GB - Need to replace the shaft??

Another journey in repairing my GB - she is a 2008 Europa model 47ft and hull #50. We are in the Northeast and had her hauled out in October 2020 at a yard up here. The hull was in really good shape and I did an inspection to see if there were any problems. The one thing I noticed was that the prop nut on the Port shaft was lose against the washer. There was about a 1/4" gap between the prop nut and the prop. The lock washer was still engaged with the prop and the one face of the prop nut. You could easily move the nut back and forth with your hand and it would go as far as the lock washer allowed. I thought this strange. First photo of discovery - Oct 2020

This shaft had been uncoupled and pulled back to replace the shaft seal (it was leaking pretty badly after the survey sea trials) and also the cutlass bearing was replaced in the strut because it too was shot. The prop was taken off the shaft for this operation. The boat was put back together at the Miami yard and we were off to the Northeast. Two photos of Miami yard efforts Feb 2020

So yesterday the yard pulled the Port Prop for an inspection. At first I had thought that possibly the prop was not torqued properly back in Feb 2020 and somehow on a reverse bell the prop had walked up the taper. But when we pulled the prop the prop had been on the taper just fine and centered on the key with not any damage to the key or the edges of the key. Upon more careful inspection two things were noticed with the threads that the prop nut engages. One thread was partially missing (about 2") and, in general, the threads under the prop nut (when in the properly torqued position) were stretched out and somewhat flatter than the threads near the very end of the shaft. Last three photos show the thread damage.

So, now I am thinking that possibly the prop nut actually popped back over several threads after being properly torqued by the Miami yard. I am not sure how a stainless steel thread disappears other than with a case of crevice corrosion over time. I am sure there are other theories as to what took place but that is the only one that I could come up with.

Right now we are thinking the only way out of this situation is to replace the Port shaft - typically corrosion does not get better with time and the threads in the bronze prop nut look fine. More to come I am sure.

Bob

Hi Kernr,

This does NOT look like a shaft machined to ASME/SAE standards for tapered and threaded propellor shafts. For instance, I've never seen a single prop nut on a shaft without a cotter pin to secure it. Nor a washer between a prop nut and the prop. Sure, lots of double-nutted installations, but never one with only a single nut, without a cotter pin. And you call this washer a "lock washer". How so? Not standard operating procedure for securing a propellor to a shaft, for sure.

I suggest you investigate different double prop nut installations, and the correct assembly procedure, such as: https://www.passagemaker.com/channels/propeller-nut-myth-busting

Should a new half-height nut properly torque down against the prop (properly seated on the taper, of course), then the assembly may work as designed. This might be a stretch, given the extent of damage to the shaft immediately behind the propeller. However, if the appropriate bronze half-height nut torques properly upon installation, all should be well.

Assuming, of course, that there are at least 1 diameter's worth of threads left to engage with the full-height nut! The thread length looks too short, at least to the naked eye. It is entirely possible that someone in the distant past miss-machined your shaft. Is the stbd shaft/prop/nut similar?

Drilling the existing shaft, and using a single castellated nut and cotter pin is a possibility. However, castellated propellor nuts are significantly thicker, and you may not have enough shafting under the "castle" to accept a drilled hole and cotter pin. Assuming, again, that the single nut will accept full installation torque.

Regards,

Pete

Take the shaft to a machine shop and weld up the threads, machine a new surface on a lathe and cut new threads. Then the shaft is reusable.

As above, a half-height nut first then a fat one to lock it. Pinning it is a good idea, machine shop can fix that. Do both of them.

Prop Nut and shaft

Pete -

Right or wrong I can almost completely guarantee this is a standard arrangement for this model of the Grand Banks with a single nut and no cotter pin. I looked at several of these boats before purchasing the one we did and they are all the same.

I only called it a lock washer in that it is tagged into the slot for the key on the prop side and on the opposite side about 1/4 of the washer is bent up against the flat of a side on the nut - thus locking the nut to the prop since the key won't allow the prop to rotate on the shaft then the nut is "locked" to the same position as the prop. This is why the nut did not come off the shaft even though it was lose.

I would be very reluctant to change the original Grand Banks design of this arrangement to a double nut design even though that is far more common in the world of trawlers.

I do appreciate and thank you for your input.

Bob

It sure looks like those threads have been stretched. My thought is weld up and machine down

Slightly away from the original question from the OP, but the attached photo shows the port prop on my GB46 which was constructed only about 18 months earlier than the OP's 47.

Completely different nut arrangement used by GB only 18months earlier, on very similar sized boats. Just FWIW.

Hamish.

BrisHamish said:

Slightly away from the original question from the OP, but the attached photo shows the port prop on my GB46 which was constructed only about 18 months earlier than the OP's 47.

Completely different nut arrangement used by GB only 18months earlier, on very similar sized boats. Just FWIW.

Hamish.

Click to expand...

It appears in the photo that this is the more standard way props are installed with the half height and full height nuts and a cotter pin. I certainly like that method much more than the OPs boat.

My 32, number 595 had the big nut - little nut/cotter pin. Just one of each!

Port Shaft continued

Hi everyone. Well to make a long story short I will just say that I found the most reputably prop shaft guy in my area and met him at the boat. He is the prop / shaft person that everyone goes to and he is a straight shooter. We looked at the Port Shaft threads and what he believes happened is that the SS washer that tabs into the prop key hole damaged the thread in one specific area do to an up and down movement of the washer. He also pointed out to me that the washer should have had two sides bent up onto the flats of the large not thus locking the nut in place and no movement possible. The small section of one thread therefore is not corrosion but a banging of the washer against the thread. He assured me that the current threads are in good shape and we should properly put it all back together as designed. First he is going to chase the threads and then re-torque the prop on the taper. He is also going to replace the Starboard shaft prop washer and put a new one on properly.

I did ask him if he could take the shaft to the shop and rebuild the threads. He basically said absolutely not - the minute you apply heat to that portion of the shaft it will not be able to get it back to the .005" tolerance that he feels is necessary - so that to him was a non starter.

When completed I will post the final pictures.

Another possibility might be to machine off a portion of the back of that wheel to reveal some undamaged threads? There is a lot of meat back there that might not be missed. A prop shop could tell you.

Thread Damage

All -

Another project behind us and we'll keep an eye on this one through the 2021 season. The prop shaft expert came down and cleaned up the threads on the Port shaft. Then he put new SS washer on and bent up two sides to the nut flats. No way is this nut moving in the near future. He felt very confident that the nut was probably not tightened properly back in Feb 2020 when cutlass bearing and shaft seal on Port shaft were replaced. Polished up the prop and ready for some paint - photo attached. Thank you all for your ideas and suggestions.

Bob