Birdseye
Well-known member
smallish boat so the keel bolts are about 19mm stainless. Hull is solid GRP so what would be the torque wrench settings?
torque settings for keel bolts?
- Thread starter Birdseye
- Start date
Daydream believer
Well-known member
You would not go far wrong with this guide
Bolt torque
Bolt torque
Tranona
Well-known member
Why do you think they need checking? If they were correctly torqued when new and the keel has not been removed there is no reason why they should have changed.
vyv_cox
Well-known member
Some thoughts and data here Keel bolt tightening torques
William_H
Well-known member
Bolts are torqued to stretch the bolt to make it more resistant to fatigue failure with cycling loads. Very necessary on a cylinder head bolt. As with the cylinder head bolt the bolt is stretched against the solid cylinder head.
I don't think your GRP hull would be capable of withstanding the crushing force needed to stretch the very large bolts. That means that you simply tighten the bolts to a torque that will stress the threads and hold on over a long period. The bolts themselves will be over sized to give a long life despite any corrosion. So IMHO opinion and I hope not contrary to Viv Cox I would do them up tight to a torque that seems appropriate with a suitable sized spanner. Certainly not too tight. ol'will
I don't think your GRP hull would be capable of withstanding the crushing force needed to stretch the very large bolts. That means that you simply tighten the bolts to a torque that will stress the threads and hold on over a long period. The bolts themselves will be over sized to give a long life despite any corrosion. So IMHO opinion and I hope not contrary to Viv Cox I would do them up tight to a torque that seems appropriate with a suitable sized spanner. Certainly not too tight. ol'will
ducked
Active member
Bolts are torqued because its impossible to put a nut on them without applytng a torque, surely?
You seem to be conflating the term with the use of torque-to-yield "stretch" bolts, a special case, and one that I'd guess doesnt apply to most bolts used on most boats.
That aside, you seem to be saying bolts should be tight enough, but not too tight. Cant argue with that, and such advice would shorten most auto manuals quite a lot.
I'd agree that torque is over-specified (as in specified when it doesn;t really need to be) and using the specified torque is of exaggerated importance. Snugging up bolts by feel is just fine for the majority of bolts on a car, but this is an unfashionable view. People like the reassurance, and they (especially Americans?) like using the shiny torque wrench they bought, and, generally this fetish does no harm, though there is the occaisional technically-assisted cockup, with a mis-read spec or a faulty wrench.
However, I do have a personal issue with the way torque is often specified assuming dry fastners. I REALLY dont like using dry fastners and wont generally do it, so I either have to apply a (rather uncertain since recommendations vary wildly) torque reduction correction factor for lubrication, or use a turn-of-the-nut procedure to "translate" a specified torque into an angle. I would probably apply keel bolts with polythene and/or grease or antiseize on the threads, so specified dry torque bets would be off.
Whether keel bolts are one of the critical exceptions (like, say, head bolts on an engine) which need to use a specified torque I'm not sure, not having much personal experience with them. They certainly seem important, but there also seems to be a fair bit of redundancy in most designs, many of which will have been commissioned long before torque wrenches were much of a boatyard thing.
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ducked
Active member
One of those tables has an accompanying note saying
"These values are for well-greased threads.", which is nice, and unusual. The others dont say, and they would probably and generally be assumed to be dry.
The same note also says "The user is cautioned to use good judgement in applying these values." It might be uncharitable to consider this a CYA cop-out, but together these two factors suggest these figures only provide rather general guidance, and that snugging up by feel might not be so very bad after all.
There's also a note from the page compiler, accompanying the first table, saying "In the event the owner was not present when the keel was re-bedded and the yard told him they just tightened them as hard as they could!", which, together with the recommendation not to over-tighten on softish glassfibre, suggests snugging up by feel yourself might be better than getting someone else to do it.
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Bobc
Well-known member
Always fills me with dread when people ask about keel bolt torquing.
penberth3
Well-known member
There's no conflation, all bolts stretch when the nut is tightened. "Stretch" bolts are a specific thing, and not relevant here.
ducked
Active member
Exactly what I said. Confusion with specific things that arent relevant is pretty much what "conflation" means.
Birdseye
Well-known member
Some of the hull bolts go through mild steel washers and the windows have at some point leaked. Rust! So the nuts need removing when the boats on the hard and the keels are on the ground. Replace washers and re-torque. Already done some without problem
Birdseye
Well-known member
What is the material being compressed to give these number?
