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Old 18-01-2010, 15:05   #1
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Question Keelbolt Torque Discrepancies - Lubricated? Dry? More? Less?

Greetings,

I've just spent the past two days glued to my screen trying to google an answer to this, but cannot come up with the goods.. I would greatly appreciate it if the CF community can shed some light on this for me.

Quite simply, I have a bolt-on high aspect fin-keel, lead, with 3/4" stainless keelbolts. Boat is 40', I don't know how much the keel weighs, but it's certainly a LOT. Boat is originally a racing design (although I'm converting to a cruiser)

Anyway, I have 13 keelbolts. The story begins two years ago when I had the boat hauled out and had the local shipyard (River Quays, Sydney) replace the washers under the nuts with larger ones...

...several years later, I noticed that one which was in a wet place in the bilge had rusted solid... after some investigation... much to my amazement, these lovely people had replaced the old stainless washers with galvanized ones!

(No, going back is not an option, asking them for advice is not an option, getting a refund is not an option - this is Australia, all too hard... "We're licensed shipwrights and have been doing this for 20,000 years... you don't question us, mate.. who do you think you are anyway!?")

After some cursing, I went out to a metal fabricator (Edcon Steel, great bunch!) and had some very nice washers (plates actually...) fabricated.. IN STAINLESS.. in about 10 minutes (!)... ahem... so here I have a bunch of nice plates, 5mm thick, way better than what I had before for about $60 (which incidentally is 25% less than what those lovely shipyard people charged per hour over some 10 hours or so to stuff up my boat... ahem..)

OK, now comes the hard part - what torque to apply?!?!?

After googling at length and reading many many many posts on a variety of forums - these are the only concrete figures I could find:

Keel Bolt Torques

http://www.tartanownersweb.org/model.../keelbolts.txt

The short of it is that according to the above, 3/4" keelbolts should be torqued to 250 lbs/ft.

My boat is not a CNC or a Tartan, however these appear to be somewhat authoritative, and I figured the torque figures given above would be standard.. right? (I'm assuming that they use stainless bolts too..)

Anyway, since I don't have much swinging room inside the boat, I invested in a very serious torque multiplier and 250 lbs/ft is a piece of cake - if required, I can even do 900 lbs/ft.

BUT!!!! Just to make sure... I googled some engineering tables etc (as I read during my original googling that bolts should be torqued to 80% of their tensile strength...?) and I found this:

Bolt Torques - Stainless Steel, Brass Aluminum Bolts - Engineer's Handbook

In short - it states that a 3/4" 316 stainless bolt should be torqued to 1582 inch/lbs - which is about 131 lbs/ft - about HALF of what CNC and Tartan recommend.

As a cross-check, I looked up the tensile strength of 3/4" 316 stainless (75,000 lbs), then took 80% of that and threw it into this tool to work out torque:

http://www.engineersedge.com/calcula...orque_calc.htm

Which came up with 9000 in/lbs, which is 750 lbs/ft... which intuitively... seems WAY too high.

The other issue here is that reading a little bit further, it appears that if the bolts are lubricated, even less torque is required to achieve the same clamping force.

To add some more complexity just for fun... my bolts/nuts have some burrs etc on the threads, so I believe there's plenty of friction there such that not all of the torque applied is being converted to clamping force.... and since cleaning/rethreading is not possible, I was thinking of lubricating the bolts too...

At the moment, prior to reading the second set of links above, I had already torqued some of the bolts to 250lbs/ft and the bolts certainly didn't snap, but I'm still not at peace - as I'm still not sure if 250 is too high or too low...

So all in all:

a: Is 250 lbs/ft dry correct for my 3/4" bolts?

b: Should I lubricate and torque to 250lbs/ft to overcome thread and nut-to-washer interface friction?

c: Anybody in favour of 750 lbs/ft (dry/lubed?)

d: Why the huge discrepancy between the engineering data and cnc/tartan recommendations?

Thank you all copiously in advance and looking forward to your advice!
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Old 18-01-2010, 16:04   #2
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There's going to be a big difference betwen the torque of a lubed vs non-lubed bolt/nut. If not lubed the friction to turn the nut is going to be high (the running torque to just turn the nut). If the running torque for example is 100 and you want to torque something to 100 the value on the torque wrench needs to be 200. Forget about the tensile strength you looked up as you would need to figure out the area of the threads that you are torquing and all you're doing is confusing yourself ( the torque charts already have this figured in if you are using the correct material, bolt and thread size). Also torque in this case will be impacted by the lead and whether the stud in the keel is just threaded in or is a "J" type stud in the lead. I would worry about going too high if they are just threaded into the lead keel with pulling them out.

