CAUTION
Tide Predictions are based on averages. Other factors can have significant effect on local tides at any particular time.
Consequently, you should always leave an extra margin for safety.
Time & Height of Local Tides
Table 2: record the Time-Diff and Height Ratios [col (3),(4)]. Identify Reference Station.
Table 1: record the Times and Heights [col (1),(2)] of High and Low tides for Date.
Compute: Time and Heights [col (5),(6)] of Tides at Local Subordinate Station.
ft.ft.h m H/Lh m H/LFactor*Factor*h m H/Lh m H/Lft.ft.h m H/Lh m H/L
HeightHeightTimeTimeHeight Height Time-DiffTime-DiffHeightHeightTimeTime
`̀
(6)(6)(5)(5)(4)(4)(3)(3)(2)(2)(1)(1)
PREDICTION OF HIGH AND LOW TIDES
LOCATION:REF. STA.:DATE:
No.
DESIRED TIME:
REFERENCE STATION SUBORDINATE STATIONSUBORDINATE STATION
Table 1 Table 2 Calculate
Actual Range: the difference between High and Low tides bracketing the Desired time.
Height at High Height at Low Actual Range
ft. ft. ft.=-Time Difference to Desired Time: Determine time interval from preceding High (or Low) to the Desired Time. Round that interval to the nearest whole number of hours.Desired Time Time of prior H/LTime Difference
=-h h hm m m
ROUNDED 1 – 2 – 3 – 4 – 5 – 6 RULE OF TWELFTHS: Look up the percentage
of change from Table below under ROUNDED hours
Calculate Height of Tide at Desired Time: apply that percentage to the Actual Range to get estimate of the change in Tide Height at desired time. Then add/substract that from prior high/low to get Height of Tide at Desired Time.
Percentage Actual RangeChange in
Height from High/Low =x% f
tft
Vertical Clearance: determine vertical clearance at the desired time (if needed). Look on chart for Table of Heights above MLLW, or calculate, using one-half of the Mean Range plus the Mean Tide height from the tide tables.
Charted Height
ft
Ht of MHW above MLLW
ft
Height above MLLW
ft
Vertical Clearanc
eft+ - =
Rounded Hours
5% 25% 50% 75% 95% 100%% of change
1hr 2hr 3hr 4hr 5hr 6hr
* If Table 2 has an * before height difference multiple Table 1 height by this factor, a +/- means add or subtract difference to get subordinate height.
Graphics on Student Form
Height of prior
High/Low
Change in Height
Height above MLLW
=±ft
ft
ft
Graphics on Student Form
Rainsford Island 831 Boston, MA
18 April 2004 1400 EST
10 29 H 10.1
16 31 L 0.3 22 45 H 10.4
04 13 L 0.0
Figure 7-2 & Figure 7-3
Classroom Exercise 7-
1
00 00
00 00 -00 02
*0.95
*0.95 *0.95
-00 02 *0.95 10 29 H
16 29 L 22 45 H
9.6
9.9 0.3
9.6 0.3 9.3
14 00 10 29 3 3175 9.3 7.0
04 11 L 0.0
9.6 7.0 16.6Charted Depth
MHWMean Rang
e
MLLW
MLW
Bottom
Mean Tide
½ Mean Range
Charted Height Vertical Clearanc
e
Height above MLLW
Time/Ht of Ref Low
Time/Ht of Ref High
Reference Station
Table 1
MLLWTime/Ht of Sub Low
Time/Ht of Sub High
Subordinate Station
Table 2
Time Difference
Height Factor
Act
ual R
ang
e
Time Dif to Desired Time
MLLW
Mean Tide Level 4.8ft
Station Name Station NumberReference StationDistance42° 19.000’ N 070° 57.000’ W
Rainsford Island, Nantucket Shoals831
Boston, Massachusetts5.1 NM
Information…
Tide Table…
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Low High Low High
04 15 10 29 16 33 22 45
0.0ft 9.6ft 0.3ft 9.9ft
Sunrise Sunset Moonrise Moonset
04 57 18 29 04 43 17 44
Apr 18 2004 1400^^ ^^ ^^ ^^
Station Local Time -5.00
^
<<Week <Day Tide Stations… Current/Tide Stream Stations… Day> Week>>
4/18/2004 1400 4/19/2004
Tide Graph X
0 2 4 6 8 10 12 14 16 18 20 22 0 2 4 6
10ft
8ft
6ft
4ft
2ft
0ft
-2ft