Birdseye
Well-known member
There is such a thing as creep. I know this happens in steel but I havent a clue how grp respond to crushing loads over 20 years
Birdseye
Well-known member
You raise an interesting question - why do we torque fasteners. I guess there are several reasons: firstly putting the nut under load makes it less likely to self undo under, for example, vibration. Perhaps less relevant in the era of nyloc nuts but not totally irrelevant. Second reason I guess is to ensure a firm joint between the surfaces being bolted - tension in the keel bolts makes the keel less likely to move and counteracts the upward movement of the hull because of waves. Thirdly tension in the bolts helps ensure a good leak resistant joint. Fourth - with dissimilar materials, working to a set torque avoids crushing one component, for example alloy wheels. But above all, using a torque wrench to a set standard avoids necking the bolts - something which I have done several times.
No doubt in the old days fitters got used to tightening bolts without overdoing it. These days we dont need lots of experience to do that - we use a torque wrench and a correct torque figure
ducked
Active member
Well, if I had necked bolts not using a torque wrench, I would probably use one more often, but I havn't, and its probably too late to start now, unless I have a sudden run of bad luck.
OTOH I have, once or twice, when using a torque wrench in marginal probably-not-really-essential-IMO situations, become alarmed at the feel of the amount of torque I was applying, and stopped doing it. This MIGHT have been because the torque was specified for a dry thread, and, as I said above, I'm not generally willing to do that, and the correction can be a bit uncertain.
Its possible, even likely, that I have been systematically under torquing for years. I have no way of knowing, but things havn't been falling off.
I do use one for head bolts and If I was torquing a TTY head bolt I'd just have to force myself to stretch it, but I've managed to avoid them so far, and they are probably relatively rare on marine engines which tend, happily, to be a bit older tech than cars.
PS You conflate "(adequate) tightening" and "using a torque wrench" together above under the term "torque" (v). I'm just considering your second meaning here.
OTOH I have, once or twice, when using a torque wrench in marginal probably-not-really-essential-IMO situations, become alarmed at the feel of the amount of torque I was applying, and stopped doing it. This MIGHT have been because the torque was specified for a dry thread, and, as I said above, I'm not generally willing to do that, and the correction can be a bit uncertain.
Its possible, even likely, that I have been systematically under torquing for years. I have no way of knowing, but things havn't been falling off.
I do use one for head bolts and If I was torquing a TTY head bolt I'd just have to force myself to stretch it, but I've managed to avoid them so far, and they are probably relatively rare on marine engines which tend, happily, to be a bit older tech than cars.
PS You conflate "(adequate) tightening" and "using a torque wrench" together above under the term "torque" (v). I'm just considering your second meaning here.
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ducked
Active member
Oh goody. Another variable.
Should the torque spec (if there was one) be modified for use afloat?
Ummm...Probably not.
But I suspect there isn't really a torque spec. anyway.
Daydream believer
Well-known member
Any minute now I can see the forum torquing b..x
ducked
Active member
Well, you might, because those figures, from a bolt seller, only consider the torque those bolts (respectively a medium carbon and an alloy steel, neither are stainless) can safely take. They can take no account of the loads the tensions implied by those torques are applying to the substrate, because the substrate is unknown to the seller.
ducked
Active member
The OP says he's "already done some without problem"
That probably implies he's already done some without a torque spec without problem, which is the norm (though an over or under torqueing problem would have to be pretty severe to be seen immediately)
The torque might change because he says hes replacing rusted and probably originally galvanised mild steel washers with stainless steel ones, which are likely to have a lower surface friction even if he didnt lubricate them.
Surface friction on fastners contributes a high percentage of the applied torque, so, IF there was a torque spec, if it were not reduced, the clamping force would be increased after this procedure.
IF the replacement washers were the same thickness, tightening through the same angle (turn-of-the-nut-method) should give the same clamping force, but I'd guess they probably wont be the same thickness.
vyv_cox
Well-known member
Several of the tables on that page of my website are specifically for keel bolts. In general it seems that the figures are downgraded from typical values for steel applications but not by very much. GRP thickness at the keel is usually quite considerable, at least on older boats, and with loads spread by the conventional large washers I doubt that the risk of crushing it is high.
Incidentally, whatever may be assumed from the tables, I have yet to see a boatyard torque keel bolts/nuts without greasing them first
