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Old 09-05-2011
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Keel Bolt Torque

I have a question for the members - does anyone know any engineering rules that would allow calculating the torque required on a given keel bolt or bolts?

The reason I ask is that the general attitude seems to be "more is better", "crank on them until they squeal", or "put a pipe on a breaker bar and pull until the suckers scream for mercy". I've never seen anything much more technical or sophisticated than that.

I've always understood that a specific torque is required in order to stretch the bolts properly to achieve proper clamping force - this varies with the size of bolt, material it's made of etc.

Also, high torque is required to compensate for powerful, high frequency repetitive or rotational forces - engine cylinder heads, road wheels etc. not static loads where the bolts are only "locating" the parts in question.

To my eye, a keel falls somewhere in between these extremes - repetitive flexing forces from the boats motion but not really high frequency. Considering that such highly loaded fasteners like cylinder head bolts, connecting rod bolts, main bearing caps etc. are only torqued to values like 80 Ft/Lbs, I don't understand the need for keel bolts to be run up to 150 - 300 Ft/Lbs, especially considering they are generally stainless steel which I understand compares to, at best, a grade 3 fastener. It seems to me that these extreme torques are only going to crush the hull material, overstretch the bolt and/or risk galling the fasteners.

Anybody??
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Last edited by SloopJonB; 05-04-2014 at 07:39 PM.
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Old 09-05-2011
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Size matters.

Steel Bolt Torque Specifications Table - Engineer's Handbook
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Old 09-05-2011
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Quote:
Originally Posted by donradclife View Post
Thanks for the table. It certainly does but those tables don't specify the usage - it isn't the same for all situations. Also, I noted the S/S torque specs for 3/4" bolts are spec'd in INCH pounds. That would put my keel bolts at about 120 Ft/Lbs.
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Last edited by SloopJonB; 09-05-2011 at 01:55 PM.
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Old 09-05-2011
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I think you also need some info as to what the hull/keel sump structure can take in terms of compression as you torque the bolts - so taking the data from the tables for the thread/size alone may not be the whole picture..
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Old 09-05-2011
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Most every boat i have seen is SOLID glass in the keel bolt area except some older Catalinas that never got the upgrade

BUT i think most boats have a user group or info available someplace on the original torque level
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Old 09-05-2011
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Yes, size DOES matter! And material type, and thread type (fine or coarse) - AND is it dry torque or wet torque? Dry torque is what it sounds like; wet torque refers to anti-sieze or oil applied to the threads AND under the head of the bolt or nut. Due to the reduced friction, a bolt torqued wet is under much more strain than the same bolt torqued dry.

If you can, I'd contact the manufacturer of your boat and get their recommendations.
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Old 09-05-2011
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It is possible to crush solid glass by over tightening keel bolts.

The link posted above is a good layman's guide
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Old 09-05-2011
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Quote:
Originally Posted by BELLATRIX1965 View Post
If you can, I'd contact the manufacturer of your boat and get their recommendations.
I would but they went out of business 30 years ago.

Actually, I used some specs that match up pretty well to the posted chart. I posed the question out of mere curiosity because I read & hear so many things like the "techniques" in my OP rather than anything technical or scientific - so unlike my other passion - cars. There everything is spec'd to the Ft/Lb.
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Old 09-05-2011
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We did 300 Ft Lbs on 1 1/4" SS with anti seize on the keel we did. The bolts are rated to 500. This was after talking to the designer about it.
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Old 09-06-2011
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I replaced the keelbolts on my Pearson 26 this past spring. I torqued the bolts to something near 80 ft-lbf. Dan Pfeiffer's P26 web page has all the details on the bolt size (3/8" - I think??) and torque for a P26.

Overtorqueing (is that a verb??) can damage the bits between the nut and bolt (i.e. fiberglass and any filler material), and serves no purpose other than to reduce the "additional" load capacity (i.e. load beyond the load created by torquing the bolt - like holding the keel in place).
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