How do generators work? Stu Davies explains…
A generator on a boat is generally powered by an internal combustion engine using an energydense carbon-based fuel such as diesel. Fuel cell generators will be coming to the fore in future years, but are outside our scope for the present.
A generator is basically a rotating magnet inside coils of copper wire. When the magnets rotate, they induce a voltage in the coils of wire. Sophisticated control systems control the output. Simplistically speaking the alternator on our main engine is a ‘generator’ which produces 12V or 24V. Th alternator produces alternating current (AC) which is rectified to direct current (DC). The ones fitted to our boat engines, simplistically speaking, are capable of producing up to 60A at a nominal 12V.
All well and good, it can keep our battery banks topped up, deal with a fridge, radar and instruments, etc. However when it comes to powering all these and an air conditioner the maths doesn’t add up and you need to consider battery capacity and type.
Running our expensive main engine to provide power is a very inefficient way of producing electricity. The full power of our engine cannot be utilised, it runs ‘light’ which causes glazing of the bores and coking up of the exhaust system.
A dedicated generator, while expensive, is more efficient fuel-wise and wear wise. The power output of a dedicated generator engine is closely matched to the electrical output of the generator it is powering, plus it is run at a higher speed than we do with our main engines.
What size generator?
When selecting the size of the generator you need to pay close attention. If it’s too big, similar issues will arise as when we use our main engines to generate power. This is why Willie Renilson’s Farymann started to suffer bore glazing and incomplete combustion.
His system is highly sophisticated. Using several possible power inputs, this generated power is controlled and mixed to produce DC or AC; whichever is required on board.
His integrated system makes the use of power seamless. It is an expensive system and perhaps not affordable or desired by everybody, but as a concept, and to him a project with an outcome that suits him, is to be admired.
There are bluewater cruisers out there using systems similar to this but built with less expensive components that he refers to in his follow up.
The storage system – Rolls AGM batteries – is a high-end product and designed for exactly a situation that Willie is using them for; many fairly deep discharge and recharge cycles.
A point that many people don’t know is that lead acid batteries are only good for half their stated capacity, so Willie’s 330Ah nominal battery capacity is only half that.
People think that air conditioning is a luxury, but it can be a necessity. A trip through the French Canals in the summer will prove that – plus his system can provide hot water too.
The aircon system itself doesn’t use a lot more power than a boat fridge, while a side benefit is that it also reduces humidity in the boat.
Would I install a generator?
It’s horses for courses. My boat in Portugal has 250W of solar power, 330Ah of battery capacity and we can live off grid indefinitely. The solar panels are more than sufficient in the summer to do this. We’ve never felt the need for aircon. At anchor, even in the sun of Portugal, the sea has a natural cooling effect through the hull. Having said that other people can and do feel differently.
What are the other options?
Fixed diesel generator
1 A diesel-powered generator is the best because of commonality of the fuel. You need the space to fit one and then the control of it. In the case of a simple diesel system, it could be plumbed into the existing fuel and mains electric system quite easily. A three-way switch: Shore power/Off/Generator would take care of the mains connection and then the rest of the mains system would work as it did before. In the marina, switch on shore power, then at anchor, switch on generator power.
A good set of batteries of whichever type and a 3kW inverter could take care of simple mains needs when you don’t want to run the generator.
Suitcase generator
2 A 2kW suitcase generator is quiet and efficient, portable and simplistically speaking can be plugged in to the mains power inlet and the existing boat’s mains wiring used to control it. Given that Willie has spent a lot of money on his system, if I was doing it I’d fit lithium batteries and more of them, as they almost double usable capacity for a given nominal capacity.
If I was doing more bluewater sailing I would consider fitting a Duogen combined water and wind generator. My friend who has done five Atlantic crossing swears by his.