Shaft material opinions?

Which is better?



A shaft that is made out of "aqualloy".

A shaft that's a little softer, like plain "stainless steel".

"Bronze" shaft or something else...

What is your opinion and why.

 

The material choice selection is principally to balance the requirements of:
1)strength
2)Fatigue
3)Corrosion

You can't just choose one for your basis of selection. Therefore you need to address what it is you want from your shaft....sounds obvious, but it is not! It requires a bit more thought to the solution, ie the question.

 

Duplex 1-4462 Stainless Steel, has a hardness 40% better than AISI 316, and 30% better tension strength. On top it has better corrosion resistance than AISI 316 and Aquamet 17 to 22.

Regards
Richard

 

I believe his boat runs in fresh water lakes etc, yes?
Therefore plain normal mild steels can be used, so long as the bearings are made the same. But a popular choice in FW applications is manganese bronze, as it is a good compromise between cost and life.

But as i said depends on what he really wants from the shaft, otherwise we just go around in circles guessing!

 

You´re right! BETTER is just nothing, better for what, is the question.

 

I'm asking for your personal opinions/preferences...

The shaft on the bottom of the picture was my first shaft.
It length was determined by a marina, who then order an installed the shaft and my original prop (don't remember the size)
The shaft was aqualloy something...
Hit a rock, broke the tapered end of the shaft off and lost the prop...

The 4 bent shafts above it are 316 stainless.
I purchased the 12' stick of 1" stainless and machined the shafts myself.
At a cost of less than $300 for all 4 shafts (back in 2000) This I thought was a good deal.

Been busy, had money (2 years ago) so I bought a "aqualloy" shaft.
Quick, easy (lazy), but done...

This year, (lazy again) ordered a new "aqualloy" shaft...
Wow, $200 now!

Is it worth it?

To me, I'm trying to decide if it is worth the extra money?
Where have you seen shaft breaks occur more often - near the coupling, by the taper, above/below strut, just random? (your experiences)

I was lucky not to lose the prop from the top shaft in the picture...
It broke at around 2500rpm and went back and hit the rudder.
I'm thinking about the possibility of losing a prop in this...

The boat is used in rivers, small lakes, also been out on lake erie, tackled 9' wakes on lake michgan, etc.
It runs nearly everyday from june to november...

apex1
Thanks for those specs, that helps a lot.

I had not considered bronze, how does it hold up?
Say compared to the 316 stainless...

Recent events have made me consider going back to a softer material...
Hence the questions...

I guess I should have asked what metals are commonly used, how they compare, there specs, etc. but I was looking for some opinions too...

 

"...I guess I should have asked what metals are commonly used, how they compare, there specs, etc. but I was looking for some opinions too..."

Well, the trouble with opinions is that they can be very subjective and sometimes ill-informed. Selecting anything on a boat is never about one simple criterion, there are usually many to consider all at the same time...

The best approach would be to go back to the beginning, to 'design' from fresh using today's methods of analysis and materials. It is not a difficult task, just requires some basic data of the engine and the prop. This then gives you a starting position or baseline, and something to aim for rather than just guessing which material is best when you don't even know what size diameter for each material is best. The two are dependent upon each other. This should also be coupled to the excellent advice from baeckmo on your other thread.

 

If you've already ruined 4 shafts of 316 SS, I do not think there is anything on the market that will not bend or snap. Manganese bronze or Monel are both softer than 316 SS.
High tensile strength steel, plated with stainless steel is an expensive solution for a shaft that will not bend easily, but when the prop strikes an unmerciful object, blades may snap off and/or you'll loose the end of the shaft.

Perhaps you should adapt the construction so that the prop is less exposed.

 

Of course hitting a rock brokes something.. but are all incident with rocks or smth softer also?
How much Hp have you in the other end of the shaft? Maybe you having too thin...
Displacement speeds.. or Planning? Prop diam, rake and rpm?

Ok found some of the details in your other thread
http://www.boatdesign.net/forums/props/props-rake-28716.html

 

No shaft will survive bad driving and hitting rocks . You could modify that old dated engineering of pinch screws. Is that all that stops it coming out in reverse? --- a pinch screw?

Going up a 1/4 of an inch (you have the space) or two with a tapered shaft at both ends would help.

What would indubitably help would be your driving technique.

