Furuno CI-68 Operator's Manual page 68

PRINCIPLE OF OPERATION
Tide differential calculations in water tracking mode
Tide speeds of layer A (C1) and layer B (C2) relative to near-surface layer (Vw) are
calculated as follows:
Tide differential based on layer A (speed of layer B as viewed from layer A)
D1 = Vw - Vwa (Speed of layer A based on near-surface layer)
D2 = Vw - Vwb (Speed of layer B based on near-surface layer)
The tide differential (Dd) between layers A and layer B is;
Dd = D2 - D1
= (Vw - Vwb) - (Vw - Vwa)
= Vwa - Vwb
= 8 - 5
= 3 (kt)
OR
Tide differential based on layer B (speed of layer A as viewed from layer B)
Dd = D1 - D2
= (Vw - Vwa) - (Vw - Vwb)
= Vwa - Vwb
= 5 - 8
= -3 (kt)
If you compare the results of calculations on this page with the ones on preceding pages,
you will find the tide differential of two layers is identical irrespective of tracking mode.
AP-12