Keel bolts

Gunni · Aug 7, 2013

Keep your bilge dry.

FastOlson · Aug 7, 2013

While it's good to have a dry bilge and be corrosion-free on the parts you can see, the main problem is always under the plate washer, out of sight.

The problem is water getting in from the interface of the top of the keel and the hull sump through which the threaded rod ends (aka "keel bolts") pass. With no oxygen present there the SS will readily corrode over time. (I have seen a 1" rod eaten most of the way through.)

That's why it's good to drop the keel every couple decades or so, inspect, and if you're lucky just clean up the mating surfaces and reseal with good-quality sealant and have the whole assembly torqued up. A competent yard can do this easily.

That's when you can get a closeup of the sump construction -- if any (!) wood products were ever used there, grind 'em out and upgrade to solid laminate. Bigger washers/plates if needed.

Loren

shemandr · Aug 8, 2013

These are the kind of things that creep into my thoughts when a boat is overpowered and heeling that 30 to 40 degrees. And, that's why it's better not to stress a boat or its rig by sailing like that.

Bob 04 H260 · Aug 8, 2013

I don't know about your boat but a friend with an Beneteau 36CC had his keel bolts changed because it was requested by his insurance company. They were were just unscrewed from the keel from the bilge and replaced one at a time while the boat was on the hard. They had little corrosion on them and I know that boat always had saltwater sitting in the bilge. I think with most boats the bolts are cast into the keel and this option is not available.

The bolts are designed to withstand a hard grounding, so are way over engineered for the stresses you will apply while sailing. The best thing would be to lower the keel and have them inspected. If that is not an option I would have them re-torqued it should give you some confidence they are not too thin ? Ours are spec'ed at 325 ft lbs, so I would expect a weak bolt would break when it is re-torqued but maybe not

. I just bought a 4X torque multiplier and intend to re-torque our bolts before we launch this winter.

good luck, Bob

bffatcat · Aug 8, 2013

keel bolt check

A few years back a friend of mine had to pump a lot of ocean water out of the bilge every day on a columbia 26. The local yard recommend new keel boats. What they did was to leave the old bolt and instal new sister bolts beside them. This cured most of the leaking. I have a vintage pearson 26w year built 1975 Ive owned it for 13 seasons The keel bolts have never been touched as long as Ive owned the boat and the boat has never leaked , The bilge is always dry unless there is heavy rain If it ain`t broke dont fix it.
My opinion.

Nom De Guerre · Aug 8, 2013

Bob 04 H260 said:
I don't know about your boat but a friend with an Beneteau 36CC had his keel bolts changed because it was requested by his insurance company. They were were just unscrewed from the keel from the bilge and replaced one at a time while the boat was on the hard. They had little corrosion on them and I know that boat always had saltwater sitting in the bilge. I think with most boats the bolts are cast into the keel and this option is not available.

The bolts are designed to withstand a hard grounding, so are way over engineered for the stresses you will apply while sailing. The best thing would be to lower the keel and have them inspected. If that is not an option I would have them re-torqued it should give you some confidence they are not too thin ? Ours are spec'ed at 325 ft lbs, so I would expect a weak bolt would break when it is re-torqued but maybe not . I just bought a 4X torque multiplier and intend to re-torque our bolts before we launch this winter.

good luck, Bob

This is an interesting theory. If true (that compromised bolts should break when final torque is applied), then this might be a poor man's acceptable alternative to dropping the keel and making a visual inspection.

Are there any mechanical engineers or experienced keel bolt inspectors who can provide some feedback on this idea?

Way to think outside the box, Bob.

Gunni · Aug 8, 2013

We have two conversations going here. Keel bolts and keel studs. Both require a different inspection process. If you have bolts, you can do the job in an afternoon, if you have the studs (threaded rod embedded in the keel, with nuts in the bilge) the inspection process is different, and may require dropping the keel.

Nom De Guerre · Aug 8, 2013

Gunni said:
We have two conversations going here. Keel bolts and keel studs. Both require a different inspection process. If you have bolts, you can do the job in an afternoon, if you have the studs (threaded rod embedded in the keel, with nuts in the bilge) the inspection process is different, and may require dropping the keel.

True, thanks for highlighting the difference, Gunny.

I'm looking for feedback on using the application of torque via nuts in the bilge to verify the integrity of the keel studs.

jibes138 · Aug 9, 2013

Think of the "studs" or "bolts" as a spring. As you torque the nut or bolt the stud stretches within the elastic limit of the material. This Stretch of the bolt , also known as strain (change in length divided by original length), puts a compressive force on the keel joint clamping the two parts together. If your sealant is absent or fractured from being old and brittle it can still leak regardless of clamping force. But back to original question, you can measure the extension of the bolt due to the torque. This will require knowing the material the stud is made out of and the thread specification. Knowing this you can calculate how much the stud "should' elongate from a given torque, and compare it to how much it actually elongated. If the actual elongation was a lot greater it will indicate a smaller diameter of the stud and you can assume corrosion is at work. There are some variables to consider such as friction of the nut plate surface, and friction of the threads but you are looking for a large discrepancy, not a small difference. If you had a large elongation difference from the calculated value then you could decide to drop the keel to better evaluate what is happening and perform repairs. It might be hard to differentiate bolt stretch from compression of the sealant if there is a thick layer of sealant acting as a gasket in the joint.
Any boat with a traditional stuffing box that is intended to drip a bit will have a wet bilge with seawater. So keeping a dry bilge is not practical for many owners. For keel stepped masts it is also common to get some rain water entering through mast penetrations like internal halyard slots and sheave opening, so some rainwater will collect in the bilge also.
Charlie Cobra had a great pictorial on re-bedding a keel. Do a search and you will get a great education from that thread.

