Articles, Catamaran Electrical Systems

By Kevin Jeffrey

You’re heading offshore and have come to realize that almost every important system on your boat depends on a steady supply of electricity. You find yourself asking, “will I have enough electrical power on board, especially when I really need it?” Good question. To answer it you first need to take a close look at your projected daily electrical load and see how that compares with the amount of electrical power you can reliably provide on a daily basis. Most multihull sailors preparing for blue water cruising these days find their electrical power situation is unbalanced-too much load for the available sources of power. The reason is two-fold: today’s cruising multihulls use much more electrical power, and most sailboat and auxiliary engine manufacturers haven’t given serious consideration to how much electricity is required for modern cruising or what it is really like for a cruising sailor to keep up with demand. The fact that most multihulls capable of ocean passages still come from the factory with only a standard alternator and a shorepower connection is absurd.

Even if you determine you’ll be able to keep up with electrical demand, as a blue water cruiser you should ask yourself if you will be doing so in a way that is acceptable. If balancing your power supply requires long periods of running the main engine, you’ll probably want to seek an alternative to that much noise, smell, fuel use, and wear and tear on such an expensive and important piece of equipment.

Balancing your electrical power supply in an acceptable manner is easy once you begin to take your role as energy manager seriously.

The Power Options – Renewable & Engine-Driven

Efficient, effective charging options available to blue water cruisers fall into one of two categories, renewable or engine-driven. Since maintaining a balanced electrical power supply is really only critical away from the dock, shorepower will not be included in this discussion.

What priority you give to renewable vs. engine-driven charging sources is up to you, but if self-sufficiency and clean power are priorities I advise thinking of the renewables as your main charging sources and the engine-driven gear as back-up.

Photovoltaic (PV) Solar Panels

It should come as no surprise that PV solar panels are at the top of my list of recommended charging sources. In my opinion every cruiser should add solar panels to their energy mix until they exceed their electrical demand, mounting space available or budget (one of these will be the limiting factor). Since solar panels convert sunlight directly to electricity, they are ideal for a blue water cruiser: no noise, smells or fuel to buy; no moving parts to wear out; incredibly reliable with a long service life (25 year warranties are not unusual).

There are three main types of panels to consider:

1) Standard panels have rigid aluminum perimeter frames and glass covers. They offer the best cost-per-watt ratio and have the longest warranties. While these panels may seem unsuitable for a rugged marine environment, the glass covers are surprisingly tough and they have been used successfully on ocean cruising boats for many years. The key is to mount them thoughtfully, out of the way of foot traffic or following seas.

2) There are also semi-flexible marine panels that can stand up to moderate foot traffic. These panels have no perimeter frame and a polymer cover instead of glass. Cost is higher and life expectancy and warranty periods are typically less, but these panels are great if a low panel profile and the ability to walk on them is important.

3) The third type of panel is fully flexible. These panels are even more expensive in terms of dollars-per-watt and they’re about half as efficient as standard panels, but they have triple junction solar cells with blocking diodes between which gives them excellent shading characteristics; if part of the panel is shaded the balance of the panel continues to produce power. They can also be sewn into biminis and dodgers or even walked upon with reasonable care.

I’m often asked, “How much of my electrical load can solar panels handle?” My response is all of it-or very little-it completely depends on the size of your load and the surface area of solar panels you have. I feel that on most cruising multihull sailboats, enough solar power can be comfortably installed to handle a large portion of the electrical load.

Renewable Charging Option 1: Add solar panels to your boat.

Accessories Needed: A charge control to prevent battery overcharge and either a simple ammeter or a complete system monitor to track performance.

Efficiency: Up to 16{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} in the conversion of sunlight to amps in the batteries

Relative Power Production: An 80-watt solar panel produces roughly 4.5 amps in bright sunshine. If the panel is in direct sunlight for 1 hour, 4.5 amp-hours of electricity will be produced. An 80-watt panel installed horizontally with good exposure to sunlight will produce roughly 25 amp-hours of energy per day.

Pros: No fuel required, works well in most cruising areas, no noise or smells, long life expectancy, low maintenance.

Cons: Initial cost, finding acceptable mounting places, for some sailors the visual appeal of solar panels is limited.

Tip: Mounting panels on davits, on a stern arch, or over a bimini frame allows for a large solar array that is out of the way and has good exposure to the sun.

Wind-Powered Generators

Generating electrical power from the wind is a natural for multihull sailors, but this option is not as simple and straight forward as solar panels. Wind-powered generators are mechanical devices with moving parts; they require some maintenance and generate sound as well as electricity (some more than others). They must be mounted safely, with the rotating blades well away from reaching hands. Finding a suitable mounting location can be challenging, even on a beamy cat or tri. In addition, wind generators produce very little power on a downwind passage due to light apparent wind.

