During our yacht surveys we rarely see shackles that have been moused. We remark that regularly in our reports, but I am not sure whether our clients all appreciate the importance of this.
Mousing is a simple and good practice to secure the bolt of a shackle to prevent it from getting unscrewed intentionally. I was reminded of this in a marina in Croatia where the pontoons had been shackled to the quay and all shackles had been moused.
Galvanized shackles tend to unscrew when they are under a pulsating load. Even if you tighten them carefully they eventually get unscrewed because the micro-stalactites of the zinc coating eventually get faired under the strain. Stainless steel shackles are also prone to unscrewing as the bolt’s thread usually is seated and rather slippery in comparison to galvanized hardware.
Using rope or galvanized wire is good for mousing, monel wire is even better, but even a cable tie will do as long as it is inspected regularly.
And by the way, the safe working load of shackles is determined by the diameter of the thread root, which is often smaller than the diameter of the shackle body.
Inspect your hardware, in particular the shackles, regularly. This way you can avoid losing your ground tackle. Losing ground tackle happens – usually at a time when it is least expected and most inconvenient, thus maximizing damages.