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NAVIGATION TRAINING
Section 9
Tides
Table of Contents
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Section 1 Types of Navigation
Section 2 Terrestial Coordinates
Section 3 Charts
Section 4 Compass
Section 5 Nautical Publications
Section 6 Navigational Aids
Table of Contents
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Section 7 Buoyage
Section 8 Position Lines and Fixes
Section 9 Tides
Section 10 Currents
Section 11 Weather
Section 9: Tides
Tides Defined
• Tides are the vertical rise and fall of the ocean level due to
the gravitational and centrifugal forces between the earth
and the moon, and to a lesser extent, the sun.
Spring Tides
• When the tidal effects of the sun and the moon act
in concert.
Neap Tides
• When the tidal effects of the sun and the moon are in
opposition to one another.
Tidal Reference Planes
• Mean high-water springs (MHWS)
– average height of all spring tide high-water levels
• Mean higher high water (MHHW)
– average of the higher of the high-water levels each tidal
day, 19-year period
• Mean high water (MHW)
– average of all high-tide water levels, 19-year period
• Mean high-water neaps (MHWN)
– average recorded height of all neap tide high-water
levels
Tidal Reference Planes
• Mean low-water neaps (MLWN)
– average recorded height of all neap tide high-water
levels
• Mean low water (MLW)
– average of all low-tide water levels, 19-year period
• Mean lower low water (MLLW)
– average of the lower of the low-water levels each tidal
day, 19-year period
• Mean low water springs (MLWS)
– average of all spring tide low-water levels
Tidal Reference Planes
Height marked on chart
Depth marked on chart
Tidal Patterns
• In general in most of the world, the tides go up and
down on a semi diurnal curve
Tidal Patterns - Semidiurnal
Tidal Patterns - Diurnal
Tidal Patterns in the Georgia
Strait
• With the tidal waters of the Georgia Strait being
restricted north and south of Vancouver Island, a
stand of higher water occurs between one of the two
high waters.
• The result is shown:
Tidal Patterns as found in
Georgia Strait
Calculating Rise of Tide
• Q. If a low water was at 0600, with a height of 0.2
metres, and the next high water was at 1200 , with a
height of 5.6 metres, what would be the approximate
rise of tide and therefore approximate height of tide if
your vessel was setting out at 0900.
Calculating Rise and Height of Tide
A.
1200 LT High water
5.6 m
0600 LT Low water
0.2 m
6.00hrs
Range 5.4 m
0900 LT
0600 LT Low water
0.2 m
3.00 hrs
Approximate rise of tide is (3hrs/6hrs) x 5.4m = 2.7 m
Approximate height of tide above chart datum, if your
vessel was setting out at 0900 would be :
Ht of LW (0.2m) + rise of tide (2.7m) = 2.9 m.
Calculating Rise of Tide
• In this case allow only 2.5 metres. Always allow less
rise of tide close to low water due to the rate of
change of height being least close to time of low
water (and high water).
IMPORTANT NOTICE
At best, tidal prediction is a
chancy business. It relies on
field measurements and data
collection that can be subject to
any of several kinds of errors.
Typographical errors,
programming errors, and
numerous weather and human
activity error sources can
contribute to a rather large level
of predictive uncertainty.