Vibration


N

nsgy11

New member
Would like to start a discussion on how to deal with vibration issues. I am getting some vibration - not bad - when I come up on plane at 2400 RPM on my 2005 40 PC, inboard with 440 Yanmars 6LY2ASTP. There is a sweet spot at 2600 RPM and then some vibration at 2800 and up. Able to get 3200 RPM on both engines at WOT but not the recommended 3300-3400 RPM. Need to check this at the shaft with a tach gun. At 3000 plus vibration and sounds like engines are not completely in sync. I do reach 30 Knots measured on my GPS. Boat Bottom is clean, had a diver check this, and I did not hit anything with my props.
When I did a pre purchase survey there was cavitation at the base of the props on both sides. Dealer Had props cleaned and pitting was removed but I do not know if they balanced the props and checked pitch.
Question is how to start systematically working out the problem - adjust engine RPM, engine alignment and shaft first or start at the prop and work back.
 
Was the vibration there when you test drove it? Have you tried single engine operations to rule out one engine or the other?
Could be a prop imbalance problem however it is strange it goes away at 2600. (There are certainly no critical speeds in this system - stiff shaft for sure)
It sure would help to get a vibration probe to determine the vibration frequency:
1 times shaft operational speed = imbalance (propeller, prop shaft run-out)
2 times speed = looseness (prop nut loose, poor prop to shaft fit, you may pick up a bad shaft bearing as well)
 
No vibration at sea trial but boat was trucked over and put in water.I had a new transmission install on Port engine. Engine mounts on port side OK. Have asked mechanic to recheck alignment and re torque mount. have not tried to run on one engine but sounds like a good idea. What I am calling a vibration is what I feel on the transom at various speeds. Not a smooth run above 2600 RPM. Have never used a vibration probe.
Thanks
 
If the vibration continues after the alignment you should check the cutlass bearings next haul out.
I have the same model, year and motors. Last season, my first with this boat, I had vibrations that progressively got worse.
This off season I replaced the cutlass bearings on both side, the old ones where worn and cracked so they were will past needing replacment. This year upon launch I had the motors re-aligned and I am vibration free.
Removing the bearings is a big job because they tend to fuse to the strut. A hydraulic press is required to get them in and out.
Also the stock replacement bearings are typically 8" long but if you have the same strut as me then you need only 7" so precutting is recommended. The parts are cheap online, the job is a bear.
Good luck. and don't run the boat for more then a trip from the launch point to the dock without the alignment in water or you'll be replacing the bearings again very soon thereafter.
 
nsgy11
Did the vibration start after the tranny replacement? Then maybe alignment. I use to get 3200 rpm sent props for prop scan. They were cut down and tuned now I get 3350 as recommended by Yanmar. I did lose a little speed at cruise.
 
I had a somewhat similar problem on my 37 PC with inboards. I was breaking the bolts where the port engine links to the V-drive (don't know the formal name!). I was getting a similar vibration as it increased at speed. Engine alignment fixed the problem.
 
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