How about a plastic sheathed stainless wire
: instead? You wouldn't need all of the ropes to be knife proof - just
: enough to deter/prevent the removal of the boat. One at each end ought to
: be enough? If they're really that keen on stealing it, then not much will
: resist bolt-cutters.

All rooftop security systems became obsolete when the cordless disc grinder was invented. With that said, all you can really do is make 'em take time, make noise and bring tools.

I know from our sailing days that the larger sizes of 1x19 stainless steel rigging wire are an absolute female dog to cut - regular boltcutters just don't "cut it", pardon the pun. You've got 20 little individual SS strands in there, all twisted around each other.