PRACTICALITIES by ANDREW BRAY
Nothing is more basic or essential to the efficient management of a yacht than its sources of auxiliary power.
YACHTS GENERALLY rely on DC electricity for cabin and navigation lamps, radios and engine starting, and often for autopilots, wind instruments, refrigeration, navigation electronics, bow thrusters, watermakers, entertainment gear, and even anchor winches. Many carry AC equipment as well, much of it powered through inverters, but engine driven AC sources are increasingly common too.
This article provides an overview of the various ways to generate and manage electrical power, starting with batteries, which remain the only way to store it.
On most yachts the DC system is based on two battery banks (starting and domestic) with the main engine’s alternator as the primary recharge source for both. A battery switch allows either or both to be selected for charging, and for connection to the domestic circuits. Diodes are sometimes added to prevent accidental discharge of the starter via the domestic circuits, although a voltage sensitive relay has advantages.
Smaller boats usually have just one starter and one domestic, both probably flooded automotive or marine starting batteries. As DC requirements increase the greater usable capacity of deep cycle batteries, and the faster recharging possible with AGM (Absorbed Glass Mat) batteries become attractive. Since electrolyte density can’t be directly measured with AGM or other Sealed Lead Acid types, accurate digital voltmeters and devices which keep a running total of power in and out of the bank become useful battery management aids.
Larger battery banks can be recharged with a given number of AmpHours more quickly than a smaller bank, but are of course heavier, bulkier, and more expensive. Batteries last longer if not cycled too deeply, and if recharged fully. But complete recharging is often impractical if all the charge comes from running the main engine.
Most marinised engines are fitted with alternators capable of generating 70 Amps (~840W) or more, but a standard Voltage regulator starts to peg back the output well before a battery is fully charged. This extends the time taken to fully recharge the batteries, so either engine hours and fuel consumption are racked up, or recharging is ended prematurely, which reduces battery capacity and life.
Faster recharging results if the voltage is raised slightly, which a battery can safely absorb provided it doesn’t get too warm or gas excessively, and the voltage is lowered to a safe ‘float’ level once recharging is complete. Some regulators can be modified to allow stepped or continuously variable manual adjustment of output. Such systems work well, but depend on someone monitoring voltage and temperature. Routinely this becomes a chore, and risks accidentally damaging the battery.
'Smart' charge regulators automate a faster charging regime in stages ‘ a high current ‘bulk’ charge, then a constant voltage 'acceptance' or 'absorption' stage. Overcharging is avoided by reverting to a lower ‘float’ voltage after a set time, or when current indicates the battery is full. Most monitor and react to battery temperature, and some have provision for deliberate overcharging to ‘equalise’ cells and ‘reform’ sulphate crystals. Charge regulators need to match the type (and capacity) of battery and alternator involved, but do cut recharge time significantly. Problems may arise when domestic and starting batteries are different.
The best approach to the different battery problem is two alternators, one nominally dedicated to each bank. This provides useful redundancy, avoids the need for accidental discharge diodes or relays, and speeds recharging. A standard regulator suits recharging the starter, so a smart regulator can be tailored to suit the domestic bank. The systems can still be interconnected via a battery switch if high power is needed, or if one alternator fails. This second alternator is usually driven from a pulley bolted to the existing crankshaft or camshaft pulley, and may be higher capacity than the standard engine alternator.
Total reliance on the main engine for charging leaves the whole boat very vulnerable to engine or alternator failure, or to flat batteries preventing engine starting. Petrol reserves are usually too limited for a portable generator to recharge a flat starting battery, so an alternative charge source should be considered. Although they may take days to recharge a flat battery, solar panels and/or wind turbines also make a significant routine contribution.
Because they can be regulated, and work unattended, photovoltaics are well suited to completing the charging process, and to keeping refrigeration systems going while crew are absent. Even a small unit can keep up with the self discharge rate of a battery bank, but ‘self regulating’ panels may damage batteries, so a solar Voltage regulator or even a Maximum Power Point Tracker (MPPT) should be used.
