An off-topic complaint elsewhere about the space occupied, 2-3 feet, by the pair of buries / splices on whoopie slings has me wonder about the Title: Has your whoopie sling slipped?
The direction I'm going in is the possibility of shortening the adjustable bury. Maybe it has been too conservatively set by the guideline based on diameter --that is to say longer than necessary -- for the Amsteel Blue (and Dynaglide.)
For the fixed eye of the sling, the recommended bury as a multiple of approximate cord diameter is about retaining breaking strength of the cord. For the adjustable bury, the length is about retaining strength without slipping right up to the point where the cord will fail elsewhere, where the load is taken by a single cord, not a pair.
That raises the possibility that some may have experienced slippage, either from very heavy dynamic loading of the sling, or because the bury was short......or not.....or for whatever reason.
On one of the arborist boards there was a lot of testing of whoopies and loopies a while ago, but one of the important differences in use there, beside serious heft in the cordage to construct the slings, is that conventional usage usually offers the opportunity to put a pinching or pressing lateral load on the bury / sleeve. The only load in our use is in line with the cord. Also the heft of the cord for guidelines may matter. Others are observing the flying behavior of crows, eagles, and geese. We are flying sparrows in comparison.
So: Would folks who have experienced slippage of whoopie slings for hammock hanging describe the circumstance? For example, do you feel you were caught cheating? Was there a slip followed by a grab? Whoopie slings just plumb wore out?