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Author Topic: Tightening keel bolts  (Read 4973 times)

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cmainprize

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Tightening keel bolts
« on: May 30, 2015, 05:45:10 AM »

I just got my new socket to fit my keel nuts.  I have read several posts saying keel bolt tourqe should be checked while the boat is in the cradle.  I don't understand this concept.  It seems to me that unless you are able to balance the boat perfectly on the keel checking the tourqe in the cradle may not be accurate.   We have a fin keeled boat and in my experience we are not able to balance boat in cradle.  The angle of the fin keel puts more weight on the bow because the boat wants to tip forward.  My understanding is this would put unequal load on the keel boats.  The front of the keel is under some compressions and the rear is under some tension. 

It seems to me that checking/adjusting the tourqe in the water is the most accurate way to ensure the tension is equal and accurate.  With the keel in its natural position the torque would be equal because the load is equal.

Just my thoughts.  Can someone offer an explanation that concurs or dissents. 

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Cory Mainnprize
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Ken Juul

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Re: Tightening keel bolts
« Reply #1 on: May 30, 2015, 07:01:52 AM »

The torque is only accurate in a static condition.  IE minimal load on the keel bolts.  In the water, the weight of the keel is pulling down on the bolts.  The torque value for that has not been determined....probably much much higher than the static value. 
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Jim Hardesty

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Re: Tightening keel bolts
« Reply #2 on: May 30, 2015, 07:41:31 AM »

To add my 2 cents with Ken.  The factory torques them out of the water.  I did start in the center and worked out, fore and aft, just looked like the right way to me.
Jim
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Jim Hardesty
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KWKloeber

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Re: Tightening keel bolts
« Reply #3 on: May 30, 2015, 01:52:44 PM »

And what torque would you use with a hunk of lead and firebrick hanging from the nuts?

Properly blocked, torque it with as much weight off the bolts as you can.

Ken
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Ron Hill

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Re: Tightening keel bolts
« Reply #4 on: May 30, 2015, 04:58:16 PM »

cmain : You think you can lift 5500+ lbs with a torque wrench on one nut  !!!!! ?????

Give me a break!!
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cmainprize

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Re: Tightening keel bolts
« Reply #5 on: May 30, 2015, 06:22:40 PM »

Would I be lifting the keel up? Or pulling the boat down?  Maybe I don't understand something.  So worst case scenario if I tourqed the nuts to the reccomened torque with the boat in the water does that mean they would still be a little loose?  If that is the case I would do it now, and then check again when the boat is hauled in the fall.  It's not a big deal either way, my keel shows no sign of movment so I can wait.  My only concern is the balance of the boat so the load on the boats was equal and being in the water seemed like the best way to do it. 
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Cory Mainnprize
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KWKloeber

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Re: Tightening keel bolts
« Reply #6 on: May 30, 2015, 06:55:37 PM »

Maybe I don't understand something.

The specified torque is not for under load, it's for as neutral a condition as you can reasonable block the boat.
if you think that in the cradle. the aft bolts are under tension a little, what do you think you would have with the keel hanging???  Under tension A LOT.

SO, with the assembly already under tension, you would need to loosen the nuts in order to set the torque to 110 ft-lbs or whatever.  That's why I asked -- What torque setting would you use?

Torquing the nuts to the factory spec, while under tension (presuming that could even do it,) will do absolutely nothing toward getting them to the proper torque.  They would be way under torqued.

Ken K
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Noah

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Re: Tightening keel bolts
« Reply #7 on: May 31, 2015, 09:57:06 AM »

I would think the factory torques the bolts (out of the water) with the full weight of the boat resting on the keel blocked balanced by cradle/jack stands?? If you are concerned, a quick call to the Catalina factory should get you the answer.
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2ndwish

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Re: Tightening keel bolts
« Reply #8 on: May 31, 2015, 10:55:28 AM »

Those bolts are monsters. A simple mechanical advantage calculation will show that you are getting a ~700:1 advantage on the 100 ft-lb torque. Even with static friction that means that each bolt will be capable of lifting near 10,000 lb. There are 8 bolts so the load from the weight of the keel is even less on each (~5000 lb/8=~600lb), which is peanuts compared to the force on each bolt from the torque alone.  A single bolt should be able to lift the full weight of the keel (ever use a jack-screw on a car? No problem).  In short I don't think it matters much whether you're in the water or not from a load perspective. The only other issue is whether the distortions placed on the keel stub by the rigging are enough to open the fore and aft ends. BTW this issue was addressed in :
http://c34.org/bbs/index.php?PHPSESSID=7b031e73257c91afd4cf6989f9e800de&topic=8027.msg55816#msg55816

It was also alluded to in Indian Falls' awesome description of rebedding the keelhttp://c34.org/bbs/index.php?PHPSESSID=7b031e73257c91afd4cf6989f9e800de&topic=6842.msg46165#msg46165, where he described "cracking sounds" at 50 ft-lb (ie the force was no longer going into lifting the keel, but rather crushing the glass and bedding) .
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cmainprize

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Re: Tightening keel bolts
« Reply #9 on: May 31, 2015, 01:31:17 PM »

2nd wish, thanks for the reply.  Can you explain how you calculated the mechanical advantage (MA).   I used a simple MA formula of circumference /pitch to calculate MA.  If the keel bolts are are 1 inch their circumference is 3.14 inches.  The thread pitch is 8 per inch or .125.  3.14/.0125 = a MA of 25.  With a applied tourqe of 100ft/lbs my math says that would apply 2500 lbs of lifting force. 

