Yacht problem

Sep 2019
The entrance to a harbour is a channel of length A which runs between two banks a distance B apart. The banks and the channel can be assumed to be rectangular. There is a current flowing with a speed v from one bank to the other. A yacht travelling at a constant speed relative to the water u wants to enter the harbour.

Find the minimum value of u for the yacht to safely enter the harbour. Express u in terms of v,A and B

Need help solving this. I keep getting told we cannot assume the yacht is perpendicular to the water flow
Jun 2016
The velocity of the yacht must be such that over the distance A the current v will push it sideways by the distance B.
Find the (equation for the) time taken for the flow v to push the yacht a distance of B
find the (equation for the) speed (along the channel) required for the yacht to move the Distance A in that time.
This gives you (the equations for) two (perpendicular) velocities
add (the equations for) these two velocities to get the (equation for the) total velocity of the yacht.
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