6061-T6 Aluminium vs 316 grade Stainless Steel

[Cheechako]

aluminum modulus is a lot lower than SS. So your problem may be that it is much more flexible. I think it is around twice as flexible, or has about 40% of the resistance to flexing. You should be able to buy SS cut to length is you search around...? You'll pay maybe a $20 cutting charge. If it's not a common size then that's a problem. Solid SS bar wont weigh that much more if that's an option.

[ForSeTi]

Originally Posted by UNCIVILIZED

There's a (perhaps) critical question to be asked about substituting solid bar for the tubing. Was tubing specified (instead of bar stock) for an engineering reason that we don't know about?

Like, for instance, does the tubing have a bending moment, which in a collision, will let it bend, where as solid bar stock, given it's increased strength & stiffness, will cause some other critical item to fail?
Say, for example, if both were used as rudder stocks, protruding through a hull laminated.

Such entirely may not be the case, but on some things, unless common sense easily rules them out, it's worth a designer consult on the question. That, or if one has the inherent knowledge to make that kind of call, then no worries.

Bending moment isnt an issue, as the top part of the rudder is attached on the transom with a pivoting point which kicks the bottom part out of the water in case it gets hit (hope I'm clear in my explanation as English aint my native tongue)
Only the bottom part rotates for direction

[ForSeTi]

Originally Posted by jimbunyard

And by the way , I have a 26" piece of Vesconite rudder bearing stock material, raw dimension of 65mm od x 35mm id, left over from when I installed my rudder. Don't remember the cost but if you're interested I can find out and could send you 10" or so (enough for 2 bearings).

Thanks for the offer
Already got my bearings sorted
Should it get unsorted I'll certainly contact u again

[UNCIVILIZED]

Originally Posted by ForSeTi

Bending moment isnt an issue, as the top part of the rudder is attached on the transom with a pivoting point which kicks the bottom part out of the water in case it gets hit (hope I'm clear in my explanation as English aint my native tongue)
Only the bottom part rotates for direction

Some of it then is likely the issue of mixing metals. As Aluminum is right next to Zinc on the Galvanic Scale,. So that if it's attached to Stainless, & then immersed, even if heavily protected via Zinc anodes, it will corrode. Most likely at an unreasonably rapid rate. - Both from an engineering standpoint, as well as that of the opinion of your wallet.

[ForSeTi]

Originally Posted by UNCIVILIZED

Some of it then is likely the issue of mixing metals. As Aluminum is right next to Zinc on the Galvanic Scale,. So that if it's attached to Stainless, & then immersed, even if heavily protected via Zinc anodes, it will corrode. Most likely at an unreasonably rapid rate. - Both from an engineering standpoint, as well as that of the opinion of your wallet.

The 316 SS (or 6061 Aloy) wouldnt be in contact with Seawater
It would only be used as a pivoting point, encased in a ply/epoxy/... rudder.
There wont be a mix aloy/SS either. Its one or the other I would use.

Also been checking for the solid bars, and they are easily found in 35mm around here, perfect match for my 35mm ID bearings.
And allmost 1/3 price of the tubing I was talking about before
So, 1/3 cheaper and tougher (I hope I aint a fool in thinking a solid bar would be tougher then a tube )
Guess I just need to upgrade the pivoting point so that can take the extra weight

G

[UNCIVILIZED]

Originally Posted by ForSeTi

The 316 SS (or 6061 Aloy) wouldnt be in contact with Seawater
It would only be used as a pivoting point, encased in a ply/epoxy/... rudder.
There wont be a mix aloy/SS either. Its one or the other I would use.

Also been checking for the solid bars, and they are easily found in 35mm around here, perfect match for my 35mm ID bearings.
And allmost 1/3 price of the tubing I was talking about before
So, 1/3 cheaper and tougher (I hope I aint a fool in thinking a solid bar would be tougher then a tube )
Guess I just need to upgrade the pivoting point so that can take the extra weight

G

How, if the metal is being used as a pivot point, will it not be immersed, or in contact with seawater? And if the 35mm bar is being used as the pivot point/pin in a bearing, then what is surrounding it to act as a bearing surface? Also, again, what is being done to prevent the aluminum from becoming the defacto anode?

[ForSeTi]

Originally Posted by UNCIVILIZED

How, if the metal is being used as a pivot point, will it not be immersed, or in contact with seawater? And if the 35mm bar is being used as the pivot point/pin in a bearing, then what is surrounding it to act as a bearing surface? Also, again, what is being done to prevent the aluminum from becoming the defacto anode?

Top part of the bar will be incased in douglas frame, wrapped with ply/epoxy/fiberglass, bottom part (the actual rudder, also encased in ply/epoxy/fiberglass )
Bottom part will be the part thats constantly in the water.
If water should get to the SS, that means the rudder is shot

[roverhi]

If this is a rudder stock/shaft wouldn't use aluminum. Pearson used aluminum for the stock on the early P30s, after only a few years they began failing. They replaced the rudders with SS stocked ones and all has been well but it was an expensive lesson for the factory.

The online metal suppliers will pretty much cut you what length you need. If the tube is an odd ball, they would probably charge a lot for it if they have to buy a longer length that may sit in inventory,however.

[noelex 77]

Here is a good link about rudder stock material from Jefa:

Rudder stock materials

[UNCIVILIZED]

Using aluminum for rudder stocks isn't a problem, if, things are properly designed & engineered.
There are plenty of both; aluminum, & composite hulled boats out there, which have aluminum rudders, with aluminum rudder shafts (and a few with aluminum shafts, & composite blades).
I owned one of the former for a while - plenty of carbon fiber in the hull, plus, S-glass, & Kevlar.

Or, witness a lot of the Dashew's boats for example. Solid aluminum rudder shafts, with fully aluminum rudders. Although, some of said rudders may have had sacrificial composite tips.

[Arthur Garfield]

ForSeTi-


The spec seems fragile and inelegant for such important location,perhaps talk with designer to re-spec/revision?

I entirely agree with jimbunyard on this element.



All the Best

[ForSeTi]

Originally Posted by noelex 77

Here is a good link about rudder stock material from Jefa:

Rudder stock materials

Great Link
It says specifically on the site that 316 SS aint stronger then 6082 alu.

Seems if I go for the 6082-T6 I should be good.
If I Use 7075 I have closer percentage tensile strength to 316, but no corrosion resistance
That could be remedied with anodizing it...
Other advantages are:
lower weight, lower price.
Seems alu is again an option

Made some calculations:
The plans ask for 316 SS, size OD 33,8mm wall thickness 6,35mm,
thats a total of 509mm² SS

If I use 6082-T6 size OD 35mm full bar I got 962mm²

Thats allmost double the amount of material in cross section

[Cheechako]

The reason many makers use thick wall tube for rudder stocks is of course to save weight, but additionally, a solid stock adds little additional stiffness over a thick wall tube. Can you live with the flex of aluminum? it's not all about strength. if the rudder flexes a lot, will the layup crack and eventually leak?

[ForSeTi]

Went on a search on the net, and compiled the following file.
It shows the specs for alu 6082, alu 7075 and 316 SS
Am I reading this wrong or, according to these specs, is the 7075 stiffest and strongest of the 3?

https://drive.google.com/file/d/0B5z...ew?usp=sharing

[Cheechako]

You need the Modulus of Elasticity for stiffness I think.