Bronze or stainless bolts?

[prv]

KS has a wooden rudder, held on by four bolts through metal straps or shoes at top and bottom. The old original stainless steel bolts have all corroded, in some cases dramatically so (I have one in front of me with less than a quarter of its diameter left in places). Unfortunately here at home I don't know what metal the straps / shoes are made of - must remember to have a closer look this weekend.

What bolts should I replace with? I'm thinking that bronze may be better in this situation than stainless, but I'm a little worried about shifting the corrosion to the metal fittings instead (which are currently fine), as these would be harder to replace than a standard bolt.

Cheers,

Pete

 

[TradewindSailor]

Stainless bolts neck like this because oxygen is excluded from the centre portion of the bolt. Oxygen is required by the chromium in the stainless steel to form the oxide that protects the bolt.

Silicone bronze bolts should be used, provided you have either stainless or bronze straps. I have only seen silicone bronze bolts fail by corrosion once ....... and I have no explanation for this (possibly stray current corrosion ..... but no evidence).

 

[prv]

Silicone bronze bolts should be used, provided you have either stainless or bronze straps.

As I say, I don't know what they are, but is there any other metal that's likely? Galvanised would presumably rot away far too fast, copper wouldn't be strong enough, and if we're going more noble than stainless and bronze there's only really monel (isn't there?) which I believe is used mostly for rivets and suchlike rather than structures. So I could order silicon bronze bolts now in reasonable confidence that they'd be the right thing?

Pete

 

[prv]

Just realised that there's some writing on the less-corroded part of the bolt head. Says "A2 80", which seems to be a grade of stainless but the only references I can find are in intimidating tomes for industry. Looks though like maybe this is the wrong grade for this use?

Pete

 

[Dan the man]

some woods have corrosive properties in them that attack stainless, like Oak. Tannin? I think it is called.

Brass or bronze or copper is what is best for wood. You could get copper rod and rivet the ends over, like roving a copper nail, maybe?

 

[VicS]

A2 is the wrong grade of stainless.

Two common grades are A2 (aka 304), which is the grade you'll find at your builders merchants etc and is unsuitable for marine use esp below the water line and A4 (aka316) which is the grade suitable for marine use.

the 80 relates I think to the strength ..I'm not familiar with the details but they're out there on the www to be looked up if required.

Even 316 stainless is susceptible to crevice corrosion so if you decide to use stainless again not only use 316 (A4) but seal the bolts into the holes to prevent water getting at the shanks of them.

Dan is right about some woods (although oak is the only one I know about) containing tannin which attacks ferrous metals.

Personally I would use bronze if the straps are bronze but probably stainless if they are stainless.

Definitely not galvanised in contact with either bronze or stainless.

Seanick, if he is around, is the best person to advise I would say

EDIT

Remembered on article by Nigel Warren in an old PBO (Feb1998)
Fastening for 316 stainless steel: 316 but sealed as above. Otherwise Monel but not silicon bronze
Fastening for bronze or gunmetal: 316 sealed as above, Monel or silicon bronze.

 

[TradewindSailor]

A2 is the wrong grade of stainless.

......
Remembered on article by Nigel Warren in an old PBO (Feb1998)
Fastening for 316 stainless steel: 316 but sealed as above. Otherwise Monel but not silicon bronze
Fastening for bronze or gunmetal: 316 sealed as above, Monel or silicon bronze

According to Nigel Warrens book "Metal Corrosion in Boats" you are correct about Silicon Bronze fasteners not being suitable for stainless steel fittings, my mistake ..... he also states that monel fasteners could also suffer pitting if used with 316. He says you should use 316 fasteners with 316 fittings and 304 or monel fasteners with 304 fittings. There is also increased potential for corrosion due to the wake.

 

[prv]

Two common grades are A2 (aka 304), which is the grade you'll find at your builders merchants etc and is unsuitable for marine use esp below the water

OK. So it looks like some idiot has used the wrong bolts (one hopes not the original builders!) at some point. I'd assumed they were original and simply decayed through age.

Even 316 stainless is susceptible to crevice corrosion so if you decide to use stainless again not only use 316 (A4) but seal the bolts into the holes to prevent water getting at the shanks of them.

If I can I'd rather use bronze, but it looks like that's not appropriate if the straps turn out to be stainless.

Would Sika 291 be an appropriate sealant? (still learning about boaty materials as opposed to building ones )

Dan is right about some woods (although oak is the only one I know about) containing tannin which attacks ferrous metals.

I don't know what wood the rudder is made of. It's not encapsulated in any way, just painted, but is still sound, so I assume not a softwood?

