Keel bolts

[damauney]

If you do a search on keel bolts you will find photos of a number of stainless keel bolts that are thinned by corrosion. When you look closely the loss of bolt cross section is at the elevation of the nuts not down in the keel stub. That means that on the hard you can remove the nuts, clean and inspect the threads, apply a corrosion inhibiting antiseize, and re-torque the nuts.

Goodwinds
DaveM

 

[jibes138]

After writing my previous response I started thinking about water trapped in the keel joint and freezing over the winter on the hard. This will apply an awful lot of stress to the joint and may be a contributing factor to the sealant in the joint failing over time. As far as thermals in the metals themselves, it should only matter if they are dissimilar materials. Aluminum expands at about twice the rate of steel. Carbon steel is about 6.5 millionths of an inch per degree F per inch and aluminum about 13 millionths. Stainless is somewhere around 9 millionths. When assembling interference fits thermal expansion is used a lot, like ring gears on flywheels are shrunk fit typically using induction to heat up the gear. If the stud and nut are the same materials they should expand and contract as one. Heat used to expand propellers to remove them from shafts is another boating example of heat used to expand metals.
Light bulbs are only installed hand tight so they aren't very tight to begin with and the threads are thin sheet metal designed for easy assembly not for super strength. When you torque a bolt to something like 100 foot pounds it is an enormous force. For a 3/4 inch bolt 100 foot pounds applies an axial force of almost 8000 pounds. So if you have say 6 bolts the keel is clamped with 48,000 pounds of force. More bolts, more force.
Sitting in water the thermals will not be very extreme but on the hard between winter and summer you could get say 130 degrees of temperature differential. Combined with the stress of freezing water in the joint it could be a lot of extra stress on the joint. I don't know the thermal properties of the FRP so how much that changes would effect the clamping force but unless the nut actually moved when everything returned to original conditions the clamping force would still be there.
Here is a really cool web site to calculate the clamping force for various torques.
http://www.engineersedge.com/calculators/torque_calc.htm

 

[Nom De Guerre]

That's a lot of info to consider, thanks guys.

So far I've distilled it down to (for my purposes on my boat) if a torque check results in no nut movement after pulling out this fall, there's not likely a problem bad enough to warrant a visual inspection via the dropped keel.

And if there is movement (on 1 or 2 out of the 10 or so that I have on my 3000 lb or so keel), then a visual inspection from the top-side for those only may suffice (assuming I can see the condition through the hull).

And if all of the nuts move, I might want to schedule a trip to the yard lift for a drop/inspect/fresh bedding for complete piece of mind.

Something to put on the check-list for her (1985 H28.5) second haul-out with me as owner, seeing as how I have no keel maintenance history records available.

 

[Oblivionboyj]

Does anybody know the torque values for the nuts on the keel studs for a Columbia 8.7?

 

[Bob 04 H260]

Another great post Dave, having some fun with the calculator..........

If you can not get a recommended torque from the original boat manufacturers literature there are tables of torque values on the internet for each bolt size and material, dry or lubricated, Google is your friend.....

Bob

 

[Oblivionboyj]

I found some standard torques online.
I have to wait till I get back up north before I can review my owners manual to see if it has values in it.