That's a gimbal bearing, mine was doing the same thing, but had a noticeable vibration you could feel in the hull whenever you'd steer hard at idle speed. As soon as it started doing that, i took it in for service, had the gimbal bearing replaced, engine winterized, oil changed, and exhaust flappers replaced for $1700. Gimbal bearing replacement is I believe around $800. It's a pretty serious job, that needs specialized tools, I'd personally leave this job up to the service department. And don't drive on it if you've noticed any of the symptoms described above. It'll only make it worse, (and more expensive to repair!)

Worn u-joints can cause a similar sound, especially if it's only when turned all the way left or right. But if you notice decreased speeds and the noise all the time, it's almost definitely the gimbal bearing. It's tough to tell, because both can cause similar sounds at similar times.

If I remember correctly, my gimbal bearing was around $600. They should be able to cut a deal on the bellows, since they must be removed while servicing the gimbal bearing anyway.

regaurdless if the u-joints or the bearing is bad, the drive has to be removed. Correct? I plan on doing this myself, not this weekend(will be in PA till next fri), but the next weekend. Will a slide hammer pull the bearing out? What is needed to install? What all should I buy "incase" I dont want to get into it one saturday and need parts and the place be closed sat at noon till Tuesday...

Based on the figures quoted here I would suspect a it's a 4 or 5 hour job minimum.

Mercstuff has a nice how to with pictures and lists what you'll need.

http://www.mercstuff.com/gimbalbearing.htm

Lifting the drive off, and re-installing the bellows are probably going to be the most difficult parts of the job.

I would look at a 19" prop. You are not turning that 21 fast enough.

a 350 V8 is plenty of motor on a 21' boat to turn a 21 pitch prop. I had this same problem with mine. 300hp and the thing would only do about 44 MPH at somewhere under 4000 RPM. The gimbal bearing was replaced and now I'm seeing 58 MPH at about 4600 RPM. I don't know how or why the bearing can cause it (other than the obvious... friction), but I'm fairly certain that's what the problem is here.

I upgraded to a SS 14.5" diameter 21 pitch, but I'm wishing it were 14" or 14.25" diameter. I'd like to squeeze out another 200 RPM and hopfully see the 60 MPH mark! I think a drop to 20 pitch would only raise my RPM, not speed. The hole shot is excellent even with the 3 blade 21 pitch. A 19 pitch would send me over the rev limiter before hitting 55 MPH.

I have a ss 21 p prop I would be willing to sell. Its to much for my 20 footer with a 5.0 motor. I wish I have a 350 mag.

Yeah, I'm with Ryan. I run a 13.75x21p Vengance with my 5.7L Shabah, and it's propped just about right for good overall performance.

If the motor can't pull that prop up to typical WOT rpms there is something definately wrong elswhere...

What's the diameter?

It is a Laser 2 14X21 no dings like Brand New. Could get some pic if you would like.