MPPTs are switch-mode DC-DC converters that automatically load the panel at whatever voltage produces peak power under present conditions and simultaneously output the optimum Voltage for the batteries at their present charge state. Units such as the illustrated SolarBoost 2000E (www.blueskyenergyinc.com) incorporate a stepping-type battery charge controller (optional temperature monitoring), with current and voltage metering and an electronic switch to prevent overnight discharge. They significantly boost overall charging.
Although plenty of power is available from wind turbines during windy weather, it can’t be stored once the batteries are full, and average outputs are way below the peaks. It’s now possible to regulate speed and output of permanent magnet rotors. Units such as the Air Xmarine incorporate a charge regulator and are able to shut down during strong winds. Noise and lower output in light winds makes turbines less suitable for base load generation when a boat is occupied in port, but they make a larger and more consistent contribution under way.
Power requirements are highest on passage, with nav lights, autopilots, radars, radios, chartplotters and watermakers likely to be in use. A water-driven turbine is well suited to meeting this increased base load, although it may shave boat speed in lighter conditions. They are usually unregulated permanent magnet alternators, but are not liable to overspeed damage, so batteries can be protected from overcharge by a voltage sensing relay.
A newly available option are Direct Methanol Fuel Cells. The process is silent, and more efficient, and less polluting than running a diesel. The Max Power AHD-100 for example (www.oceantalk.com.au) weighs about 7 kg, measures 380 x 155 x 260mm, and produces about 100 Amp hours (at 12 Volts) of electricity per day from 1.4kg of methanol. It's designed to interface automatically with a boat's battery system – generating power on demand.
Battery Chargers While a yacht has access to shore power it can run and recharge its DC system via a battery charger. Capacity, quality, and price vary enormously, from cheap automotive types that may have no regulation and are electrically noisy, right through to powerful and sophisticated systems that incorporate multi-stage charging. Some operate in reverse as an inverter.
Hot Water The cumulative cost in wear and fuel of running the main engine for battery charging and hot water is worth avoiding. Non-engine sources can provide all or most of the electrical power, but not enough to heat showers. If solar-heated portable showers won’t do, diesel-fuelled water heaters are more practical in most situations than bottled gas water heaters, and can also heat the boat via pumped hot water radiators.
A small donkey engine can drive an alternator while also heating water. The smallest are based on single cylinder hand-started agricultural diesels, which generally lack the accurate speed governing needed to belt drive even a small 230V AC generator, but can provide a poor-man’s genset if run in conjunction with a substantial inverter.
Permanent AC systems should be installed by electricians familiar with local regulations and practical boating realities. The prospects of lectrolysis, falling overboard with a live power tool, and perhaps on-board AC generation all introduce risks that are not present on shore. Details such as double-insulated multistrand wiring, circuit breakers, double pole switches, polarity detection ( and perhaps reversal correction) and voltage monitoring are important.
An extension cord through a hatch may be fine for an occasional power tool or hair drier, but a proper inlet receptacle is neater, provides the opportunity to install an earth leakage safety cutout, and an electrolysis blocker (diodes that block small fault currents but pass larger ones so that circuit breakers and safety cutouts can function), and allows locking the boat with power connected. The receptacle should be in a protected location, have a sealing cap, and the cord should be secured at the boat so the live plug can’t fall overboard.
Isolation and Grounding
Isolating transformers remove the voltage between the boat’s AC and shore power, including its earth. This improves safety and reduces potential marina electrolysis problems. Traditional transformers are expensive and heavy, but there are now much smaller switch-mode units. Devices that produce AC on board such as gensets and larger (ie non-portable) inverters are naturally isolated with respect to shore power, but will usually be required to be installed with one output wire internally grounded, allowing safety cutouts to work, and simplifying shore power switching. Smaller inverters and petrol gensets sometimes have ‘floating’ outputs which won’t trip safety cutouts and are better restricted to use with double insulated tools and small insulated appliances such as laptop rechargers via extension cords, not via installed wiring and power outlets.
