What was the existing gearbox ratio and prop? Did it move the boat well enough?
It is notoriously difficult to get this right. The designer normally puts quite some time into doing so.
It is similar to trying to match the engine, gearbox and
wheel size (prop) on a motorcycle. If the
wheels are too small, they spin all the time (and your prop is like a food whisk). If the
wheels are too big, they labour the motor, (and your prop is like the paddle on a paddle steamer and too much drag on the motor... it never gets up to speed).