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Old 08-05-2003
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Understanding Tidal Currents

 
Tidal currents have minimal effect on the open ocean, but out here you need to watch out for temporary, permanent, and rotary currents.
 
Tidal current is the one thing that affects your dead reckoning (DR) more than anything else. Without the tidal current effect, your DR would be a lot more accurate and your boating safer. Of course you still have leeway and steering error which can change the accuracy of your DR, but it is tidal current that will throw it off the most. Before I tell you how to account for the effects of the tidal current in your DR, it is best to discuss the theory. First I will concentrate solely on tidal currents and then touch briefly on ocean currents.

In two previous articles I talked about the relationship of the sun and the moon with respect to the earth and their effects on the ocean's heights. It is this change in tidal heights that is the cause of the tidal currents. The flow of water to and from a location results from the rise and fall of the tides and is called tidal current. Remember that tide is the vertical rise and fall of water levels and current is the horizontal flow of the water. Tidal current is the horizontal flow of water as a result of the tides.

The normal type of tidal current in coastal bays and rivers is the reversing current, which flows alternately in one direction and then the other. The direction, or "set," of the current is a true direction toward which the current is flowing. A current that flows from north to south is called a southerly current and has a set of 180 degrees. This is just the opposite terminology of wind direction which is always referenced regarding the wind's origin. For example, a wind blowing from north to south is called a north wind.

 
Tidal currents are not only a concern for vessels underway, but for those berthed, moored, or anchored as well.
 
The strength of ebb and flood of a current is its speed, normally measured in knots and is called "drift." A tidal current is said to flood when it flows in from the sea, resulting in higher tidal stages. Conversely, a tidal current ebbs when the flow is seaward and the water level falls. For the most part, the ebb is usually stronger and lasts longer than the flood. Tidal current used in navigation is always referenced using both set and drift.

As these currents reverse direction, there are periods of no flow, called slack, or slack water. The time of slack water is not the same as the time of stand, which is when the rise or fall of the tidal height has stopped. Also the time of high or low water will not necessarily correspond with the change of tidal current direction.

A few miles offshore, and in some large bays, you can expect to find rotary currents. These currents flow continually with the set rotating through 360 degrees during the period. The speed of the rotary current varies throughout the cycle, passing through the two maxima in opposite directions and the two minima 90 degrees from the direction at the time of the maximum speed. However, the change of speed is relatively small.

 
Wind-driven currents can be as strong as two percent of the wind velocity, which is important in areas of limited maneuverability.
 
One other special tidal current is the hydraulic current, which occurs in most canals and some passes. Differences in the time and height of high water cause a strong flow of water from one end of the canal to the other and back again. A prime example of this is the Cape Cod Canal, which connects Massachusetts Bay and Buzzards Bay. If your boat's maximum speed is six knots or less you will need to get there when the current is running with you. Otherwise you can find yourself making only one knot or less over the ground as the flow runs against you.

Tidal currents are due primarily to tidal action, but other causes are often present. Following heavy rains or a drought, a river's current prediction could change considerably. Set and drift may vary significantly over different parts of a harbor because differences in bathymetry (the depths of the water) from place to place will affect the current. Since this is usually an area where small errors in a vessel's position are crucial, a knowledge of predicted currents, particularly in reduced visibility, is important.

Strong currents occur mostly in narrow passages connecting larger bodies of water. Currents of more than five knots are sometimes encountered near the Golden Gate Bridge in San Francisco Bay, and currents of 13 knots or greater sometimes occur at Seymour Narrows, BC. In straight portions of rivers and channels, the strongest currents usually occur in the middle of the channel. In curved portions, the swiftest currents and deepest water usually occur near the outer edge of the curve. Countercurrents and eddies may occur on either side of the main current of a river or narrow passage, especially near obstructions and in bights. In general, the range of tide and the velocity of tidal current are at a minimum in the open ocean or along straight coasts. The greatest tidal effects are usually encountered in estuaries, bays, rivers, and other coastal indentations. A vessel proceeding along an indented coast may encounter a set toward or away from the shore; however a similar set is seldom experienced along a straight coast because the flood and ebb are usually parallel to the shore.

Most computer programs for tidal currents calculate the direction and strength of the current for your present location based upon the computations of current constituents for the nearest reference station and the selected sub-station. But sometimes the nearest sub-station may not be the one to use, such as when the station is on the opposite side of a spit of land, large island, or downstream from your position.


Differences in the depth of the water of a particular region will greatly influence the tidal flow.

In many boating areas, current conditions can be critical. There are many ocean inlets that are difficult or dangerous to navigate when the current is strong, and opposed by the wind and surf. In many narrow bodies of water, the maximum current may exceed your boat's maximum speed. In such cases, it is wise to wait for slack water.

Non-tidal currents include the permanent currents in the circulatory system of the ocean and the temporary currents arising from sustained winds. The Gulf Stream and the California Current are the most important permanent currents for sailors in North America. The Gulf Stream generally flows northeastward up the Atlantic coast from Florida toward New England where it turns eastward. However, there are so many eddies, back eddies, and meanderings that an offshore sailor will need up-to-date weather charts to compensate for its effects. The California Current generally flows southward along the North Pacific coast. Off Baja California in Mexico this current turns westward. The California Current is slower and not as well defined as the Gulf Stream.

Wind-driven currents can be as strong as two percent of the wind velocity but the set will not be in the same direction as the wind, due to the earth's rotation. This direction may be offset from the winds direction by as much as 15 to 45 degrees to the right in the northern hemisphere. The amount of this offset is dependent on the latitude and water depth.

All currents will have a significant effect on your boat speed and direction of travel and you must know how to determine and correct for it. All computerized navigation and electronic charting programs have the capability of showing the tidal current on your electronic chart. They can also take the set and drift into account when calculating from your course and speed to get Speed and Course Over the Ground information. These are great aids to safe boating and the prudent mariner will take them into account when on the water. Remember, tidal current predictions are for a specific location, and the set and drift of the tidal current just a few hundred feet away may be substantially different. Current may also vary according to the depth of the water. As with the tidal predictions, the currents predicted by all computer programs and by government current tables may often be upset by abnormal local weather conditions of sustained wind or rain. In such cases, use the predictions with caution, both during and after the abnormal weather.

A Glossary for Tidal Currents

 
Current
horizontal movement of water.

 
Tidal current
horizontal movement of water caused by gravitational interaction between the sun, moon, and earth. Tidal currents are a part of the vertical rise and fall of the sea which we refer to as tide.

 
Non-tidal current
horizontal movements of water that are not the result of tidal effects. Examples include river currents, ocean currents, and wind-driven currents.

 
Set
the direction toward which a tidal current flows.

 
Drift
the speed of a tidal current, which is normally expressed in knots and measured to the nearest 10th of a knot. River currents are measured in mph.

 
Ebb
tidal current moving away from land or down a tidal stream.

 
Flood
tidal current moving toward land or up a tidal stream.

 
Slack, or Slack Water
the state of a tidal current when its speed is near zero, prior to reversing direction. The term is also applied to the entire period of low speed prior to and after the turning of the current when it is too weak to be of any practical importance in navigation. Not to be confused with stand.

 
Stand
the point when vertical movement stops at both high or low tide.



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