Rudders..bronze vs stainless

[Tom Curran]

Hey group Ill be brief..I cannot decide between the 2 choices of oversize rudders
The stainless or bronze
I thought stainless wasn't suppose to be submerged under water all the time.. corrosion from Oxygen starvation etc..
Im looking at the ones from High tide Bertram products,(bronze) and the folks that build the Capt Patrick design(stainless)
I have read almost all the threads on this topic..lol
They are both a bit of cash,,
Thanks

[John Nardi]

Not an expert Tom, but my understanding is that the main difference ill be that you cannot use the new dripless technology rudder seals with bronze...John

[Tony Meola]

Tom

I would say that of those that have upgraded, have used Capt. Pat's rudders. Stainless with I believe a monel shaft. No issues, if I strip the anti following off of them, they still look great. No pitting at all. Mine have been submerged during the season, April through end of Oct. since 2009.

Remember the shafts are stainless, until we repowered, the originals were in the water from 1975 until the repower started in 2008 and finished in 09. No issues at all.

So if you go stainless nothing to worry about.

[Howesounder]

Stainless construction allows for thinnner web thickness like it does on props. If they take advantage of that in the construction of the rudders, I like stainless for them. The oxygen starvation issue is not there unless you really starve it. I have used old 316ss pump shafts for my moorings out in front of my cabin here, and they stand up perfectly.......except.......if I tie the moooring line tight, with a couple of wraps to the stainless "hoop" the stainless gets a cavity, and corrodes like nothing I have ever seen. My centre consul 19 foot aluminum boat the "good dog" was beached in a storm and badly damaged from the hoop breaking off at "the cavity" created by oxygen starvation where the rope wrapped around the hoop. So I lifted my second mooring and found that one similarly corroded, and I was just in time. A 1.250" 316 SS shaft had been eaten 90% through and the very sharp edges of the cavity had nearly worn the rope through. New 55 gallon plastic drum moorings have no metal. Full of rocks and concrete, and a 4" plastic pipe through the plastic barrel sideways, cast in place with the concrete and rocks. Then I just thread a rope through, and leave a nice big loop, and splice it. I used an old bit of tug tow line that wash up around here for the line. 2" in diameter, so I think I have a lifetime mooring now. That being said, if I were to have used a big rope loop on my stainless hoop, I suspect I would have zero corrosion.

Steve

[Tom Curran]

Thanks for input..I believe Pats rudders use A22 shafting and 316L stainless blade
according to supplier
Ill let you know what I decide..It may be this summer before I push the button on the purchase
Thanks

[Dug]

I don't mean to be a goof, but water is two parts hydrogen to one part oxygen, hence h2o... there is oxygen present in water, and you will not starve stainless if it is submerged. Unless I am missing something...

Dug

[Tom Curran]

Hey Doug..you're probably right..but I talked to my marine surveyor and he suggested the following article.
Thanks
Im a marine electrician and tend to overthink things..

[Tom Curran]

http://www.passagemaker.com/channels/st ... corrosion/

This is a good read if interested..

[Tony Meola]

Tom

While the article is spot on, non of us have ever experienced it and I know of a lot of boats with Stainless running gear.

[Tom Curran]

Thanks Tony..Ill probably run with that idea..I mean Capt Patrick knew his stuff..
Happy Holidays

[Carl]

I don't mean to be a goof, but water is two parts hydrogen to one part oxygen, hence h2o... there is oxygen present in water, and you will not starve stainless if it is submerged. Unless I am missing something...

Dug

Dug-
Look up pitting and crevice corrosion.
Most of it is above my head..but my simplistic understanding is:

Stainless Steel needs and uses the Oxygen in water to make a protective oxide layer. Not a problem replenishing used up oxygen when water is moving by.

Now if water cannot circulate...like between a Cutless Bearing and a Prop shaft, in particular the rubber strakes.

That area where shaft sits does not allow water to circulate and becomes stagnate. The SS keeps taking the oxygen to keep that protective oxide layer going...that causes oxygen depletion. Fresh water cannot get in to replenish oxygen.

What is left when the oxygen is gone...hydrogen, salt and water.

This is where I get confused as several things start to happen and never clear as what happens first.

Hydrogen makes water solution acidic which is corrosive.
Salt breaks down into chlorine which is highly corrosive.
Unprotected SS (due to Oxygen depletion) layer becomes Analytic and the protected layer of SS remains cathodic causing galvanic corrosion.

Someplace in there the salt water, which is already an electrolyte becomes a better one as the reaction goes on.

I do know as a result a cycle is started that eats thru SS...you'll see pits eat down into the material like a termite eats wood.

This can be seen where shaft sits on cutless bearing, stuffing box. On boats that have SS shaft logs, where the rubber stuffing box tube is attached (popular on old Silvertons and Luhrs) those tubes rot thru. SS butt joints, SS trim tabs riveted together, around SS welds. Welded pipes and joints with small opening left for water to get in...but not flow.


It is a very large reason recreational boats go with A-22 shafts over other grades of SS. A-22 is the highest corrosion resistant SS grade. On commercial boats many will go with cheaper, less corrosion resistant grades that are similar in strength...but as boat is used all the time they don't have to deal with this kind of corrosion resistance as the boats always on the go...not sitting week after week at a dock.

If you want to see for yourself...wind a piece of rope around half a piece of 304, 316 and a piece of A-22 SS, toss into water behind your boat in the beginning of the season, pull at the end.



As to SS for rudders...316L Plate with 316L filler, correctly welded to A-22 shaft then all ground smooth will last a good long time without a problem.