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11-17-2003
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Contributing Authors
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Join Date: Jan 2000
Posts: 147
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Basic Thoughts on Tides
The expression "Time and tide wait for no man" is certainly on the mark—in some ways they are connected. Since ancient times mankind has used the celestial bodies and their movements to determine the passage of time. One rotation of the Earth was a day, from one full Moon to another represented a month, and one orbit of the Earth around the Sun marked a year. Later on these concepts were refined into our present calendar and time system. With the use of the sundial, the day was sub-divided into hours, and after the clock was invented, the hour was separated into minutes and seconds. Even before the sundial was invented, mariners knew that the relationship between Earth, Moon, and Sun controlled the tides. Today, thanks to our refined knowledge of celestial movements and how land masses affect the ocean's movements, we can quickly and easily predict the height of tide and flow of tidal currents. It is well to remember that these are theoretical predictions only, and may not reflect the actual values. | | I think we would all agree that without tides and currents, boating would be a lot less complicated and perhaps safer. We could aim the boat at our destination confident that we would get there without any further corrections. Our speed through the water would be the same as our speed over the ground, and the depth of the water at our anchorage would remain constant. Unfortunately, this is not how things work. On the open ocean, tidal heights are not noticeable. It is only in coastal areas where the tidal ranges and currents become apparent, and their effects can often be significant. The height of tide determines where you can travel and anchor safely, while the tidal current will affect your boat speed and complicated course calculations. Mean Lower Low Water (MLLW) is the reference datum for all National Ocean Service (NOS) tide heights on US charts. Bridge clearances are measured from Mean High Water (MHW). The difference between these two heights is the tidal range. Tidal predictions are based upon actual observation of the past tidal ranges over decades. This is because the complete lunar cycle is approximately 19 years (actually 18.6). Among some boaters the terms "tide" and " tidal current" are often used incorrectly. To avoid any confusion I will use them in their formal sense. Tide is the vertical rise and fall of the ocean's level due to the orbits and gravitational relationship of the Earth, Sun, and Moon. Tidal current is the horizontal flow of water from one place to another due to a difference in height of tide between them. Non-tidal current is the horizontal flow of water from any cause such as a river flowing to the sea, or a major ocean current like the flow of the Gulf Stream. It is important to understand the difference between current and tidal current. For example, have you ever been in the mouth of a river as the tide is rising? The clash of tidal current against river current creates standing waves which will give you quite an exciting ride. Tide rises and falls while tidal current floods and ebbs. At most places, the tidal change occurs twice daily. The tide rises until it reaches a maximum height, called high tide or high water, and then falls to a minimum level called low tide or low water. This rate of rise and fall is not uniform. From low water, the tide begins to rise slowly at first, but at an increasing rate until it is about halfway to high water. The rate of rise then decreases until high water is reached, and the rise ceases. A falling tide behaves in a similar manner. The period at high or low water during which there is no apparent change of level is called stand. Range is the difference in height between high tide and low tide. Earth, Moon, and Sun Basically, as the Earth rotates, high tides are created on opposite sides of the Earth by the combined gravitational relationship of the Earth, Sun, and Moon. Centrifugal force causes the oceans to bulge on one side of the Earth, while the gravitational attraction of the combined Sun and Moon causes them to bulge on the other side. Because the Moon is so much closer to the Earth than the Sun, its affect on the tides is almost twice as powerful. The rotation of the Moon about the Earth takes 24 hours and 50 minutes. As a result the tidal highs and lows occur about 50 minutes later than the highs and lows of the previous day. It has been said that "the tides follow the Moon." When there is a new moon and a full moon, the Earth, Sun, and Moon are lined up (in conjunction) and the resultant gravitational affect causes greater tidal ranges than normal. These tides are called Spring Tides. When the Moon is in first and third quarters, the Sun-Moon relationship is at right angles (in quadrature) and the resultant gravitational effect is smaller tidal ranges than normal. These tidal ranges are called Neap Tides. To further complicate matters, in its monthly orbits around the Earth, the Moon travels north and south of our equator. Mixed and diurnal (once daily) types of tides occur when the Moon is further north or south