Low Voltage Cabin Electrics

System

Advantages

Disadvantages

12v

Full range of equipment readily available
Generally cheaper than 24v equipment		 Voltage drop problems in long cable runs

24v

Voltage drop is not really a problem

Initial cost of the equipment is higher
Not all equipment available in 24v

Generating 240V

 

There are 3 main ways of producing mains 240v electricity whilst out cruising:

  • Separate generator
  • Engine driven 240v alternator
  • From the batteries via an inverter

The important factor to be taken into account is the size, in watts, of the appliances that you are likely to run on board and the total number of watts being run simultaneously.

Remember that it contravenes BW's licence conditions to run a generator or an engine between 8pm and 8am when moored up .

System

Advantages

Disadvantages

Portable suitcase style separate generator
  • Cheaper option
  • Easy retro 'fit'
  • Sine wave supply
  • Tend to be small output - sometimes less than 1000w
  • Petrol driven: petrol not safe on board - needs dedicated gas-tight locker - BSC not keen
  • Gas driven : acceptable to BSC - just - but awkward to set up
  • Can only recharge batteries through a mains charger

Built-in separate generator - diesel combustion engine
  • Plenty of power - usually 3000w or more
  • Just switch on and run
  • Fuelled from the main or separate tank
  • If main engine broken can still have 240v power and, if a mains charger is fitted in your boat, charge the batteries.
  • Sine wave supply
  • Expensive to fit
  • Often raw water cooled so need to check the filter regularly
  • Air cooled motors are noisy
  • Often not possible to retro fit
  • Can be noisy - motor runs at constant speed either 1500 or 3000rpm
  • 2 engines to service etc
  • Uses almost the same amount of fuel regardless of the load

Built-in separate generator - diesel Sterling engine
  • Almost silent
  • 'Free' hot water & central heating
  • Plenty of power - usually 3000w or more
  • Just switch on and run
  • Fuelled from the main or separate tank
  • If main engine broken can still have 240v power and, if a mains charger is fitted in your boat, charge the batteries.
  • Sine wave supply
  • Very expensive to fit
  • Often not possible to retro fit
  • 2 engines to service etc
  • Uses almost the same amount of fuel regardless of the load

Engine-driven 240v alternator
  • Cheaper than separate, built-in genny
  • Running and servicing costs for 1 engine only
  • Usually 3500w supply when engine running at 1500 rpm - 1000w at about 1100rpm
  • 'Free' mains power when travelling
  • If main engine breaks down, no means of charging batteries & no 240v
  • Not always compatible with modern 3-stage mains battery chargers
  • Older units not pure sine wave

Inverters
  • Silent mains power any time
  • Cheaper than built in diesel generators
  • 'Free' mains power when travelling
  • When using high power equipment with a large inverter, running the engine at the same time, especially if there are 2 alternators, means the unit acts like a 240v alternator
  • Supply limited to life of the batteries
  • Outputs vary - 300w to 3000w
  • May need a second alternator to reduce recharge time
  • Cheaper units are not pure sine wave

Paul's thoughts:

My own choice has changed with time and the developments in technology. It is worth noting that those boats with some form of generator also have an inverter or inverter/charger so that they have silent 240v power for the evenings. With this in mind I now recommend a large inverter (1500w+ continuous) with a large (90amp+) 12v alternator or two fitted to the main engine and a decent sized battery bank (330 amp/hours+).

Effectively this means that when using a low power consumption, say in the evening with an entertainment system & table lamp, you use the inverter without the engine. When using a high power consumption (washing machine or drier or iron or vacuum cleaner or modern hair drier etc - one at a time of course!), you do so with the engine running ie the inverter & alternator operate as an engine-driven 240v alternator with much of the power being supplied by the alternator.

The main advantage of the inverter & 12v alternator combination is that when stationary and not using much 240v power, the large alternator(s) will re-charge the batteries quickly thus reducing the running time.