Originally Posted by Omatako
Having said all that, there doesn't seem to be any comment in this thread on the concept of simply putting a lock nut above/below the turnbuckle.
Locknuts create significant risk of galling the threads if tight enough to rely on. Heavy rigging strains will stretch the turnbuckle enough to let the locknut loosen and turn. To prevent this, you must have them tight enough to be right up against the yield stress of the metal, not good.
You'll probably get away with locknuts in casual weekend duffing around but not if you drive the boat hard and frequently as you will in deep water sailing.