perfboat

I've got a merc 502 mpi/b1 combo single engine. My problems started with a bad gimble bearing. So I replaced the gimble bearing and u joints. Now there is a vibration or shudder that feels like it is coming out of the prop area on acceleration just before the boat rolls over on plane. It is rpm specific and only occurs at about 2500-3500 rpms, drive all the way in. The boat runs fine at speed and in the no wake zone. I replaced my slightly dinged mirage plus 23p with a brand new bravo one 22p and the vibration changed rpms with the prop. The mirage was around 2000 -2500. I was stumped, so, I pulled the prop and put a dial indicatior on the prop shaft and came up with .003 runout. With the boat on the hose and in gear if I accelerate, the hub area of the prop wobbles some. But, looking at the center of the shaft it is perfectly straight. I was told this is normal as long as it does not contact the bullet area? There is no binding in the drive at all and this problenm has me completely stumped. I even tried a never used mirage 21p just to have some guidelines to go by in order to rule out a bad prop or hub. It wobbles slightly also but not as bad as the bravo one. The prop appears to turn true when viewed from the rear. I have not had a chance to try the 21p in the water yet. Also, could I just be experiencing ventilation? If the prop shaft is bent how hard is it to replace? I'm very mechanically inclined and have access to the tools needed. Anything special I need to know to replace it? Any ideas would be greatly appreciated.

bowtie

Try trimming out a little when the vibration starts and see if it goes away.

articfriends

iTrader: (1)

I doubt .003 runout is the culprit. What your seeing when you turn prop or watch prop running is the imperfect body of the prop,you have to remember it is a casting and won't be perfect.Your problem sounds like slight cavitation as boat tries to get on plane,Smitty

perfboat

This site is the best. Thanks guys. I'm going to go out tonight and try what you suggested and see what happens. The funny thing is that the new bravo one prop wobbles alot compared to the old mirages? Julie at throttle up (awesome people) mentioned the prop could be out of balance. So, i'll have it checked. Artic friends, you mentioned cavitation...could be, because it feels like the vibration/ shudder is beyond the shafts and gears and at the prop. Bowtie, I'll try what you mentioned. Plus I can feel it as I turn the steering wheel slightly. Once again, at the prop, not the ujoint area. I just assumed that I should never feel any vibration or shudder at all. None of my other boats have ever done this.

vandeano

Only thing that concerns me is that the vibration occurred after the rebuild. I would be double checking the repairs and other possibilities.

jpclear

I agree with Smitty. Merc. says that up to .005 runout is acceptable. --- Jer