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My question was: Do the captive pin bow shackles really work as advertised? And prevent a shackle from being an un-shackle?
Couple things...
Unless I'm mistaken, on a 16, there technically is no "bow shackle". The bow tang should secure directly to the eyelet in the bridle wire- a shackle is not required. So if you have shackles there, your best bet is probably to just get rid of them. However there are plenty of other places that use shackles.
When I see the term "captive pin" shackle, I think of a shackle that has the pin flared at the end so it can not be separated from the main body of the shackle. You unscrew the pin and slide it back so the shackle is fully opened, but you can not actually remove the pin from the shackle. This type of shackle can come unscrewed the same as any other type of shackle, you just can't lose the pin (unless you lose the whole shackle).
The shackle you show in the picture has a hole in the pin which would allow you to pass a safety wire through to wrap around the main body of the shackle. This type of shackle would not be able to come undone under any circumstances unless the wire is cut. Typically these are only used on Hobies where all the shrouds are connected to the mast tang. So this type of shackle is definitely more secure than a standard shackle.
I use loctite on all threaded shackles that are not typically loosened. Also crank down on them using a good set of pliers (multi-tools are nice, but they generally don't provide as much torque as a good set of pliers).
Another option is to use shackles that use a clevis pin rather than a threaded pin. Then tape the pin to prevent it from snagging.
sm