There is a plate that connects to the cavitation plate on the motor. You lift or lower the unit slow or speed up. I haven't seen them here in Canada but the boats that I have seen them on were all from the states.
P.S. If that doesn't work if he tunes his bait less (less curve) he should still be able to catch fish. I troll up to 3 knots and catch large chinooks they can swim very fast if they want to.

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Fish early Fish hard