What grade st/st below waterline?

Porthandbuoy

Porthandbuoy

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The title says it all really. What grade of stainless should I use to fasten the trim-tab gudgeon and pintle straps?
I used 316 last time and they corroded to nothing in < 2years. The old trim-tab was oak.
The new trim-tab is iroko and the gudgeon & pintle straps are 316 st/st, the fastenings are 8mm.
Or can I use bronze with st/st?
 
Porthandbuoy said:
The title says it all really. What grade of stainless should I use to fasten the trim-tab gudgeon and pintle straps?
I used 316 last time and they corroded to nothing in < 2years. The old trim-tab was oak.
The new trim-tab is iroko and the gudgeon & pintle straps are 316 st/st, the fastenings are 8mm.
Or can I use bronze with st/st?
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Your problem may have been due to the oak...... Ordinary steel is not used in oak but the suitability of stainless has been discussed before with no firm advice emmerging. http://www.ybw.com/forums/showthread.php?102528-Stainless-Steel-and-Oak.... Or it may simply have been due to crevice corrosion of the stainless steel

You need advice from a wooden boat expert .... where is Sea Nick when you need him ?

Monel is said to be suitable as a fastener for stainless steel underwater but I dont know where you will get the Monel fastenings.
 
Inconel and Hastelloy are higher grade stainless steels that have improved corrosion resistance but this is very much dependent on the conditions. Perhaps need to consult a metallurgy guide on the internet.
 
I did. None of them really catered for a fastening tightly embedded in salt-water saturated oak. That's probably an acidic/saline/anaerobic environment. I don't know whether I need an electrical engineer or a chemist. :confused:

Jaguar 25 said:
Inconel and Hastelloy are higher grade stainless steels that have improved corrosion resistance but this is very much dependent on the conditions. Perhaps need to consult a metallurgy guide on the internet.
Click to expand...

Found a link a link to a thread in the Woodenboat forum. Opinions vary as to suitability of 316 st/st. http://forum.woodenboat.com/showthread.php?106878-Stainless-Fastenings-Below-Waterline
 
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With the salt water present in the wood, I would bed bolts in epoxy to remove the possibility of stagnant/trapped sea water causing corrosion from de oxygenated water. We use this for 316 A4 bolts in Aluminium, bronze and stainless.
 
You might find this link useful http://www.classicmarine.co.uk/Articles/metals.htm I believe that silicon bronze fittings are widely used in wooden craft nowadays. Bronze and stainless steel are not too far apart in the galvanic table and if you are only getting two years from your stainless ones I would risk it with a bronze. Bedding in plenty of sealant will always help, regardless of the materials.
 
In my experience, any grade of stainless will dissolve in water-saturated oak under the waterline. Stainless simply cannot cope with the acidic, anaerobic conditions. The best material would be monel, if you can get bolts or threaded bar and nuts. If not, you could use silicon or aluminium bronze with electrical isolation from the stainless. You over-bore the stainless and either fit a plastic/nylon sleeve or fill with epoxy and then drill to the shaft diameter, in combination with nylon washers under the bolt heads and nuts and some mastic. This effectively forms a non-conductive mushroom washer to prevent the two metals forming a galvanic couple. The bronze is resistant to the acids in wet oak and should last for years.
 
Found that a short bit of heat shrink is very effective as well. some times the clearence hole was slightly on the larger side so fitted without any further fettling.
 
Thanks to all for the tips and advice. I'm going to try and source some monel machine screws. If that fails I'll try 316 again in bored out epoxy.

I'll report back in two years and let you know how it worked. :)
 
Monel in oak ? I thought the navy used iron bolts on oak for many years. Victory is still afloat 200 years on . Is iron a suitable low tech material for your application - ??
 
The sea discharge pipe from Sellafield is made from mild steel as it is less susceptible to corrosion in sea water than stainless. Not suggesting that you use mild steel though!
 
Jaguar 25 said:
The sea discharge pipe from Sellafield is made from mild steel as it is less susceptible to corrosion in sea water than stainless. Not suggesting that you use mild steel though!
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Most pipelines are made from carbon steel, but with a corrosion allowance of at least 3 mm, maybe more. When the Fulmar gas line was being planned, 24 inch and 100 plus miles long, it was suggested that maybe it could be made from 18/8 stainless steel. Someone calculated that do so so would consume the whole of the world's known resources of chromium.
 
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