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Introduction to Tides
Background
Information
Whenever your ship enters or leaves port, one of your most important
tasks will be to calculate how much water will be available along your
route of transit. The importance of accurate tide calculations cannot be
overemphasized. If your ship attempts to pass beneath a bridge without
adequate vertical clearance, you could lose the ship’s mast. If you pass
over a shoal with an insufficient depth of water, your ship will probably
go aground, losing sonar dome, rudder, and propellers. All navigational
charts reference the depth soundings in mean low water.
Definition
Tide is the vertical rise and fall of the ocean level caused by the
gravitational forces between Earth and the Moon, and Earth and the Sun.
Generally speaking, these interacting forces between the planets cause
the tides to rise and fall twice daily, this is known as a tidal day. The
period of one high and one low is referred to as a tidal cycle.
Terms Associ-
Use the following table to learn the meanings of terms that are
ated with Tides
associated with tides.
Term
Definition
High tide or high The maximum height of the water resulting from the
water (HW)
rising tide.
Low tide or low
The minimum height of the water resulting from the
water (LW)
outgoing tide.
Duration of rise
The period of time measured in hours and minutes that
and fall
it takes the tide to go from low water to high water.
Range of tide
The distance between HW and LW.
Stand
A brief period where no rise or fall occurs; this occurs
when the tide reaches its maximum or minimum level.
Mean high water The average height of all high-tide water levels,
(MHW)
measured over a 19-year period.
Mean low water
(MLW)
Mean lower low
water (MLW)
The average height of all low-tide levels, observed
over a 19-year period.
The average of the lower of the low water levels,
observed over a period of 19-years. This is the
reference plane currently used on almost all charts
covering U.S. waters as the basis of measurement of
charted depths and height of tide.
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