Mixing bronze & steel rudder fittings?

[LittleSister]

Can anyone offer any informed advice about how unwise/pragmatic it might be to mix steel and bronze rudder fittings? Or point me in the direction of a supply of off-the-shelf steel fittings?

My heavy wooden rudder is hung from the GRP hull by 3 sets of steel gudgeons and pintles at present, but the central one is worn, apparently primarily the pin (which in this case is the hull fitting). I can buy an off the shelf replacement of the same dimensions in bronze for the worn fitting, but haven't been able to find steel ones.

I would have thought that it would generally be advisable to avoid mixing metals in a salt water environment, to avoid galvanic corrosion. However, I also notice that one often finds a bronze propellor on a steel shaft, and that one of my steel transom fittings for the rudder has a brass or bronze spacer behind the steel fitting but no obvious sign of corrosion in the immediate vicinity (this is the fitting with the worn pin, but the wasting of the pin seems consistent with the combination of predominantly sideways wear and some rusting, rather than galvanic corrosion).

I could replace all the fittings in bronze, but this is a rather larger task than I want to deal with before the season. (I am wondering about doing this sometime later as the existing fittings look rather on the small side to me, though it seems to have held OK for the last 20 years, and neither of two surveys raised it as an issue.) I could have a piece custom made in steel to replace the worn one, but is this necessary when I can get a bronze one through the post?

I am not sure if it makes any odds, but the rudder straps appear to be stainless steel, while the transom fittings, and the two pins on the rudder, are non-stainless. None is wired up to the hull anode.

Any thoughts?

 

[ccscott49]

No probs with bronze and steel. Thats why they often use bronze bearings in steel pulleys on steel shafts, even in salt water enviroments. IMHO.

 

[VicS]

Bronze and <u>stainless</u> steel are fine in contact with each other. If you look at the galvanic series you will find that they are not widely separated. As you observe bronze props on stainless steel shafts are common.

Bronze in contact with ordinary mild steel will lead to rapid corrosion of the steel because they are far apart in the galvanic series. A mixture of the two in close contact with each other under water would stand very little chance of any cathodic protection being effective.

There is I suppose no objection to having one set bronze and another set steel provided there was no connection between them just as so long as gudgeon and pintle in each set are the same.

An insulating bush between gudgeon and pintle if they are of dissimilar metals would solve the problem.

You should also not mix mild steel and stainless steel for the same reasons.

 

[ccscott49]

Sorry I meant stainless!! I guessed the pintles would be stainless not mild!

 

[wotayottie]

Every metal has a different electrochemical potential so the only safe answer is to use the same metal for all underwater fittings. The difference between some metals is small however, and if the distance between them is great then the problem will also be small. Add into that equation that wood itself can suffer from attack.

Have a word with MG Duff - they are very helpful. But whatever you do, do not use stainless steel embedded into wet wood.

 

[boatmike]

Yet again Vic, you beat me to it. Absolutely agree on all counts. As it has now been determined that the parts are stainless it shouldn't be a problem. If it's an old wooden boat I would check carefully that they are however by running a magnet over them. Not a conclusive check but a good indicator. As you say, if parts are seperated, like top and bottom pintles of a rudder it should not be a problem apart from the fact that there is probably a shaft through the rudder that connects them. However if they are stainless this is not a problem, carbon steel would be.

 

[VicS]

[ QUOTE ]
As it has now been determined that the parts are stainless it shouldn't be a problem.

[/ QUOTE ] Actually it hasn't been. That was Englander clarifying something he said earlier. Carvel has not yet responded to any of the points made in answer to his original question.

This might now be the time though to draw attention to the problems that anodes can cause on wooden boats .... There is a bit on MGDuff's website about it.

 

[wotayottie]

having re-read your initial post Carvel, I would certainly check the stainless straps. The side that faces the wet wood of the rudder could easily be badly corroded, and if the pintles are some other material that may well create a problem of its own. Nor would I recommend stainless pins in bronze pintles - the rubbing as the rudder moves will damage the protective oxide coat on the stainless and will allow corrosion. Had exactly this experience on a boat with stainless rudder fittings and the bearing surfaces looked for all the world as if they had been attacked by giant worms. The bolts that went through the rudder and were therefore damp and air free were 75% eaten through.

The comparison with a prop and stainless shaft isnt valid - the prop isnt working against the shaft surface, done properly the joint face should be water free and most shafts have anodes anyway.

 

[LittleSister]

Thank you all for your comments, and taking the trouble to respond with advice.

Just to clarify - this is a GRP, not wooden, boat, but with a wooden rudder; there is no rudder shaft - it is transom hung on gudgeons and pintles; there are no stainless pintles, they seem to be non-stainless (mild?) steel; and I've found no evidence of galvanic corrosion.

I agree that in an ideal world one would not mix metals (or indeed, allow one's boat to get wet!), but given the boat, budget and time I've got available I'm aiming for a safe, working compromise.

On the basis of the replies above, it seems that bronze would be less of a mismatch with the stainless steel rudder straps than the existing (mild?) steel transom fittings and pintles (which in any case do not appear to be at all galvanically corroded). I am therefore tempted to fit a bronze transom fitting to replace the one worn (mild?) steel one for the time being. This is above the waterline (though only by a few inches) so I can keep an eye on it during the season and replace again if necessary.

I will have a closer look at the stainless straps for potential hidden corrosion, as suggested, but from the outside they seem very smart and sound, and this potential issue was not raised in either of two recent professional surveys. (The rudder pin wear was.)

Though I'm not planning to use them, I'm not entirely convinced by the suggestion that stainless is a no-no for bearing surfaces such as pintles. Yes, the oxidised surface may be worn off, but in the presence of oxygen, won't the stainless steel just oxidise (in a non-rust way!) again?

Anyway, do let me know if I've misunderstood anyone, but thanks for all your help.

John

 

[wotayottie]

I'm still not completely sure about the construction of your rudder and attachments. My comments come from many years working in the steel industry and in particular the manufacture of stainless steel. That doesnt make me an expert ( technically educated but not a metallurgist) but has made me very nervous of common stainless steel grades in marine use. If the oxide coat is damaged, and there is insufficient oxygen available to allow replacement, then the corrosion of even A4 stainless can become bad. Worse than mild steel. So the part of your rudder straps that would worry me is not the shiny outside (lots of O2 available) but the side facing the wood where any trapped water is likely to be stagnant. All depends on how the straps were bedded. As I indicated earlier, 12mm A4 bolts through the skegs of my boat and which were poorly installed by the builder were so badly eaten away after 7 years that some came out on pieces. The pivoting bearings of the rudders were similarly eaten away where the rubbing of movement removed the oxide despite nylon bearings.

In your position I would remove one of the stainless straps (assuming this isnt a major job) to check. They might be well bedded and perfect - or they might be corroded.

personally, I'd replace what was worn with identical since at least you know that works OK.

 

[VicS]

It's a very valid point you have made about corrosion of stainless steel where water is trapped but oxygen is excluded. (Crevice corrosion)
I would say that it is vital that stainless fittings are bedded on a layer of sealant and that it is not all squeezed out as the fastenings are tightened. Equally, probably more, important is the need to seal fastenings so that water cannot penetrate. that applies to GRP as well as to wooden boats.

Those inverted U bolts and the like often used for the shrouds are very vulnerable.