What are Tides and Currents

Whether you are an avid sailor or a recreational boater, being out on the water can be safer and more enjoyable when you know common nautical terms.  The language used in the boating world is different from what is used by land lubbers.  While it is not necessary to know all of the “salty” language, knowledge of basic terminology can save valuable time in emergency situations. 

The term tide is a commonly misused term.   Many people mistake currents for tides.  The tide is not the inflow and outflow of water as a “change in tides.”  The tide is simply the rise and fall of water as a result of the gravitational pull of the moon and the sun.  This vertical movement of water is the only movement that is considered a tide. 

A current is the horizontal movement of water.  Currents have strength and direction.  The direction that the current flows is called the set.  Its strength is called the drift.  A current that flows from south to north is a southerly current because the set of a current is described by the direction that the water is moving towards.  This is the opposite way that wind direction is described. 

There are a number of types of currents.  River currents, upriver from areas affected by tidal currents, vary based on the width and depth of the river.  Rainfall can also greatly impact tidal currents.  Ocean currents result from constant winds, like the trade winds and prevailing westerlies, as well as the rotation of the earth.  Wind-driven currents are caused by temporary weather conditions that cause the surface water to move to the extent that it creates movement. 

Combining the two, a tidal current, describes the flow that results from the influence of tides.  An incoming tidal current is moving, or running, towards the shore.  This is often referred to as a flood, or “flood tide.”  The outgoing movement, or retreat of the tidal current, is called the ebb.  Slack tide is the period of time between flood and ebb currents is when there is no flow in either direction.

Range is the difference between the height of the high water and low water.  The range can differ day to day as the gravitational pull of both the sun and moon change.  A spring tide occurs during a full or new moon, increasing the range.  Neap tides occur at quarter moons and have a lesser range than other tides.