an advancing and receding flow of sea water.

Tidal Cycles

High and low tides occur in predictable cycles. Most coastal areas in Canada experience two high tides and two low tides in a 24-hour period. The two high tides are almost equal in height as are the low tides, and the time between high and low tide is roughly six hours and twenty minutes. This is a semi-diurnal tide and it is common on the eastern seaboard of Canada and the United States.

Mixed tides are high tides or low tides of unequal height. This is typical on Canada's Pacific coast. A diurnal tide cycle has one high tide and one low tide each day.

Differences in Tidal Height

The difference between the depth of water at high tide and low tide is called tidal range, and it varies with the position of the earth in relation to the moon and sun. When the range between high and the low tide is at its maximum, the tide is said to be large. Most tides are average, or mean.

Tidal range also depends on location. The largest tides in the world occur in the Bay of Fundy on Canada's east coast where the range can be more than 14 metres!

The point at which tides stop increasing or decreasing in height is the stand of the tide, a point of no vertical movement.

It is essential for a navigator to know and understand the times and heights of tides in coastal waters. Fortunately tides are predictable and reliable tide tables are available.

Tidal Current

As the tide rises, ocean waters flow from the sea towards or along the coast and into bays, harbours and river estuaries. This creates a flood current. Falling tides generate an ebb current as water reverses direction and flows back to the sea.

The difference in the direction of flow between a flood and ebb current is usually, but not always, 180°.

The speed, or rate of the current is affected by a number of factors including the size of the tide, and varies by location. Currents in and around British Columbia's Gulf Islands can run at nine knots.

The rate of the tidal current also increases and decreases during the tidal cycle. At high and low tide, the current reverses; and for a brief period the flow stops. This is the turn of the tide, also known as slack water. From this point, the rate increases until it reaches a maximum rate approximately half way between high and low tide.

Predicting Current

Current is a significant factor in coastal navigation. As with tidal heights, the rate and direction of tidal current can be predicted. These predictions appear in current tables. In Canada tide and current tables for a given area are found in one publication.

Measuring the Tidal Current

Tidal currents are measured in knots and degrees of direction.

Measuring the Tidal Height

Tidal heights, low tide and high tide, are measured in feet or metres. As in soundings on a nautical chart, tidal heights are measured from datum, the level below which the water seldom falls.

So at a location where the charted sounding is 5 metres, this depth would be added to the tidal height to find the total depth. Note that because tidal height is measured from datum, it's rare that it has a negative value.

Anchoring in Tidal Water

When anchoring in tidal water, it's important to make sure of two things: at low tide, will there be enough water depth to float the boat? At high tide, is the length of the anchor line, or rode, adequate.

The amount of rode is dependant on the depth of the water. This is called scope: a ratio of the length of rode measured from the height of the vessel's deck, and the depth of water. A scope of five-to-one (5:1) is typical for overnight anchoring in calm conditions. If the water depth is 3 metres, and the boat's deck is 1 metre above the water, then at 5:1 the amount of rode would be five times the water and deck height, or 20 metres.

When determining the amount of rode to use in tidal water, the scope is calculated using the highest level of water in that location over the period the boat is at anchor.