TIDES

Guiding Principle

Tides contribute to the “texture of the sea” and the biological productivity of intertidal zones. Understanding the effects of Earth’s tides is essential to good seamanship.

Performance Objectives
  1. Paddlers should demonstrate where to find tidal information.
  2. Paddlers should predict periods of spring and neap tides.
  3. Paddlers should explain impacts of tidal ranges on kayakers.
Materials
  • Eldridge/Reed’s.
  • Weather radio.
  • Other sources of tidal information.
  • Locations for showing tidal range (outside boathouse, beach).
  • Models of earth, sun, moon.
Tides: The periodic variation in the surface level of the sea caused by gravitational attraction of the moon and the sun. 

CONTENT

Explanation of a Tidal Cycle

The gravitational force of the moon raises a bulge in the ocean on the moon side of the earth and a corresponding bulge, due to centrifugal force, on the opposite side of the earth. High water corresponds roughly with those bulges. 

It takes 24 hours and 52 minutes for the moon to orbit the earth. Therefore, times of high and low water are approximately 52 minutes later each day. 

The sun has a gravitational effect on the ocean as well.

Spring tides are times of greater tidal range, when the gravitational pull of the moon and the sun are in line, lagging one or two days behind full and new moons. 

Neap tides are when the gravitational pull of the moon and the sun are at right angles.

Not all spring tides are equal. The moon’s elliptical track around the earth and the earth’s elliptical track around the sun mean that during certain times their gravitational pull on the ocean is greater. When they are further away from the earth, gravitational pull is less. Perigee corresponds to times of unusually great tidal ranges; Apogee corresponds to times of small tidal ranges. 

For a given tidal cycle high à low water or vice versa, we refer to The Rule of 12ths:

Hour after high or low water - Fraction of total tidal range - Total rise or fall given tidal range of 12 feet.

  • 1st hour - 1/12 of total range - 1 foot.
  • 2nd hour - 2/12 of total range - 3 feet.
  • 3rd hour - 3/12 of total range - 6 feet.
  • 4th hour - 3/12 of total range - 9 feet.
  • 5th hour - 2/12 of total range - 11 feet.
  • 6th hour - 1/12 of total range - 12 feet.

Notice that ½ of the vertical movement of water occurs in the middle two hours of the tide (3rd and 4th hours).

Some useful tidal terminology
  • Chart datum - The numbers on charts correspond to the depth of the sea at mean lower low water (MLLW,) or the mean of the lower low water heights over a 19 year period.
  • Spring tides - tides of increasing range, occurring twice a month, around times of the new and the full moons. 
  • Neap tides - tides of decreasing range, occurring twice a month, around the times of the waning and the waxing half moons.
  • Diurnal inequality - The difference in height of the two daily low waters or of the two daily high waters.  It is a result of the moon’s (and the sun’s) changing declination.
  • Mean High Water - the mean of all high water heights; the charted clearance of bridges, etc. is measured from this height.
  • Mean High Water, Spring - the mean of water heights for spring tides.
  • Mean High Water, Neap - the mean of high water heights at neap tides.
  • Mean Low Water - the mean of all low water heights.
  • Mean Low Water, Spring - the mean of low water heights of spring tides.
  • Mean Low Water, Neap - the mean of low water heights of neap tides. 
Where to find tidal information for your area
  • Weather radio marine forecasts (times but not heights)
  • Calendars at marine stores
  • Internet
  • Eldridge Tide and Pilot Book (current year)
  • Reed’s Nautical Almanac (current year)

Tidal information is generally given in relation to a standard port, such as Portland Harbor. Many other locations are given as corrections in relation to the standard port. 

For example:

Portland, Maine tidal information for January 1, 2002 as given in Reed’s Nautical Almanac on p. T24: LW 0549  0.0 ft; HW 1203 11.0 ft; LW 1831 –1.3 ft; Tidal range from 1203 – 1831 is 12.3 ft.

Bath, Maine tidal information is based off of Portland and is given as:

HW in Portland +1:01  à HW in Bath, ME occurs at 1304; LW in Portland +1:17  à LW in Bath, ME occurs at 1948.

Actual tidal ranges may vary significantly from predicted values due to weather patterns. Strong storm systems (low pressure) centered off the coast may cause significantly greater tidal ranges. Strong high pressure depresses tidal ranges. A barometric pressure of 980 mb will allow the tide levels to rise a foot, while a reading of 1030 mb will depress levels about a foot.

Significance of tides to kayakers
  • Local topography – A sandy beach at low water may be a cliff at high water. A cobble beach at low water may be a cliff at high water. Several islands at high water may be one island at low water.
  • Mud flats – Imagine leaving from a nice launch site, only to return from a paddle five or six hours later with a slog through a mile of mud flats back to the car. 
  • Intertidal zones – These are areas of amazing biological productivity and diversity.  Kayakers can have a huge impact on an intertidal zone when walking across them at low water.
  • Ledges – Cover and uncover depending on tide. Ledges can make great surf spots but need to beware of a changing tide making the ledge unsafe. They can also break up a swell enough to make a passage close to the rocks possible.

Activities

  1. Determine time of high and low water for a given location.
  2. Understand dates of spring and neap tides from a tidal atlas.
  3. Find dates of especially strong spring and neap tides.
  4. Locate on a chart areas that are only passable or accessible at high water.
Assessment Tools
  1. Paddlers should record times and heights of high and low water for a specific day and location.
  2. Paddlers should explain the physical characteristics of a specific location from information on the chart and tidal information.
  3. Paddlers should find locations accessible only at high water.

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