What is Minimum Diameter for a Propeller Shaft

[foolishsailor]

Originally Posted by Dockhead

Is selling the boat an option?

I can't recall ever seeing such a kludge in any boat design.

Me either - you're dead right, it is a serious design flaw. But i dont think it warrants selling the boat. This is a 30 year old aluminum boat that is built unbelievably well and that hash no other problems, except this monster. I have owned for 5 years and sailed 20k miles in her. We have already rebuilt her interior from the skin up, redesigned the sail plan, even redesigned some of the deck and the addition of a hard dodger...so yeah at this point we are irrationally in love with the boat...

...regardless - it is not an insurmountable problem. The problem is merely that of oscillation of the shaft. The cost to replace the stern tube is not that large - cutting out the old and replacing it is doable as I have good friends who are aluminum welders willing to take the job on. it is the 40mm shaft that I am afraid to price...

Since I am going to go ahead one path or the other it really comes down to

1. Will a 40mm shaft in a sterntube with ID of 100mm whip less? Or is this a stupid solution?

2. Or if above is not a good solution then I need to find a way to create and access point from the exterior of the hull through to the stern tube midway down to allow the insertion and removal of a bearing. (This would be an engineering nightmare for sure)

3. Or lastly does anyone have any bright ideas on how to slide in and lock a middle bearing that wont creep or spin and that is also removable (even if difficult to remove) - this is the real solution but for the life of me I cant think how to do this?

thanks for the continued input everyone

[foolishsailor]

Originally Posted by GordMay

“

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.

thanks for this Gord. I looked at the specs - My measurement was to the aft end of the prop - but even so I am still approx 9-10 cms - which according to your spec is 3 times too long. This is one thing I can easily address.

Originally Posted by Dockhead

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.

I am beginning to see this as well <sigh>

So now the question is - how to install a removable bearing?

Could I use a phenolic bearing with paper backing as the middle bearing? Would this swell and "lock"? Could I epoxy /glue it? It could be drilled out in the future should it need replaced?

[GordMay]

Originally Posted by foolishsailor

... it is the 40mm shaft that I am afraid to price...

An 80" x 1 3/4" shaft should roughly cost between $625.87 and $858.05
➥ http://www.deepblueyachtsupply.com/prop-shafts

2 metres is equivalent to 78.7 inches
40mm = 1.57"

[Dockhead]

Originally Posted by foolishsailor

Since I am going to go ahead one path or the other it really comes down to

1. Will a 40mm shaft in a sterntube with ID of 100mm whip less? Or is this a stupid solution?

2. Or if above is not a good solution then I need to find a way to create and access point from the exterior of the hull through to the stern tube midway down to allow the insertion and removal of a bearing. (This would be an engineering nightmare for sure)

3. Or lastly does anyone have any bright ideas on how to slide in and lock a middle bearing that wont creep or spin and that is also removable (even if difficult to remove) - this is the real solution but for the life of me I cant think how to do this?

thanks for the continued input everyone

1. Of course -- the thicker the shaft, the more rigid it will be, and the less it will whip. This is a good idea. But it is not enough, by itself.

2. Why do you think it is an engineering nightmare? It's very simple -- remove the tank and create tankage elsewhere.

3. The bearing needs something structural to mount it to, otherwise it will not function as a bearing -- pretty obvious when you think about it, no? This cannot be done inside the tank. So see the answer to Question 2, above.

[Dockhead]

And here is a great source for new tanks:

Tek-Tanks - Custom made or standard fit water, waste and fuel tanks

I can't think of any 30 year old boat which would not benefit from new fuel tanks -- they really don't usually last that long. So you will get a double benefit from ripping out your old tank -- you will solve your prop shaft problem, and you will have lovely new tanks. Make sure and design them with a sump and pick up for draining off water.

[osirissail]

I think you need to think "outside the box" of conventional marine motor and shafting. Solid shafts unless really perfectly aligned and supported (and all the other requirements/limitations posted by others) will "whip" or bow when one end is moved/oscillated laterally - such as a mis-aligned engine or an engine that "moves" a lot in its engine mounts.

If you go to an auto junkyard and look underneath some really old cars, you will see "drive shafts" running for meters from transmission to rear wheel differentials - and they are made of tubing. Tubing flexes less than solid rod.

So I would suggest using "Sailvayu's" idea of a fixed shaft mount at the engine end of the shaftlog with a pillowblock. Then a tube running down the inside of the fuel tank and a short piece of solid shaft through to the propeller. You might be able to build a more creative economical version of the "Evolution Shaft systems" to solve your problem. See: The Evolution Company Inc. - Product Specification

One thing I didn't see is where is, and what is being used the "packing gland" or shaft seal for that long run through the fuel tank. If it is on the engine end of the shaft log/tube through the tank you may be inducing uneven drag on the shaft if you are using a classical packing gland. A dripless might be better.

