Renovation of the propeller shaft and bearings

The first priority this year is to get the propeller running properly. Last year we were somewhat bothered by vibrations and shakings, partly from a worn stern tube bearing and partly from a damaged propeller. So now it will be fixed properly.

 

It was no big deal to take it apart once I had figured out how it was constructed - it was just a matter of fabricating a puller pull the propeller and the flange off. But to start with, it looked a bit problematic, because on the propeller shaft there is a bronze bushing close to the propeller which goes into the bearing. It took me a while to figure out that it was actually screwed onto the shaft - so there are threads on the propeller shaft. The bushing could be turned further in so that I could get a 7 mm plate in between the bushing and the propeller to pull on. Then the propeller was easy to pull off.

 

The flange at the other end looked more problematic at first glance. It was secured with 2 quill screws - I know from experience that such screws can rust so they are impossible to loosen. But that was not the case, they were easy to loosen, and with a custom-made puller and a little heat, it actually went quite easily.

 

The stern tube bearing itself has a lining of some fibrous, hard material, I think it's called Etronax, which is very difficult to find for sale. Today it's mainly used for electric isolation. Therefore, the new bearing will be made of graphite nylon.

 

At the other end of the shaft, a Bullflex coupling will be fitted, therefore the shaft must be turned down to 25 mm on the first 150 mm.

 

I wanted to get the propeller repaired, so I sent it over to Korsør Propeller A/S to get an offer for repair. But unfortunately, it will be too expensive to repair it - it is simply worn out, so a new one is needed. That's DKK 3850 extra out of pocket.

 

May 26, 2011
When the boat came into the water on May 9, I was very anxious to see how the newly renovated propeller and shaft would perform. My expectation was that it would run noise and vibration-free. But I was disappointed - the annoying vibrations are gone, but instead, there is now a deep resonance, which is present at all revolutions, and it is not completely vibration-free. Over 1700 rpm it makes a lot of noise, and over 2000 rpm it is very bad. I have checked the alignment of the engine and it 100% where it has to be. When I ran a test before the launch, there was no vibration, even at high revs.

 

So I think the root of the problem is the propeller itself. The sound changes when I turn: Stronger when the aft end swings towards the direction of rotation of the propeller, weaker the opposite way. I have the feeling that there is a kind of resonance in the propeller itself. This is a problem we've never had before, not even with the damaged propeller.

 

Another problem is that the new propeller has a slightly larger surface than the old one, and it is difficult to get over 2200 rpm. From approx. 2000 rpm the engine works hard. With the old propeller, I was able to rev it up to 2400 rpm and the engine only started to strain at 2200 rpm. The speed at the different rpm is pretty much the same as before, so to get a 5% saving with a modern class 1 propeller is definitely not something you can count on.

 

I have of course contacted Korsør Propeller, and as expected, they do not believe that the propeller is to blame for the noise, but instead the shaft. I do not believe that, because the only thing that has been done to the shaft is that it has been turned down at the front end so that the Bullflex coupling could be mounted.

So I'll take the boat out of the water and mount the old propeller - if this removes the noise, is it the propeller, if it is still there, is it the shaft.

I hope that Korsør Propeller will handle the matter in a sensible way, but we will see when I get the propeller replaced.

 

June 5, 2011

As I expected, the noise is completely gone with the old propeller - so it is the new propeller that is the cause of the noise. I simultaneously checked the shaft for skew and it runs 100% straight. With the old propeller we have a bit of shaking, but not nearly as bad as before the stern tube bearing was replaced and it is no more than we can live with it until the problem with the new propeller is solved.

The difference in the noise level at 1750 rpm is 3-4 Db, which is quite a lot, as 3 decibels is perceived as a doubling of the noise level by the human ear. I did not measure the noise level at higher speeds, but the difference is even greater - with the new propeller we have to shout to communicate, with the old one we can talk normally.

 

June 6, 2011
I spoke with John from Korsør Propeller A / S, and I must say that it's nice to get someone on the phone who knows what it's about. We agreed that I will return the propeller, they will send me a 17" with a slightly larger pitch, which should eliminate the problem.

June 20, 2011
The problem is solved, with the 17x14 "propeller it runs perfectly, the noise is gone. I think that Korsør Propeller A/S has handled the case with excellence. No discussion, they solved problem with expertise, so I can definitely recommend them without hesitation.