304 vs 316 on deck

[Chotu]

My machine shop is FINALLY doing the attachment points for my crossbeam.

I have attached a picture of this type of thing.

The plans specify them in stainless. I don’t know how to change the material to aluminum. I’m not sure what thickness aluminum would have the same structural properties.

The 316 stainless is $700 just for the sheet of stainless to make these.

Would 304 last?

Lastly, what about changing them to aluminum? How can I change the specs so they would have the same physical properties?

They are made from 3/8” 316 stainless in the plans.

[ItDepends]

In the marine environment, 304 has one advantage over 316: it is cheaper.

304 is MARGINALLY stronger, but the actual specification ranges overlap by a lot, so you can have a piece of 316 that is stronger than a piece of 304 and both still be "in spec". So you COULD change to 304 without changing the part dimensions for strength.

304 is MUCH more susceptible to corrosion.

If you want to build a cheap boat that has rust stains down the side from the attached hardware, 304 is your go-to material.

Both grades are essentially similar to each other in terms of resistance (or rather LACK of resistance) to stress corrosion cracking in chloride environments.

If at all possible, 316L would be the better choice for parts that need to be welded, but if 316 is too expensive, then you are not likely to like the cost of that either...

[formosa aus]

If it designed for 316 why not use it? To change to aluminium would require lots of calculations and be more prone to corrosion.

[thinwater]

Notice the increased corrosion abound the critical bolt. Aluminum seems wiser to me. They are aluminum on most cats and my PDQ.


Thickness would depend on the alloy; increase thickness in direct proportion, keeping the other dimensions the same (add an 1/8" if the bolts are near the edges). I would also add about 15-20% to allow for the slightly greater fatigue vulnerability of aluminum (if stressed below that limit the problem goes away).


There is also the matter of anodizing. That will run the cost up.

But unless you get someone to look at the plan and run the numbers it's on you.

[Chotu]

OK. Thanks for the input.

I didn’t think I could use 304. Just wanted to really double check.

When it comes to aluminum, very preliminary check says that I have to have three times the thickness for the aluminum sheet that I would for the stainless. So it doesn’t save any weight.

So I guess I’m just going to have to bite the bullet here.

My fabricator is the one that told me not to get stainless if possible because the price was so stupid. He’s never seen anything like it.

Needs a 30“ x 30“, 3/8 thick stainless 316 sheet.

$717

[phorvati]

You can use 304. Its a matter of how much elbow grease you put in to it to polish it. Mirror finish 304 works very well. My split backstay chainplates are 304, 1.5" wide by 1/2" thickness from McMaster with certificate. 4 years into it they look just as shiny. I have about 5k NM on them.

[Chotu]

Originally Posted by phorvati

You can use 304. Its a matter of how much elbow grease you put in to it to polish it. Mirror finish 304 works very well. My split backstay chainplates are 304, 1.5" wide by 1/2" thickness from McMaster with certificate. 4 years into it they look just as shiny. I have about 5k NM on them.

No kidding!

How often do you have to watch the rust?

Would polishing it once a year do the trick? Maybe every six months?

I use this stuff on stainless steel usually. It works great. And it leaves a little protective barrier.

https://www.amazon.com/Collinite-850...26039990&psc=1

[s/v Jedi]

I have some 304 tubing that is more corrosion resistant than some 316 tubing but I also have 316 tubing that is far superior to both of them. I agree that the polished finish is a major factor.

Whatever you use, have the finished parts electro polished

[Matt Johnson]

I can give you the specs for our aluminum ones. Then just figure out what's needed from there.

[s/v Jedi]

Originally Posted by Matt Johnson

Machine polish is better than electro. Electro still leaves micro scratches and shows corrosion quicker than a well done hand polish.

You will not be able to machine polish all the nooks and crannies, especially not for this part as shown in the OP. Same for the welds.

Send it in for electro polish, then polish and wax after installation and keep that up once a year. Much depends on the conditions the boat is kept in. When we were in Panama, the environment was much more corrosive than in Florida where we are now; I blame it on continuous waves slamming the Port Cristobal breakwaters in Panama, but the stainless was salty a day after washing it.

[Chotu]

Originally Posted by Matt Johnson

I can give you the specs for our aluminum ones. Then just figure out what's needed from there.

OK. That’s definitely worth checking out. It’s always nice to compare these types of things between boats. Thanks. Can you send on DM/PM?

If I can switch to aluminum, that greatly simplifies things.

[s/v Jedi]

I believe aluminum only needs double the thickness to achieve equal/more strength.

[wingssail]

Originally Posted by ItDepends

In the marine environment, 304 has one advantage over 316: it is cheaper.

304 is MARGINALLY stronger, but the actual specification ranges overlap by a lot, so you can have a piece of 316 that is stronger than a piece of 304 and both still be "in spec". So you COULD change to 304 without changing the part dimensions for strength.

304 is MUCH more susceptible to corrosion.

If you want to build a cheap boat that has rust stains down the side from the attached hardware, 304 is your go-to material.

Both grades are essentially similar to each other in terms of resistance (or rather LACK of resistance) to stress corrosion cracking in chloride environments.

If at all possible, 316L would be the better choice for parts that need to be welded, but if 316 is too expensive, then you are not likely to like the cost of that either...

We have had stainless fabrication done for our boat, on deck and below deck, many times, usually in third world countries. Like Chotu, we've often had a hard time finding a fabrication shop to do the work. When we have found someone willing to do it, we are plenty happy that they can find ANY stainless steel and we don't often throw a monkey wrench into the works by demanding a particular grade of stainless steel. "Just do it, please, and soon, my boat is laid up without it".

So we've gotten 304 at times, maybe we even got 316 once or twice (other times, we have no idea).

Are there rust stains all over our boat? No, but we do see rust. But a quick clean up with metal polish makes it nice again. Say half a day every few months. We have a guy who does it for us for $40.

And we have had crevice corrosion and breakage, mostly on original hardware which was put on the boat when built. We have no idea what type of stainless it was; could it have been done in 304? Horrors to think that those rednecks in New Orleans who built this bucket would have done such a thing.

So maybe it is not worth stressing about.

[Celestialsailor]

Originally Posted by s/v Jedi

I believe aluminum only needs double the thickness to achieve equal/more strength.

No...Yield strength of 6061 Alum is appr. 6,600psi and 304 stainless is appr. 30,000psi. A factor of appr. 4.5.

[Celestialsailor]

Originally Posted by Chotu

No kidding!

How often do you have to watch the rust?

Would polishing it once a year do the trick? Maybe every six months?

I use this stuff on stainless steel usually. It works great. And it leaves a little protective barrier.

https://www.amazon.com/Collinite-850...26039990&psc=1

Contrary to the usual conjecture these questions bring up, 304 is used on boats with 316 more popular and yes, more expensive. Either one of them will need electro-polishing after weldments or machining which will expose iron to the surfaces. The method will remove to a depth of .0005" or if you like, 1/15 of a human hair. However, the rust stains will reappear on both without regular maintenance. How soon? Well...316 will last longer all things being equal. If you want the best possible scenario for each, the surfaces would have to be at least an 8 finish. The rust bleeding is also accelerated buy salt water in the air.

Also Aluminum would also need some sort of coating. Hard anodizing would help but over time could lift. As I stated earlier, stainless is approximately 4.5 times stronger than Aluminum.