Re: 20 hp nissan vibration?
You will probably find that it trolls down much smoother and quieter (and less smoky) than the old 9.9/15 2-stroke 'rude.
If the motor is for trolling-only, consider getting a high thrust prop. That will let the motor troll at a somewhat higher RPM, (with excellent control) which the 4-strokes prefer -- reduces occurrence of oil dilution. But... of course, that's not the best prop if you want high speeds as well.