I replaced the shaft to my 20hp beavertail. The motor still seems to vibrating really bad at high rpms and rattling at both low and high rpms. My prop was a little loose, would this be causing the noises and vibration, and would it be better to tightened it down with a air wrench? Will a full shaft housing of grease relieve some of this vibration?

You might want to check all of the welds. Especially near the prop on the rock guard.

another thing as to why your prop is loose im not sure how the setup is on a beavertail but on mine i have a prop nut, washer, prop, washer, and prop nut then jam nut you cant just tighten the nut then the jam nut you need to get 2 wrenches on there and tighten the nut in front of the prop and the one behind it together if you just tighten the one after it it will come loose every time just my 2 cents

Have you looked at your bushings? Im not sure how a beavertail is set up but there aught to be bushings in the shaft at least at the bottom. My setup has a roller bearing at the top and a bronze bushing at the bottom with a grease zerk. I know some people put a bushing in the middle of the shaft to eliminate "shaft whip" which could be whats happening? I got a washer up against my shaft housing with a little bit of play, lock nut, prop snugged, nut up against it, lock nut snugged against it and haven't had problems yet.

I would guess the bushings are the issue. Most people replace them if/when the replace the shaft, that way everything is newly replaced and has the same wear.

Wow, where have I heard this before? What year is your motor? I just went through replacing everything from my prop nut to my u-joint on my 2000 beavertail. If you grab the prop nut, are you able to shake the shaft? Is there play? I will tell you if you have a older motor that has a bronze bushing as opposed to the (3)SS bearings they make you will not be able to find the bronze bushing.

The shaft does move a little bit when i rotate or wiggle the prop back and forth, the keyway is a little loose at the shaft to the motor, but it vibrates really bad and makes some ridiculous noises. It is a little older motor.

you may have bent one side of your prop. if so, your top bearing housing will get HOT. run another prop and see if the problem goes away, if not, it's bushings or bearings, or rarely, a bent shaft.

bent shaft should be easily spotted by turning prop by hand.

a full shaft tube of grease may quiet down the noise, but is just masking the problem, not fixing it.