Bob 04 H260 · Aug 9, 2013

thanks Dave, great explanation ! I suppose if the bolts are corroded then when you re-torque them they should move as the smaller bolt will have stretched and released some of the pressure on the keel joint. As you mentioned they would also move if the keel joint sealant has compressed, so maybe moving does not tell you anything, but you do learn something if they don't move at all ?

We have a dry bilge, (pss shaft seal and deck stepped mast). The owners manual for our boat does not mention checking the torque as a maintenance procedure. Last year an old boat yard worker suggested to me that keel bolts should be re-torqued every few years. Our boat is 10 years old and I saw this torque multiplier on sale so thought this will be the year I would start before we launch for the winter,(and being a tool junky it was the only reason I could think of to justify buying it

).

Bob

jibes138 · Aug 9, 2013

Torque on a thread is translated into an axial force on the parts being clamped. The thread is modeled as an inclined plane and exerts enormous force. Think of a wedge splitting a log. A hit on the wedge exerts a huge force on the angled sides of the wedge causing the log to split. As torque is applied the bolt (or stud) stretches just like you were pulling on a very strong spring. As long as the stress (force divided by area) is below the elastic limit of the material it will return to its' original condition when the torque is removed. If it is torqued too much it will permanently deform (called plastic deformation) and no longer spring back to its; original shape. So if you loosen the nuts measure the height of the studs with a caliper depth gage or a depth micrometer (you will need to measure in thousandths of an inch). Then torque them to specification and measure them again. The difference is the elongation of the stud due to the stress created by the torque. Now calculate for the size and thread type and torque how much they should elongate. Note that a fine thread is stronger than a course thread.
Regarding torque multipliers. I had to torque something to 6000 foot pounds once. Had to use a hydraulic torque wrench. If your torque is very high then you need a long arm on your torque wrench. For a typical 18 inch long torque wrench it is very hard to apply say 150 foot pounds especially using an extension and a deep socket as the wrench wants to put a bending moment on the nut. Best to have a longer arm on the torque wrench and someone to hold the shaft vertical while applying the torque.
I think most would be surprised just how strong a keel joint is. Just one bolt will hold up a keel no problem. Since it is typical to have 7 or 8 bolts there is a lot of strength. Lets due some quick math. Say you have 1/2 inch bolts. Area = pi x R**2 = 0.196 square inches. For low strength stainless steel the yield strength is probably around 40,000 PSI.
Tensile Yield Strength (.2% offset):
276 MPa / 40 kpsi
So 40,000 PSI x 0.196 square inches = 7850 pounds.
So each 1/2 inch bolt can hold up 7850 pounds in tension. Most small boats like ours have maybe a 4500 pound keel or less. Since you have maybe 7 bolts and they might be larger than 1/2 inch there is an awful lot of strength. Now the bolts are subject to dynamic loading in bending, shear, fatigue, etc. but the biggest would be a hard grounding exerting an impact bending moment on the keel. From what I have seen of hard groundings the fiberglass fails before the keel bolts break and you get a big crack in the hull.
Maine Sail has photos of corroded keel bolts with very little area left so it happens and there is risk of failure but as I note a small cross section is still pretty strong and there is a lot of bolts involved. Say your bolts corroded to the size of a 1/4 inch screw (very small). 1/8 x 1/8 x pi = .049 sq. inches. x 40,000 = 1,962 pounds. So my keel can be held up by three 1/4 inch screws with some safety factor left over. In reality I have 7 or 8 maybe 5/8 or 3/4 inch studs.
Once a keel joint is properly torqued and you find the nuts are loose it might be a good idea to ask why? What would make the nuts get loose? It would take say 100 foot pounds to loosen the nuts if they were torqued to 100 foot pounds in the first place. This is very unlikely in everyday life that they would just loosen up all by themselves, so something else is happening. The sealant is failing and causing a loss of preload in the joint. The fiberglass is compressing (maybe a rotten wood core in there) or the bolts are corroding and getting smaller in diameter and thus elongating more causing a loss of preload. So if the joint is good the bolts should not get loose. If the bolts do get loose it is for a reason. That reason needs to be investigated and fixed.
Good luck with your project.

Bob 04 H260 · Aug 10, 2013

thanks Dave, you are a great source of information. Our boat has 6 s/s bolts 1" dia. torqued to 325 ft lbs and 2, 1/2" bolts torqued to 45 ft lbs, for a keel that weighs about 5000 lbs. So I guess it is well fastened to the bottom of the boat !

Bob

rpwillia · Aug 10, 2013

Is it not true ...

that that majority of failures in this area have been due to the fiberglass keel area failing?

shemandr · Aug 10, 2013

jibes138, thank you for your detailed explanation of how keel bolts function and how much they are over engineered. I guess I just don't realize how strong a material steel is. I'll enjoy my next windy sail more!
Does thermal expansion and contraction play any role in loosening of threaded connections? I've wondered if that is why light bulbs loosen.