6061 t3 aluminium vs 316 ss bowsprit considerations..

[Eastward ho 24]

This winter I'm hm getting rid of my wooden bowsprit and replaceing with what i was considering 316. But the weight of the aluminium has peaked my interest.
I've got a downeaster 32 cutter rig. As far as sizeing I'm going off of the westsail 32 square tube bowsprit that bud sells. Which is 4x4" by .120 thickness. I'm not sure if the westsail replacement is grade 316 I can only assume so. I'm refitting with asperations of ocean crossings and circumnavigation so my number one concern is strength and longevity. So I have been trying to figure out what a 6061 t6 square tube would need to be to equal the strength of the 316. I've got roughly 5" of room between the bulkwarks at the stern to pass through. But more likely 4x4 is the magic number to make this easy. Any one able to figure the thickness I'd need and the weight savings? Also any real world advice as far as corrosion saftey issues with the alu? The sprit has bobstays and will be roughly 8ft long.
P.s I weld ss and carbon for a living, just never touched the alu. So I'd be welding and building the SS myself, so I'm gonna really need a good salesman to sell me on the alu. The SS is roughly 10lbs ftsq. I can only assume it'll be about the same as the solid wood one I'm replaceing. As I type I'm talking myself out of the alu! Haha but the question is still valid if any one cares to weigh in.

And one more thing. I've never polished ss. Only when I'm done welding shine it up w a 80 grit tiger paw which is hardly a proper finish. And I've tried in the past to find a place to electropolish in the past to no avail. I know painting SS is a no-no but probably easier then polishing.
If my suspicion is correct I could paint it and it'll be fine may only last 50 yrs as opposed to 5000 yrs?

[stainless guy]

If you can find linear buff stainless go with that, mirror polished or second best #2B which is easy to polish up nice

Aluminum to takes more design work to make it stong for longevity.

I work with both in my business and would opt for stainless

[Eastward ho 24]

The stuff I was looking at is from online metals looks like all they have is mill finish..

[stainless guy]

Yes linear buff is usually thin walled tube though there are some suppliers that make specialty stuff it may be cheaper to buy what you need and get it polished before you Fab and weld. I suggest going to a Harley or speed shop this guys usually have lots of polishing done

[model 10]

I'd make it out of stainless flat stock. That way you could taper it and have very little buffing to do, the sharp corners would be easy to polish. You could fit it so that it would be a seam weld and go pretty fast.
Another note, think about getting your anchor chain out in front of your bobstay, headstay.

[bongo]

I made our bowsprit from a 20' length of 2", schedule 40, 316L pipe. Had it bent by a Boiler Works outfit. Getting the angles just right, and polishing were the hardest parts. It started with a mill finish, and ended up with close to a mirror finish. It was messy to do, but do not regret it. It is in the irregularities in the surface that corrosion/cracking is going to start. Got polishing supply's from an auto body paint supply store. All MIG & TIG welded.

[wsmurdoch]

You might take a look at 5052 or 5083 aluminum in place of 6061 for better salt water corrosion resistance, but any of them would probably be fine. I have found 304/316 stainless much easier to weld.

Bill

[Cheechako]

That's a tough one. Aluminum with crevices and spaces on deck may corrode from seawater trapped. .. that ugly white powder style corrosion.
SS, even 316, will have rusting at welds unless you anneal it after welding, or super polish the welds, but may rust inside the tube.
Aluminum will be about 1/3 the weight (A little over actually) of SS in the same dimensions. Going for 2 x thickness in Aluminum will still be lighter and stronger. But aluminum is much more "bendy" (modulus of elasticity is much lower)
The bow gets plenty wet with seawater and, as offshore sailors can attest, SS lifeline stancions etc will start to rust quickly if not cleaned and polished regularly. However, bare aluminum sloughs grey powder on things that touch it after a while .
My gut says 1/8" wall on the SS is a bit thin, but I'm an "overkill" kinda guy.
If I could have aluminum heavily anodized it would likely be my choice. But that big a piece may be hard to do.
On either material I would build it so it doesnt sit flat on deck, but has an air gap under the sprit between it and the deck.

But like Bongo's boat above, making a SS sprit/pulpit instead would eliminate a lot of on deck stuff!

[Cherod]

dont be afraid of aluminium welding if you already are a welder and have a compatable AC machine , pure argon, an ordinary SS / carbon lead and tungsten are fine , clean material and go fast , no prob, i am sure google / youtube is full of advisory videos.

[Cheechako]

If you are not experienced with aluminum welding, get someone who is to do it. It can be beautiful, but is a bit of an art. I manage a shop building aluminum boats for 3 years. And engineered aero space builds and welding for 30 years prior. Sometimes beautiful welds can be barely stuck on, properly done they can be rooted in deep as they should be.

[LeeV]

I don't know about the look on your boat, but I'd be tempted to make a carbon sprit. We had one on our Corsair 36 trimaran with big loads, and never had a concern. Painted it Awlgrip black on black.

[desdestiny]

If you wish to understand the relative strength between SS and aluminium, and the weight, send me a drawing of the proposed parts with the SS material thickness, I can analyse both designs and show relative strength and weakness areas.
I do this for a living running Finite Element Software and high end CAD.

I will run a simple analysis no charge.

BTW Stainless steel is only stainless if it is unpainted. Stainless steel forms an oxide layer over the surface which prevents further rusting. In marine environment #316 SS is best for the salt water immersion. #304 suffers crevice corrosion in small gaps where there is insufficient oxygen to maintain the oxide protective layer. This is why sometimes you see a rust stain around the base of a staunchion of bolts.

email me des@edgetec.net

[fabricator-ken]

That statement leaves a bit to desired. 316 is not good for immersion. Only should be used for deck hardware. For immersion use duplex. For prop shafts, rudder shafts etc. Both 304 & 316 will suffer from crevice corrosion. Sometimes called tea staining also. Stainless needs to be flushed with water to remove the fallout/ contamination. If it is highly polished it will remain in excellent for years.
I used t6/6061 alloy for my prodder. Very strong. This material will not bend. It is also much lighter then SS. I used a plug of a plastic material inserted in each end. The tack strop runs through center of the alloy tube to adjust the tack. Approximately 98mm dia OD with a 6 to 8 wall thickness. My prodder has two ss sleaves which it runs in. So it can be extended or withdrawn. Has been in constant use for 8 years around the World.