Stainless steel nuts on Galvanised mild steel.

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BurnitBlue

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I am replacing the keel bolts on my Moody. I have 24mm galvanised studding but having dificulty buying mild steel nuts to fit. I can buy stainless steel nuts (and 24mm stainless studding) from Cleopatra chandlers. I need to get this job finished before I launch so, as a temporary measure, can I use stainless nuts until I can get some mild steel ones.

Just to make sure that there is no short term damage does anyone know if there will be any damage to either the stainless nuts or the Galvanised studs (bought in Greece so maybe Chinese manufacture) . Would it be better to go with all stainless studs and nuts? How long would a mixture of stainless nuts and galvanised studding last?

I am still trying to source mild steel nuts but no luck so far.

Thanks
 
Assuming the nuts are inside the boat above the keel they should be fine. Coat them and the threads thoroughly in waterproof grease. Using stainless for keel bolts may be less wise due to crevice corrosion.

Corrosion rate to the galvanised stud will depend on how wet the bilge is kept and how good a coating of grease you apply. I would check one after say 3 months and if no noticeable deterioration should be ok to continue.

However, Zinc plated nuts on eBay here. 300730551959
 
As a temporary measure it should be fine. Just don't put off getting the right nuts.

Personally, I would be reluctant to change the studs to stainless but others may have different views.

Do you want mild steel nuts/studding? Are you sure that a higher spec material isn't needed? Just asking, I have no idea what is needed for a Moody.
 
Moody yachts are somewhat unusual in using carbon steel bolts, but all of them do. The majority of others use stainless steel. Putting stainless steel nuts and washers on steel bolts is perfectly acceptable provided they are kept dry. Even if wet it would take a very long time for significant galvanic corrosion to take place. Use plenty of sealant down the bolt holes and if you see rusty water in the bilge after sailing you may need to consider rebedding the keel.
 
I'm going to buck the trend and disagree here! I think it's unrealistic to expect to be able to keep keel bolts dry. At best, I think maybe "damp"? Although I'd not worry as a temporary measure, I'd have thought the zinc on the studding would sacrificially donate it's electrons to the stainless? I agree that there needs to be an electrolyte present, but I'd just be a bit worried that this might be the case on occasions. Once it's done that, your studding will loose it's protection in that area. Also, (and I'm not at all sure here!) wouldn't covering it with grease to exclude the oxygen lead to crevice corrosion in the stainless? I thought it needed to be exposed to oxygen?
 
I tend to agree that hoping to keep the keel bolts dry is no way to prevent problems. There are so many variables in materials and geometry that for each installation you need to weigh the pros and cons of each option. Although S/S is more prone to crevice corrosion (which is a specific example of differential oxygenation, I believe) it can be offset by keeping ALL the componnets in the assembly waterproof by greasing, i.e. exclude the electrolyte. But unless your better half does the maintenance as some do the hoovering (daily, unless it gets dirty), you can't guarantee that the bolt assembly will remain properly greased... I wonder whether Stockholm Tar would do the job? Best simply to get the recommended galvanized nuts and then do all you can to keep them dry/waterproof.

Rob.
 
I would be very wary of using galvanised anything. If the galvanising should break down for any reason you will be left with 'sloppy' threads. Were the original studs galvanised? I would suspect not, probably just plain steel.
 
Lots of 'pseudo-metallurgy in this thread!

Keel bolts may well get damp, but their material is the same as was originally specified. The joint between the stainless and the galvanised steel should remain pretty well dry and even if it did get wet the corrosion rate would be low. The bolts and keel are the anode but they are vastly bigger than the nuts.

Once again, crevice corrosion is a form of corrosion and has nothing to do with oxygen being excluded from the environment. The crevice creates the lower oxygen content. See my website under 'metallurgy'.

Grease? There are specialist bedding sealants for keels, although Sikaflex 291 seems just as good. Nuts and washers are normally flowcoated in the bilge.

Galvanised bolts are widely used in a variety of wet conditions. Why should the zinc 'break down' in the threads? It's only a few microns thick anyway. I saw a programme about repairing old piers recently. All the bolting was galvanised.

What is the mechanism for zinc 'donating its electrons' to the stainless steel? A diffusion mechanism is known, not at all the same thing, but that is a high temperature mechanism.

Galling occurs between stainless nuts on stainless bolts. One way to avoid it would be to have one component in carbon steel, the other in stainless.

A method that used to be used on tankers to prevent nuts from corroding onto bolts was to put brass nuts on carbon steel bolts. Effective and far cheaper.
 
Thanks for all the replies. It would appear that for the summer season I will be OK with Stainless steel nuts on galvanised mild steel studding.

