Power On The Hook

By J. Rodall

Boaters who prefer to be on the hook, such as ourselves in our Islander 36 sailboat Holole’a, greatly extend their cruising experience. There are many more bays and nooks and crannies available when using the anchor. And it is free! However, the one big issue is electrical power. The boat has to be self-contained for storing electrical power (batteries), recharging the batteries, and providing 120 Volt electrical power (main engine with alternator or dedicated genset). Solar panels can help recharge batteries also.

Most boats use deep-cycle batteries for the house battery system. These are batteries that can tolerate hundreds of cycles of a 50% discharge. Without getting too technical, they are generally robust batteries of lead-acid, gel-cell, or AGM (absorbed glass matt) construction. The common physical sizes for 12 volt systems can vary from Group 24 (common car size), to golf-cart 6 volt batteries (connect 2 in series for 12 volts) up to massive and very heavy 4D and 8D. Battery banks can be added in parallel for more capacity.

Charging systems (when not on shore power) generally run off an engine of some sort. The only other options are solar power and wind turbines, but those are different topics. For a sailboat such as mine, typically the only engine has the secondary duty of battery charging through its alternator. A big improvement over the stock 60 amp (or so) alternator is a 100 to 150 amp externally regulated unit such as made by Balmar. These alternators are controlled by a “smart 3 stage” regulator mounted nearby to properly charge batteries, but of course require the main engine to run. This system is strictly 12 volts. A better alternative if space and budget permit is a proper genset, using whichever type of fuel the main engine(s) use. Gensets provide household quality power of 120 or 120/240 volts to receptacles and to the same battery charger that is used when on shore-power. This hopefully is a smart 3 stage charger. Electric stoves and such notwithstanding, only run gensets at appropriate times of the day.

Now, seriously consider adding a battery monitoring system. These are essentially a very accurate fuel gauge for the batteries. These units accurately track outgoing power and incoming charging power.  They are a voltmeter, an ammeter, a temperature sensor, a small computer, and display all in one unit, but are simple to read. They need to be programmed initially for the type and size of the batteries. Batteries are more complicated than they appear, and using a battery monitor will tell you the state of battery discharge. When charging the battery, it can be determined when to shut down the genset or main engine. This will minimize engine running time.

Adding a high quality (true sine wave) inverter of at least 2000 watt size, but more preferably 3000 watt size, will provide quiet AC power any time. Some of the better units combine a good battery charger with an inverter. When shore power, or genset power is available, they automatically become a battery charger. When the shore power or genset is disconnected, they switch to being an inverter which provides 120 volts AC to receptacles. A true sine wave unit provides high quality 120 volt AC power not unlike the utility power ashore. A true sine-wave inverter will power anything up to their rating. Bear in mind that motors such as a vacuum cleaner and large power tools have a large inrush current when starting, and may temporarily overload an inverter. Most good inverters will tolerate this.

Cheaper inverters are available and provide modified sine-wave AC power. This will work for devices such as vacuum cleaners, toasters, and hair dryers, but are not suitable for electronic devices such as microwaves, televisions, computers, etc. Electronics can overheat and be damaged when subjected to the poor quality AC power of cheap inverters.

The one big advantage of inverters is providing silent AC power; the big disadvantage is the large power requirement from the battery bank. Inverters require very heavy gauge cables to work properly. A rule of thumb is roughly 10x current out for current in. Simplified, a 1200 watt toaster draws about 10 amps from a receptacle; the inverter draws about 100 amps from the battery bank to provide this. Fortunately this heavy power drain is only for a few minutes at a time. Electrical appliances and tools that run for only minutes are very practical, as are electronics used for extended periods. High draw devices like electric stoves and hot water heaters are not practical with inverters.  The aforementioned battery monitor is almost mandatory when using an inverter.

An obvious but often overlooked energy saving scheme is…turn off the lights and appliances when not needed. Also consider using LED bulbs, including the anchor light. These bulbs are getting less expensive and the power consumption is considerably less than incandescent bulbs.

Always remember, electricity can be dangerous and life-threatening. Even 12 volts, while not likely to electrocute, can cause a devastating arc-flash which can seriously burn and blind anyone nearby. And 120 volts can definitely cause electrocution. Batteries for the most part are filled with highly dangerous sulphuric acid. Always wire devices to applicable standards for wire size and fusing using the ABYC standards. A fire on a boat can be deadly. If you are not competent to set up marine electrical systems yourself, many good marine electricians are available.

A good reference: Boatowner’s Mechanical and Electrical Manual, Nigel Calder, ISBN 0-07-143238-8.