Liveaboard dependence on electrical appliances has increased enormously on yachts. So have the headaches about how best to provide sufficient onboard electrical power to satisfy the ever-growing demand.
At one time, the primary solution was a generator or alternator mounted on the vessel’s engine(s) to charge a house battery underway, and on a shore-power connection to charge and provide electricity at a dock. When the average yacht’s hotel current draw began to exceed what a shore-power connection could provide, the use of onboard gensets steadily gained in popularity, eventually becoming a mainstay.
The problems with standalone gensets are that they’re heavy, they occupy a lot of space, and they’re noisy. Truth be told, they are often oversized for the loads they are required to meet, which means they tend to carbon-up and smoke, which is bad news in a quiet anchorage.
An increased awareness of the need to abate carbon and other emissions, matched by heightened interest in alternative and hybrid forms of power generation, has led to a proliferation of new power-storage systems and significant strides in load reduction.
Energy Savers
One example is the reduction of current draw from marine air conditioning and refrigerators. Until recently, reciprocating compressors were almost universally used in both. Although the running-current draw of a traditional reciprocating compressor can be moderate, the compressors are either on or off. When they’re off and a thermostat or other controller tells them to turn on, the current draw surges to seven to 10 times their normal running current. That surge affects the specification of a yacht’s house-power system.
Inverter-controlled, variable-speed compressors not only use less current on average than traditional reciprocating compressors, but they do not produce high-transient, starting-current surges, with the energy savings amounting to as much as 70 percent. And, since a/c and refrigeration loads are two major power sinks on a yacht anchored overnight, changing to more power-efficient technology in these two areas alone significantly reduces the total demand for onboard power generation.
Additional reductions in electrical house-power demand can be achieved by installing LED interior lighting, and by converting from radiant to inductive range cooking. These steps not only reduce total electrical load directly, but they also reduce a/c loads by lowering the amount of heat released into a yacht’s interior space.
What Else to Add
When you combine a reduction in total current draw with a high-capacity battery suite, you have the beginnings of a perfect storm of improvements. So, let’s talk about what else can be added.
Today, there are a number of ways to provide alternative electrical power generation. Perhaps the best-known method is to fit the main propulsion engine with a high-capacity alternator. These are available with outputs reaching 300 amps at 12 volts, 200 amps at 24 volts, and 100 amps at 48 volts.
A potential obstacle is that your main propulsion engine(s) must have excess or reserve horsepower available when running under a propulsion load. Power production can be modeled by the following equation: horsepower equals rpm times torque divided by 5,252. We can also reasonably model the relationship as horsepower equals rpm times fuel burn. The variance between the horsepower an engine is actually producing and the maximum horsepower it is capable of producing can be called excess or reserve power.
An engine-mounted alternator absorbs this additional power. It’s generally specified to recharge the starting batteries in a reasonable amount of running time. So, if you want to use an engine-mounted, extra-high-output alternator to charge a house bank rapidly while running the boat, you first have to make sure that your propulsion engine(s) can accommodate the upsized alternator. You also have to determine that the physical space exists on the engine, and in the engine room, to mount an ultra-high-output alternator.
And, your battery bank must have sufficient capacity to carry you through at least 24 hours without being recharged or falling below critical voltage level. In addition, the bank must accept a high rate of charge, because charging house batteries with your main propulsion engine almost always means that you have less time to complete the task.
That, in turn, means you have to charge at a higher rate (with more current, or higher voltage, or both). Luckily, recent advances in battery technology have made that feasible.
Intelligent Design
Durable systems that fulfill the identified mission aren’t just cobbled together. They must be researched and designed.
“Every energy system is unique,” says Bruce Schwab, president of Woolwich, Maine-based Ocean Planet Energy. “An assessment must be made of [all the relevant] factors to determine the energy storage requirements, recharging options, energy monitoring needs, and the safe configuration of it all, to prevent irritating surprises, system failures, injuries or worse.”
Ocean Planet Energy designs and sells propulsion-engine-mounted and solar-charging systems, in custom and standard packages. The company’s Integrel genset–replacement system mounts on an engine and monitors loads based on the amount of reserve horsepower available. The system ensures that propulsion power gets first priority and that only unused horsepower generates electricity, with no danger of overloading the propulsion engine(s).
Nature’s Way
Other options for charging house batteries include wind, water and solar.
Wind-driven turbines for boats have been around long enough to get a reputation for being generally noisy and fragile. They are not efficient in sizes that are practical for mounting on board a yacht, producing only around 0.4 kilowatts of power in a 28-knot breeze. Compare that to a 300-amp, 12-volt alternator that produces about 3.6 kW of power.
The average output of marine solar panels in conditions of subtropical sun can be as high as 15 watts per square foot of panel. On most powerboats between 40 and 50 feet length overall, you should be able to arrange for 30 square feet of panel area. That means an output of 0.45 kW of power on a sunny day, or less than half as much when it’s cloudy. And none after dusk.
An output of 0.45 kW maximum means that for 30 square feet of panels, the most you can expect out of solar panels is 3.6 kWh per eight-hour day. You could get the same amount of power to charge batteries in a single hour of running with a 300-amp, high-output, engine-mounted alternator. Even if you could find space to double the solar panel area, the engine-mounted alternator output would still be four times greater. And the charging time would still be 4:1 in favor of the alternator-based charging system.
Solar power generation does have a distinct edge in terms of being silent and lacking moving mechanical parts. It also compares favorably in regard to carbon footprint. But it usually cannot approach engine-mounted alternators for supplying convenient, silent, overnight house power.
If your electrical loads cannot be trimmed to levels that solar power can support alone, then the best alternative may be a solar-mechanical hybrid approach. Reduced fossil-fuel use may not be a complete win for the planet, but it moves you upwind toward the goal.
This article was originally published in the October 2023 issue.
