Sea Knot

A great beach haul  today... not often you come across one of these sea knot things - looks like it's been sloshing about in there for a while.
Anyone know the proper name for them?
All the rusty bits were from our Blast beach - the one just to the south of town where they filmed some of Alien 3 - such was the pollution there. It's cleaning up very quickly now - even some seaweed among the rusty bolts and screws nowadays.
https://www.flickr.com/photos/interzone-inc/5303912229