I need to re-rig the rudder on a friends sea kayak. Can anyone suggest what line to use? appears

stainless cable was
used on my Necky rudder system. Needs to be something that does not stretch. Suggest contacting the kayak manufacturer, or any serious sea/touring kayak dealer.

You never finished your post !
Are you asking about the line that raises and lowers the rudder?

Or are you asking about the steel cables that control it?

Jack L