Hard knock in forward gear at low RPM

[Claude L.-Auger]

Good morning folks.
Been looking at the archives but don't really know where to start and hence looking for advice.
My boat has a 3GM30F engine with the Kanzaki KM2P-1 tranny (2.62/1 ratio). Recently I started having a severe knocking and some vibration at low RPM while in forward gear. No vibrations or knocking at all RPM while in neutral or in reverse. As I did not hit anything and the prop is clear and clean, my first thoughts are:
1) Misalignment: Everything checks out and less than .003" variance on the coupling. No vibration at high RPM !
2) Bent shaft: No vibration in reverse and none above 1500 RPM in forward gear.
3) Damaged prop or line caught: Nothing at high RPM and I have a line cutter.
4) Loose zinc on shaft: I think I would also hear some knocking when shifting in reverse.
5) Damaged or worn engine mounts: That is a big ??? They are very old but my strongest friends cannot shake engine manually from either side.
6) Worn cutlass bearing: It is about 10 years old and could be an issue but would it not create the issue at all RPMs and also in reverse ?
7) Worn clutch cone slipping ? Seems possible particularly when seeing that everything runs A-OK when above 1500 RPM and there are no knocking or vibrations.
Given all of these ideas, I am wondering if I'm missing anything. Boat is coming out of the water on the 19th and I want to get everything fixed before next season as I will be planning a very long cruise so do not want any problems. Given the high cost of motor mounts and tranny, what would be your recommendations as to what I should look for beyond the above points or the highest probability of the cause ? Thanks for all suggestions. Claude

 

[ggrizzard]

Cutlass bearing. It may only knock at a specific RPM.
It's old enough to warrant replacement anyway. And, it's a cheaper fix if it is the problem.

 

[kloudie1]

The hard metal of the top of the mount, should be 3/8" or greater above the bottom plate.. If both ends of all 4 mounts are ok, I'd think the mounts would be OK. I changed mine a few years ago for an identified bad mount and the problem went away.. You can troubleshoot by having someone push or pull on the valve cover (side to side) to see if the knock gets better or not.. The shaft hitting the inside of the stern tube is another possible but you've probably checked that.. and of course, the forward cone clutch which was said to be the problem in a recent post with similar symptoms..

 

[NYSail]

Have you dove on the boat to check underwater?? Also have you gone down below while knocking was occurring to try and locate. If its that loud one would think it would be obvious and identifiable.... you would see engine moving it it were mounts. Your friends can't move engine might not be good... as the engine is designed to have a little bit of movement and bounce.... Also if you suspect the shaft or prop just look at shaft while spinning from inside the boat.... you should be able to see a wobble or at least feel it. And diving you should be able to check prop, zincs and cutlass..... (make sure you turn engine off when diving..... haha). A friend just had a similar problem and continued without really checking. Turned out his prop was damaged and the strut wobbled loose..... big issue.


Good Luck!!

Greg

 

[datasailor2]

I had similar issue. Found problem was worn out Cone Gear in tranny

 

[markwbird]

That sounds like a worn clutch cone. I've just been through this myself. Heavy knocking vibration only in forward gear. I replaced the shaft, cutlass bearing, engine mounts and balanced the prop and it did not go away until I disassembled the tranny and resurfaced (lapped) the clutch cone. That did it.

 

[walmsleyc]

As kloudie mentioned, but worth double checking, is knocking against the stern tube. My shaft was not well centered in the tube, and at low RPMs, quick deceleration, healing while switching gears, and such (things that make the engine shudder), I would get knocking of the shaft on the stern tube.

Checking for centering can be checked visually with goggles under water. Alternately, if you grab the shaft just in front of the stern tube (engine not running please!) and push up/down/port/strbd and see if you can make the shaft touch the stern tube. If you can in one direction and not others, then you may have a stern tube alignment problem.

Chris

 

[Benny17441]

Your 3 cylinder engine firing sequence is not inherently balanced. The engine requires dampening counter forces and sound motor mounts to tame the unbalance which is more evident at low RPM. To the determine if the fault is engine or drive train related run the engine in neutral and slowly accelerate it to see if the knock is present. If it is then check the motor mounts, not just look at them but jack the engine up to check if they are too soft or cracked. If no nock then you know it is drive train related. With the boat tied at the dock engage the transmission in forward and accelerate the engine while observing the engine and the turning shaft. If the engine don't shake and the shaft turns concentric then the fault is in the cutlass bearing or fouled prop. If the engine shakes then the motor mounts are likely soft or if the shaft wobbles then it is damaged or the cutlass bearing is worn. If everything checks out good then look at the tranny like others have suggested.

 

[Claude L.-Auger]

Sorry not to have come back quicker guys but the weather has been real nasty and I didn't get to the boat until yesterday. Hopefully the week-end will allow me to perform more tests.
- NYSail: Water is now too cold for a dive (I don't have diving suit). Besides, the boat is coming out of the water in 2 weeks so I'll have a close look at shaft & prop, and be able to check for wear on cutlass bearing.
- ggrizzard: You are right that cutlass bearing has been there long enough to warrant a change. In the scheme of things, it is not too expensive when you figure it has to have been at least 10 years !
- Kloudie, at this point, from a close inspection with a flashlight and looking at the measurements on the front part of the front mounts reveals a sad shape. Pix of port side front mount included shows top and bottom plate distance different from left ( < 1/2") to right ( < 3/8") of mount, and the back side of same mount barely at 1/4". The others are not in any better shape ! Plus the rubber seems to have delaminated although it does appear very firm and the engine is hard to move manually side to side and front and back. No wobble on shaft between coupling and stuffing box. No knocking sound at stern tube. Does feel like I'm going to have to bite the bullet and replace them. I suppose I should not complain as I've had the boat 19 years and put on more than 25,000 miles on her. I took the engine out and rebuilt it in 09 and used the same mounts !
- Benny: I have already done some of those tests and plan on doing all of them this week-end when I have a friend available to help. Checked from the inside and strut is strongly attached. Noise and vibrations do seem greater towards the back of the cockpit when engine in forward gear (below 1500 RPM; nothing above that and boat runs smoothly). No vibration and no noise other than engine rumble when increasing RPM in neutral. No noise or vibration in reverse.
- Proponents of cone clutch wear: You may be right but at this time, because of the age of cutlass bearing and engine mounts, I feel it is probably smart to replace them before taking transmission out. I am planning a long cruise for next summer and will have a safer frame of mind knowing this is all new. It can all be done while the boat is still on the hard and tranny checked upon launching in the spring. Which would still leave me time to have the cone clutch looked at.
Thank you all and please don't hesitate to come back with any other idea that might help ! Claude