Fastening for 316 stainless steel: 316 but sealed as above. Otherwise Monel but not silicon bronze
Fastening for bronze or gunmetal: 316 sealed as above, Monel or silicon bronze.

Brilliant. Next time I'm at the boat I'll have a closer look at the rudder fittings and see what they are. Any surefire way to tell? I guess just scrape the paint away a bit and look for any hint of yellow for bronze? Here's a picture of the bottom shoe in situ; I don't have one since dismantling it.

Pete

 

[TradewindSailor]

Take a strong magnet. 304 tends to be a little magnetic, particularly where it's been cold worked ..... but it's not definitive though. 316 is not magnetic.

 

[VicS]

I'll have a closer look at the rudder fittings and see what they are. Any surefire way to tell? I guess just scrape the paint away a bit and look for any hint of yellow for bronze?

Surefire way to tell ... send samples to an analytical chemistry laboratory ... but I am sure you will at least recognise stainless steel if that what they are.

Would Sika 291 be an appropriate sealant?

Yes but it'll make them very difficult to remove again. Maybe someone will recommend a suitable non setting sealant.

 

[xtiffer]

Surefire way to tell ... send samples to an analytical chemistry laboratory ... but I am sure you will at least recognise stainless steel if that what they are.

Yes but it'll make them very difficult to remove again. Maybe someone will recommend a suitable non setting sealant.

Red lead putty.
Cheers,
Chris

 

[boatmike]

Have just cought up with this post and I am glad I read all the way through before posting. Without a doubt 316 underwater but recognise that crevice corrosion will still happen. If it's a wooden boat it will happen even faster. What you need to do is make sure the hole is a good clearance on the bolt and ensure that you get a good coating of Sikaflex all over the bolt shank not just the head. insert the bolt and rotate to make sure there is no gap between the bolt and hole. I usually use an undersize bolt or dummy to ensure the inner surface of the hole is fully coated before covering the bolt with a liberal smear of the stuff and inserting that. If you have a hexagon head on the bolts (which it looks as if you do) then Sikaflex will break with a spanner turn of the head when set so no worries. Sikaflex is the most waterproof sealant I know and it does not degrade underwater. If it is a flush rivet or countersunk bolt probably sikaflex would make it difficult to ever remove so some other sealant may be a good idea but in your case I would use sika.

 

[Bosun Higgs]

Wouldnt consider stainless steel for bolts through a wooden rudder - there's too much moisture in the timber / too little oxygen and you will get the sort of corrosion you describe whether you use 304 ( A2) or 316(A4) types. Would suggest you use silicon bronze.

You might get away with 316 if the rudder was grp and you used a sealant all round the bolt since then they would be protected against moisture, but even then I found exactly the same problem with the grp rudders on my prout.

Stainless is fine under water provided the water is flowing / there's lots of dissolved oxygen.

 

[vyv_cox]

Stainless is fine under water provided the water is flowing / there's lots of dissolved oxygen.

But in a bolt, by definition, it is impossible to supply flowing water to all parts. So crevice corrosion, which is the type we are talking about, cannot be eliminated. It often occurs in cut threads anyway, where the crevice at the thread root is enough to reduce local oxygenation. So stainless steel propshafts rarely have crevice problems, except beneath cutless bearings on occasions, but many other fittings do.

As boatmike says, eliminate the water by using a good sealant and the problem will not occur. However, if the rudder straps are not stainless steel I would use bronze bolting, keep an eye on things and maybe fit an anode if it all goes wrong. But note that an anode will not prevent crevice corrosion.

 

[boatmike]

But in a bolt, by definition, it is impossible to supply flowing water to all parts. So crevice corrosion, which is the type we are talking about, cannot be eliminated. It often occurs in cut threads anyway, where the crevice at the thread root is enough to reduce local oxygenation. So stainless steel propshafts rarely have crevice problems, except beneath cutless bearings on occasions, but many other fittings do.

As boatmike says, eliminate the water by using a good sealant and the problem will not occur. However, if the rudder straps are not stainless steel I would use bronze bolting, keep an eye on things and maybe fit an anode if it all goes wrong. But note that an anode will not prevent crevice corrosion.

I agree on all counts. My boat is also a Prout which I built myself. The rudder lower pintle bracket is 316 and I mounted it in just the way I suggested. 7 years later I "parked" in the boatyard next to a guy with a similar aged Prout built by the factory. His lower bracket has almost fallen off due to the cross pin rivets (in this case) wasting away. His was a factory built boat and they were not properly sealed. It worried me enough that I extracted one of mine (with a valve compressor clamp) It came out reasonably easily and was like a new one. No corrosion at all. I replaced it in similar fashion. Who said "amateur" built boats were no good!

