Is it not correct to use silicon bronze all thread 1/2" for keel bolts?

Someone said don't do it, not the best idea. I was thinking If I did get some rod, JB weld the nuts on at the keel bottom and fill a few inches of the lower thread with epoxy, then use some T plus 2 rector seal to seal bolt in at the base. Or wrap some greasy yarn around base of bolt. All I could think is the cut threads are going to want to let water spiral up the bolt?

 

The only issues as you said would be related to the threads. The downside of allthread is that the keel may work and the threads can chew up the holes. Threads are also less resistant to fatigue. If you can thread solid rod or have someone thread the ends, you will be better off. The bolts should be snug in the holes because it is hard to get enough clamp load on wood to prevent any motion in the joint. Is this a ballast keel? a lot of the argument goes away if you aren't swinging a ton of lead down there. JB weld is nice, but they may never come out again. Tar still has a place in boats.

I see no major issues with silicon bronze fasteners. Steel is tougher until it rusts away. Silicon bronze alloy 655 is about as strong as SAE Grade 2 steel fasteners, and 651 is close to SAE Grade 1. That link was chosen for the properties table. I am not familiar with the company. McMaster lists the yield strength as 50 ksi, so they have alloy 651, both solid rod and allthread for twice the price. There are less expensive sources that don't list the alloy or strength, so you have to either ask or assume alloy 651 to be safe. When you start comparing 30 ksi yield to the transversely loaded wood, you need to worry more about spreading the load with big thick washers to avoid crushing the wood than the strength of the bolt. A nylon washer between the bronze and the galvanized iron wouldn't be a bad idea. Fatigue in a fastener is only a problem when the clamp load is too low to prevent things from moving.

 

The issue that hasn't been touched on is that besides the risk of the allthread wearing the holes a bit larger and the effective strength being that of the root diameter,the thread itself forms a very effective leak path if moisture does intrude.Finding somebody to cut some threads on the ends of plain rod shouldn't be that hard in most towns.

 

You can buy a cheap tap and die set at Harbor Freight for $12.
https://www.harborfreight.com/carbon-steel-sae-tap-and-die-set-21-piece-61398.html

 

I completely agree with the principle of getting hold of a die and doing the job yourself,but I don't think I would use those dies as the force needed to cut a 1/2" thread in one pass might be a bit excessive.A split die would be my preference.

 

A 1/2" takes little effort. I have done more than I can remember in my life.

 

A new sharp die, or properly re-sharpened one, squared up on the vertical rod, and good tapping lube copiously used, back off every 1/4 - 1/2 turn to break the chips, and it's an easy job. Have the die split opened up for the first cutting, so you don't cut under size, then adjust for the nuts so they aren't too loose on the thread, (and not too tight). I don't suppose anyone uses Whitworth threads any more, but they are least likely to fail from crack initiation at the thread root. Nuts and big washers of the same material as the rods; poss nylon too if contacting other metals. And marine grade thread locking compound probably wouldn't hurt either.

Is it a re-furb. or a new installation ? Slightly bigger diam. threads than previously used should be more than adequate if things are worn, unless causing other issues. Find out the recommended tightening torque and go over all the nuts several times in stages when assembling, trying to not crush any wood. Many sealants to choose from, just ask here when you have your boltup and keel ready to install.

 

Good information Seasquirt. Now I know what to do with my Whitworth tap and die set.

Torque recommendations are not as easy as they would seem. Friction plays a major role, and no two tests get the same answer. Torque tables are generally intended for bolting metals, not wood. You can break a grade 8 bolt without damaging aluminum, but you can pull the head through a piece of wood without damaging the bolt. Hence my previous recommendation for large, thick washers. The clamp load spreads through the washer at a 45° angle. Thin washers bend as the wood is crushed, so they spread the load more, but not without damage.

The Janka hardness of dry white oak is 1350 lb which is what it takes to bury a .44" steel ball halfway into the wood. A torque table for bolting metal tends to shoot for 75% of the yield strength of the fastener, but some go 90 and some go to yield. 75% of yield for 655 Si bronze is 5500 lb, which needs to be spread out quite a bit. Fortunately torque and clamp load are linear, so you can calculate the torque and load for lower torque values. If I were to try to test this, I would torque down on a piece oak to see what it takes to start crushing the wood and back off enough to be safe. Enough is not a term I can define here.

Background for the insomniacs among us. Read at your own risk: I have run and had run several torque tension tests using load washers. It takes about 5 trials to start getting consistent results on each bolt. That tells you that the torque table is a nice guideline, but there will always be a lot of scatter because no one ever slathers on a ton of lube and retorques each fastener 5 times to burnish all the burrs and polish the asperities. The coefficient of friction given for Loctite's lubricated grades is 0.24 for Si bronze, and .09 for brass. I find both coefficients suspicious. They were taken from a Loctite publication 20 years ago, and hopefully they have revisited the numbers since. The torque to 75% of yield for a Si bronze 1/2-13 bolt with a yield strength of 50,000 psi using those two values is 69 and 29 ft-lb respectively. I would assume a generic coefficient around 0.15, calculate (I have a spreadsheet) the torque for 75% yield of a grade 2 (45 ft-lb with a clamp load of 5500 lb.) and back off a bit from that. This assumes lubricating under the bolt head, which sees just as much frictional drag as the threads, but at a different radius. With no friction, the torque value would drop to 5.6 ft-lb and the nut would spin off as soon as you let go of the wrench. (Friction is important)

 

Do you have 1/2" Whitworth nuts in bronze? Whitworth 1/2" is 12 TPI.
Or will you need to buy nuts which in the US will be UNC, not Whitworth threads. UNC 1/2" is 13 TPI. (Pitch numbers corrected.)

 

Nowadays it is possible to use thread locker or nylock nuts to prevent them from loosening. In the past it was usual to have a nut with a locking nut after.

 

DC I think that you may have inadvertently flipped the numbers.........

American standard pitch for half inch is 13TPI. SAE, fine thread for half inch rod is 28 TPI. Whitworth has 12 TPI. Metric makes more sense. That system uses the distance between adjacent threads. For 13mm rod the space is 2.0 mm. For we American primitives, that works out to a pitch of about 12.7 TPI.

 

Looks like I did. Thanks for the correction.
Original post now corrected.

 

I find it easier to count the number of threads in a set distance (such as an inch) than to accurately measure the distance between threads, particularly for smaller threads. But the easiest method of checking pitch is using a an appropriate set of thread gauges.

 

No, just a tap and die set left over from a 1929 Springfiled Rolls P1. It had a mix of SAE and Whitworth fasteners.

FWIW, 1/2" allthread comes in 13 and 20 tpi. 28 tpi would be 1/4".

 

Messabout is not to be trusted late at night, either the bourbon influence or the approach of senility. Half inch SAE threads are 20 threads per inch, not 28. My bad! (Twenty eight is used on quarter inch rods and cap screws)