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Tide Differential
Tide differential is a relative movement of tides at different depths.
It can be measured in the ground tracking, water tracking and nav-aided modes.
:Base of measurement
w
e
Layer A
Start of
trip
End of
trip
(Moving)
10 NM (1 hour trip)
Layer B
(Moving)
2 kt
5 kt
Assuming that the ship, layer A and layer B are moving in the same direction, and ship’s speeds
based on layer A (Vwa) and on layer B (Vwb) are measured as;
Vwa = 8 kt (Ship’s speed based on layer A)
Vwb = 5 kt (Ship’s speed based on layer B)
Tide differential calculations in ground tracking mode
As an absolute ship’s speed (Vg) is available in the ground tracking mode, tide speeds of layer A
(C1) and layer B (C2) based on ground are calculated as follows:
C1 = Vg - Vwa (Speed of layer A based on ground)
C2 = Vg - Vwb (Speed of layer B based on ground)
Thus, the tide differential (Cd) between layer A and layer B is;
Cd = C2 - C1
= (Vg - Vwb) - (Vg - Vwa)
= Vwa - Vwb
= 8 - 5
= 3 (kt)
or
Tide differential based on layer A
Speed of layer B viewed from
layer A