In general the rule of thumb is 3 times the water depth equals cable required.
This assumes a roughly 3-4 knot towfish speed through water. Notice I did not say vessel speed. When towing against a strong current the vessels’ speed over ground may be 2 knots but speed through the water equals vessel speed plus current.
If your driving 2 knots you can get away with less cable but cover less area per unit time.
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