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.
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.
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.
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
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