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Old 15-01-2012, 13:02   #1
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What is Minimum Diameter for a Propeller Shaft

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?????
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Old 15-01-2012, 13:07   #2
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Re: What is minimum diameter for a propellor shaft

i would consider a pillow block bearing on the shaft to stabilize it. This is often done on long shafts.
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Old 15-01-2012, 13:19   #3
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Re: What is minimum diameter for a propellor shaft

Quote:
Originally Posted by sailvayu View Post
i would consider a pillow block bearing on the shaft to stabilize it. This is often done on long shafts.
Agreed!
Or second cutless bearing just aft of the stern tube ("shaft tube").
A larger dia. shaft would be complicated in your situation.

I see this a lot on older power boats.
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Old 15-01-2012, 14:32   #4
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Re: What is minimum diameter for a propellor shaft

Thanks for the replies.

How and where would I install the pillow block bearing?

The stern tube is 1.9m long and not accessible its entire length as it is inside the fuel tank.



The engine sits at the front in the engine compartment. The fuel tank starts just aft where the stern tube begins. THe entiner length of the stern tube goes through the fuels tank. I guess I forgot to mention my boat is aluminium and the tank is integrated into the hull...

As i mentioned presviously the middle bearing is not an option.
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Old 15-01-2012, 15:00   #5
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Re: What is minimum diameter for a propellor shaft

With that a larger or better quality shaft material would be in order. Your stern tube would need to be, at least, 1/4" larger in diameter then the shaft. But that would also mean a larger - cutless bearing, transmission coupler, stuffing box and prop.

Or an Aqualoy 22 material, if you don't already have it. If so, then I'd say your in a bind.

A fool and his money are soon parted.
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Old 15-01-2012, 15:27   #6
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A bind, ugh yea i know.

I was hoping a 40mm/ 1.5in shaft inside a stern tube with a 100mm/4in inner diameter would work?

I was thinking of having the ends of the shaft milled down so that i could use my existing and expensive flexible coupling and propellor?

Will this work or am i crazy?
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Old 15-01-2012, 15:35   #7
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Re: What is minimum diameter for a propellor shaft

Quote:
Originally Posted by foolishsailor View Post
A bind, ugh yea i know.

I was hoping a 40mm/ 1.5in shaft inside a stern tube with a 100mm/4in inner diameter would work?

I was thinking of having the ends of the shaft milled down so that i could use my existing and expensive flexible coupling and propellor?

Will this work or am i crazy?
With that much extra tube space, all you can do is try. But just the ends. And go for a full radius for the transition of sizes, and go for a larger stuffing box. The shaft will be heavier! So, no shifting gears too fast!!!!!

Also, getting the shaft into the cutless bearing maybe a problem, w/o moving the motor or strut.
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Old 15-01-2012, 17:32   #8
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Re: What is minimum diameter for a propellor shaft

Hi,

In our boat with MD7 (13 h.p.) the shaft is 25mm the unsupported length is roughly 3 ft.

40mm shaft sounds thick. Will you find a matching prop for your boat?

I remember Vetus catalog gave shaft recommendations once. Look them up.

b.
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Old 15-01-2012, 17:36   #9
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Re: What is minimum diameter for a propellor shaft

Quote:
Originally Posted by barnakiel View Post
Hi,

In our boat with MD7 (13 h.p.) the shaft is 25mm the unsupported length is roughly 3 ft.

40mm shaft sounds thick. Will you find a matching prop for your boat?

I remember Vetus catalog gave shaft recommendations once. Look them up.

b.
Here ya go........ Western Branch Metals: Marine Propeller Shafting
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Old 15-01-2012, 17:45   #10
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Re: What is minimum diameter for a propellor shaft

I wonder if you could find a bearing with the same O.D. as the I.D. of the tube. Then slide the bearing down the shaft and into the tube. I am just not sure how to hold it in place once in the tube. You would want to make sure the bearing race did not spin inside the tube as well. In the old days a builder might use Lignum vitae as a bearing. That might be a thought here as well.
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Old 16-01-2012, 04:00   #11
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Re: What is minimum diameter for a propellor shaft

Quote:
Originally Posted by barnakiel View Post
Hi,

In our boat with MD7 (13 h.p.) the shaft is 25mm the unsupported length is roughly 3 ft.

40mm shaft sounds thick. Will you find a matching prop for your boat?

I remember Vetus catalog gave shaft recommendations once. Look them up.

b.
I would love it if we only had 3' unsupported to worry about.

It is not just he unsupported length that is an issue but the fact that the unsupported length runs through a 2meter stern tube.

