strut vibration
Off-center, out-of-plane thrust may cause a throbbing vibration. Your prop may be innocent of wrong-doing. A string looped around the keel at the hull tied to a centered object aft of the boat may give you a center-line, but designs sometime incorporate off-center counter-acting components to provide stability and control in some conditions. When I replaced my prop, I noticed the shaft seemed to be slightly off-center, but parallel to the boat center-line. I did not notice the boat turning under power, though. I figured the shaft alignment and rudder counteracted the rotational wash and "walking" that you can see in reverse.
I would make sure that each strut is the correct length and angle (from a specified reference). The particulars may be available from someone more experience with this boat.