The reason I am changing them is because they were replaced a few years ago by a marine engineer organised by the previous owner after an insurance survey declared them with rusty nuts. Unfortunitely the job was interrupted when Ionion Marine boatyard demanded a consideration of some percentage which the engineer refused to pay. The boatyard then made him persona-non-grata. The boat had to be shipped out of the boatyard (I don't know how) across the channel to Preveza, probably Megollis yard where the job could be completed. I learned about this only late last year so I became suspicious about the bolt integrity.

A building surveyer from a friendly boat examined the visable part in the bilge and told me the job was OK but unconventional. The bolts have been locally manufactured from studding with the nuts welded to the cut-down studding. During the winter lay-up I removed one bolt and was surprised that the depth of penetration of the manufactured bolt into the keel was only 28 mm measured with a vernier. Back to the internet where it was stated that a stud/bolt should penetrate at least 1.5 times the diameter of the bolt. That would be 36 mm. The hull/keel interface was 30 mm of solid GRP making a bolt 58mm long (2.5 inches total under the welded nut). I therefore pulled another bolt (the forward most one) and that was much worse with a penetration of only 20 mm not even equal to the bolt diameter.

I reckon that the engineer cut down the original bolts from the rusted bolts or attempted to manufacture 11 bolts from a single one meter length of studding.

In my opinion this low penetration may be OK in benign conditions but as they are obviously out of spec I must replace them with the conventional system of a nut plus locknut on studding penetrating to at least 1.5 times the bolt diameter. If only for peace of mind. The thought of pulling the thread out of a cast iron keel in bouncy conditions could be expensive.

I welcome your thoughts (those that have the patience to read this explanation).
 
I think you are wise to follow the route proposed. The studs should be tightened down to bottom in the tapped holes in the keel. Two nuts per stud would be perfect but on my boat, as built, there is only one per stud.
 
346s don't use stainless. Only problem I see with stainless nuts on galv steel studs, long term, is that they won't stay dry as the chain locker drains into the bilge. Our 346, like the 376, always had some salt water in the bilges as we often live on anchor.
 
BlueTwo,

Do think seriously about joining the Moody Owners Association. Membership is only £20 and the website has much valuable information and the many members are very willing to help with first hand advise.
 
BlueTwo said:
I am replacing the keel bolts on my Moody. I have 24mm galvanised studding but having dificulty buying mild steel nuts to fit. I can buy stainless steel nuts (and 24mm stainless studding) from Cleopatra chandlers. I need to get this job finished before I launch so, as a temporary measure, can I use stainless nuts until I can get some mild steel ones.

Just to make sure that there is no short term damage does anyone know if there will be any damage to either the stainless nuts or the Galvanised studs (bought in Greece so maybe Chinese manufacture) . Would it be better to go with all stainless studs and nuts? How long would a mixture of stainless nuts and galvanised studding last?

I am still trying to source mild steel nuts but no luck so far.

Thanks
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You will be perfectly OK using stainless steel as long as you make sure that the keel / hull joint is well gunged with sealant so that the stainless is not sat in stagnant water. A bit of water in the bilge itself ned not be a worry. My Bowman built boat ( and Bowman werent a low grade builder) has stainless studs and nuts as standard. The Moody only uses galvanised because its cheaper ( stand by for incoming but its likely true - my Moody 336 had softwood battens behind the headlining and plenty of other evidence of cheapness elsewhere - like the mild steel fuel tank).
 
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theoldsalt said:
BlueTwo,

Do think seriously about joining the Moody Owners Association. Membership is only £20 and the website has much valuable information and the many members are very willing to help with first hand advise.
Click to expand...

I get asked this a lot. Marieholm, Westerly, Nicholson, and many many others do not charge for access to their web pages. I have gained quite a lot of information about Moody yachts due to my experience commissioning the one I presently have. With other makes of yacht I have contributed tips and hints and cruising experience. With Moody I am not allowed to contribute information unless I pay them £25.

Quite frankly they can get stuffed.
 
I read recently not to over tighten galvanised shackes as this can damage the thin layer of zinc leading to premature corrosion. Surely stainless nuts are much harder than the zinc galvanising and very likely to damage this thin protective coating when pinched up hard.
 
winsbury said:
I read recently not to over tighten galvanised shackes as this can damage the thin layer of zinc leading to premature corrosion. Surely stainless nuts are much harder than the zinc galvanising and very likely to damage this thin protective coating when pinched up hard.
Click to expand...

The fix is only temporary. Moody do not use galvanised bolts or nuts anyway. Studding is cheap. A meter length of 24 mm studding from a Preveza tool shop cost me 7.5 euros. Unfortunitely they didnt have any nuts. The most expensive item was a 36 mm socket with a 3/4 inch to 1/2 inch socket adaptor that cost me 40 euro.
 
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