420AC: Vibration while underway

jeffsuwanee

jeffsuwanee

Member
Jan 21, 2012
77
Lake Lanier, Ga
Boat Info
1997 420AC
2000 Nautica 11DL Rib
Engines
Cat 3116
This is the first "large cruiser" I've had since my express cruiser with stern drives...so, I'm not sure exactly what to expect, but here's the story:

My 420AC with Cat3116's run very smoothly up to about 1800rpm's. Between 1800-2300, there is noticeable vibration. From 2300-2900, the boat runs smoothly again. I recently had the cutlass bearings replaced, so the shafts are very smooth and can be turned easily with one hand. I have had the shafts run and they are within 2/1000 at full rotation, so we know it's not the shafts. I've just had the 4 blade props tuned by a very reputable prop shop in Wilmington, NC...so we know it's not the props. The 420AC has the tunnels over the props...one of the techs that has been working on the boat suggests that the cavitation I'm feeling is due to prop wash within the tunnels at those rpm ranges.

The truth is, I don't know where to go from here...maybe with this style boat, I should expect that this is normal.....any thoughts would be great.

Thanks in advance,
Jeff
 
I had vibration. It was due to a combination of prop and shaft out of true/balance. There is nothing in the design of the boat that will cause vibration on its own. Mine is almost 100% vibration free though I did pay a fair chunk of change to get the best prop balance I could.
 
Your situation is the classic symptom of the engine/transmission out of alignment with the shaft. The shaft is in a blind because the engine is not aligned. That causes the shaft to be slightly deflected at the transmission end. As the engine develops rpm's and torque, the prop end of the shaft begins to whip, mirroring the deflection at the other end. For you its occurring at cruise speeds, which is unfortunate because you can't live with it there. If the alignment fault were less, then the whipping would occur at higher rpms. The fix it to realign the engines, and if you have never done it, this is one to hire done by an experienced mechanic. My last one took 15 minutes....but this ain't my first rodeo. I've seen it take 4-5 hours because you are working in very small increments (.001") while you move very heavy objects (engines coupled to transmissions) with a pry bar.
 
Thanks Frank...Our service manager said that he would uncouple the shafts and check it from there. I dont really understand what that means. Anyway, I didnt think that on the 420AC with diesel motors, that there was room for side to side alignment, only vertical. Seems like Iam in a bit of a conundrum. A completely different tech said the problem was caised by the underwater exhausts and that the cup and pitch of the props needed to be adjusted to account for the air bubbles generated by the underwater exhausts...I guess I am just not sure what to do at this point.
 
Check with another boat yard and ask their opinion. You don't want to go on a wild goose chase. Bring the mechanic out on the boat and let him feel the problem first hand.
 
The underwater exhausts may induce some bubbles in the water under the boat, but they occur well after the props have passed the bubbles.......in other words, there is no way exhaust will travel 3 ft sideways and 4 ft forward to enter the water ahead of the props, and if you are running props with the same specs as the OEM props, cavitation/ventilation caused by excessive cup is very unlikely........chalk that one up to a wild a$$ guess by someone who doesn't understand your boat.

There have been some complaints about exhaust vibration on smaller gas boats, but never on a diesel this size.

The 420AC has always been known as a smooth and stable boat. The only one I have spent some time on was new and I was moving it 35 miles from one store to another for the dealer and it was silky smooth and vibration free.


The alignment thing is really pretty simple. THe engine/transmission is coupled to the prop shaft with an engine coupler, The coupler is made in 2 halves. When the engine and shaft are aligned, the mating surface on those 2 halves are parallel and equidistant from each other. To check the alignment, the coupler halves are unbolted and separated a small amount and the mechanic measures the distance between them with a feeler gauge in 4 places 90 degrees apart. They should be within about .0015, depending upon the size of the coupler diameter. If the top and bottom distance needs to be adjusted, it can be done by raising or lowering the engine mount nuts, but if the side to side distance needs adjusting, you loosen the jam nut on top and scoot the engine one way or the other at the front or back to achieve t he right clearance between the coupler halves. So........we are only talking about thousandths of an inch of sideways movement.

Hope that helps you understand the process..........
 
+1 for the engine/transmission alignment. I have this checked each year. As Sea Salt noted the boat should be in the water at least 24 hours. This lets the hull settle to its natural position after being blocked, if it has been out of the water for a long period. The first time I had this done it cost $187. Another time it was $800 (the most). It is the labor time to get it done which adds up the cost.
 
Sometimes they are tough; other times easy. What usually runs up the cost is the mechanic not understanding that he only needs to make slight adjustments at a time. Once a pry bar is needed, they kind of default to not-so-delicate movements. With mechanics hourly rates around $100-$125/ hour, I don't want to see a lot of trial and error going on, hence my recommendation that you look for a mechanic that has done this before and has some gray in his hair.

Sea Rays are well engineered and well built so a lot of folks use the 24 hours rule and get by with it. Old timers, me included, feel a boat should rest on her own bottom several days before doing an alignment. In fact, the entire process should be done in the water, so I like to do it after the boat has been run several hours and in the water at least 3 days.
 
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I don't have any gray, but a lot of it is falling out....

Dale
 
OK Frank...I really appreciate the advice. I may have to seek out a technician off of Lake Lanier that has experience with this type alignment. Because I can see the $$ adding up with a greenhorn.
 
Outside observation/question from someone who never had inboards. Can you tell if it is port starboard or both? Or do you just have to align them both anyway? Good thread with lots of knowledge.
thanks Rob
 
Have a 420 AC with 3116 350hp Cats engine/transmission alignment done last year when I upgraded props to the new DQX motor has plenty of adjustment. Like frank said the 420AC is a very smooth running boat A bit heavy for its power at this time running 21 x 23 DQX with cup 2350 rpm will run 22-23 mph WOL 2850 will run 29-31 very smooth at all rpms 420ac with the 350 hp cats should have come with 21 x 24 props from SEA RAY The DQX have more blade area so had to lose 1” pitch . Exhaust has nothing to do with your vibration too far away and behind props. When did it start this and why Im with Frank on this think engine/transmission out of alignment with the shaft
 
You normally do both sides at the same time. Vibrations are sometimes transient and seem to move from side to side as you increase or decrease throttles. You really have to know the boat and have a very acute feel to be able to pinpoint a vibration. ALso, getting a competent mechanic and then getting him to the boat is a good part of the job here. Spending another .25 to 1.5 hours only makes sense and removes an unknown from the list if suspects.
 
fwebster said:
getting a competent mechanic and then getting him to the boat is a good part of the job here.
Click to expand...

BINGO! Same problem. In my neck of the woods. Best of luck with the vibration and hope you are back up and running smooth soon.
 
I don't feel this is an appropriate forum to discuss my hair loss. Besides, South Carolina is a bit far for me to travel...

As Frank stated, it is always a good idea, if you're in the engine room anyway, to do both sides. A slightly misaligned shaft may not manifest with an acute vibration, but it would place additional loading on the transmission bearings, shortening their life. Misalignment also reduces efficiency, and with fuel prices on the rise, this element becomes more and more important.

Too bad I can't re-align my hair follicles...

Dale
 
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