What material to make sail boat rudder shafts from

camojoe · Mar 2, 2013

Guy needs three rudder shafts made for his sail boat.
All three shafts are the same dimensionally, but the sample I have is eat up in corrosion.
This makes me think the material is not stainless as he thought, or if it is, a very low grade.
The question is, what is the best material to make the shafts from?
Bear in mind that this vessel will be making a trip from Savannah, Ga., around the Keys, and back up the west coast of Florida, so it will be in a marine environment.
Any suggestions will be greatly appreciated.
Camo

Limy Sami · Mar 2, 2013

Most if not all the stainless ones I've come across have corroded / pitted, those made out of nickle aluminum bronze have not.

jCandlish · Mar 2, 2013

Admiralty Bronze.

camojoe · Mar 2, 2013

Thanks for the replies fellas.
I was already looking at using 464 Naval Brass.
Hadn't even thought about aluminum nickel bronze.
Camo

Capp48 · Mar 2, 2013

Naval Bronze or Naval Brass. N Brass contains 60 percent copper. N Bronze has about 90 percent. Monel 400 or the more expensive but vastly superior K-500. If the shaft is exposed I would forget stainless. It will rust even in fresh water. Ciao.

72bwhite · Mar 2, 2013

Bronze will hold up better all stainless will eventually corrode in a marine environment, but how long did it take to get that corroded?
Not knowing more just make him what he asked for out of stainless unless he asked for bronze.
And the bronze would need sacrificial zinks just like a prop shaft.

rmsfla · Mar 2, 2013

Just finished one today!!

Changing from out board to a inboard built a whole new rudder
Customer supplied materail C-655 bronze 1" dia.

Using Aquamet 19 for the drive shaft

Robbie Machine Service

Raul McCai · Mar 2, 2013

The problem with SST underwater is the fact that there is very limited free oxygen for the chrome to form the oxidixed passive layer so when the water strips the old chrome oxide away ( which it does) a new layer does not form so well and you get corrosion.

Inconell might be the best of all possible choices save for the expense of it. Bronze is cheaper and is mostly copper and survives salt water quite well.

Trboatworks · Mar 2, 2013

Lots and lots of stainless is used in rudder stocks for marine environments.
One always looks for corrosion failure on survey but it is not frequently found.
If seen, it is more typically a grade or site issue- not the use of stainless per-se.
There is plenty of free oxygen in sea water to prevent corrosion.
Anoxic environments such as inside of leaking FRP foil shells, fasteners or stock buried in structures, electrolysis etc can lead to failures.

This page is as good as any on the subject:

http://www.jefa.com/products/materials.htm

gnorbury · Mar 2, 2013

In the "old days" rudder posts were made of bronze, but that got too expensive so most boats these days use stainless. If it was made in China or Taiwan it could well be 304, but that pits and corrodes unmercifully when submerged in salt water. 316 is better so long as it's not welded, and if you have to weld then use 316L.

Aquamet 22 is top of the line for shafting. Not sure whether it's ever used for rudders though.

Trboatworks · Mar 2, 2013

Bronze is no free ride either though- galvanic corrosion will turn the stuff to chalk.

Forrest Addy · Mar 2, 2013

The stuff I favor is Everdur silicon bronze. Almost bullet proof but like most metals immersed in satl water it needs to be cathodically protected either passively via direct connected sacrificial metals or by active electro-galvanic protection. A failed bonded ground system may play a part in your corrosion problem.

BTW most stainless steel is not suited for immersion in salt wather. It requires atmospheric oxygent for its surface metallurgy to kick in. Above water in the spray, no problem; lasts for years. Underwater shafts and hull fittings not much better than galvanized steel or plain brass.

Bill D · Mar 2, 2013

strange that no one has mentioned that stainless steel is stronger then bronze. I would think there may be issues replacing steel with brass unless the material is a bigger cross section.
Bill D.

Winker · Mar 2, 2013

The biggest issue is electrolysis. Proper bonding and sacrificial zincs or an active system will eliminate the problem no matter what metal you use.

seiner · Mar 2, 2013

As aluded to earlier ss needs to have an intact oxide layer for durability, it also suffers from crevice corrosion which is exacerbrated by warm temps and brackish water so careful manufacturing practices are required that all being said 316 holds up very well if the two previous points are followed.

Any bronze with zinc will fail and in fact any bronze with zinc is brass and should be avoided. Admaralty bronze is a trade name for a decent brass, maganese bronze is also a brass and both should be avoided.

I would use Monel or Aguamet which in reality are just very high nickel content bronzes.

And last the definitive word on marine corrosion is a book By Nigel Warren called "Metal Corrosion In Boats".

Sea Farmer · Mar 2, 2013

316 SS is probably cheaper than bronze. 304 isn't much use in the ocean in my experience. Monel would most likely be prohibitively expensive.

I wouldn't worry much about strength, because I think of a shaft as an extra shear pin

I'd rather have the shaft snap than see torque from a "mishap" (translation: "Hit something")
transmitted into the hull. Much cheaper to replace a shaft. I'd put more effort into making sure there was some sort of galvanic protection than in looking for the perfect material.

Eventually shafts have to be replaced. Everything metal in the ocean eventually has to be replaced. Your guy bought a sailboat large enough to require three (3) rudders for his auxiliary power. Did he think this boat would be cheap to maintain?

If it was my boat I'd use 316 or 316L

keithmech · Mar 2, 2013

I just finished up a couple made from 6061 al. 3 1/2 dia.
These are going into a 50 foot cat sailboat.Time will tell how
they will work.

oldbrock · Mar 3, 2013

One word. Monel

boatwelder · Mar 3, 2013

Lots of good info but spread through out the replies. Brass or bronze high in sink will fail because any part immersed in the water will become cathodic. I be seen it in brass through hulls electrolizing to the point of sinking a boat. Aquamet for shafting is used extensively however if welding is involved on the shaft ive heard aquamet doesn't weld very well. I have used 316 stainless for building rudder shafts and never had a problem. The reply above not to weld 316 stainless should consider the welded areas and heat effected zones can be passified to make them corrision resistant again. We are talking about sea water not hot nitric acid baths.

steve-l · Mar 3, 2013

The material of choice depends on what other metals attached to the hull are below the waterline. Electrolosis is the enemy to avoid. That means dissimilar metals. Brasses (zinc alloys must be avoided at all costs) Bronzes can work, as long as there are no steels or aluminum below the water line. Aluminum will also work, as long as it is 5000 series and ONLY alumininum is below the waterline. (6000 series doesn't work at all in a seawater environment. Stainless works well, as long as it is a 316 variant (Ti is the best), as long as ONLY steels are below the waterline AND it is not exposed to stagnent water. There is plenty of disolved oxygen to satisfy the surface oxidation requirement of SS in seawater, but only if it is constantly flushed. Please remember that the use of anodes is not a cure, anodes are a band aid to apologize for other poor material choices. One more very important point, observed corrosion can and often is, caused by poor wiring in the boat and not just material choice. The hull and all attachments must be isolated from electrical earth, both AC and DC. I always install an LED between battery and the hull as an alert to a ground fault.
Steve

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