I was thinking of having both ends ot the shaft milled down, and via advice above tapered, to 25mm just long enough to have the aft end threaded and keyed to use my existing prop and the fore end so that I can use my existing flexible coupling. We would only mill down the bare minimum length to allow this to work.

Quote:
Originally Posted by sailvayu View Post
I wonder if you could find a bearing with the same O.D. as the I.D. of the tube. Then slide the bearing down the shaft and into the tube. I am just not sure how to hold it in place once in the tube. You would want to make sure the bearing race did not spin inside the tube as well. In the old days a builder might use Lignum vitae as a bearing. That might be a thought here as well.
We tried this and could not find a satisfactory way to secure the bearing. It not only has to be secured but also removable. How to guarantee that it does not break free and start spinning?

Lignum vitae is an interesting suggestion as it would swell in the water to lock itself tight into the stern tube - but how to determine the I.D. of the bearing as the swelling here would cause it to tighten on the shaft??

I am open to any ideas on how to get a middle bearing - as that would solve everything, I just cant think how to do it which is why I was going the direction of a thicker shaft and larger stern tube to remove the ossilation of a 2 meter unsupported shaft.

I main issues with middle bearings:

1. No access to stern tube as it is inside (goes through) the integrated fuel tank
2. How to secure a middle bearing when you will be required to slide it up a meter of stern tube
3. How to secure it in a way that it will not creep and eventually break free and start spinning
4. How to secure it so that it can be removed some day as well

help...
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Old 16-01-2012, 04:55   #12
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Re: What is minimum diameter for a propellor shaft

“...The propeller is balanced and sits 15cm (150mm) aft the exit of the shaft tube...”

“...Shaft diameter is 25mm...”

Unsupported shaft length, (cantilever, last support/strut to prop’), should not exceed 1.5 times shaft diameter. In your case, 3.75 cm, not 25 cm.

See also:

ABYC P-6 ➥ http://www.abycinc.org/committees/P-06.pdf

Propeller Clearance Diagram ➥ Propeller Clearance Diagram - Cruisers & Sailing Photo Gallery
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Old 16-01-2012, 05:31   #13
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Re: What is minimum diameter for a propellor shaft

Quote:
Originally Posted by foolishsailor View Post
Thanks for the replies.

How and where would I install the pillow block bearing?

The stern tube is 1.9m long and not accessible its entire length as it is inside the fuel tank.



The engine sits at the front in the engine compartment. The fuel tank starts just aft where the stern tube begins. THe entiner length of the stern tube goes through the fuels tank. I guess I forgot to mention my boat is aluminium and the tank is integrated into the hull...

As i mentioned presviously the middle bearing is not an option.
Is selling the boat an option?

I know that will sound quite unhelpful, but you have an engineering nightmare on your hands -- designed in my the original designer -- which will be difficult and expensive to solve. I can't recall ever seeing such a kludge in any boat design. I would have to love a boat with an irrational passion to be willing to spend the time and money to correct that.

If you do love the boat with an irrational passion (many of us do), then if I were you I would not waste time with half-measures that will cost time and money and will be unlikely to really solve the problem. I would rip that tank out and do the shaft properly, with proper access and with no compromises. Then start from scratch with tankage, maybe in a different location. Trying to accomodate both a long propeller shaft and a fuel tank in the same volume is the original sin, in your boat's design, the consequences of which you have experienced the hard way, and will continue to experience unless you get at the root cause.

You asked:

"I main issues with middle bearings:

1. No access to stern tube as it is inside (goes through) the integrated fuel tank
2. How to secure a middle bearing when you will be required to slide it up a meter of stern tube
3. How to secure it in a way that it will not creep and eventually break free and start spinning
4. How to secure it so that it can be removed some day as well

help... "

Answer: rip out the fuel tank. It cannot fundamentally co-exist with the propeller shaft in the same hull volume.
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Old 16-01-2012, 06:14   #14
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Re: What is Minimum Diameter for a Propeller Shaft

25mm is too thin for a shaft this long. It will whip. According to the handbook 30mm is just adequate so I would err on the side of safety with the 40mm that you appear to have settled on.
Stanley
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Old 16-01-2012, 06:22   #15
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Re: What is Minimum Diameter for a Propeller Shaft

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25mm is too thin for a shaft this long. It will whip. According to the handbook 30mm is just adequate so I would err on the side of safety with the 40mm that you appear to have settled on.
Stanley
Unsupported length between two bearings should not be more than 40x the diameter of the prop shaft.

That would be 1.6 meters for a 40mm shaft. The OP says he has 1.9 meters of length inside his tank. I don't see how he can make this installation work without a center bearing.
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