If it was me I would:
assuming the keel is supported by the other nuts/bolts etc; I would would lube the threads and bearing surface of the nuts and the new plates. Install the nuts and once they contact the plate would measure the torque to turn the nut and add that to 131 ft/lbs from the standard chart.

This isn't from any boat experience I may have. But does incl;ude my years in nuclear power mechanical systems including quality assurance where I used to have to approve torques for things we didn't have a manufacturer's spec for.
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Old 18-01-2010, 16:38   #3
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Make sure the plates or washers have their perimeters relieved and rounded off where they contact the glass surface. Resin, though hard to the touch, has a flow character when compressed, and the rounded edges help relieve stress.
There was a lengthly article on this in Professional Boatbuilder some years ago.
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Old 18-01-2010, 17:46   #4
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I would remove each nut individually, clean the threads carefully, lube them with an anti-seize lube to prevent galling, and tighten the nut to say 150 ft lbs. I would do that individually to each nut--not remove them all at once--and service the bolts in a cross pattern like tightening head bolts or auto wheel lug nuts. After all the nuts were redone with the stainless plates, I would cross tighten the nuts in the same pattern to around 250 ft pounds.

Good luck,
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Old 18-01-2010, 18:22   #5
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Quote:
Originally Posted by Don Lucas View Post
There's going to be a big difference betwen the torque of a lubed vs non-lubed bolt/nut. If not lubed the friction to turn the nut is going to be high (the running torque to just turn the nut). If the running torque for example is 100 and you want to torque something to 100 the value on the torque wrench needs to be 200. Forget about the tensile strength you looked up as you would need to figure out the area of the threads that you are torquing and all you're doing is confusing yourself ( the torque charts already have this figured in if you are using the correct material, bolt and thread size). Also torque in this case will be impacted by the lead and whether the stud in the keel is just threaded in or is a "J" type stud in the lead. I would worry about going too high if they are just threaded into the lead keel with pulling them out.

If it was me I would:
assuming the keel is supported by the other nuts/bolts etc; I would would lube the threads and bearing surface of the nuts and the new plates. Install the nuts and once they contact the plate would measure the torque to turn the nut and add that to 131 ft/lbs from the standard chart.

This isn't from any boat experience I may have. But does incl;ude my years in nuclear power mechanical systems including quality assurance where I used to have to approve torques for things we didn't have a manufacturer's spec for.
Interesting indeed - although I have a bedding compound underneath the plates - which will obviously compress - how would you suggest this is factored in?
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Old 18-01-2010, 18:27   #6
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Quote:
Originally Posted by Blue Stocking View Post
Make sure the plates or washers have their perimeters relieved and rounded off where they contact the glass surface. Resin, though hard to the touch, has a flow character when compressed, and the rounded edges help relieve stress.
There was a lengthly article on this in Professional Boatbuilder some years ago.
Unfortunately the edges aren't "round" as per se, but not 90degree-sharp either... I tried to round them myself with a dremel tool and grinding stones etc - however it was quite amusing to watch the stones disappear and the metal stay completely "as is"... I ruined 3 stones before I made the decision that on the rockwell hardness scale, the metal was way up there and the stones were probably somewhere on par with a sausage.

In the end, I took out a proper grinder and made an approximate tiny 45degree chamfer.. unfortunately a grinder is not exactly a delicate tool to work with, so it's not art, but better than "sharp".
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Old 18-01-2010, 18:29   #7
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Originally Posted by FloridaWriter View Post
I would remove each nut individually, clean the threads carefully, lube them with an anti-seize lube to prevent galling, and tighten the nut to say 150 ft lbs. I would do that individually to each nut--not remove them all at once--and service the bolts in a cross pattern like tightening head bolts or auto wheel lug nuts. After all the nuts were redone with the stainless plates, I would cross tighten the nuts in the same pattern to around 250 ft pounds.

Good luck,
Yep, I'm trying to keep doing "opposite" ones as much as is possible in a straight line..

