‘Works’ would mean enough passengers to make those services pay for themselves....so worth continuing without subsidy.
Stepping way back up thread ... as hinted at further down, you raise a number of further questions in that answer, Chris
1. Are you looking at a service which pays for itself financially - i.e. the company running it breaks even at least, or a service which pays for itself economically - i.e. where the lack of the service would hit the area / places served / wider business?
2. Are you looking at the late service stand-alone, or also considering the traffic reduction on services the opposite way earlier in the day? One of our factors doing late train work is that running a late train help increase loads the other way on afternoon services as many/most journeys are returns
3. Are you looking at immediate operational costs (the difference between the boat sitting moored up and running), or are you looking to add a proportion of fixed costs (depreciation, insurance, etc) to your late evening run figures?
4. If you are also running the car ferry, are you also considering the loss of income from the late service on that route when you consider the passenger ferry? There may not be as many using the car ferry because they have no choice, but there WILL be some and they'll be bringing income to that boat, even if it's inconvenient ...
Mention in-thread of the Hovercraft too. Not close to rail inks in Portsmouth as I recall (so arrivals there by train have an extra bus transfer) and not quite as convenient at Ryde Esplanande to the train (especially with luggage or reduced mobility) as the Pier Head transfer is.