One thing is the terrible Swaging job. Not enough gap between swage and thimble. And the tail is way too long. Should terminate just outside the swage? And it's not crimped fully

Also the wire is the wrong type. It doesn't look like its conforming to the thimble so it's not flexible enough?

lets see if any of that sticks.

Again great ideas from all and on the right track.
First and most importantly are the turnbuckles which are commodity grade stainless, fine for a shade sail but not holding a rig up.
The best way to tell these are on ypur boat us the thread, metric (course) is commodity and UNF (fine) is rigging grade.
Second is no lock nuts or any form of locking.
Finally is the galv shackles. You can use galv shackles but they need to be the lifting type with the SWL on them. Again these are commodity grade, now considered not even good enough for your trailer chain!!

shaggybaxter said..
Thanks Rumblefish, this is good fun. I'm learning stuff!
UNF for rigging grade, eh? Bugger me.

Yup, means less force required on the spanner to tighten turnbuckles and more threads to share the load.

Should add these were stainless threads on a stainless body, another nono due to stainless galling.
Rigging uses stainless threads on bronze bodies to eliminate this