Articles, Catamaran Electrical Systems, Engines and Mechanics

Part I – Do I need a high-put alternator?

If your multihull has an inboard auxiliary engine and you want the most charging amps for the least expense and maintenance, you should consider replacing your standard alternator with a high-output model.  It’s not unusual for sailor’s to get four or five times the power output of their existing alternator for a relatively modest investment. 

It helps to know that standard alternators are mass produced, adapted from the automobile industry with minor modifications for marine use.  They’re fine if you use your boat like a car—that is, you go aboard, motor to your destination, then either leave the boat or plug in to dockside power.  But most multihull sailors and a large percentage of power boaters need more out of their engine-driven charging source; they need to produce the most electricity they can during the times when the engine is on, especially those cruisers who spend a lot of time at anchor. 

As I’ve mentioned in other columns, high-output alternators aren’t just alternators with more amperage, they are precision alternators made to perform reliably in harsh conditions such as those found in small engine rooms on multihull sailboats.  In addition to much closer machined tolerances of their internal components, they have heavier wire windings, better cooling characteristics, and higher output at lower engine rpms.  Unlike standard alternators, they are made to run at their rated capacity indefinitely in a hot environment.    

There are a number of models currently on the market to choose from.  Many independent power system suppliers have their own brand of high-output alternators, although in most cases they are privately-labeled versions from one of several high-output alternator manufacturers such as Lestek and Powerline.  Before adding it to your Christmas shopping list of new gear to buy, however, determine if you really need to upgrade to a high-output alternator, and if so what you should look for.

Here are a few simple guidelines to follow are when making the decision.  You probably don’t need to upgrade if you spend most of your time at the dock, if your electrical load is light, or if you don’t want to run your engine for battery charging and have sufficient alternative power sources to do the job.   You probably would benefit from upgrading to a high-output alternator if you spend large amounts of time away from a dock, if you are planning some serious cruising, if you have a sizeable electrical load (refrigeration usually elevates you to this category), or if you simply want a high-power, engine-driven back-up to your renewable chargers (solar, wind, & water-powered gear).

It would be a great help in the decision-making process if you knew just how much (actually, how little!) power your standard alternator produces.  An accurate ammeter on the output line of the alternator is needed, yet few boats are equipped with this simple device.  Typically, a 35-amp alternator driven off an engine running at idle speed for battery charging produces about 1/3 to 1/2 its rated capacity for a few minutes as starting current is replaced.  As voltage rises, alternator output quickly diminishes.  After 15 minutes or so, output is usually down to a small percentage of rated capacity, mostly due the type of voltage regulation inherent in standard alternators.  I estimate that a 35-amp standard alternator driven off of a 25-hp engine at idle speed for battery charging has only about a 2{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} energy-conversion efficiency from liquid fuel to stored amps in the battery.  High-output alternators with performance charging controls can increase this efficiency up to 10{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986}, but compare this with readily available solar panels with 16{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} energy-conversion efficiency from sunlight to amps produced, then decide for yourself which technology still needs to be perfected!

Part II

In recent articles I’ve stressed the importance of including renewable power sources such as solar, wind,and water generators in your energy arsenal where possible, especially if you plan to cruise extensively.  Now it’s time to review engine-driven power sources and the contribution they can make toward energy independence. In this issue we’ll focus on alternators powered by auxiliary marine engines.   

Alternators, named for the alternating current they initially produce, actually have internal “rectifying diodes” that change their electrical output from alternating to direct current so it is suitable for battery charging. Charging batteries from an alternator while motoring is a good example of efficient power cogeneration—using one power source to perform multiple tasks. When using the auxiliary engine solely for battery charging, however, care must be taken to ensure that your time, equipment and fuel are being used wisely. The standard type of alternator supplied with most automotive and inboard marine auxiliary engines is fine for use on powerboats, and even on sailboats where time spent aboard is limited or the electrical load is light, but they are not suited for the performance battery charging needed by most cruising sailors. In fact, standard alternators are grossly inefficient—on the order of 1-2{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} total energy conversion—when an auxiliary engine is used just for battery charging (compare this to currently available PV solar panels that are over 16{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} efficient!). To illustrate the point, consider that in the typically battery charging scenario a 25-45 horsepower auxiliary engine is operating to produce an average of about 10-15 amps—about 1/4 horsepower—of electricity! 

Most sailors who invest in an ammeter to read alternator current are shocked to discover that the average output of their standard alternator on an inboard engine is so low. Poor performance is partly due to the type of voltage regulation used on standard alternators, a mediocre compromise between effective charging and protecting batteries from overcharge during long periods of motoring, and partly due to the mediocre design and construction of mass-produced, automotive-type alternators. Actually, it is a good thing that voltage regulators prevent standard alternators from operating even close to their rated capacity for very long, since they simply aren’t made to work that hard.

High-output alternators are reasonably priced (the best power vs. cost ratio of all charging sources) and can make a dramatic difference in battery charging performance.  These heavy-duty units, used in all police and emergency vehicles, are quite different from their standard counterparts, and the difference is not just in rated output.  Because of their high-quality components, close internal tolerances, improved cooling characteristics, higher output at lower engine rpm, and altered voltage regulation, a 100-amp high-output alternator will typically charge batteries 4 to 5 times faster than a 35- to 55-amp standard alternator.

High-output alternators are classified by rated output and case size.  Small case models, direct replacements for almost all standard alternators, are the least expensive and range from about 80- to 150-amp output.  Large-case models, slightly bigger in diameter, are a good bit more expensive and range from about 160- to over 200-amp output.  When comparing units, look at the charging curves showing rated output at various engine rpms.  Small case, 120- to 125-amp units with good output in the low to moderate rpm range are by far the most popular with cruising sailors.  Be advised that most manufacturers rate their alternators’ output when the unit is cold; actual output when the unit is hot is typically about 15{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} less.  And if your high-output alternator is being fitted to a Yanmar engine, you’ll need the “saddle mount” version, which has a double-footed mounting bracket instead of the more typical single foot.

While high-output alternators are a good option for those with inboard engines, many multihulls have outboard engines with notoriously low output from the battery charging circuit (typically in the 8- to 14-amp range), making high-performance, engine-driven charging options more limited.  Several entrepreneurs have attempted to adapt high-output alternators for use with outboard engines with only limited success, and until now the outboard engine manufacturers themselves haven’t taken this issue seriously.  As a result, many multihull sailors with outboard auxiliaries have opted to include a separate engine-driven charger onboard.  The choices here are 1) a portable AC gas-fueled generator (Honda, Kawasaki, etc.) operated on deck and plugged into the shorepower outlet to run a high-capacity AC-to-DC battery charger; or 2) a diesel-fueled DC Charger, which mates a compact diesel engine to a high-output alternator, permanently installed belowdecks.  The first option is less expensive, the second option much better for heavy electrical loads and routine use.  


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.