[foolishsailor]

Originally Posted by Dockhead

1. Of course -- the thicker the shaft, the more rigid it will be, and the less it will whip. This is a good idea. But it is not enough, by itself.

2. Why do you think it is an engineering nightmare? It's very simple -- remove the tank and create tankage elsewhere.

3. The bearing needs something structural to mount it to, otherwise it will not function as a bearing -- pretty obvious when you think about it, no? This cannot be done inside the tank. So see the answer to Question 2, above.

Re 2. The tanks are not tanks in the normal sense. My boat is aluminum and the bilges have basically been covered and turned into fuel and water tanks. i have 140gal of diesel - there is not other place to put the tanks - and if I took the aft lazzerette for example it would change my centre of gravity and center of effort. There is no additional bilge space as it is all consumed by my diesel and water tanks.

re 3. The middle bearing would need to be a cutlasss bearing and would have support - the stern tube. It is not a question of how to mount or where to mount the middle bearing it is how to secure a cutlass bearing into the middle of a stern tube when you cant access the stern tube from the outside to add locking screws for example.

Originally Posted by osirissail

...You might be able to build a more creative economical version of the "Evolution Shaft systems" to solve your problem. See: The Evolution Company Inc. - Product Specification

One thing I didn't see is where is, and what is being used the "packing gland" or shaft seal for that long run through the fuel tank. ...

thanks for this - interesting product. I dont have a packing gland I use a manecraft dripless system - works very well.

Doesnt look like there is a definitive solution but at least i build off the info you all have given.

1. Increase the diameter of the shaft
2. Increase the diameter of the shaft tube
3. Need a middle bearing redardless of shaft diameter
4. Look into building/buying evolution system

I spoke with a mate and he suggests sliding a tapered thread that threads the opposite direction of the forward gear into the stern tube and welding it in place and then threading the middle bearing and adding a notch on the back so that you can slide it in place and then lock it. Even the idea of epoxy or lock tite on the threads. As it is the middle bearing and we will have the engine dead aligned it should last a looong time.

I know this may be kickinig the can down the road 5-10 years but if it works well I would be delighted to only have to deal with it in 5 years.

[foolishsailor]

Originally Posted by GordMay

An 80" x 1 3/4" shaft should roughly cost between $625.87 and $858.05
➥ Propeller Shafts for sale. Buy your Prop Shaft online and save.

2 metres is equivalent to 78.7 inches
40mm = 1.57"

Gord,

Thanks for sourcing this for me. Do you know a supplier of comparable price that I can use in trinidad - as the boat is on the hard there now?

EDIT: Nvrmind - I took the time to check out the shipping rates on the site. for my shaft works out to $181 - good price compared to whats out there.
ahhhhh, american prices. I hope all of you in the states know how good you have it.

Here in Ireland they would laugh me off the island if I went in with that price. it is more than double that here, in the UK it would not be as bad but still substantially more expensive.

[Dockhead]

Originally Posted by foolishsailor

Re 2. The tanks are not tanks in the normal sense. My boat is aluminum and the bilges have basically been covered and turned into fuel and water tanks. i have 140gal of diesel - there is not other place to put the tanks - and if I took the aft lazzerette for example it would change my centre of gravity and center of effort. There is no additional bilge space as it is all consumed by my diesel and water tanks.

re 3. The middle bearing would need to be a cutlasss bearing and would have support - the stern tube. It is not a question of how to mount or where to mount the middle bearing it is how to secure a cutlass bearing into the middle of a stern tube when you cant access the stern tube from the outside to add locking screws for example.

Won't be a cutlass bearing unless it's under water. Isn't it in the middle of the tank where you need the middle bearing?

A middle bearing inside the tube inside your tank will not work unless the tube is structural -- capable of handling the loads imparted to the bearing. I suppose it would be possible to take out the existing tube and install a much stronger one, with strong mounts at either end. But how will you align it? How will you get to the bearing? I can't really picture how this could possibly work. A bearing inside a tube which is not structurally adequate will do more harm than good.

I guess you could divide your tank into two parts, with two shorter (and thus already much stronger) tubes through each remaining half. Leave a gap between the two tank halves wide enough to create a strong, accessible, and adjustable mount for your center bearing. Join the tank halves with balance pipes. You'll lose some tank capacity, but at least you will solve your driveshaft problem. Lot of welding will be required.

[foolishsailor]

Originally Posted by Dockhead

Won't be a cutlass bearing unless it's under water. Isn't it in the middle of the tank where you need the middle bearing?...

