Choosing the Right Generator: Research Pays Off
After the main engine, the expensive single piece of equipment on a boat can be the generator, also known as the marine genset. Additionally, a generator can log two or three times as many hours as the main engine(s), so choosing the right one is an important decision. A boat owner wants a generator that’s reliable, offers longevity and delivers a comfortable time on board.
Choosing the right marine genset can be easy if a captain works with a generator dealers and analyzes a boat’s requirements carefully. This buyer’s guide to choosing the right generator will familiarize readers with a few terms and provide a basic understanding of the different types of generators and how they operate.
AC or DC
The choice of AC or DC depends on the application. If the primary goal is charging the batteries, many people put small generators in just to charge batteries. A specifically designed DC charging battery charger will have higher efficiency, the size and weight will be smaller and comes with the benefit of being a variable-speed generator.
Many early DC generators were simply a diesel or gas engine with an automotive alternator, which are inefficient and noisy by today’s standards. New technology has seen the development of more efficient alternators.
Inverter or Generator
Inverters change DC electricity from a battery bank into AC power to run AC equipment. Inverters work well for vessels that have relatively low power demand (1000 to 3500 watts) for short time periods. Boats with larger, consistent power demands and electric motors require a generator or a generator plus an inverter.
Engine, Hydraulic Drive or Both?
Powered by a hydraulic pump on the main engine, hydraulic drive generators are best suited to boats with small, intermittent power requirements or long-range cruisers. Generally, it’s best to rely on a hydraulic generator when only a small amount of power is necessary.
Operating Speed of Marine Gensets
Electronic equipment is designed to consume electrical energy with a fixed frequency. The international (SI) unit for frequency is hertz (Hz), which is equal to one cycle per second. The United States and Canada use 60Hz power. Europe, Australia, New Zealand and most Asian countries use 50Hz.
Choose the frequency used in the region where the boat is most often used. The frequency output of a generator depends on its fixed engine speed, unless a new variable-speed generator is being used. To produce 60Hz electricity, the engine operators normally at 1800 or 3600 rpm. Generators rated at 50Hz run at around 1500 or 3000 rpm. Four-pole generators running at 1800 rpm for United States power standards or 1500 for European standards are most common.
They offer great benefits and are extremely fuel-efficient, relatively quiet so long as they have a sound-absorbing shield, inexpensive to operate and dependable for the long haul. Two-pole generators small and light, but these engines run at twice the speed of four-pole units, which increases noise and decreases engine life.
A modern 3000 rpm engine, however, is fuel efficient and comes with a long lifespan. It is significantly smaller and lighter than a 1500 rpm generator. While the engine life is probably a little shorter, the chances of running enough engine hours to wear out a unit is unlikely. The average genset hours on a cruising boat are probably around 100 to 150 hours per year and the typical life of a modern two-to-three cylinder high-speed diesel is probably between 3000 and 4000 hours. With a proper sound capsule and engine, isolation mounts, a 3000-hour genset is as quiet and smooth as a 1500-rpm unit.
Gas or Diesel?
If a boat’s main engine is a diesel, the generator should run on the same fuel. Keep in mind that the explosive nature of gasoline requires a spark-free generator. This could make a diesel genset a safer bet even if the main engine runs on gasoline.
Cooling Systems
Liquid-cooled generator engines are engineered to be used in a marine environment and they are available in three configurations: heat exchanger, keel-cooled or direct seawater. The generator should have the same type of liquid cooling as the main engine.
Marine gensets that cool with a heat exchanger feature two cooling-water circuits. They include a rubber impeller or centrifugal pump that moves water from outside the boat through a heat exchanger and back overboard, often through the exhaust elbow. The “jacket water (also called freshwater) circuit” has a circulation pump that moves a coolant mixture continuously through the engine block and exhaust manifold (where it cools them) and through the heat exchanger where it is cooled by the seawater.
Keel-cooled generators have only the jacket water circuit. A circulation pump moves the coolant through a cooling grid on the bottom of the boat. Keel cooled generators require their own keel cooler so they are not tied to the main engine’s grid.
Engine Type
Marine gensets with in-line 4-stroke engines, whether diesel or petrol, are easy to install and service. Since four-pole generators operate at low rpm, the engine needs to produce its maximum torque near or below the operating speed. Automotive engines produce maximum torque at higher speeds. For example, when they run at 1800 rpm, almost all automotive engines are working at a point below peak torque. This will limit the engine’s ability to pick up extra loads such as watermakers, air conditioners, or refrigerators. Engines that are made for heavy-duty, industrial applications offer strong, reliable low-end torque and provide the power to handle supplementary electrical loads, even when running at full power. The penalty is extra size and weight, which can be significant.
Those looking to produce less than 10 kW with a 3000 rpm engine can still run a two- or three-cylinder unit. If a user wants the same from a 1500-rpm system, the requirements are generally for a four-cylinder engine. As engine technology has developed in recent years, the negatives against high-rpm small diesel engines has certainly been diminished.
