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3. TIDE, NAV-TIDE & TIDE DIFFERENTIAL
Tide
(Absolute tide)
Absolute tide can be measured in the ground tracking mode.
:Base of
measurement
w
q
Layer A
Fixed
Start of
trip
End of
trip
(Moving at 10 kt on ground)
(Moving at 3 kt on ground)
10 NM (1 hour trip)
Assume that the ship and layer A are moving in the same direction, and ship’s speeds based on
ground (Vg) and on layer A (Vwa) are measured as;
Vg = 10 kt (Ship’s speed based on ground)
Vwa = 7 kt (Ship’s speed based on layer A).
Speed of layer A based on ground (C1) can be calculated as follows:
C1 = Vg - Vwa
= 10 - 7
= 3 (kt)
Nav-Tide
(Absolute tide)
Absolute tide can be measured in the nav-aided mode.
Nav-aided ship’s speed (Vn) is equivalent to ship’s ground tracking speed in the ideal conditions.
That is, the nav-tide can be calculated by simply replacing Vg with Vn in the above equation.