For a permanent join a C link is the traditional way. However, not all C links are made equal, so I would read this first before buying
C-links – Cox Engineering
I took Viv's advice and ordered an expensive C link. Bashing the little nodules together on the pontoon was really difficult, but it isn't coming apart without an angle grinder.
Note you are probably using 6mm or 8mm chain in the UK, but measure it before buying
anything in case it's imperial. The C link for 8mm from crosby was actually an imperial size and only just fits in the 8mm chain if you gently open a link up a bit.
For a temporary solution when needing extra length for deep mooring
, well I am still pondering that and there are some good recent threads on here about chain. Currently I using one of these which is rated to 1000kgs, again they are not all made equal and I have seen cheap
ones with no rating.
Nick on S/V Jedi chastised me for it about a decade ago, with the best intentions, but its still there only because we don't often need to go beyond the 45m of chain on to the rope