On the lube front - can I just use normal grease or should I go and buy a proper nickel-based anti-seize compound?
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Old 18-01-2010, 19:19   #8
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Quote:
Originally Posted by akio.kanemoto View Post
Interesting indeed - although I have a bedding compound underneath the plates - which will obviously compress - how would you suggest this is factored in?
The bedding will just compress. Till it does you will not get any torque, you should be able to tell when you get to that point.
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Old 18-01-2010, 19:25   #9
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Originally Posted by akio.kanemoto View Post
Yep, I'm trying to keep doing "opposite" ones as much as is possible in a straight line..

On the lube front - can I just use normal grease or should I go and buy a proper nickel-based anti-seize compound?

On the chance that you might want turn the nuts again (tighter or looser), get the correct anti-seize!
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Old 18-01-2010, 21:10   #10
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I think the 80% of tensile strength is for bolts in shear loading, which would only happen when you go aground hard. There should be tables for compression/tension loading too. Also, I have tables somewhere that list the correction for lubricating.

There's one factor you missed: if you use nylock nuts, it's different again.

I would make 1/16" thick neoprene gaskets for between the fiberglass and the steel washers/plates. Search this forum to find my post on how to make those. They will compress to almost nothing (note the word "almost"... bedding compound will not work at all).

ciao!
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Old 18-01-2010, 21:13   #11
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I think the 80% of tensile strength is for bolts in shear loading, which would only happen when you go aground hard. There should be tables for compression/tension loading too. Also, I have tables somewhere that list the correction for lubricating.

There's one factor you missed: if you use nylock nuts, it's different again.

I would make 1/16" thick neoprene gaskets for between the fiberglass and the steel washers/plates. Search this forum to find my post on how to make those. They will compress to almost nothing (note the word "almost"... bedding compound will not work at all).

ciao!
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Howdy,

Why would bedding compound not work at all? Not compressible enough?

As for the nuts, nope, not nylock - just straight 316, no inserts etc.

Meanwhile, I found this tool:

FUTEK Bolt Torque Calculator | Bolt Torque Calculators

and putting in the numbers yields close to 250 lbs/ft, although it doesn't state whether it's based on dry or lubed....
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Old 18-01-2010, 22:23   #12
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A couple of points: Stainless, either common 18-8 or 316 is great for what it does which is the anti-corrosion properties. Stainless is not all that strong! It is brittle, hard to machine but not all that strong. All in all, an acceptable compromise. I would be far more concerned with over torque than under torque.

A couple of things could occur if you over torque the bolts: You could start to pull the bolt out of the keel and you ain’t ever getting it back in. If you fatigue the bolts they will break when stressed, such as a grounding. If you over torque you can stress fracture the plastic and it will fail over time.

Be gentle. If it doesn’t leak but snags down nicely, leave it at that. I like the idea of something with a little crush under the washer but if it compresses it will need periodic re-torqueing until it is stable.

Bottom line for a ¾ stainless bolt and nut with a load of say 700 lbs. (700 x 13 = 9000 LB keel) just a rough guess. Don’t torque more than 180 lb/ft. and don’t go that high all at once.

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Old 18-01-2010, 22:33   #13
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To add to my last post: Use lock washers over the flat washers.

If this is down in the bilge area, which most are, I would use waterproof grease on all. If you have any corrosion it will be where the stainless passes through the FRP and it is wet with an o2 deficit. Best to dry out all as much as possible and get grease down into the opening around the bolt, if there is any.

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Old 19-01-2010, 06:12   #14
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Lubricated nuts/bolts have a lower torque spec*, than do otherwise identical dry nuts/bolts. Baseline torque is usually (but not always) calculated/specified for a dry non-lubricated bolt.
As a rule of thumb, you can reduce dry torque specifications by about 15 - 30% when lubricating. (depending upon the lubricant’s coefficient of friction).

* It requires less torque to achieve the same face force.
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Old 19-01-2010, 08:27   #15
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I am with the previous post regarding not over torque is a bigger problem.
Let’s look at one reason torque is necessary in let say torqueing a head. You want a stud that is pre stretched greater than the force of the upward force of the compression stroke. You do not want a stud that is under torque which will end up work hardening every time the upward force stretches and relaxes the stud. Now in the case of a keel all you want is a keel that is held firmly in place with studs that will not stretch when subjected to heeling loads and I would suggest that this is much less than the pre stretch required on an engine head which is constantly resisting the compression and intake stroke
2 cents from a land based Millwright.
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