# Quick Keel Bolt Question

Discussion in 'Sailboats' started by LTDboatdesign, Jun 24, 2013.

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### LTDboatdesignJunior Member

I am calculating the number and size of keel bolts I will need for a boat i'm designing. What i'm wondering is if I use say 3/4in bronze and I have threads cut into it on both ends, can I still consider it a 3/4 in bolt or should I calculate the area remaining at the valleys of the threads. I am using Nevins rule of 1 sq. in per 1500 lbs of ballast.

Thanks for the help.

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### jehardimanSenior Member

Use the minor thread diameter. It's not so much the reduction in area, but the stress concentration at the thread root. So for 3/4-10 UNC 2A, minor dia is 0.6255; for 3/4-16 UNF 2A, minor dia is 0.6718.

Remember to have at least 5 threads or 1 diameter engaged. Torque depends on what the internal thread material is. Also you should figure out the SM of the bolting pattern and look at the moment; really important for narrow fin keels without flanges.

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### LTDboatdesignJunior Member

Thanks, thats what I suspected, but its good to be reassured. Well written reply, thanks so much.

Cody

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### PetrosSenior Member

this is normal to use the minor dia, plus a safety factor for the stress concetration, for bolts in tension. if the bolt is in shear than you use the full diameter assuming the full diameter is at the mating/shear interface. On a keel you possibly have both shear and tension, the one that dominates determines the size and number of bolts.

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