This is all good stuff but, as far as narrowboats are concerned, we're off down the usual anchoring rabbit hole again. Most of the experience and writings, and hence available data, about anchoring are from those involved with sea going vessels.
However, in the OP's case, and most other threads on the subject here, we're interested in an anchor only as an emergency brake. Narrowboats are not likely to be anchoring in a tideway whilst popping to the pub in the dinghy and then back onboard to sleep it off, and the OP isn't asking that. The scenario is that the engine or steering goes duff and we need to stop the boat heading downstream with the current whilst we fix it or summon help. Very different requirements I'd suggest.
I'm not meaning to be pedantic or have a pop at anyone who's generously giving their experience and opinions, just seeking to clarify the requirement.