Pulsing vibration while crusing

2001 4.3L alpha 21' aluminum deck boat. Spent some time just cruising at 3600-3800 rpm last Monday and I noticed a vibration that pulsed in and out every 1-2 seconds. Not strong, just something you could feel through the deck. Had the out drive pulled and checked last fall. The mechanic said the gimble looked fine. He did use OMC HPF XR oil in it. I have been having to add a little oil a couple of times to the drive reservoir this summer. Is this oil ok for my drive and any ideas on the vibration?

Re: Pulsing vibration while crusing

Did the prop get damaged or bent? Would explain feeling it at upper rpm's

Re: Pulsing vibration while crusing

chcliffb said:

2001 4.3L alpha 21' aluminum deck boat. Spent some time just cruising at 3600-3800 rpm last Monday and I noticed a vibration that pulsed in and out every 1-2 seconds. Not strong, just something you could feel through the deck. Had the out drive pulled and checked last fall. The mechanic said the gimble looked fine. He did use OMC HPF XR oil in it. I have been having to add a little oil a couple of times to the drive reservoir this summer. Is this oil ok for my drive and any ideas on the vibration?

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I am not sure about the OMC oil but, I sure that will not create any vibes . What you have is a harmonic type of vibration . Something is setting it off and it is traveling down through the hull and gets dissapated and keeps repeating . I had a freinds 69 Dodge Super Bee that would do this about 75 MPH or so . The viberation would , like you say roll in and roll out and keep reapeating . It could have been a driveshaft or even the engine out of balence . In your case you could have a prop the rebuilt too many times and is out of balence or the prop shaft could have a slight waver to it . . Another thing could be the rear motor mounts . There is a 1/4 inch thick fiber washer that goes between the block and the transom plate . If one of these are missing the engine could be torquing over and and making contact with the transom plate and sending normal engine viberations through the boat .This is quite remote but I have seen it happen if someone took the engine out and missed these washers while putting the engine back in

Re: Pulsing vibration while crusing

I agree, check your prop.

Re: Pulsing vibration while crusing

Might want to check the prop shaft for excessive end play and that the prop is tight.

Re: Pulsing vibration while crusing

About a month ago, I did bump through something at the launch at idle hard enough to knock a chip out of the back side of the skeg. Something was rolled between the prop and the skeg. I smoothed down the rough edges on the prop with a file, but removed very little material. I will check and see if the prop shaft is bent. Can I check the prop or is this better done at a prop shop? Thanks.

Re: Pulsing vibration while crusing

Well just great. The prop shaft is bent. I will get a quote from the mechanic on Monday. I just imagine that this will be a little over my head. What do you think?

Re: Pulsing vibration while crusing

chcliffb said:

Well just great. The prop shaft is bent. I will get a quote from the mechanic on Monday. I just imagine that this will be a little over my head. What do you think?

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Best taken to the shop on that one. Sorry to hear it but at least you know where it was coming from.