Launch Success.

 Tern Raft in Transit!

 Steady as she goes.

 Touch Down.

 It floats, as if there was any doubt!

 Half blood knot, remembered from my fishing days.

 I think it was 14mm polypropylene rope , which has a breaking strain of over two tons.

 Thanks Graham.

 Just need to add some 20mm washed gravel and a couple of half pipes and it is all ready for some Sterna hirundo.