Arne, the offending cable ties were white, and therefore, as you said, probably unsuitable for outdoors in the sun The shackles and mousing were renewed less than 12 months ago. This time the top shackles will only be moused with wire! Tony and Sally

Locking wire, as David describes is probably the ultimate method.

However, there are cable ties and there are cable ties. In my job we used them everywhere (all made by Panduit). The white or transluctant ones were restricted to indoor use only. The outdoor versions were black. We used them for tying cables to radio masts etc. and they never failed due to sun, frost or rain.

My mast top shackles are too small for cable ties, so I use to mouse them with ordinary, waxed polyester twine. My thinking is that if it works in a sail, it probably works in a shackle as well. I look after them each time I enter the mast top (on Johanna and now Ingeborg) to check that all is well, and it looks good.

Arne

PS: May I ask  when the faulty cable tie was fitted, and what colour it had?

Hi all, Ron GLAS has just finished a passage of exactly 1000 miles from Seychelles to Kenya. Conditions were lumpy, with 4+metre swells from two different directions. No problems when the wind was the forecast 15 to 20 knots, but at times it dropped to 8 knots or less for long periods. Lots of slatting and banging with the rolling and insufficient wind to hold the sails out on any point of sail. Early one morning we sailed across a line of up to8 big whales.... Then a mighty bang, thought we'd hit one... Looked around the boat, nothing to see except the Main sail was neatly in its sail bag and the halyard and block were swinging around wildly at the masthead! 

The shackle pin was gone, DO NOT MOUSE YOUR IMPORTANT SHACKLES WITH plastic cables ties. They suffer UV degradation in tropical sun!!! The jury rig and replacing broken lazy Jack's took nearly 2 days due to extreme motion(more so without damping effect of the main) even then we could only hoist 30% of the main as the aforementioned wrap at the masthead made hoisting the tackle any higher impossible.

The foresail mousing was as bad but soon sorted, as the emergency halyard was already rigged for hauling the dinghy on deck. 

So a slow sail for the next 600 miles with a few lessons learnt. Preparation is all, if we had not been thrown out of The Seychelles at the last minute on visa expiry, due to their bureaucratic incompetence we would have been up the masts and seen the problems..... Perhaps!!