Shaft layout issues

[SeaJay]

I’m trying to lay out my drive line for my 46’ motorsailer (new construction) and am coming up with a million questions. Hopefully you guys can help answer some of them. Here are the “knowns” of the situation.

1. Beta 75 hp w/ ZF 2.71 reduction gear. Max shaft rpm = 960.
2. 1-1/2” Aqualoy 19 shaft – prefer not to go larger
3. Python P80-M CV Joints and Thrust Bearing
4. Per Gerrs’ formulas, shaft max distance between bearings = 85 in.
5. Actual shaft length between stern bearing and aftmost location of thrust bearing = 90 in.
6. Shaft enclosed for entire external length in stern tube / skeg. (optional)
7. Would like to avoid an external intermediate bearing

Here are some of my questions.

1. I had planned to use a flexible stuffing box at the inboard end of the shaft just prior to the thrust bearing. This gives me an unsupported shaft length of 90 inches. Do you think I am going to be asking for trouble by pushing the bearing spacing 5 inches? Conversely, if I was to use a rigid stuffing box, I’d have a fixed bearing six or seven inches in front of the thrust bearing which violates minimum spacing rules. I could possibly get back into spec by installing an extra long (12”) stern cutless bearing. Suggestions, thoughts?
2. Do rigid stuffing boxes utilize a cutless bearing or is it just the packing material coupled with the fitting being anchored that makes it act as a bearing?
3. Are there any problems associated with a stainless steel shaft (Aqualoy 19) in an enclosed sterntube? It seems to me that this is can be an oxygen depleted environment that seems to promote corrosion issues with ss. On the other hand, it is done all of the time…just curious.
4. Do stern tubes tend to foul internally? If so, what are recommended preventative or maintenance measures?
5. When or if, is water injection recommended for flexible stuffing boxes? Rigid stuffing boxes?

I know this is quite a laundry list, but as always, any input is greatly appreciated.

Regards to all,

SeaJay

[gonzo]

It seems like the installation is on a rather slow boat, so you won't likely need to add water to the shaft.
Shaft logs do foul, but normally not enough to seize the shaft. That would take a really long time. You can paint it with antifouling.
Are you calculating the shaft at the indicated RPM's. 90 inches don't seem too much.
A rigid stuffing box that close to a soft mounted engine will put too much stress on the gear.

[SeaJay]

Thanks Gonzo.

Yes, this is strictly displacement speeds...8 kts. or so., and the bearing distance has been calculated at a shaft speed of 960 rpm.

I'm using Gerrs' as the basis of my calculations. I know he tends to be conservative (not necessarily a bad thing) and I was trying to see if anyone had another set of calcs or emperical evidence that might provide some sense of how far over the line I am with the extra 5". It doesn't seem like a lot but on the other hand the rules are there for a reason.

I probably wasn't too clear, but if I took the option of installing a rigid stuffing box, it would be just aft of the thrust bearing for the Python CV unit, not the engine itself. It doesn't seem like a good idea in any case and I would really like to avoid it if possible.

Good idea about the antifouling in the stern tube. I'll be sure to do that.

[PAR]

The extra 5" will not be a concern.

[Bglad]

Why not Aquamet 22 to maximize corrosion resistance?

[SeaJay]

Thanks Par. I was hoping you would comment.

Bglad - Actually I originally planned on Aqualoy 22 just as you suggested, but saw that 19 gave me a slightly larger bearing distance. However, looking at the calculations again, the increased distance is meaningless. Thanks for the nudge...Aqualoy 22 it is.