I have a Beneteau 343 that I have bought from Sunsail, ex charter boat so the engine a Yanmar 3YM30 has done a few miles. It starts and runs fine with no smoke. The problem is at 2100rpm it makes the boat vibrate badly. I have checked the prop alignment and it was slightly out, I have adjusted it and I think it is well within tolerance. There is no play in the cutlass bearing. The 2 rear engine mounts are a bit soppy but I dont think they are the problem. When the vibration occurs there is no noticeable movement on the engine. Before I tried the boat in the water it was lifted out for antofouling etc, while out I could not see any obvious damage to the prop, it has 2 fixed blades. Difficult to tell at 2100rpm wether there is any misfire. With prop disconnected there is no vibration but then it is not under any load. I am now retired but was a car mechanic for most of my life. A bit stumped on this one. Any Suggestions greatly appreciated. Ray