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Cory Mainnprize
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cmainprize

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Re: Tightening keel bolts
« Reply #10 on: May 31, 2015, 01:45:56 PM »

I just read the post from Indian falls from a couple years ago when he lowered and then re bedded his keel.  Interestly enough he reports he lifted the keel using the bolts with no problem.  I guess mechanical advantage works!
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Cory Mainnprize
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KWKloeber

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Re: Tightening keel bolts
« Reply #11 on: May 31, 2015, 09:17:03 PM »

I just read the post from Indian falls from a couple years ago when he lowered and then re bedded his keel.  Interestly enough he reports he lifted the keel using the bolts with no problem.  I guess mechanical advantage works!

We're not saying that MA doesn't work.  What we're saying is that the setting of 110 ft-lbs of a hanging keel, won't have the same net effect as 110 ft-lbs of torque on a supported keel.   We'll call 'net effect' the psi compression of the keel joint.  What torque would you use to get the same net effect?  

kk
« Last Edit: May 31, 2015, 09:17:17 PM by KWKloeber »
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Twenty years from now you'll be more disappointed by the things you didn't do, than by the ones you did.
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cmainprize

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Re: Tightening keel bolts
« Reply #12 on: June 01, 2015, 04:42:09 AM »

KK, I agree the net effect will not be the same.  And I do not know how to compensate for the difference.  I am going to boat today to check bolts.  My plan is to start at say 50 lbs see what moves.  I will note the tourqe at which each nut turns and work my way up to 110.  I will post my results.  I will check again when the boat is blocked in the fall and note the outcome again.
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Cory Mainnprize
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2ndwish

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Re: Tightening keel bolts
« Reply #13 on: June 01, 2015, 08:28:53 AM »

Just got back to this- was out sailing yesterday. I like to do MA calculations the way I would for a block and tackle. Fin*Din=Fout*Dout. So for the bolt and conveniently say a 1 ft wrench on the bolt, one full turn with 100 lbs (so the torque is 100 ft-lb) would be 100*2*pi*1 ft=628. The bolt has a pitch of 8 per inch (I think it might actually be 9 but it doesn't matter), so in one full turn Dout=1/8 inch =0.125in=(.125in)/(12in/ft) =0.01 ft
so 628 ft-lb=Fout*.01, divide both sides by .01 and Fout=62800 lb.

A few interesting things here a) the bolt diameter never came in b) it assumes a frictionless twist. You could have put the equation entirely in terms of "bolt diameters" as units (of course the force at the bolt radius is greatly multiplied so 100 ft-lb at 0.44 inches is 2727 lb at the bolt radius). As to the friction, these bolts are dry, so I would assume a worst case scenario of 0.1 or 90% goes into friction and our answer would have been 6280 lb. Anyway you can find calculators on the web that work all this math (adding in pitch angles, tensile strength etc) and you end up with a simple relationship:

T=c*D*F where T is torque(in-lb), c=0.1-0.2 (friction), D is the bolt diameter (in), F is force (lb) [now the pitch is gone!-darn math]

This equation gives the answer 14000 lb.- only about a factor of 2 off of my simple minded approach (hmm maybe I've misplaced a factor of 2-sounds like a diameter/radius thing? might also be in the friction definition- close enough though)

Ok now let's turn to preloading... Preloading is black magic, but I would argue here that the bolt that CY used was not a complete POS- let's say grade 2. That bolt has a dry torque spec of 165 ft-lb with a net clamp force of 11400 lb [hmm somewhere in between the two] - this is at 75% of max, so 110 ft-lb has margin even on a grade 2 bolt (higher grades are much better). So the difference in preloading 600 lb of dead weight on each bolt can easily be compensated by a 10% increase in the torque applied . In reality, most torque wrenches are only good to 20% (or worse) and the friction term is uncertain at 50% level, so I doubt this 10% matters much.


All that said, I'd rather be on the hard when snapping a keel bolt than in the water!  

 
« Last Edit: June 01, 2015, 12:11:26 PM by 2ndwish »
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Bobg

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Re: Tightening keel bolts
« Reply #14 on: June 01, 2015, 09:00:40 AM »

wow! now I got all that to think about when torquing my keel boats to 105 ft lbs on the hard, my head is going to explode lol   
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Bob Gatz, 1988 catalina 34, Hull#818, "Ghostrider" sail lake superior Apostle Islands
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