What is Minimum Diameter for a Propeller Shaft

[4HMainer]

Your other option is to fit something like a Maritex bearing at each end of tube (they can be finished to any size) but longer than normal. You can check with the manufacturer on what they think. But I'd guess at a 200.00mm bearing each end and you'll have a very rigid and quiet shaft.

We've done this on some similar long tube set ups, just make sure you have a good water feed. To get a bearing fitted half way down the tube you can epoxy them in as a clearance fit in the tube.

Standard bearing lengths are only standard for normal unsuported lengths, longer bearings will support longer lengths.

[Philsboat]

Two words.

Hydraulic drive?

Phil

[2007.pierre]

Originally Posted by foolishsailor

Need some advice:

My boat has integrated water and fuel tanks as part of the bilge. The propellor shaft goes through a shaft tube that goes through the main fuel tank before it exits the boat. The shaft tube is 1.9 meters long.

Problem:
THe prop shaft is moving too much and knocking the inside of the shaft tube. The knocking was at low rpms and generally disappeared after 1,000rpm only to reappear around 2200 or when changing from forward to reverse. It was annoying but didn’t seem dangerous and the advice I got from many including an engineer who specialized in making shafts, was to let it be. After 11,000 miles the knocking has cracked welds on the shaft tube and possibly worn out the tube as there was a massive influx of salt water into the diesel tank on the way into Trinidad.

Current Setup:
Vovlo MD17c (its a 3 cylinder - they are known for alot of vibration)

New motor mounts designed for 3 cylinders...(at least thats what they say )


Vetus Bullflex Flexible Coupling



Graphite impregnated vesconite cutlass bearings where the shaft enters and exits the shaft tube.


The propeller is balanced and sits 15cm aft the exit of the shaft tube
The engine sits 50cm in front of the shaft tube.


Shaft diameter is 25mm

For various reasons I cant add a middle bearing. The shaft is not accessible without cutting into the fuel tank.


The previous owner somehow slid a phenolic bearing with epoxy into the shaft tube as a middle bearing. It detached while I owned the boat and almost caused a catastrophic failure of the shaft.


Proposed Solution:
I feel that having a cutlass bearing so close to the engine acts as a fulcrum enlarging the unavoidable motion of the engine. I would like to remove this and only have a cutlass bearing where the shaft exits the boat. This means I will have 1.9 meter of unsupported shaft.



I understand that a 40mm shaft is rated for this span. Is this true?

I will replace the current shaft tube with one that has a 100mm inner diameter.


That’s as far as I have gotten.



Any ideas?????

I had a 25mm shaft on my first sailing boat, a Melody from Jeanneau. I experienced some vibrations at certain rpm, probably due to the flexibility of the shaft. The day I installed a shaft generator betyween the stuffing box and the Hurth gear box, I never had any problem and went that way for twelve thousand miles. The ratio of the main pulley to 13A alternator pulley was 3to1.

[hellosailor]

I'm wondering if you couldn't attack this a bit differently. instead of trying to mount a conventional center bearing, could you buy a teflon rod, drill out the center to insert your prop shaft, and literally sleeve the entire prop shaft with a teflon cylinder that acted as a full-length bearing down the entire shaftway?

Or, alternately, fit a series of discs on the shaft, and insert large Delrin bearings, of the type used in jib rollers, to act as live ball bearings along the shaft? Eventually those would develope some slack, but you could simply pull the shaft and replace them as needed.

Somewhere along these or similar paths, I think you should be able to find an adequate way to control any lash on that shaft.

[delmarrey]

[QUOTE=hellosailor;891339]I'm wondering if you couldn't attack this a bit differently. instead of trying to mount a conventional center bearing, could you buy a teflon rod, drill out the center to insert your prop shaft, and literally sleeve the entire prop shaft with a teflon cylinder that acted as a full-length bearing down the entire shaftway?

Or, alternately, fit a series of discs on the shaft, and insert large Delrin bearings, of the type used in jib rollers, to act as live ball bearings along the shaft? Eventually those would develope some slack, but you could simply pull the shaft and replace them as needed.

Somewhere along these or similar paths, I think you should be able to find an adequate way to control any lash on that shaft.