Despite these practical considerations, many sailors would benefit from having a wind generator in their electrical power inventory. These units are efficient, affordable, and produce power anytime the wind is blowing. They are also a good complement to other renewable charging sources.

There are half a dozen marine wind generators suitable for cruising sailboats on the market, both small prop units (less than 4 ft. rotor diameter – Aerogen, Air Marine, Ampair, LVM) and large prop units (over 4 ft. rotor diameter – Ferris, Fourwinds, KISS, Windbugger). The small prop units can be more easily incorporated into your boat while the larger units produce more power.

You can mount wind-powered generators on a pole at the stern, as a fixed or rotating mount on a mizzen mast, or rigging-suspended in the fore triangle. The pole mount seems to be the most popular option these days, but there are several advantages to the rigging-suspended mount. First, the unit is only up in port, which allows for clear decks at sea. Second, very little noise and vibration are transmitted below decks. And third, the Aquair, Ferris and Fourwinds units can be converted to water generators during a passage, which essentially allows you to continue to capture wind energy for electrical power generation on a downwind passage; conversion from a rigging-suspended wind unit is the most convenient and typically the least expensive to purchase.

Renewable Charging Option 2: Include a wind-powered generator in your energy mix

Accessories Needed: Charge control if optional, method of monitoring performance, overspeed protection if optional, mounting system (pole and mizzen mount), spares kit as recommended by supplier.

Efficiency: Up to 30{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} in the conversion of wind energy to amps in the batteries
Relative Power Production: A large prop wind generator can generate 10 amps in 15 knots of wind, or up to 200 amp-hours in a 24-hour period. In the typical windspeeds experienced while cruising, wind generators can produce an average of 50-150 amp-hours per day.

Pros: Relatively high power output, good cost-per-watt ratio, no fuel required, work well in most cruising areas, no smells, 24-hour operation when deployed.

Cons: Finding acceptable mounting space, appearance on yachts with traditional lines, potential noise and vibration problems, potential safety issues, not effective on a downwind passage.

Tip: On pole mounted wind units make sure to use rubber isolation at all contact points between mounting pole or struts and boat deck, railings, etc.
Water-Powered Generators

Water generators are the most underrated pieces of marine gear I know of. They can supply a tremendous amount of electricity for their size, weight and cost.

There are several types of water generators, the traditional “trailing-log” models and “outboard leg” style. Trailing-log units have a gimballed electrical generator mounted at the boat’s stern. A long line with a spinner attached to the end is connected to the generator shaft. When trailed behind a boat under sail the spinner and line rotate rapidly, turning the generator shaft and producing electricity. In an outboard leg model the electrical generator is submerged at the bottom of the leg, or it is mounted on top of the leg and connected to the spinning rotor through a shaft and angle gears.

A good package for cruisers is a combination wind- and water-powered generator that allows for effective battery charging under sail and in port. However, many of the trailing log type water generators have a serious boat speed limitation, which affects their appropriateness for fast cruising multihulls. The faster the boat speed, the faster the generator shaft spins, and some generators are unable to cope with much over 7 knots of boat speed.

Renewable Charging Option 3: Include a water-powered generator in your energy mix (if you can find one that is compatible with your average cruising speeds).

Accessories Needed: Charge control, method of monitoring performance, spares as recommended by supplier.

Efficiency: Roughly 20{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} in the conversion of the energy in moving water to amps in the batteries

Relative Power Production: A water generator can produce up to 200 amp-hours per day with average boat speeds of 5 knots.

Pros: High power output, good cost-per-watt ratio, no fuel required, works well on all points of sail, no noise or smells, low maintenance, high degree of reliability.

Cons: High boat speeds and certain sea conditions may prevent use of a water generator, trailing log units cumbersome to deploy and retrieve and the spinner occasionally becomes a meal for a large fish, may get in the way of fishing lines and other gear at the stern, slight decrease in boat speed (roughly 1/4 knot).

Tip: Mount a trailing-log water generator on the port side of the stern; the direction of the prop makes the spinner angle slightly to port.

High Output Alternators (HOAs)

One of the most cost effective changes you can make is to swap your standard alternator for a high-output model. While standard alternators work well on powerboats and cars, they are unsuitable as the main charging source on a modern cruising multihull. If you don’t believe it, consider that a standard alternator on a 35 hp auxiliary engine is only about 2-3{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} efficient at converting the energy in a gallon of fuel into electricity stored in the boat’s house battery bank. No, that’s not a misprint. Clearly an alternative is needed.

High-output alternators aren’t just standard alternators that can produce more current. They are specially designed and built for the job with heavier wire windings, closer internal tolerances, and better cooling characteristics. Created for the police and emergency vehicle markets, they are made to run continuously at their rated capacity in a hot environment without harm, something that would quickly kill most standard alternators.

HOAs come in small and large case designs. Small case models are available in power ratings up to 150 amps and are typically a direct replacement for standard alternators. Large case models have a slightly larger body diameter, so if you want to replace your existing alternator it’s a good idea to check clearances before purchasing.

