Drive Line Knocking

[morgantheship]

First some background. I have a 27' Hunter. It originally had a Renault 8 Hp engine in it. At the beginning of last season I repowered with a Yanmar 2GM 16 Hp engine. The engine runs fine while not engaged in forward or reverse (idles at 850 rpm's and can run it up to 3300 rpm's without any issues). But when engaged in forward gear, I can run it up to about 2200 rpm's and it seems fine vibration and noise wise. But above 2200 rpm's I begin to hear this knocking sound. It is not coming from the engine but I believe from the drive line. It is a consistant sound that will increase in sound signature as I increase the rpm's.

My thought is that when I installed the engine (on the hard), I lined up the output shaft of the gear box with the prop shaft. I used feeler gauges and I believe it was well aligned.

Someone then mentioned to me that I should re-align once it is in the water as the engine is supported differently and the alignment while in the water vs. on the hard could be different.

Does this make sense? Could that be why I am hearing the knocking noise due to a misalignment in the output shaft and the prop shaft?

I don't feel comfortable running it faster then the 2200 rpm's due to the knocking so I can only obtain about 4 mph speed which makes it quite difficult in a stiff breeze and current.

Any thoughts??? Thanks in advance.

 

[Don12364]

Morgan,
There are TWO alignments necessary for smooth running.

Everyone is familiar with the second one which is to have the two flanges parallel before bolting them together; but, before that, it is also necessary to ensure the prop shaft is centred in the log tube. If it is not then it can clout the inside of the tube when there are only quite minor vibrations present from the engine. The noise is usually loud and gives rise to great concern.

There is usually a short length of rubber hose between log tube and stern gland so it is not easy to see whether the shaft is close to the wall of the log. Matters are sometimes worse if one has a dripless stern gland with a face type seal as there is less to restrain a 'whirling shaft'.

Remember also that at higher revs the prop is pushing the boat along with considerable force and this same force is pushing the engine forward on its mounts, so any misalignment is likely to be magnified and loose or damaged mountings will surely be found out.

This log tube alignment is best checked with the boat out of the water; but as you are already afloat another way to check is to uncouple the gearbox and propshaft flanges and separate them slightly so that they are not in contact. Then see how far you can push the shaft out of alignment using hand pressure only. Use some force though as you will be bending the rather stiff hose. If you can move it roughly equally in all directions from the normally connected position then you are okay - i.e. up and down and side to side.
If you find that it hardly moves in one direction but has plenty of movement in the opposite direction then the engine needs to be moved bodily up, down or slid to one side until the shaft is centred in the log.

Only then does one do the smaller tilting movements on the engine mounts to get the flanges parallel.

 

[David in Sandusky]

Realign in the water

You definitely need to realign while the boat is floating, since the strain in the hull is totally different than when on the hard.

 

[capejt]

You said you get this noise/vibration in forward at a higher speed, do you get it in reverse? I have the same noise, but not in reverse.
I'd love to hear what you find as a remedy.
Thanks

 

[Alex Dare]

Definitely has to be aligned in the water. On the dry doesn't cut it. A boat generally needs a couple of days in the water before it takes it's proper shape and the engine and shaft can be aligned. Was the prop changed and cutlass bearing replaced when you re-powered?

 

[amcmahon]

Hey Morgan, I have a 1984 h27 and re-powered with a Yanmar 2YM15 about 4 years ago. I won't comment on the in/out of the water issue since I did the re-power while boat was in water on mooring. However, getting alignment just right has been a slow process. I did use a feeler gage to get the flanges aligned, but this does not help centering the prop shaft in the tube. When I experienced the clatter, I tried to establish which side of the tube was being affected. I found that by grabbing the flax packing nuts and pulling, or pushing or raising/lowering the shaft, I could determine which side was closest and banging. Then I made minor adjustments to better center the engine and shaft. After a few tries I was able to completely stop the banging. Tony

 

[morgantheship]

Drive Line Knocking - Up date

To up date you on my ‘drive line knocking’ issue. I finally was able to get down to the boat and attack the problem. I first checked to see that all of the bolts on the hub/coupling were tight as well as the engine mounts. Everything was tight as they should be. I wanted to make sure that a loose bolt was not causing the ‘knocking’.

I then removed the 4 bolts from the hub/coupling and the output flange of the engine and loosened the motor mounts so that I could adjust the height and angle of the output shaft of the engine as needed. I found that the engine output shaft was a slight bit lower then the prop shaft. I also found that it was at a slight angle to starboard. I then spent the next 3 hours adjusting the height and angle of the engine output flange. When I finally was satisfied with the trueness of the hub/coupling to the output shaft of the engine (using a feeler gage for reference), I reconnected the 4 bolts mating the hub/coupling to the output flange of the engine. I then tightened all of the motor mounts. Finally I rechecked the tightness of all bolts to insure I didn’t miss anything.

The real test now was to run the engine and see if my adjustments eliminated the ‘knocking’. I started the engine and with the boat still in the slip I tested both forward and reverse by running the engine up to 3600 rpm’s. (if you remember I began hearing the knocking above 2000 rpm’s). To my great delight the ‘knocking’ was gone. I then repeated my test two more times. The ‘knocking was gone.

I want to run one final test by taking the boat out into the harbor to see if anything changes while the boat is actually moving through the water. I hope to do that this weekend if the weather abates some.

To be very honest I was surprised that I was successful at doing this. But I am glad that I took the time and effort to try. Although, I must admit that laying on top of an engine for 3 plus hours was not the most fun position to have one’s body in.

Thanks for all the advice. I will provide a second report once I run the boat through the water.

 

[capejt]

Glad to hear it!!

So glad you made so much progress! I too am facing the knocking. I tried the other day to check the engine shaft / prop shaft alignment, but couldn't get the hub separated. I think it's about time for new engine mounts anyway (31 years), so I think I'll just "baby" it for the rest of the season and tackle the alignment after I replace the mounts this fall/winter.
It IS encouraging though to hear you did so well.