.

[hellosailor]

Delrin ball bearings, that is. Used in furling drums, not to be confused with the ones folks use to pop jibs over the lifeline!

[Nicholson58]

There is a lot of good advice in your replies. The question that come to mind for me is what has changed since the thing once worked correctly?

If the engine & coupling are new as I suspect then consider: The Vetus bullflex accomodates the vibration comming from the torsional pulses sent by the pistons and explosions in the engine. It is not great for parallel offset and 2 degrees for angular offset. If the shafts had ONLY angular offset - OK. You can almost guarantee the shafts will also have parallel offset. Both parallel and angular misalignment of the engine shaft and the prop shaft will impart a bow to the shaft, especially that long between supports. Once a bowed shaft reaches a harmonic rpm the lateral deflections will become large. Assuming the shaft does not have a permanent bend from some event, the only way to make this work is to have very good alignment betweent he two shafts. We do this in our shop with lasers and targets clamped to the shafts. The allowed errors are typically in the thousandths of an inch. It would be great if you can perfectly align the two shafts but this is difficult and sometimes nearly impollible.

There was a thread here http://www.cruisersforum.com/forums/...nce-70411.html

There is a coupling, Aquadrive, discussed here. It is essentially a miniature constant velocity joint with a cardan joint in combination like your see on the front axle of a front wheel drive car. This allows both parallel and angular offsets and coincidentally has a thrust bearing (pillow block) integral. Welcome to Aquadrive There are a lot of advantages with this system including that the prop pushes the boat rather than the engine. This means the motor only supplies torque and its crank bearings see no thrust or offset loads. The short double flex CV shaft accomodates a lot of sin.

[capngeo]

Welding aluminum that has had fuel oil in contact with it for 30 years is in and of itself no easy task. Of all the suggestions so far, the idea of hydraulic or electric drive, seems the path of least resistance

[foolishsailor]

Thanks for the continued replies

I looked into the aquamarine and spoke with a rep. The issues are

1. I don't have space between my engine and the stern tube for the first CV joint
2. I don't have a location for the aft cv joint at all.
3. I would need a huge diameter stern tube to go through the fuel tank to contain the tubular shaft

To make their system work I would still have to meet marine standards for minimum shaft diameter for that unsupported length which is 40mm, I still have to have a larger stern tube, and have to do even more cutting and welding to create space for both the fore and aft cv joints. It's too much, it turns it to an even larger project.

This idea was presented up thread a ways and seems a more viable for my situation

The Evolution Company Inc. - Product Specification

http://www.evolutionmarine.com/detai...29731984185645

They are completely different systems and frankly it is just the fact that this doesn't require an aft cv joint and that the stern tube is integral to the design that makes it appealing...

[foolishsailor]

Originally Posted by hellosailor

I'm wondering if you couldn't attack this a bit differently. instead of trying to mount a conventional center bearing, could you buy a teflon rod, drill out the center to insert your prop shaft, and literally sleeve the entire prop shaft with a teflon cylinder that acted as a full-length bearing down the entire shaftway?.

I had considered this pathway but was concerned on how one would ensure adequate water flow over the surfaces to prevent friction generated heat, especially since I couldn't find a shop that had the capacity to make a sleeve 1.9m long with groove for water flow in a single piece. Also the idea of pounding in a 1.9 meter cutlass bearing seemed like a nightmare.

But it did make me think maybe I could hold a middle bearing in place using teflon sleeves fore and aft of the middle bearing. The sleeves od only just less than the id of the stern tube and just a couple mm thick so they don't touch the shaft, just to prevent the centre bearing from creeping.

[captain58sailin]

I am sure you have considered this, but I'll fly it anyway. If you are going to the larger shaft size, check your coupler and see if it has enough material where you could enlarge the hole for the shaft rather than turning the shaft down to 25 mm., on the prop end, I think I would look for a new prop with right diameter and pitch, and shaft taper for your 40 mm shaft. You will probably be able to find a used one in the shipyard at a decent price. I would check the face of your coupler on the transmission and the shaft to insure that they are true and have not gotten warped over time. Of course the alignment between the transmission coupler that the shaft flange is important, shaft, prop, are true as well. The short term solution for the mid bearing would be to take a cutlass bearing with the fiberglass exterior and put a bunch of epoxy on it and drive it mid way up the tube and let it sit. When it eventually wore out it would be a real head ache to remove. I have had to put a hack saw blade on a stick and cut the bearing into sections and then pry the sections loose to remove the old bearing. As previous posters have mentioned, what has changed in your system that is giving you the vibration in the first place?

