Would this article (pdf download) have the timber hitch info you want?
Breaking Load of Hitches and Ropes Used in Rigging
http://joa.isa-arbor.com/request.asp?JournalID=1&ArticleID=3218&Type=2

Thank you very much. the article indicates that, while a properly tied timber hitch is adequately strong, it is not the very strongest knot. Somewhat surprisingly, they did not test the unknotted breaking strength of the ropes they used, and hence did not provide a measure of "efficiency," the percentage decrease in strength that occurs when the rope is knotted.

The article does indicate the importance of a properly tied and dressed knot and that there is a degree of randomness in any lash up,so don't push to the very limit of these numbers!

I would say that probably the very strongest anchor know would be a rewoven figure eight, properly tied and dressed,closely followed by the bowline and its variants. There are more variations of the bowline that there are cross ties on the railroad or stars in the sky.

More stuff on knot strength.

"A figure eight follow through, for instance, is often cited as having 80% of the strength of the rope. In our tests this ranged from 100% – it did not fail at the knot in climbing ropes in about a quarter of cases, including in a UIAA fall test that we observed – to near 50% in 5.5 Spectra/Titan cord. "

"Nor does it matter if the eight knot is “sloppy” or “neat”. In fact we tested quite a few figure eight knots, in a variety of ropes, that were improperly tied so that the last pass of the “follow-through” was not made. Did it matter? Drum roll please: sometimes it tested stronger than the “correct” eight and in a third of our tests it did not break at the knot."

http://efclimbers.net/wp-content/uploads/2013/02/Knot-and-cord-strength.pdf

Yeah, but try to untie after falling hard on a figure eight knot...

Not related to strength, but if people do not dress the knot correctly or tied is rather sloppy, I'll just tell them to redo it. Got to keep people from getting too sloppy with everything. You can often see one sloppy thing leading to another.

This statement caught my eye in this very worthwhile report:

" It would be interesting to perform a few of these “messy knot” tests on UIAA drop tests and see how they perform in dynamic loading situations."

After all, the reason we tie into a rope in the first place when climbing is so that it will catch us in a dynamic fall. Certainly a messy knot is better than no knot at all....

I've always felt that obsessive concern with knot strength is somewhat wasted effort. Modern climbing ropes have a rather large safety margin with regards to strength. People die when anchors rip out of the rock. People die when ropes get cut over sharp edges of rock. But one simply doesn't hear much about real instances of people dying because the rope broke at a knot.

I think the key factors for knots are can they be easily tied correctly, and are they easy to check? Are they secure when tied correctly, and can they be untied once they've been loaded? I once saw a climbing partner come untied from the belay in the middle of a pitch. He had somehow miss-tied his tie in knot, and it simply came untied as he was moving up the pitch. Fortunately, he didn't fall, so the incident was very scary, but not catastrophic. Note that after that he became extremely methodical in checking all of his knots!

Excellent points, but some of us (I won't name names) began climbing before modern climbing ropes were in vogue. my first use of a rope was a 1/2"manila rope fixed by our leader who free climbed the pitch (today rated at 5.6) and we clambered up hand over hand. Belay, what's a belay? I have no idea how the rope was fastened to the stout tree that anchored our ascent.

Fortunately, someone arrived in town with nylon ropes, pitons, carabiners, and the knowledge to use them correctly and we lived to tell tales.

I do think it is useful to be aware of the changes in strength wrought by knotting, running sharply over a carabiner, wear and tear, etc., and be mindful of these factors when climbing or rigging.

I have receeently had occasion to apply my knowledge of rope craft to rigging large paleontological specimens of unknown weight (but estimated to reach around 1200 pounds) for extrication. The work is much more methodical than the usual rock climbing situation, but care is needed. Failure would b embarrassing, to say the least....

Sounds like a mammoth job! Especially for an old fossil such as yourself. I hope you were able to channel the forces correctly? Just remember, no man is an island.

But many are atolls.

A lot of what passes for gospel in climbing is based on the untested or anecdotal.

Things like connecting two loops of cord. In the old days we did a strop hitch or girth hitch and thought no more about it. We just figured any knot would reduce the strength of the cord by up to half. Since we overbuilt everything in the first place, no problem.

Now, I get told when connecting two loops of cord or web to use a carabiner, that it is much stronger. (Bull pucky, it is one more thing that can go wrong, either by the added chance the biner is damaged or by getting cross loaded, and it uses up one piece of gear you might need later on the climb.)

All that because John Sherman reported a girth hitched sling broke at low load.
Suddenly girth hitches are only for connecting a leash to your harness. (If they are not strong enough on one piece of an anchor system, how in heck are they okay as your ONLY connection as a leash?)

This supposed new safety rule is still being passed about years after tests showed Mr Sherman's broken sling had been badly damaged beforehand.