Yes. I neglected to mention that detail. The tanks are below the waterline mostly and the whole length of the sterntube is under the waterline - so a middle bearing and the forward bearing are/would be cutlass bearings.

Originally Posted by Dockhead

..A middle bearing inside the tube inside your tank will not work unless the tube is structural -- capable of handling the loads imparted to the bearing...

This is a good idea - the tank is baffeled alread so it wouldnt be a huge stretch to weld the plates onto the stern tube to add extra structural rigidity. This may already be the case as I cant see into the tank and dont know the current situation.

Originally Posted by Dockhead

..I guess you could divide your tank into two parts, with two shorter (and thus already much stronger) tubes through each remaining half. Leave a gap between the two tank halves wide enough to create a strong, accessible, and adjustable mount for your center bearing...

Im glad to hear you say this. I had this idea as well but thought it might be crazy/difficult so wanted to see if anyone else would mention it. I think this would probably be the "correct" solution but it also would be the most work. Isnt that usually the case?

The only issue is that the "access hatch" would have to be on the hull as the tank only has limited access. That said this would not be a huge deal as I have 4 small acess hatches already for the pivot pins on my two centerboards and they are flush, water tight and have been working well for 30 years - so i can use them as models for this approach...

[foolishsailor]

just thinking...

really all i need is an ability to reach in and secure two set screws. If drill and tap two sets of threads into the sterntube approx 60mm apart - inline with the sterntube, then slide an 80mm long cutlass bearing into place from the aft end of the stern tube, I can then tighten two set screws to secure the bearing.

It is the creation of this access tube/hatch that is going to do the welders head in...

[Dockhead]

Originally Posted by foolishsailor

just thinking...

really all i need is an ability to reach in and secure two set screws. If drill and tap two sets of threads into the sterntube approx 60mm apart - inline with the sterntube, then slide an 80mm long cutlass bearing into place from the aft end of the stern tube, I can then tighten two set screws to secure the bearing.

It is the creation of this access tube/hatch that is going to do the welders head in...

Two questions:

(a) Can you create a mount which will allow you to align the cutless bearing inside the tank? If it is not aligned, it will be worse than no bearing at all. Can you reach it to align it?

(b) Can you reinforce the tube inside the tank so that it will take the loads? If not, it will shake loose and create all kinds of problems.

If you have access from the top of the tank around the middle -- or if you can create access -- you might think about dividing the tank in half with an air gap in between the two halves, and moving the stern gland back to the new forward wall of the new aft half-tank. Then install a regular pillow block bearing in front of that, on a good solid, adjustable mount. The pillow block bearing will live in the air gap you created between the two halves of your fuel tank.

From there, the shaft will run in air, rather than water, through a tube inside the forward half-tank you created.

If you don't have or can't create access from the top of the tank near the middle, then I'll be damned if I can think of how to do this properly without ripping the tank out.

Another piece of advice -- do it properly or not at all. A poorly engineered solution will be worse than doing nothing.

[Dockhead]

Another reason to be extremely careful with putting a bearing inside a stern tube inside your bilge -- this tube is a barrier between your boat and the sea. If you get a vibration and shake the tube apart, you can sink your boat.

This is probably a good reason not to use the tube to carry loads of this type, at all, even if you have reinforced it, even if you can align it somehow.

I think more and more that you can't solve this without either (a) chopping up the tank; or (b) removing the tank altogether. The original installation was an incredible bodge -- the designers deserve shooting for that -- blatant violation of obvious rules about unsupported length of propeller shafts.

The designers were too strongly attracted to the appeal of using one bit of hull volume for two different purposes. Well, those purposes may be simply irreconcilable. Don't succumb to the same temptation to do a bodge yourself, for the sake of the same values. It may simply not work, and you might have to simply face that fact and put the tankage elsewhere.

[foolishsailor]

dockhead,

You've given me alot to think about. Thanks for the back and forth helps me to crystalize what I need to do.

Until I am looking at the boat again I wont be able to decide the direction i am going to go - but now I can definitely see this is a lager project than I had originally envisioned.

I will be going down to trinidad to start this project and some others at the end of Feb. I'll post photos of the project and the decisions we made here once we get started.

Thanks for the advice everyone - and if you ahve any more ideas let me know

-jc

[delmarrey]

I'm probably a little late here. But since your tube thru the tank is so large couldn't you just insert another tube the same length, in from the bottom end, with a cutless bearing already installed. Adjusting it around at the ends until it's on center, and lock it in place either with flanges, welding or whatever.

If you do have a center cutless you'll need to inject water in from the shaft seal area, just like on a PSS, to keep everything cool and lubed. And you will not need a larger shaft w/a center support..