One long shaft, or stay with two and the muff?

[johnjen50]

All,
I have a vibration on the strbd side and have replaced all cutlass bearings (I have 3 as they are on a 58 Yachtfish) had props computer balanced and trued, and engine aligned. Stuffing box still wobbles at lower rpms and vibration still there on the high end.

I currently have two shafts joined by a muff coupling. I am going to have new shafts cut, but am wondering what people think of going with one long shaft and getting rid of the muff coupling. Any advice is appreciated. The shafts are about 20ft and 10ft long, and 2 inches in diameter.
Thanks
John

[bostonhatteras]

I have heard that the coupling takes one knot of speed from the boat. when I do it I will have one long shaft. only issue is shipping the long shaft and of course installation as the boat has to be higher to install it. repair away from home port also is compromised but everything is a compromise right?

[GJH]

For something that long, you'll need to dig a pretty deep and long trench to get the things on and off. I suppose you could also do it while in the slings in the well, if the well is deep enough and you can find a diver or two to do it. And it pretty much eliminates the ability to keep a spare on board. Performance wise, to my knowledge from talking to local shaft specialists, a well built single will always run smoother than a muff set up. We don't go fast so it's not something that's on The List.

[johnjen50]

One knot would seal the deal for me by itself if that is true. Certainly seems like it could be. As far as compromise goes, without cutting two separate shafts and mating to the muff coupling, the associated labor savings may offset the other compromises involved.
Does anyone know why they designed it this way to begin with? Must be the ability to change the aft most shaft easier in the case of a grounding. Anyone have any knowledge on this?

[johnjen50]

Thanks George. Wasn't on my list either, but now that it is clear it needs doing, I want to do it right and be done with it. I'm thinking if I do it right now, hopefully the thing will last 20 years and I wont have to take it out at all!

[Boatsb]

If the boat is in the slings its a breeze. Blocked it will need to be up high but one piece is always better.dont worry about replacing half a shaft later on. Don't run aground and you'll be fine

[rsmith]

30' ??? Mine were 15' 10" total I got rid of the muff couplings and went 1 piece 10 years ago picked up 2 kts and the boat has 0 vibration now. Huge difference. I have a keel mounted fighting chair and it always "buzzed" no mater how well balanced the props were, now you can sit in the chair at cruise speed and not feel a thing.best thing I ever did on the boat!

[dsharp]

We had a stretched 46' for years. The two piece shafts were replaced with 2" Monel shafts years ago. I don't know that I would spend a lot of time worrying about not having a spare. I don't remember ever seeing a 2" shaft bend after a grounding and I've seen boats run up on the bank all the way to the rudders. Of course everything on the Texas coast is mud. You may want to take a look at the rear motor mounts if you are running detroit s with the original vulcanized rear mounts. They start sagging and shear causing all kinds of alignment issues.

[dsharp]

Getting a one piece shaft out is not a problem. Just tilt the bow down a little when you block the boat.

[Glory]

Have you checked to see that the key, at the trans end, of the long shaft is set correctly, a tought job in a 53/58MY, YF due to the space, or lack of it. It is likely a taper with a lock nut.
the easy to check is to seperate the coupling, and set up a dial indacator, first check the shaft, if it is OK, less than .010 runout, check the shaft coupling face.

JM

All,
I have a vibration on the strbd side and have replaced all cutlass bearings (I have 3 as they are on a 58 Yachtfish) had props computer balanced and trued, and engine aligned. Stuffing box still wobbles at lower rpms and vibration still there on the high end.

I currently have two shafts joined by a muff coupling. I am going to have new shafts cut, but am wondering what people think of going with one long shaft and getting rid of the muff coupling. Any advice is appreciated. The shafts are about 20ft and 10ft long, and 2 inches in diameter.
Thanks
John