 

[Tranona]

Agree with using 316 if the fittings are also stainless. Would use polysulphide sealer rather than Sika as easier to get apart and just as sticky and waterproof. If the hole in the rudder is fully coated in sealer then there is little chance of water getting in and any crevice corrosion is likely to be on the thread - again if well sealed that should not be a problem. The shoe on the keel may well be a bronze casting (if you are lucky!) in which case bronze bolts. I have a similar bronze heel fitting which I designed and had made nearly 20 years ago. It is fastened with 5 bronze bolts through an iroko keel and set in Boatlife Caulk. I have removed and re-fitted 4 times since without any sign of water getting into either the recess or the bolt holes.

 

[Bosun Higgs]

As boatmike says, eliminate the water by using a good sealant and the problem will not occur. However, if the rudder straps are not stainless steel I would use bronze bolting, keep an eye on things and maybe fit an anode if it all goes wrong. But note that an anode will not prevent crevice corrosion.

Thats fine in theory Vyv and its what I did to the bolts on the GRP rudders of the Prout. In that case you knew that if you got a good seal under the head and another under the nut / washer that no moisture was going to get at the metal in the middle of the rudder.

That doesnt apply with wood because the wood itself will be wet and you cannot be sure that a bolt liberally smeared with goo will take that goo right along to the point where the threads re-appear and you wind on the nut. Having seen what moisture diod to the bolts I took out of the Prout ( and after 30 odd years in the special steels industry) I wouldnt take the risk of using 316 through a rudder under water.

 

[prv]

Would use polysulphide sealer rather than Sika as easier to get apart and just as sticky and waterproof.

Sounds good - but where do I find polysulphide? All the various sealers I've found for sale are very coy about what's in them, so I might know I want polysulphide or polyurethane or whatever, but I can't identify a tube of it.

I wouldnt take the risk of using 316 through a rudder under water.

That's all well and good, and I'm quite happy to buy bronze instead, but other posts suggest that if the straps are stainless then using bronze bolts will cause the straps to corrode instead. I'd rather the bolts went - at least I can replace them.

Pete

 

[Tranona]

The brand I use is BoatLife Lifecaulk. Comes in the usual small tubes, or more economically in larger tubes for use with a mastic gun. Any decent chandler will stock it.

Sounds like you are going to take the rudder off completely. If so, take it home, prop it up in a nice warm place until the holes in the rudder are completely dry. Then soak them in neat epoxy applied with a bottle brush until no more will soak in. When it is cured line the holes further with epoxy thickened with a filler - mix of silica and microballoons will do. When that lot is cured re-drill the holes to a clearance for the bolts. Your bolts will then be going through an epoxy tube and will get nowhere near the wood. As previously advised, bed your fitting in sealer, sealer on the unthreaded part of the bolt and under the head and nut. Tighten up almost there, leave to go off for a day or so then finally tighten and clear up the excess that has oozed out. Bit long winded but from my 30 years or so of using epoxy on a wooden boat, well worth the effort.

 

[prv]

The brand I use is BoatLife Lifecaulk. Comes in the usual small tubes, or more economically in larger tubes for use with a mastic gun. Any decent chandler will stock it.

OK, thanks, I'll keep an eye out. This is less adhesive than Sika 291, is it? I'd been kind of working on the principle of Sika 291 for sealing and 292 for bonding and nothing else - what further nuances would people add when choosing a gunk for each use?

Sounds like you are going to take the rudder off completely. If so, take it home, prop it up in a nice warm place until the holes in the rudder are completely dry.

It's already off and sat in my Dad's garage. I think it was all pretty dry when I took it off - the boat's been ashore for many months and the wood in the rudder is perfectly sound.

Then soak them in neat epoxy applied with a bottle brush until no more will soak in. When it is cured line the holes further with epoxy thickened with a filler - mix of silica and microballoons will do. When that lot is cured re-drill the holes to a clearance for the bolts.

What's the aim of these steps? It's not going to keep water away from the wood, as the rest of the rudder is not sealed or specially protected, just painted. And I don't think the bolts need to be protected from the wood - I'm satisfied that the corrosion is due to using the wrong stainless for the job, not tannin attack. In any case, the holes are pretty tight-fitting, so while I'd be able to soak a bit of plain epoxy into the edges of the wood, any thickened paste I applied would be removed again as soon as I drilled a hole big enough for the bolts.

Cheers,

Pete