[foolishsailor]

Originally Posted by captain58sailin

I am sure you have considered this, but I'll fly it anyway. If you are going to the larger shaft size, check your coupler and see if it has enough material where you could enlarge the hole for the shaft rather than turning the shaft down to 25 mm., on the prop end, I think I would look for a new prop with right diameter and pitch, and shaft taper for your 40 mm shaft. You will probably be able to find a used one in the shipyard at a decent price. I would check the face of your coupler on the transmission and the shaft to insure that they are true and have not gotten warped over time. Of course the alignment between the transmission coupler that the shaft flange is important, shaft, prop, are true as well. The short term solution for the mid bearing would be to take a cutlass bearing with the fiberglass exterior and put a bunch of epoxy on it and drive it mid way up the tube and let it sit. When it eventually wore out it would be a real head ache to remove. I have had to put a hack saw blade on a stick and cut the bearing into sections and then pry the sections loose to remove the old bearing. As previous posters have mentioned, what has changed in your system that is giving you the vibration in the first place?

Good idea on enlarging the coupling, I will look into that...

Regarding shaft taper, seems you can order shafts with a taper to 25mm as standard...surprising to me as well so this is not an issue. However I will be replacing my prop regardless as it turns out to be seriously pitted and was probably one of the contributors to the problem. If I can get it rebuilt up and balanced cheaper than a new one then I will try that route as well.

The engine and shaft were lined up prior to my last passage and done so using normal techniques but also to check we used a 10" deep 25mm id cylinder attached to the gear box in place of the coupling so that the shaft had to perfectly in line to be able to slide all the way up the cylinder...

Regarding what has changed...there was a middle phenolic bearing that had been in there that had over time had melted and seized onto the shaft and prevented and water from coming up to the inner cutlass bearing. This is another reason I am looking to put in a larger diameter stern tube so that if I decide to put back the middle bearing then I could have the machinist cut larger water grooves...

[captain58sailin]

Sounds like you have all the bases covered then. Good luck! I hope all goes well for you.

[Nicholson58]

Whatever path you choose please consider that plastic bushings are nice and self lubricating but are limited to a PV value. That is the product of the pressure (force per area) times the velocity. You can get away with greater velocity if the area of the bearing is increased - up to a point. Exceed the PV or max V and the bearing begins to melt or smear. Some plastics are better at this than others. I like Moly filled Nylatron. you can look up the PV values on line.

If your shaft whips as you noted and strikes the tank, I am sure you know you could risk breaching the tank.

Any way to move the engine forward to make some space?

You may ultimately have to change out or modify the tank - yuk!

Is a sail drive and option?

Good luck.

[4HMainer]

There is a lot of good advice and some questionable ideas here as Nich 58 has noticed, for a prop shaft Delrin rollers will get to warm, as delrin is a soft melting material it will creep and buckle and wipe off probably ending in a horrible mess. teflon is also a soft material so no good at shaft support with any load at all. When it is used for higher speed applications it needs a thin layer of Teflon backed by a more rigid material.

The best composite bearing materials currently are probably Maritex (non melting) and for less performance some of the softening elastomeric materials may be OK. Essentially for a bearing you want a hard material that resists movement. Whatever you use in a long tube you will have to have it lubricated and have some way of getting heat out. I've done long tubes with plain bushes for shafts (bronze initially and then Maritex) but you need to circulate the oil and if it's enclosed have an oil cooler. You do get a lovely quiet smooth drive though.

Coupling wise there is a new coupling from Norway that is more appropriate than an aquadrive. The "Marine joint" from Power train, they also make the well known Hydradrive.

Be careful what you read on forums and talk to manufacturers, there are many pitfalls in bearing choices.