Your Stern Tube Assemblies

I looked over the forum for coverage on this topic, and really did not find what I was looking for.

I am rebuilding this old 12m steel ketch from the keel up, and I am looking at the options for my stern tube assembly, specifically seals and bearings.
I happen to have something I am not familiar with currently, and even though I think it is an interesting idea, it failed miserably. The damage from spraying sea water in the engine room shows it had been an ongoing battle that for whatever reason, was not resolved.

I am no expert, so I read and listen to everything. I would like some real world input on the options for seals and bearings. Whats working, what's not. Solutions to problems with what you have.

Our intentions are to spend 10 years sailing the world with no set plans and no hurry to get anywhere. We have a robust budget and our plan is to build a boat that will safely accomplish that goal.

 

Since this is your post and your boat that needs help, why don’t you start by posting an accurate description of your existing system with photos and measurements, then detail the problems you’re having with it?

 

Ours was set up with a Tapered roller bearing at the front of the stern tube, with what looks like and oiler, although it was not there when I disassembled it. At the prop end, we had a large bronze nut with a rubber seal that screwed to the end of the stern tube. The shaft rode in a short 2 inch stave. I have never seen a set up like it before, and it was built in Sweden..
It was seized up solid, and would not turn. Bearings are rusted away to mulch.
Purpose of this post was more geared towards finding out the pros and cons of typical stern tube assemblies available.
Of course if what I have is familiar to anyone, it would be helpful to know in an effort to determine how it may have failed and if its worth resolving... My unprofessional opinion is no..

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As is often the case,,, I jumped before I looked... I have question posted helping me identify what we have and here it is.

 

The simplest (KISS) solution is to go to a wet tube with cutlass bearings, and traditional packing gland on the inside.
If exposed shaft length requires additional bearings, add pillow blocks at appropriate spacing along the shaft.
The original system looks like a scaled down version of something off a large commercial vessel!

 

I agree with "kiss". However, since I posted this, I have done a ton of research, and I am convinced that the oil lubricated stern tube assembly has advantages where I need it most.

I am working with the folks who now have the "evolution shaft" system. They provide shaft systems for large ships and they saw that the evolution shaft designed by Mike Visconti was perfect for the small vessel market, so they bought the patent.
( i'm not 100% sure of this but that is what I understood)

This system provides a completely sealed stern tube, two bearings at the head, one at the lower end and one mid shaft. It has an auxiliary oil tank and a pump to circulate the oil. The oil is circulated from both ends of the tube which allows you to pump out the oil from the lower end of tube for oil changes. We will add a filter as well. A sight tube quickly allows visual confirmation of oil level and if water intrusion does occurs.

It also works great with the CV joint shaft and soft engine mounts for low vibration and reduced noise, but the best part is zero water in my bilge which on a steel boat is critical.

 

So you strike a log, several hundred miles from anywhere, and suffer a slightly bent prop and shaft, soon loose the rear seal and all the oil, but must press on and soon loose all the bearings in the tube and inner seal, now what?
If you had chosen the cutless bearings and rubber mounted packing, your chances of limping to port would be greatly improved, in my opinion.
I do not agree that the Evolution system is perfect for the small vessel market, and the condition of your system is proof of that! You only showed half of it in the drawing above, there are still more complications along the way to the transmission. This is the type of system looks very industrial, and while it has great potential to last al long time, a small amount of salt water can render it useless in short order.
My current vessel sports a hard mounted motor and an oversized shaft which enables alignment to be tuned to within a gnats *** of O. Vibration is not an issue. It maintains alignment at any throttle setting, torque variations do not change the relationship of the motor to the driveline. A drivesaver is installed between the coupling halves just in case, with spare onboard.

 

I restored my fiberglass boat years ago and relaunched her in 2014. The engine, v-drive and transmission were removed during the process. My boat is much smaller than what you guys are discussing here. I can tell you that I have absolutely no water intrusion from my stern tube. None, zero, you can leave a paper towel in the bottom of the bilge all day and it's dry. I replaced the engine stringers, used a Packless Shaft Seal and installed a new cutlass bearing in the strut that supports the new propeller shaft. I don't understand why one would want to have (what looks to me like) roller bearings in some kind of waterproof assembly under the waterline. Maybe this is the preferred approach in a commercial boat where someone is tending to the power system all the time but for a pleasure boat? I understand the desire for a dry bilge but I've found that condensation and water leaking down from above are my sources for small amounts of bilge water. A decent ignition protected ventilation fan takes care of that. There are a number of seals on the market, I'll post a link to what I used below.

PYI Inc. | PSS Shaft Seal https://www.shaftseal.com/

 

All good points.. I must admit, I had not considered my prop being hit in my calculations. Also, to qualify, the boat is a heavy steel motor sailer which will see more motoring than a typical lighter sailboat.

My first thoughts on a damaged prop, would be that I would feel that vibration, and not continue to motor. In that case, I would lock the shaft regardless if it was oil or water lubricated, and I would be looking to get her repaired asap. I can not imagine needing to "power" to the point of damage.

I will run through my pros and cons again, adding that new contingency.

 

Murphy lives on every boat, and he will dictate that your prop strike or grounding or whatever failure will occur on a windward shore at night in high wind and seas, where the possibility of sailing away is nil.
Having a “robust budget” on a project like yours is rare, suggest you utilize some of that to consult with a reputable engineer who also has experience with long distance cruising.

 

You leave no room for argument. You are absolutely correct, and Murphy is what I call my shadow.

Its funny, at 53 with numerous projects under me, where I was involved from concept to construct, and I still consider Murphy only as an afterthought.

I have Bruce Roberts as my design engineer, however, there are certain items which become personal preference, and compromise. The boat was built with a sealed, oil bath unit, which in 1982 was not common. The design at the time was sufficient but had its quirks, and required diligence, which it never received.

I was hoping for more feed back on what others think about their setups, but this side of Boat Design does not seem to get many viewers.

 

This is a great example of one of my concerns...

 

Best commercial boat setup is stern bearing, wet tube, longer than normal heavy weight wire reinforced hose then two post packing gland. Put a zerk mid gland and use gortex packing. We did the conversion with guidance from sbar marine in 2013 and just clicked over 9000 hrs this summer. It's been virtually dripless, and absolutely pain free. Longer than average hose gives some flex for when we ding a prop or suck up line, takes up some of the movement over the rigid option. The buck Algonquin bearings have been good for at least 10k hrs in the last three boats I've had. When the pss seals go, they go. Had a fluke depart on a long liner in my youth, ended up being a casting flaw on a new wheel. Due to tide and wind we had to keep rpm up to make it home. Trip got a little to exciting when the new fancy pss packing bellows decided to give up the ghost...

If shaft is long throw a pillow block bearing of robust material after the packing.

 

Cutless bearings are robust and trouble free, I'd chose them over a sealed tube anytime.

I moved away from a gland seal to an oil seal (from Rexmar in my case) as the boat at the time was keel cooled so no access to sea water to lubricate a PSS type seal. The main benefits are that they are completely dry and that they're not reliant on sea water for lubrication. I have since installation added an oil temp gage so I'll now if the bearing is about to quit (before it seizes up and causes a water intrusion). Maybe that'll keep Murphy out of the equation...

 

UPDATE: took all this into thought and opted for a wet tube... Installing now... Thanks all for the suggestions...