Vibration issue

My Tieara 3100 with twin 454 Crusaders and velvet drive transmissions has developed a bad vibration at the rear of the boat that starts at approx 1800 rpm seems to peak at approx 3000 rpm and goes away at 4000 rpm and above. The vibration has slowly been getting worse over the past year. I plan on checking the engine alignmentthis week but a cursory review of the engine mounts indicates that they are in bad shape and so I suspect that realigment will not be easy. I have also read on this forum that vibration like this can be caused by the rudders being out of alignment. Also it has been approx four years since the props were last tuned. My question is that with so many possible causes of vibration, does the rpms at which the vibration is worse give any clue as to the possible cause?

Possibilities: Dinged prop, worn cutlass bearing in strut, alignment / shaft not straight. At the high rpm it spins so fast you can't feel the the vibration.....typical.

-Mark-
85 31 Tiara Open

I had a similar problem a few years back but at different RPM's. It turned out to be a couple of dings on one of the props. The dings were not very large. I never thought that a couple of small dings could set up a vibration like I had. I'd start your search for the problem by taking a very close look at those props.
Good Luck,
Nightstalker

Also, rotted stringers allowing the motor to torque out of alignemnt.

You have your work cut out for you.

Jeff

The rpm won't be as good a clue as where you can feel the vibration. Based on your description, I'd be inclined to start with the props. Worn out or broken mounts will cause issues because alignment isn't maintained.

Thanks for all the input. I plan on checking the engine alignment this weekend and see what that brings. I had the diver take a look at the props, shaft, strut etc and he says that as best he can tell everything appears ok. No bent props or major play in the cutless bearings. So my thinking at this point is that if the engine alignment is ok, I will pull the propos and have them balanced. With respect to the location of the vibration I notice it most at the very rear of the boat right around the steering gear. It feels like something in that area is trying to shake the boat apart. While underway I opening the engine room cover to look at the shafts spinning and they appear to spin straight however I am not sure that you can tell anything about alignment from the ***** eye.

Vibration from a bent shaft, mis-alignment or rotted stringers (same thing) will show up at the props, for that's where the force of all that horsepower is being applied. Your diver can determine if a prop blade is bent, but no way could he determine a bent shaft or truly bad prop. Before spending big $$ to play with the props, the smart thing to do would be to take the boat for a ride and see "What's shaking."

Have someone else run it through that vibration zone and see if the motor moves to one side. Note the following:

1. If you can see ANY movement, that's too much.

2. If the motor is rock solid, that eliminates rotted stringers and loose mounts. That leaves misalignemnt and/ or a bent shaft.

3. If the shaft is bent, you should be able to see it wobble/ run out at the coupling, possibly moving the rear of the trans.

Note: You can NOT align the motor with a bent shaft!

Jeff

Checked the engine alignment yesterday and the port egine appears to be spot on but the starboard side may have the engine a little high. One this I am a little confused about is centering the shaft in the shaft log. After unbolting the transmission coupling from the propshaft I attempted to center the shaft in the shaft log so I could then check the alignment however I found that I have very little movement in the shaft. Maybe a fourth of an inch up and down and nothing at all side to side. Not sure if this is a problem or not as most of the posts on this isuue seem to induicate that there should be some play in the propshaft log. The side to side alignment seems to be perfect but I need to lift the shaft up about an eight of an inch to get it to line up with the shoulder on the coupler. Does this sound like an acceptable situation. I have no visual way to see if the shaft is centered in the log so I am having to rely on feel to get it centered. Any thought?