 

size for size absolutely nothing measures up to 2205,
tensile is way up corrosion in tepid salt is nothing like that of other ss,s
I use it , every time, it allows me to use smaller dia. for likes rudder stocks, , and prop shafts

 

"What would indubitably help would be your driving technique."
I agree with Thurston J. Howell lll here but hate the thought of breaking shafts (I have not in near 8,000 days on the water). The best way IS a taper at the forward end of the shaft. (except that the bolt is hidden by the coupling and not easily inspected). I don't know what a pinch screw is.
Especially in this day of aluminum housed trannies, Apex certainly has a big point here, tho. Since I watch where I'm going, I opt for stuff that doesn't break and will eat the $10,000 tranny cost, if it comes to that.
Many breaks happen at the keyway. Your prop/shaft doesn't need a key anyway. The convenience of not having to lap a prop to a shaft is the only reason for having this weak link, as I see it. If one is going to have a keyway, the edges should be rounded, perhaps the end of the keyway spooned, as they used to do, or better, a short, shallow keyway nearer the thick end of the taper.

 

There are two areas where you have had trouble, the end by the prop and the end by the coupling.

At the prop end if you have a material with high ductility it will be more likely to bend than break off like your first shaft did. 316 SS isn't very strong, but it has the ductility of taffy (50% elongation at tensile failure) and while the 2205 is better in terms of strength, it has only 25% elongation. Aquamet is three times as strong as 316 but only has 18% elongation at failure. The high ductility of the 316 coupled with the low strength allowed the shaft to bend and not fail. Typically what you gain in yield strength you lose in ductility, so unless you go to something exotic (like Inco 718 nickel alloy which has 170ksi yield strength and still has 21% elongation) and if you are balking at paying $200 for an Aquamet shaft, an Inco shaft would cost a lot more than that for just the material.

The Aquamet 17, 19 or 22 are all pretty good stuff. 17 and 19 have a yield of 105ksi and the 22 is 130ksi. The Aquamet 22 is three times as strong (yield strength) as the 316 that you had and bent. Aquamet 22 is better in both yield strength an ductility than 4340 (normalized) steel so you are probably better off with the Aquamet than going with a high strength steel since you get the better corrosion resistance and as good or better material properties. I'd pick the Aquamet 22 over the other higher strength steels, but your choice is more basic than that.

If you don't want to lose the prop maybe the softer shaft is worth doing again, provided you are willing to replace the shaft every time you hit something. If you go with something like Aquamet 22 you are less likely to lose the shaft, but may bend the prop. But if you hit something really hard you are going to lose both.... Neither is very nice, but you are the only one who has an idea what makes sense for you.

It also depends on what you do for a prop. If you have a softer prop material you will absorb some of the energy in the blades and won't be so hard on the shaft. A soft (bronze as opposed to stainless) prop coupled with an Aquamet 22 shaft may be a way to go.

With regards to the coupling end, as others have noted you should get rid of the coupling that you have now and replace it with an SAE tapered joint with a flange bolt pattern that you have now. The flange can likely be bought off the shelf and the shaft will bolt thru the flange (like the prop does on the other end) and seat on the taper. A keyed tapered coupling won't have the fatigue issues that you have with your current coupling, and most importantly the shaft will be properly centered in the flange.

What you have now is a rotaing fatigue machine. Misalignment of the shaft and engine are creating bending loads in the shaft and that is what is causing the fatigue. It is made worse by the pinch screw setup which forces the shaft off center in the flange, and concentrates the bending under the pinch screws. A tapered SAE flange will fix both of these issues and if you align it properly and use a flex joint you will never have an issue with this end of the shaft no matter what shaft material you chose. Somebody offered to help you get the engine and shafts properly aligned, and you absolutely need to do that.

Lastly don't listen to Frosty, his advice in this case is really bad (as usual). You don't want to increase the shaft diameter at the flange end of the shaft. If you do you will increase the bending stiffness of the shaft and that will increase the stress from any misalignment. If you have enough strength to transmit the torque, you don't need to increase shaft diameter or you will just make things worse. More strength at the prop end wouldn't hurt, but you have to fit the existing prop strut and once you do that the diameter is pretty much set.

I trust all this is helpful, it doesn't solve all your problems, but hopefully it lets you make an informed decision as to what your choices are. You are the only one who knows what you may or may not hit where you are running your boat, and that makes all the difference in the world.

 

The dia is not pretty much set. Did you know that you can buy different sizes of cutlass bearings. The shafts are tearing up, and bending, you say that going bigger will make it worse!!!!.

Look at the pics of the prop??? tiny shaft, what is it 3/4.

He 'can' turn out his original drive coupling to an sae. This would improve vibration which must be terrible.

How do you know my advise is always bad?--You have only been on the forum 3 months, you hav'nt been here long enough to know.