All steamed up!

At this point, I'm sure a lot of the traditionalists will throw up their hands in horror and think I've gone completely off my trolley.

I’m thinking seriously about fitting a boiling water tap. Why? Well, the main reason is that I'm installing a an oil-fired range, and although it's very quick to heat, it will still take around 20 minutes to boil a kettle - and I won't want to wait that long in the morning. I also thought it would be great for when I want to make a quick cuppa, like when in a lock. When I've been on previous boats with a gas hob, I've often put the kettle on, then forgotten it, only to go back down in the cabin later to find it full of steam.

All of this got me thinking. Somewhere along the line, someone questioned how much power it would take to keep the boiling water store hot for each 24hr period. A fair comment, but let's think about the issues with the accepted standard solution - a kettle on a gas ring. First off, most people inevitably put far more water in the kettle than they actually need, so you're wasting a lot of energy heating up the surplus. Second, how much of that hot gas by-passes the kettle without doing any useful work? Third, every Kg of propane burned produces roughly a litre of water vapour. Then there's the cost of gas.

Now, the boiling water tap is basically a small, insulated container with a heating element in it. That's going to be very efficient, because the energy is heating the water directly. There will be a small heat loss, but it will be only a few percent. The electricity will be produced by burning diesel, either in the main engine or a generator.

So let’s look at the relative costs of propane and diesel. Propane is currently around £28 for a 13Kg cylinder = £2.15 per Kg. Bulk non-propulsion diesel is 57.5p per litre which equals 68p per Kg.
The calorific value of propane is 12.88KWh per Kg; diesel is 11.83KWh per Kg. This means that for the same energy output, propane costs around three times the amount of diesel! There are of course, inefficiencies on both sides. With an electric system, there’s the generator, charger/inverter, batteries, etc. With the gas hob, there are losses due to inefficient heating and over-filling the kettle. So they are probably similar at around 40-50% losses. The real difference arises with the cost of the fuel, with diesel costing one third that of propane.

The benefits of a boiling water tap are that it’s instantly available and can’t boil away unattended, unlike the kettle. Plus you only reheat what you use.

The other big issue is the amount of water vapour produced by burning propane. (I’m talking about a gas hob, oven and grill here – a gas water heater vents to the outside.) So for every cylinder of propane burned, around 13L of water is being released into the boat as vapour, which will then condense on any cold surface, usually windows and their frames. This can easily give rise to mildew where there are cold spots and damp clothes and bedding. By contrast, the oil-fired range exhausts combustion products to the outside. Even steam from the oven goes up the flue. So my boat will undoubtedly be much drier.

I think all of this adds up to a reasoned argument in favour of the boiling water tap! However, there are also two downsides: one is the cost; even the 'cheapest' ones are nearly £500. The other is the space taken by the water reservoir under the sink.

Maybe I'll just buy a small electric kettle. We'll see...