Originally Posted by lakelandterrier
But the point is that they won't - or shouldn't- undo or give under load!
Depends on what you're using it for. For, say, an anchor, then yes, you don't want it coming loose. For other things (as in my example above), you may want to be able to free it while some amount of load is still applied. Granted, there are other bends or knots that could be used with better release characteristics while loaded, but that is another matter. The original comment was that an anchor bend can be nearly impossible to get loose once a load has been applied.
A RTTHH or fishermans bend by being tight around their fastened point will exert a steadytension on the line. If you use a bowline, the slack inthe loop will result in tension coming & going & the line snatching, which you may not want (e.g. towing).
Not quite sure what you're saying; tension coming and going will cause snatch in the line in either case; the difference in your example is that the bowline will be subjected to a higher overall stress delta, as it will be absorbing the difference between slack and full shock, whereas the tightened hitches maintain some of the tightening force without load applied. Not sure that really makes any difference in your example though; I've never seen a bowline come loose from varying loading (at least, not in any kind of standard marine cordage.)
If you're talking line derating due to the knot, well, that's a whole 'nother kettle of fish [and no, I have no idea where that particular saying came from.]
I use a bowline for many things, but not when there will be planned sustained tension on the line.[/QUOTE]