engine knocking

[Bob]

I stated on a previous thread that I thought the zincs were bad because of what the diver told me. This turned out not to be the cause of the knocking. A mechanic told me that one of the injectors is not working properly. This could be true, however, when the engine is in neutral and brought up to 3,000 rpms there is no noise. Once I put it in gear and apply throttle the knocking starts. The engine does not appear to be out of alignment with the shaft and I have not hit anything. This knocking started on its own. Any thoughts or similar experiences?
Thank you.

 

[jviss]

A little more?

Please describe "knocking." Is it like pre-ignition knocking in a gas engine/ Is it top-end rattling, like tappet clacking noise/ Is it drive-train thumping, like a low-pitched banging?

Etc.

Have you gotten your head near the engine to localize it?

 

[Bob]

The noise acts like there is something wrong with the drive train. It makes a thumping action that shakes the boat. However, the shaft moves freely when in neutral.

 

[VIN]

This has happened to several of our boats in the Club.
The knocking could be caused by engine mounts. We found this to be the case in several boats where the engine shifts around when put in gear and shaft hits boat entering thru bottom. Also check cutlas bearing and straightness of shaft when pulling the boat.
Good luck
vin

 

[jviss]

That's not engine knocking!

I know the sound well. Vin is pointing in the right direction. Causes are misalignment, bad engine mounts, worn cutlass bearing, out-of-balance (shaft, hub, prop combination), or some or all of the above.

 

[RichH]

Several cuses

You stated that the engine runs smooth in neutral .... therefore the problem is from the transmission back.

Most probable causes (all imbalance problems)
1. barnacles on the shaft or prop - go overboard with a scraper and clean the barnacles from the prop/shaft.
2. bent prop or propshaft
3. broken motor mount

think simple
Barnacle growth on the east coast in past few years has been 'ferocious', especially in brackish water due to the higher water temps. Use a 'barnacle barrier' spray paint on the shaft/prop. I prefer Petit Zinc Rich Barnacle Barrier.

http://www.defender.com/product.jsp?path=-1|10918|296162|1017093&id=157662

 

[Carl]

I have a knock too..

I've never been able to figure it out.. New engine mounts,new cutlass bearing,
correctly pitched prop.. aligned drive shaft and so on.. Mine is almost like
a loose piece of wood hitting something at different engine rpm's.. It's also
known as a Hunter knock.. I've had this knock since day 1 of ownership about 7
years ago now.. I can get this knock to stop at about 1200rpms but any higher
and there it is.. I've gotton use to it now as the engine is running great.
I would love to know where this is coming from but it seems it was build that
way from the factory.?? At cruising rpm 2600 or so you can't hear at all so I
live with it.