When selecting a HOA it’s best to compare the power curves of various models. A power curve shows alternator output as a function of engine rpm. If efficient battery charging is a priority (and it usually is when purchasing a HOA), select a model that gives maximum output at low engine rpm. Keep in mind that a model with a lower output rating may actually give more electrical power at engine idle speeds than a model with a higher rating.

Once you decide that a HOA is in your future, how do you determine what size to get, and whether it should be a replacement for your existing alternator or a second alternator that supplements it? This depends on many things, among them your electrical power requirements and how much renewable power sources contribute, your engine size and the clearances available, and your budget. Most cruisers replace their standard alternator with a small case HOA (usually around a 125 amp model), or leave the standard alternator in place to charge the starting battery and add the HOA to charge the house bank if there is the physical space to mount it on the engine. On a catamaran with twin inboards there are several options: a) add a HOA to the engine that has the water heater and refrigeration compressor on it and use the other engine more often for propulsion to balance the hours of run time, or b) put a HOA on each engine.

Engine-Driven Charging Option 1: Replace or supplement your standard alternator with a high-output model.

Accessories Needed: A “smart” control for an efficient charging sequence that stores the most power and extends battery life, a voltage spike suppressor to prevent damage to alternator diodes during an accidental circuit disconnect, and spares as recommended by supplier.

Efficiency: Roughly 10{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} in the conversion of fuel in the tank to amps in the batteries

Relative Power Production: A 125-amp alternator can produce roughly 70 amps at engine idle speeds (depends on battery state of charge), or roughly 70 amp-hours of energy for every hour of engine run time. A HOA will produce power any time the engine is running.

Pros: Greatly reduced engine run time, high return on investment, easy to install, long life expectancy, low maintenance, high power output on demand.

Cons: Can’t think of any.

Tip: Upgrade both the output wire and ground wire when replacing a standard alternator with a HOA.

Diesel chargers

You can also achieve high charging power without wear and tear on the main engine by installing a Dieselcharger, a small diesel engine mated to a high-output alternator. The engine size and alternator output are well matched for most efficient performance. To illustrate the point, a 4-hp engine is all that is needed to drive a 100-amp alternator at full output.

Dieselchargers produce DC power for direct battery charging. They are a practical choice if AC loads on board can comfortably be supplied by an inverter when away from a dock. Dieselchargers can be mounted in a locker or other convenient location, and sound-proofed to reduce noise.

Engine-Driven Charging Option 2: Add a DC Diesel charger .

Accessories Needed: Components to complete the installation, spares as recommended by supplier

Efficiency: Roughly 20{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} in the conversion of fuel in the tank to amps in the batteries

Relative Power Production: A Dieselcharger with a 100-amp alternator can produce roughly 85 amps when hot (depends on battery state of charge), or roughly 85 amp-hours of energy for every hour of engine run time.

Pros: High power output, small size, relatively lightweight, easy on fuel, reliable, power available on demand.

Cons: Another engine to maintain, additional weight and space on board, initial cost.

Tip: For best efficiency, try to use high-draw appliances during times when the Dieselcharger is running.


Charging batteries with a small gen-set matched to an efficient high-current battery charger can also reduce main engine wear and tear. Gen-sets are a better charging option than Dieselchargers if high-draw AC loads such as air conditioning are required away from a dock. With this set-up the batteries will be recharged anytime the gen-set is running to supply AC loads.

Efficient battery chargers, such as the power-factor-corrected units from Mastervolt, can produce a high amount of charging power with a relatively small gen-set. Most of the compact gen-sets on the market come in a soundproof housing for quiet operation.

Engine-Driven Charging Option 3: Small gen-set matched to an efficient battery charger

Accessories Needed: Components to complete the installation, spares as recommended by supplier

Efficiency: Roughly 20{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} in the conversion of fuel in the tank to amps in the batteries

Relative Power Production: A gen-set powering an 80-amp battery charger can produce roughly 80 amp-hours of energy for every hour of gen-set run time.

Pros: High power output, small size, relatively lightweight, easy on fuel, reliable, power available on demand.

Cons: Another engine to maintain, additional weight and space on board, high initial cost.

Tip: Make sure gen-set output and battery charger are compatible.


My ideal cruising battery charging package would an array of solar panels (as large as practical), a combination wind- and water-powered generator, a high-output alternator on the main engine, and a good system monitor to track performance. This gear and the necessary accessories could be owner installed for roughly $4,500.


About the Author

Kevin Jeffrey is a long-time multihull sailor, independent energy consultant, author and book publisher.  He is the author of Independent Energy Guide, a valuable resource for cruising mutihull sailors, and is the publisher of Adventuring With Children by Nan Jeffrey and the first three editions of the Sailor’s Multihull Guide.