This is very much an approximation, but it
Table 3
does give you a sense of how the tides accelerate
Read the footnote to Table 3 on the next
and decelerate; and it gives you some tools with
page first. If confused, you may wish to try
which to approximate intermediate heights.
this example.
For example, if the tide will rise 10 ft during
We’ll find the height of tide at 0755 on a day
its 6-hour cycle, the law of twelfths suggests
when the predicted tides before and after 0755
that after 2 hours it will have risen 2.5 ft:
are given as
(1/12 + 2/12) = 3/12 = 1/4 x 10 = 2.5
morning low: 0522, 0.1 ft.
If a tide will fall 7 ft in its 6-hour cycle, after
morning high: 1114, 4.2 ft.
4 hours it will have fallen 5.25 ft:
Therefore, the duration of rise is 11h 14m
9/12 = 3/4 x 7 = 5.25
– 5h 22m = 5h 52m. The time from the near-
Graphic Method est high or low water is then 7h 55m–5h 22m
You can graph a typical tide using the one-
= 2h 33m (from low). The range of tide is
quarter, one-tenth rule:
given as 4.2–0.1 = 4.1 ft.
Plot the high- and low-water points in the
We then enter the left-hand boldfaced col-
order of their occurrence, measuring time
umn of the table and find the value nearest
horizontally and height vertically. Draw a
our value for the duration of rise and fall (5h
light straight line connecting the points.
52m)--in this case 6h 00m. Following across
Divide this line into four equal parts. At the
that row, we look for the tabular time that is
quarter point adjacent to high water, draw a
closest to 2h 33m, the time from the nearest
vertical line above the point; and at the quar-
tide—in this case 2h 36m. Staying in the 2h
ter point adjacent to low water, draw a vertical
36m column, we move into the bottom sec-
line below the point, making the length of
tion of the table (Correction to height) and
these lines equal to one-tenth of the range
look left across to rows to find the tabular
T
ides
between the high and low waters used. Finally,
value in the left-hand boldfaced column that
draw a smooth curve through the points of
is closest to our range of tide value (4.1 ft)—in
high and low waters and the intermediate
this case 4.0 ft. Matching the 2h 36m column
points, making the curve well rounded near
and the 4.0 row, we arrive at a correction of
high and low waters. This curve will approxi-
1.6 ft. Because the nearest tide was low, we
e
s
mate the tide curve, and heights for any time
add the correction to the low:
u
r
v
of the day may be scaled from it.
0.1 + 1.6 = 1.7-foot tide height at 0755.
An example of the graphic method is illus-
trated below. Using the same predicted tides
i
d
e C
as in the above example, the approximate
height at 7h 00m is 1 foot.
o
n/T
u
c
ti
od
I
n
tr
351