Shimmy Wobbled 1990 40 DC

[Cinken007]

Hopefully someone can add new info to my mystery shimmy/wobble. I have a 1990 40 DC with D.D. 6V53ti engines - 1650hr. At 1500rpm’s to approximately 2000rpm’s there is a shimmy/wobble at the helm chair and at the aft deck. Outside of those rpm’s the boat runs good. This is what I’ve done to date chasing this issue down, starting with the props. Reconditioned props two years ago and hade them checked again the following year. Each time they were within specs.I had the shafts checked (dial indicator) both were under 5k I had the struts checked, all are good. The cutlass bearings had some (not much) play so I replaced them. I had an alinement completed this week and found the port side good. The starboard side was way out and adjusted. The motor mounts appeared to be in good condition. No movement under load. The port side transmission (twin disc) has what they call “neutral drag” where the shaft rotates but goes away right away. Now I’m thinking about a tune-up on the engines. Maybe injectors? Engines rune good and reach WOT rpm’s Synchronizer? It works. Wobbles if engaged or not. Your opinion matters! Please respond with your ideas. Thank you!

[racclarkson@gmail.com]

Frustrating I’m sure. Was WOT no load or underway? Possible to make it unloaded.

[jim rosenthal]

Does this occur with the boat in or out of gear, and is it specific to one side being in gear?

[Cinken007]

The boat is in gear and I felt it more on the port side. WOT and under way. When not under load there is no shimmyWhat’s interesting is that it happens right at 1500 rpm’s and it shimmy’s when accelerating and decelerating. Maybe I/we missed something...... I have another set of reconditioned spare props and I’m thinking about changing them out.

[rustybucket]

The boat is in gear and I felt it more on the port side. WOT and under way. When not under load there is no shimmyWhat’s interesting is that it happens right at 1500 rpm’s and it shimmy’s when accelerating and decelerating. Maybe I/we missed something...... I have another set of reconditioned spare props and I’m thinking about changing them out.

That should at least help rule in/out the props.

[oscarvan]

Sounds more like a harmonic vibration than an out of balance condition. Hard to trace down. You could get a vibrometer and measure a set of locations, increase 50 RPM, measure all and keep going until you find where it starts.... But I'm thinking outside the box and not speaking from experience. Cost you a few hundred though instead of throwing parts and labor at it in the hope of maybe eliminating it.

https://www.amazon.com/VM6310-Digital-Vibration-Vibrometer-VM-6310/dp/B00KCO798K/ref=sr_1_8?dchild=1&keywords=vibrometer&qid=1595686185&sr=8-8

[racclarkson@gmail.com]

Sounds more like a harmonic vibration than an out of balance condition. Hard to trace down. You could get a vibrometer and measure a set of locations, increase 50 RPM, measure all and keep going until you find where it starts.... But I'm thinking outside the box and not speaking from experience. Cost you a few hundred though instead of throwing parts and labor at it in the hope of maybe eliminating it.https://www.amazon.com/VM6310-Digital-Vibration-Vibrometer-VM-6310/dp/B00KCO798K/ref=sr_1_8?dchild=1&keywords=vibrometer&qid=1595686185&sr=8-8 Read the Q & A on Amazon. No need to ask where it’s made.

[oscarvan]

Read the Q & A on Amazon. No need to ask where it’s made.

Didn't go that deep and I'm sure you are correct. So a few more $$ for a better one? I think the idea is valid.

[rustybucket]

Didn't go that deep and I'm sure you are correct. So a few more $$ for a better one? I think the idea is valid.

Knowing the frequency would at least help you separate the vibe from the engine and the trans->shaft->prop as they would be spinning at different rates.

[Nonchalant1]

I had that in a 53MY and it turned out to be worn rudder bearings and a loose rudder shaft top collar. Particularly likely with the aft deck vibrations like mine. I ran it at the "wobble" speed and then went down and looked into the lazarette where the rudder posts are and I could see and hear the wobbling. I replaced the rudder shaft bearings and tightened and aligned the top collars and Voila, no wobble. Note that the rudders are not supposed to be exactly parallel. There is some small intentional offset per specs.

[racclarkson@gmail.com]

I had that in a 53MY and it turned out to be worn rudder bearings and a loose rudder shaft top collar. Particularly likely with the aft deck vibrations like mine. I ran it at the "wobble" speed and then went down and looked into the lazarette where the rudder posts are and I could see and hear the wobbling. I replaced the rudder shaft bearings and tightened and aligned the top collars and Voila, no wobble. Note that the rudders are not supposed to be exactly parallel. There is some small intentional offset per specs. Now isn’t that interesting.

[kelpy]

Worn out 30 year old front engine mounts are another possibility. Observe the engines and rudder linkage the next time the condition occurs.