Engine Benefits
- Cast-iron, liquid-cooled exhaust manifolds increase safety. Dry manifolds can be a fire hazard. If there is a turbocharger, make sure it’s liquid cooled.
- All service points should be gathered on one side for easy maintenance. This permits installing the non-service side of the generator directly against a bulkhead to save space in the engine room.
- On larger machines, choose one with liquid-cooled, replaceable cylinder liners. This lowers rebuild costs. If it’s a smaller high rpm diesel it may be cheaper to simply replace the engine.
- Safety shutdowns for high water temperature and low oil pressure should be on a generator.
- Look for a design that eliminates unnecessary, troublesome equipment such as hoses, belts and gaskets.
Single-Phase or Three-Phase?
Marine gensets produce either single-phase or three-phase power. Three-phase motors are less expensive than single-phase motors. And while three-phase power is better for motor starting and running, 20kW generators and smaller usually feature single-phase motors.
Variable-Speed Marine Gensets
Until recent times fixed rpm generators have been the only technology available. Regardless of whether they are low speed (1500/1800rpm) or high speed (3000/ 3600rpm) they have the same drawback. The engine can only deliver the power available at one rpm. In the case of the low-speed generators, this results in a large engine being required to develop relatively small amounts of power. In the case of the high-speed generators the compromise is on engine noise and life expectancy.
An ideal scenario is for the engine to be able to adjust its rpm to match the load power required at any given time. This is not possible on systems where the frequency is related to engine speed. Hence a more sophisticated approach is required.
This involves generating a high frequency (nominally 400Hz) at a high voltage (nominally 270VAC) and then converting this via some solid-state rectifiers into a high-voltage DC power source. This DC power is now fed to a DC to AC inverter that has a fixed output of 230VAC at 50HZ. Since the actual AC output of the generator winding is converted to DC, the engine rpm can now run at varied speeds without affecting the inverter-output frequency and the engine now only works at a speed sufficient to supply a given load requirement.
The advantages are a large reduction in overall generator size and weight (high frequency generator windings are much smaller than lower frequency (50Hz) windings). A lower powered engine is also possible since for peak loads the engine can be operated at higher rpm. Also since the loads are often significantly less than the maximum power rating of the engine, the engine can be slowed down to an appropriate speed to deliver the lesser amount of energy without altering the output frequency of the inverter. Engine life is increased, noise levels decreased, and fuel economy optimized.
Advances in power electronics have made the whole conversion process highly efficient with the entire system having efficiencies of more than 90% A further adaptation of the technology can be used for DC battery-charging generators. These provide a quiet and energy-efficient method for battery charging and find frequent application in cruising boats where the prime requirement of a generator is battery charging.
The Right Size Marine Genset
Selecting the right size generator for a vessel is critical. If it is too small, it will wear out quickly, produce excessive exhaust smoke and potentially damage electrical equipment. If it is too large, it will run under-loaded, lead to carbon build-up in the combustion chamber, leave unburned fuel in the exhaust and operate inefficiently. A generator should never run continually with less than a 25% load. 35% to 70% is optimal.
Two generators may be the best answer for boats with varying power requirements. Use a higher kW generator for high demand periods and a lower kW generator for times when power demand is minimal. Another option is to use a medium-size generator that runs singularly or together with paralleling switch-gear or a simple split-bus distribution panel.
It is best to have a generator dealer perform a load analysis of your vessel to determine what proper size generator. The dealer will need the wattage requirement listed. Use this formula to calculate wattage: amps x volts = watts. Turning on appliances that use electric motors produces a current inrush, which can cause voltage and frequency dips and lights to dim. Depending on the quality and size of the motor being started, the amount of power necessary to start an appliance can be up to ten times its running wattage. This is why it is so important to supply the dealer with the starting and running wattages of each motor. The dealer can calculate the electrical load of all the equipment you will run at one time.
Accessories
Once the generator size has been determined, make a list of the optional equipment that may be added to a boat. Front PTO (power take off) is a great way to take mechanical power off the front of the engine. Front PTOs engage with an electric clutch. Pump-mount PTOs can have hydraulic pumps attached to run hydraulics, deck machinery and bow thrusters.
To reduce engine noise and increase on-board comfort, sound enclosures come optional with many generators. Some sound enclosures make accessing the generator more difficult, so when choosing a sound enclosure, make sure it features panels that are easy to remove. It is recommended to use circuit breakers to protect the generator from short circuits. Mount the circuit breakers as close as possible to the generator.
Once the generator size has been determined, get independent advice because each supplier will have a slightly different take on what works. This will ensure that a boat will have a great power system that will provides years of reliability.
Special thanks to Northern Lights and Enertec for the basis of this article. For more, go to www.northern-lights.com or www.enertec.co.nz.