On-board AC generation
Inverters are electronic devices that convert 12 or 24 Volt DC power from a battery into 115 or 230 Volt AC power and are an affordable and quiet way of powering a range of smaller tools and appliances. Modern switchmode inverters are compact, light, reliable, nearly silent, quite efficient, and indifferent to heeling. They are often ‘softstart’, meaning the output voltage ramps up gradually to protect electronics against spikes. It’s worth considering two units, one for smaller long term jobs like running a charger or a laptop, and another to handle larger short term loads such as a grinder or a microwave. Some stand themselves down between uses.
The main points to watch with inverters are that they’re more than adequate for the intended loads (power, but also starting current and power factor); that the output waveform (sine, square, and modified square wave etc) is suitable; that output is electrically isolated from input; and that battery, wiring, fusing, and recharging have sufficient capacity. Although the amount varies with waveform, most have some ‘surge’ capacity that helps with starting motors.
Inverters are available from less than 100 Watts up to several kilowatts; some incorporate battery chargers; and others can be remotely installed, with flush mounting digital control/display. Sophistication varies, but most have at least automatic shutdown on low battery, and their own thermal overload protection. Very large inverters are available, but producing 2kW will soon flatten most battery banks.
Engine + Inverter
If high power is only required occasionally for relatively short periods, simultaneously running the engine may supply some additional power for the inverter.
Main Engine Generators
An AC generator can be belt driven by the main engine, but to produce the standard frequency and voltage the engine must be run at the correct speed, which can be inconvenient. This can be overcome by using self-governing variable speed pulley systems such as the Autogen, which are bulky, but allow AC to be produced over a range of engine speeds. A more compact option is generators that combine a high voltage alternator and switchmode electronics to produce controlled AC at most engine speeds. This approach is now used with some portable generators which regulate their own speed to suit the present load.
Diesel-driven generators are common on larger boats, and there are trends towards units suiting smaller boats, and having PTO shafts from which additional equipment (DC alternator, refrigeration and diving compressors, hydraulic pumps, watermakers etc.) can be belt driven.
Most smaller generators are ‘two pole’ machines, rotating at 3000 R/min for 50 Hz. Four poles rotate at half these speeds and are often chosen for higher output gen-sets. Most diesels are water cooled, and some also water cool the generator. Fresh water cooling with a salt water heat exchanger is preferable to raw salt cooling, but heavier and more expensive. Some smaller units can be started by hand, and are compact enough to go under a bunk or seat, but installation should take into account heeling, and the need for daily checks and occasional maintenance.
Modern equipment is usually supplied inside a sound-insulated enclosure, has electric start, and will shut down automatically if water or exhaust temperature or oil pressure problems develop. Exhaust gas/water separators can be fitted to quiet exhaust spurting. Lighter, air-cooled units are available, but tend to be noisier, and despite probably requiring a water pump for exhaust cooling may not be able to heat shower water.
Starting currents and load power factors need to be considered when calculating required generator capacity (rated in kVA rather than kW). Voltage regulation under varying load is important, as excessive sag, or momentary over-voltage can affect or even damage some equipment. Governor quality is a factor in voltage regulation, and also effects frequency stability.
Compared with running the main engine the advantages of a ‘gen-set’ include lower fuel usage and less noise, making it relatively painless to run the system longer or more frequently. They allow larger devices to run without drawing on batteries. One approach in cruising yachts is to combine gen sets in the 4 to 8 kVA range with powerful 1-2 kVA inverters to provide 24 hour AC power availability. The gen sets are sized so that various daily tasks can be accomplished with just a few hours running per day.
Two and four stroke air-cooled petrol generators range from very portable 10kg units that put out a few hundred Watts through to 40 kg luggables in the 1 to 2 kVA range. They’re useful as temporary AC sources for maintenance but not really a practical basis for routine AC power on board. If the fuel is drained off smaller units can then be stored below, but larger units spending their lives on deck require an enclosure. The latest inverter-based units have advantages over conventional constant-speed machines.