Whilst it is certainly true that there are several gas-free boats on the canals, by and large, most boats do still use propane in some form or another, mainly for cooking but occasionally for heating too though this is a very costly way of doing it as the gas heaters seem to burn through huge amounts of the stuff.
It is also true to say that of those boats that do use it, the vast majority will have the “Calor” gas system which is where the bottle(s) are owned by the Calor gas company and what you’re paying for is the propane itself. When the bottle runs empty, you hand the entire thing over to the supplier and they issue you with another full one.
I do know of one boater who eschewed Calor for “Gaslow” cylinders which you own outright and can be taken to roadside petrol stations for refilling but then you have the hassle of carrying the empty cylinder along the towpath and up a road to the petrol station (and note that stations that supply LPG are becoming more and more scarce) but worse, then lugging the full and very heavy cylinder back to the boat afterwards. A trolley would be a wise investment if this is your choice.
Typically a narrowboat will have 1 or 2 13kg bottles, though in some cases the smaller 6kg size is used for reasons of space. Be warned, these are not appropriately cheaper than the 13kg ones (ie not half the price) so it works out much more expensive with the smaller bottles. And Calor recently announced they’re discontinuing the 6kg size anyway – though because of an uproar in the boat and camping community, they have apparently just said they’re reversing this decision – so be warned if that’s what’s in use on a boat you’re looking to buy.
There are several legal requirements relating to gas storage, connections and use, and you are extremely unwise to attempt any modifications to the system yourself unless you have not only “Gas Safe” accreditation but specifically the “boating” sub-category.
The only thing boat owners typically will do themselves with gas is unscrew the cylinder from the pipe leading to the pressure regulator and screw a new one on.
Gas is highly explosive stuff so bottles must be kept in a separate enclosure (usually a locker at the bow, but also often one or more lockers on the stern. For example, what appears merely to be seating in a semi-trad boat will often turn out to have gas bottles inside) and that enclosure must have vents to the outside at its base. This is so that any escape of propane will leak to the outside and not into the boat. It’s a key item checked in your four-yearly Boat Safety Certificate examination. It’s crucial that these vents are not blocked in any fashion.
It is prohibited from keeping other items in your gas locker in case their movement might cause a spark and ignite any escaping gas, but many boaters merrily ignore this and stick loads of boating miscellany down there (fenders, ropes, anchor etc). Just ensure it’s removed when it’s time for the BSS check.
Bear in mind that most boats, somewhat oddly, are not fitted as standard with any kind of propane detector or alarm, which means that in the worst case there could be a leak which accumulates in the bilges, eventually exploding when a stray spark sets it off. It’s rare – but it has happened.
For this reason, some boaters prefer to keep their craft entirely gas-free and cook using diesel ranges (like an Aga stove in a house) or, in more modern craft, on electric induction hobs. Such cooking does require substantial amounts of power though, so you’ll need a large battery bank, a huge inverter, and a generator.
The latter is required even if you slather the boat in solar panels because outside of peak summer time, you simply won’t generate enough to keep the batteries topped up unless you only ever do short trips out before returning to a mooring where you can plug into shore power (ie the National Grid)
Inevitably though, just like diesel and coal, gas is going to have to be phased out one day because of its environmental impact.