Replacing keelbolts on 32 ft boat - how big a job is it?

[Sandmartin]

Did this on a boat I bought 20 years ago. Cut the internal GRP cover away to reveal bolt head, then, using a basic welding kit, spot welded a bar to the bolt head.

 

[vyv_cox]

So, how does the bolt get corroded when the water that is corroding the heads is in the bilge and the bolts are encased in the GRP and then the keel?

If you have ever seen corroded keel bolts you will know that the corrosion occurs where the bolt enters the keel where water has come from a failed joint, not from under the head downwards. Corroded heads are common in boats with mild steel bolts or nuts and leaky so water rests in the bilge. It is not a good indicator of bolt failure.

Lots of boats change heads with rusty keel bolt heads - goes with the territory on boats of this type. Clean it up, rust prevention and paint and try to keep dry so it does not get any worse.

When we bought our boat the two most forward studs and nuts were quite rusty, all the others still covered with flowcoat. Some investigation showed that it was the carbon steel washers that had rusted, the stainless studs and nuts being perfectly OK. A long time after that we found the cause - any leakage from the heads compartment ran along its floor and straight into the forward end of the saloon, under the sole board and into the bilge compartment where the corroded washers are. I made a small threshold modification that has now prevented this from happening. No more rust.

 

[LadyInBed]

Would you grease the replacment bolts before insertion or what?
(Mine has encapsulated keels, so this is just out of interest)

 

[lw395]

Lots Of "possiblies" there - hence the earlier advice to ask the surveyor why he thinks they should be withdrawn when an earlier sample proved OK. What has changed in 4 years? There won't be any crevice corrosion as they are mild steel.

They will probably be M20 bolts - takes an awful lot of corrosion to have any impact on strength of such a bolt - even if the head is corroded. The washers (or more likely plates) will be substantial and bet that even if the exposed parts are corroded, the bit under the bolt head will be fine.

In four years, they'll have rusted some more I should think...

And crevice corrosion is not unique to stainless steel.
I'm not a gambling man, I don't take bets on critical parts.
They're M20 bolts or whatever because M16 wouldn't be strong enough, not for the weight.

 

[lw395]

If some of the heads are corroded to the point where they are difficult to remove, I would replace them before they are impossible.

Exactly.

 

[doug748]

Exactly.

Quite so.

Lets face it the bolts are not going to fail in a sound joint. If it was an aircraft it would be held on by two of them. I suspect the failure mode is a bit of movement in one followed by it's neighbour and so on, leading to a failure of the hull/keel joint seal and then flexing etc.

In this respect, rusty washers may be of equal concern to the bolt head. Taking them out in pairs, fitting stainless washers and replacing with new as necessary may well be doable without disturbing the seal.

 

[Modulation]

Many thanks for these replies- very thought-provoking!
Before I take any action I'm going back to the surveyor to establish his reasoning for the recommendation. I think he'll say it's to do with age of the boat, the state of the bolt heads etc. And then I'm going to concentrate on the 2 corroded heads, which happily are above the same bearing plate. If I can get the 2 out I'm going to replace that plate and the bolts - even if it shows no corrosion along its length i.e. no evidence of water penetration at the keel/hull joint. I plan to do this without lifting out as there are - from memory -10 or 12 bolts in total.
Does that sound a reasonable way forward?

 

[bernie_ni]

Would you grease the replacment bolts before insertion or what?
(Mine has encapsulated keels, so this is just out of interest)

I use a bit of copper anti-seize on the threads, then a 'lot' of sealant underneath the head to prevent future water ingress. Too much grease would affect the torque figure

 

[Daydream believer]

Would you grease the replacment bolts before insertion or what?
(Mine has encapsulated keels, so this is just out of interest)

Stockholm tar - obtainable from tack shops

 

[pmagowan]

I removed a couple of my old bolts which had rusty heads. Absolutely perfect everywhere else, you couldn't know they were half a century old. I needed to use a cold chisel to get the nut off but it didn't take long. I replaced like for like and the new nut is already a bit rusty like the old one. I think a bit of rust protects it! My bilges are a bit wet, as in always got some sea water in them. (Wooden boats, love em!)

 

[PaulRainbow]

I removed a couple of my old bolts which had rusty heads. Absolutely perfect everywhere else, you couldn't know they were half a century old. I needed to use a cold chisel to get the nut off but it didn't take long. I replaced like for like and the new nut is already a bit rusty like the old one. I think a bit of rust protects it! My bilges are a bit wet, as in always got some sea water in them. (Wooden boats, love em!)

Mine are studs and nuts. The nuts are encapsulated in resin and look like new.

 

[Tammany]

Don't know what size bolts you have but this set removed a sump bolt for me that was completely mauled.

http://www.screwfix.com/p/erbauer-bolt-nut-remover-set-6-pieces/6083d

 

[lw395]

Many thanks for these replies- very thought-provoking!
Before I take any action I'm going back to the surveyor to establish his reasoning for the recommendation. I think he'll say it's to do with age of the boat, the state of the bolt heads etc. And then I'm going to concentrate on the 2 corroded heads, which happily are above the same bearing plate. If I can get the 2 out I'm going to replace that plate and the bolts - even if it shows no corrosion along its length i.e. no evidence of water penetration at the keel/hull joint. I plan to do this without lifting out as there are - from memory -10 or 12 bolts in total.
Does that sound a reasonable way forward?

On the one hand, I'd rather have the hull-keel join in compression to do the job.
OTOH, the tension on the bolts is way higher when you are heeled over.

The worst that can happen would be finding one bolt is so dodgy it snaps. Very unlikely, but you'd then need to lift.
Second worst is somehow it seeps and you have to re-do the job over the winter.
2 bolts out of 10 does not seem unreasonable to remove.

I'm assuming the holes are normal blind holes in the keel.
There are boats out there with tapped through holes in a flange around the top of the keel. Undoing one of those afloat would not be advisable!