Fabricate new rudder post and fingers, what material to use?

[lostsoul]

I am back at it again. Finally took my rudder completely apart. The post is shot. The post is 1 7/8 inch OD and about .15 wall thickness. 49 inches long and made of stainless. Welded to the post are three 1 1/2 wide fingers made of plain steel. (as I hear, this was common). I will use the same alloy as the post, if possible.

I am seeking advise on what alloy to use for the assembly. I have no way of knowing what the original alloy was, but that does not really matter. Rudders like this are ubiquitous.

I know that SS can suffer crevice corrosion when in a low oxygen environment, so this is my main concern. The post has some deep pitting at the point where it exited the rudder and this is the reason I need to replace it.

I also have to decide on whether to copy the original design and use fingers or to come up with a new design. For instance, rather then use 3 fingers I could use a single plate, (like a mini rudder).

I did a quick search of the online metal suppliers and found 316 tube, welded and seamless.

There are a few decisions to make:

what material 316, 316L, 2250 Duplex SS, or something else
what wall thickness
welded, seamless...
where to get the materials from.

HELP! (lol)

Thanks guys...
Hopefully there are 2 pictures attached

[Cheechako]

If possible 316L. I would use a solid bar instead of tube/pipe. There are better modern alloys than 316L too, but not sure what shapes they come in or how much more money.
A couple guys here know what they are , will probably pipe in .
Also, on my rudder years ago I filled it with lightweight epoxy slurry/microballoons instead of expanding foam, eliminating any future water ingress. Not really that heavy and it weighs less in the water anyway.

[lostsoul]

I measured a bit better. Finally got my good caliper out.
The od is 1.90. Wall is .15 or maybe .145. This does correspond with 1 1/2 pipe. I can feel a bit of a flat spot inside the pipe.

I looked around at metal suppliers. Interesting stuff out there.
There is inch and a half 316/316L pipe available for a lot less then the tube. Without shipping, it prices at just under 100 dollars for 4 feet.

TW metals had a choice of welded and seamless and schedule 40, 60 and 160.
80 is .20 thick and 160 is .28 thick.

Other alloys are available, like aquamet 22 and 2250 duplex, but only by email quote. I sent a few emails, but I doubt it is worth the extra expense.

At this point, I will most likely go with the pipe, probably schedule 160. My bottom bearing (pintle) needs to fit inside the pipe. I need to do some machining and checking for this. Schedule 80 would be fine too.


Then there is the ribs or plate decision. Ever price 316 SS plate? For about $250 I can get a nice 1/4 inch plate, wow. 3/16 is about 175. If I go with 1/4 inch ribs it is about half that price. I can make 3 ribs with a piece welded across the aft ends.

Next is deciding whether to weld center line or edge (tangent). If I weld it center line it is easier for my to rebuild the rudder. I do wonder if it would be strong enough, especially if using the 1/4 by 1 1/2 ribs.

With all my research I am going further down this rabbit hole. I am learning about what happens when SS is welded and how the welds are not corrosion resistant. Apparently pickling has nothing to do with spices. I have done a little bit of brazing and welding but not for years, so I will be having a shop do this work.

[Cheechako]

Welding centerline should be fine. Trying to weld on one side tangent is often not a strong weld. If you are sealing/filling the inside of your rudder with epoxy etc, you could use 304 SS for the plates, (If it's any cheaper!) it welds fine to 316. It's buried and not exposed to corrosion inside anyway if you seal it.
Can you get it in flat bar? Is that cheaper?
It's amazing how much SS is now days. I don't get it. I think it's a controlled market. Back when steel was maybe $.50-.60 a lb, SS was $1.50-2.00 a lb. SO a 1/4 x 3 x 36" flat bar of SS would have been maybe $16 + a cut charge...
Yeah...or tube is real expensive...

[sailah]

I'm doing a similar project, not a rudder though. Welding 316 stainless. The welds will corrode unless you passivate them. You can do it with a spray or dunk it in citric acid. I was going to do this but I ended up opting for electropolishing which is not really an option for the rudder. But yes you want to pickle/passivate the welds or they will rust just like steel.

I would also seriously consider getting your webs laser cut. I buy literally tons of stainless and brass for work a month. It made no sense for me to buy it and send it to the laser shop. With CAD you can draw out the webs and send that DXF file to a laser shop. The money you spend will be subtracted from the many hours of hand labor it will take to shape the curve.

The laser shop I used was very reasonable and they had the 1/4" 316 in stock so their number was inclusive of material.

Don't know if you have access to someone with Cad skills though.

Google Keith fenner on YouTube. He recently had a video series on building full stainless rudders for a large powerboat. He goes over many of the topics you're interested in. Obviously his skins are stainless whereas yours will be glass but much will translate.

Found it, it's a long series but a good watch, called wing it

He's a real pro too.

https://youtu.be/277VGi7XbZs

[Celestialsailor]

As a Machinist/Toolmaker, I would use 316L (passive). It should outlive you. I would do a perimeter frame 1 1/2"X1/4" maybe a gusset in the center.
I use McMaster-Carr for my stainless needs.

[Toaster]

If you happen to be near Port Townsend, Washington, talk to Pete at Port Townsend Foundry. He built a rudder shaft for us out of Nickel Aluminum Bronze - we handed him the old one, and he handed us a perfect copy in about a week. It has a higher strength than SS, and better corrosion resistance - it does not suffer from crevice corrosion. The SS shaft we took out of the rudder (35 years old) had scary crevice corrosion.

Actually, even if you don't happen to be near PT, consider talking to Pete anyway - shipping is cheap. He's a craftsman and really understand boats. He even knew what a white metal bearing is, and made us one.