of the equator in its orbit. When the Moon is over the equator, the tides are semi-diurnal, but the daily cycle of tides varies widely from place to place and depends on the latitude of the location as well as the Moon's north-south location (declination) in its orbit. Generally speaking, tidal ranges decrease as you approach the equator. The Moon's orbit is also elliptical, and when the Moon is closest to the Earth, the tidal ranges are greater. Conversely, when the Moon is furthest away, the tides are smaller. Yearly variations in tidal ranges are also caused by the Earth's Elliptical orbit about the Sun. When the Earth is closest to the Sun in the winter, the Sun's gravitational effect is greater, resulting in higher tidal ranges than in the summer when the Earth is farthest from the Sun. If you would like to read a more detailed explanation, I refer you to Bowditch, and his classic, The American Practical Navigator. Calculating Tides For semi-diurnal tides along the US east coast, high tide can be determined for a local area by first noting the relative position of the Moon at high tide. From then on when the Moon is in that same relative position you are at high tide. For a new Moon use the Sun. For boaters in Maine in the summer months, on the day of a new Moon or a full Moon high tide will occur at approximately 1200 EDT, plus or minus 15 minutes. For each day after the new or full Moon the time of high tide will be 50 minutes later. | 
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Every tide reference station has a different range of tides and time of high and low water. This is due to their particular coastline configuration, ocean depths, and obstructing land masses such as islands and peninsulas. These actual measurements are taken and used by NOS to calculate the harmonic constants, tidal constituents, used in making the predictions. All computer programs compute the height of tide based upon mathematical computations of tidal constituents for the nearest tidal reference station and sub-station. It is well to remember when using computer programs to predict tides that, sometimes, the nearest subordinate station may not be the one to use. This can occur when the station is on the opposite side of a spit of land, isthmus, or large island from your location. Always use a subordinate station which is in the same body of water as your boat The height of tide computed should not be confused with depth of water. Soundings shown on your chart are the vertical distances from the selected reference level (MLLW) to the seabed. The actual depth is this charted depth plus the height of tide. For example, if the chart depth is 20 feet and the height of tide is 5.7 feet, the actual depth is 20 + 5.7 = 25.7 feet. When the height of tide is negative, such as -1.3 feet, and the charted depth is 15 feet, the actual depth is 15- 1.3 = 13.7 feet. It is important to understand that the actual depth may be less than the charted depth several times a month on neap tides. Remember, heights found in the Tide Tables and computer programs are predictions. When conditions vary considerably from those used in making the predictions, the heights shown may be in error. Heights lower than predicted should be anticipated when the atmospheric pressure is higher than normal or when there is a persistent strong offshore wind. Glossary of Tidal Terms | |  | Diurnal tides Only a single high and low water each day. | | | | Semi-Diurnal tides Two high water and two low water levels in approximately 24 hours. | | | | Mixed Tide A twice daily tide of unequal high and/or low waters. | | | | High tide or High water The highest level reached by an ascending tide. | | | | Low tide or low water The lowest level reached by a descending tide. | | | | Range The difference between high tide and the following low tide. | | | | Mean Low Water (MLW) The average height of all low water at a reference station over a 19 year cycle. | | | | Mean Lower Low Water (MLLW) The average height of the low water spring waters over a 19 year cycle. | | | | Mean High Water The average height of all high waters over a 19 year cycle. | | | | Tidal Datum A reference level from which heights and depths are measured. On a chart this is called the Chart datum. In the US, Mean Lower Low Water is used as the tidal datum. This datum is normally low enough so that the majority of low waters won't go below it. | | | | Height of tide At any time this is the vertical measurement between the surface of the water and the tidal datum. Do not confuse height of tide with depth of water. | | | | Mean Sea Level (MSL) The average height of the surface of the ocean for all tidal stages. | | | | Stand The point when vertical movement of the water ceases. | | | | Charted clearance The difference in height between mean high water and the underside of an overhead obstruction, such as a bridge or cables. | | | | Actual Clearance The difference in height between the bottom of an obstruction and the actual surface of the water. | | | | Actual Depth of Water The charted depth plus the height of tide. When the height of tide is a negative number, the actual depth of water will be below the charted depth. | | | | Charted depth The depth of water as shown on the chart with reference to the chart or tidal datum. |
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