by Gavin Sorrell
  Of all the methods designed to provide electricity to a boat, the most potent is the alternator.
  An alternator consists of two components. One is the stator which is effectively the case structure with a network of copper wires. Inside spins the rotor which is a shaft with steel laminates and another copper wire network. Spin the rotor all day and nothing happens until a current is fed to energise the rotor windings. This is done via brushes and slip rings in the same way the old generators worked.
  The major difference is that the generator produced the full direct current to charge the battery via the brushes, whereas the alternator requires a small amount of direct current from the regulator to energise the rotor, resulting in three phase alternating current produced by the stator. This is why the brushes will last much longer in an alternator.
  The AC is rectified by a bridge diode array resulting in direct current to charge the batteries.


  • Size the alternator to 30-40% of battery capacity.
  • Ensure that mounting brackets are rigid.
  • Fit single belt drives for alternators up to 100A; dual belts drives for larger alternators.
  • Minimum alternator speed for effective charging is 5000rpm, so on a diesel engine running about 1600rpm, a pulley ratio of 4:1 is required. Don’t make the alternator pulley too small, resulting in belt slippage and wear, rather increase the crankshaft pulley size.
  • It’s imperative that a marine alternator should run as cool as possible. Adequate ventilation is essential, e.g. fit a high flow fan.
  • Never operate the alternator open circuited as the diode bridge circuit will be damaged.
  • Use large cables to prevent voltage drop. Because of engine vibration, it is not uncommon to see cable connections to the alternator falling off or broken. Secure the cable loom to a strut attached to the engine and then leave a length to absorb vibrations. Use Nylock nuts to secure the alternator connections.
  • Connections to alternators vary but can include: positive output(B+), negative output(B-), external regulator field connection(DF), tachometer output(W) and excitation input(D+).
  Most marine inboard engines have an automotive type alternator with a built-in regulator. These will perform quite adequately and reliably for many years if correctly installed and are relatively cheap.
  Dedicated marine alternators are available, with larger fans, more heatsinking, corrosion resistance and robust bearings. Importantly, they are sealed and ignition proof which is essential when fitted to petrol engines. Avoid running automotive alternators and starter motors on enclosed petrol engines. If you do … make sure you are using your bilge blower effectively!  In a vehicle, the alternator is only required to quickly replenish the battery energy expended in starting the engine and then supply a low current to keep the vehicle circuits operational. A low cost internal regulator is all that is required for this purpose. But such regulators are quite unsuitable for recharging deeply discharged house batteries, as the automotive regulator will quickly reduce output to a fraction of the alternator’s capacity and the motor will have to be run for extended periods to recharge the house battery.
  By fitting a smart three-stage external regulator, the alternator can be driven to full capacity, thus reducing engine running time and costs. External regulation will ensure that the batteries are fully charged, but not overcharged, and allows accurate control of bulk charge, absorption and float voltages and absorption timing.
  To use an external regulator with an automotive type alternator, the internal regulator must be removed from the alternator or bypassed, a relatively simple operation in most cases. Achieving good cooling and correct drive ratios becomes more important due to increased loadings.
  Battery recharge times can be reduced to less than half by the fitment of a smart regulator.
  Peeking into the future, I suspect that the common alternator may be superseded for many vessels. One engine manufacturer already has a 3kW 240VAC generator which fits between the engine and gearbox, perfect for running air conditioning, kitchen equipment and entertainment directly from the engine. Hybrid technology is now being used on European catamarans to power electric propulsion motors from diesel powered generators up to 52kW.

*Gavin Sorrell works in collaboration with Aquavolt Electric Boat Parts. Tel: 02 9417 8455