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AC Specifications of a Catamaran System

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When laying out our electrical power system we often neglect the AC side things. AC  here refers not to air conditioning, which many modern multihulls choose to include in their list of appliances, but to alternating current, the same type of power found in a house or dockside. In this series of articles we'll review the various options for providing AC power and the system components needed to make your system functional and safe.

Let's begin with the Shorepower supply, since almost most cruising multihulls have provisions for hooking up to dockside power. The simplest type of system has one shorepower inlet, which is usually mounted in a convenient location on the transom or vertical section of the coach roof adjacent to the cockpit. On a catamaran the shorepower inlet can be mounted on one of the hull transoms well above waterline or on port or starboard coach roof, or there can be a shorepower inlet to port and starboard to provide a choice depending on how the boat is aligned with the dock. If possible, reduce the shorepower feed wire run inside the boat by placing the shorepower inlet near where you locate the AC Distribution Panel. Shorepower inlets should have a strain relief feature that holds securely the wire leading into the boat.

A special shorepower cord is used to connect dockside power with the boat's shorepower inlet. Shorepower cords typically have end connectors with a twist-lock feature so they can't be pulled out accidentally. Straight blade connectors are available but not recommended.

Shorepower inlets and cords are available in various amp and voltage ratings: 20A, 30A, and 50A ratings are available for 125VAC systems, and 50A rating for combination 125/250VAC systems. The most common system for boats with moderate AC loads is a 30A 125VAC set-up. Boats with larger loads may need a 50A 125VAC rating, and if the boat has 250VAC loads on board then a 50A 125/250VAC system is needed. Many marinas in desirable cruising grounds have only 30A 125VAC circuits available. In these situations a Reverse Y Adapter is handy for boats with larger AC loads. This device allows you to plug into two 30A 125VAC outlets and draw 125/250VAC power into your shorepower inlet. This system will provide 50A at 125VAC and 30A at 250VAC.

Once inside the boat your AC system should have a Main Shorepower Disconnect Breaker that allows for convenient disconnect of incoming AC power and protects the main incoming circuit. On a 125V system the 2-pole main breaker breaks the hot (ungrounded, black) and neutral (grounded, white) conductors, while on a 250V system it breaks the two hot (ungrounded, black) conductors. ABYC guidelines stipulate that the Main Shorepower Disconnect breaker should be located within 10 ft. (3 meters) of the shorepower inlet. Main Shorepower Disconnect breakers are typically mounted on a panel with LED indication for power on (green or amber) and reverse polarity (red).

At this point in the Shorepower circuit you have some options for additional safety and galvanic corrosion protection. The most cost effective and lightweight method of dealing with stray current that causes galvanic corrosion is to install a Galvanic Isolator. This device breaks the grounding conductor (green wire, not to be confused with the white "grounded conductor" on a 125V system) to prevent stray current from passing along this wire from dockside to the boat's system. These devices are effective at reducing or eliminating galvanic corrosion, but do not provide personal safety protection.

An Isolation Transformer, a more sophisticated (and much heavier and more expensive) device is needed for this. Isolation Transformers electrically isolate the incoming shorepower from the boat's AC system. This isolation provides a high level of protection and peace of mind that for those that spend a lot of their time at marinas with dockside power. In addition, some Isolation Transformers can receive both 125VAC and 250VAC input and provide either 125VAC or 250VAC output to the boat's AC system.

Part II

 A shorepower connection can usually supply a multihull's complete electrical power needs when the boat is shoreside at a marina or other location with utility power available. After leaving the dock, however, the boat's AC loads must be supplied by an on-board power source. This onboard power source can be either an inverter that takes battery power and "inverts" it to alternating current similar to what is found dockside or in a household, or a genset that matches the available power of a diesel or gas engine with the electrical capability of an AC generator in what is essentially a matched "set". In this article we'll focus on inverters and their use on board cruising multihulls. I place emphasis on inverters over gensets since almost all cruising multihulls can benefit from having an inverter on board, whereas relatively few boats have need of a genset.

What Is Available

Inverters are now commonplace, and most sailor's are aware that they come in a variety of power ratings. On one end of the scale are pocket-size models in the 50-300 watt range suitable for powering computers, entertainment appliances (TVs, VCRs, DVDs, stereos, etc.), small power tools and low-draw appliances, and rechargeable devices. At the other end of the spectrum are models in the 2-5kW (2000-5000 watt) range capable of powering the AC needs of a large multihull. If a multihull's AC loads are too large for a single inverter, some models allow multiple units to be connected in series to give the desired power output. Since most large inverters aren't very efficient when powering only small loads, it often makes sense to have a second, smaller inverter for small yet critical loads like computers and other office or navigation equipment.

Size of Boat vs. Size of Inverter

Size of boat doesn't necessarily correspond to size and power rating of an appropriate inverter. I've designed electrical power systems for owners of large multihulls who had relatively small AC power requirements, and in those cases a small to modest-size inverter matched to the loads was incorporated into the design. And some modest-size craft may have inordinately large AC loads, necessitating a larger inverter. For those who are uncertain about what AC will be run onboard and, therefore, what size inverter to get, think about choosing an inverter with a 1500 watt rating. This size unit can handle any load that runs on a standard household circuit, which is typically 15 amps (15 amps x 110 volts = approx. 1,650 watts maximum available power). You might not be able to run some loads simultaneously, but chances are if you are unsure about your AC requirements this size unit will be adequate for your needs.

Do You Need A Battery Charger?

Most larger inverters on the market are "combi" units that serve as inverter and can also serve as the boat's AC charger when either shorepower or genset power are available. Combi units are convenient, but there are some advantages in selecting stand alone inverters and chargers. For one thing, you can select the appropriate rating for both units independent of the other one. For example, you might need only a small inverter but require a charger with a high power rating, or vice versa. There's also the redundancy of two separate pieces of gear; if one goes down and has to be sent somewhere for repair the other item is still available.

Size of Inverter vs. Energy Consumption

Having an inverter with a large power rating on board doesn't necessarily mean you'll have a large total AC energy load. Since power x time = energy, it is apparent that the time an electrical appliance is in use is as much a factor of total energy consumption as its power draw (this is true for both AC and DC loads). Running high power appliances such as a hair dryer, which has a 1500 watt power draw—that 1500 watt inverter mentioned above would handle this load nicely—doesn't create a large total energy draw if the appliance is on only occasionally for a few minutes at a time. In contrast, appliances with small power draws that are on often and for long periods of time can create surprisingly large total energy consumption.

Wiring An Inverter

Wiring the DC side of an inverter is straight forward. Positive and negative cables are brought from the house battery bank to the positive and negative connections on the inverter. A properly sized high-amp circuit breaker or fuse is placed in the positive wire as close to the battery as possible. This protects not only the inverter but also the wire between inverter and battery. Regarding the AC side of things, small pocket inverters typically have built-in AC outlets right on the front face of the device, while larger models are "hard-wired" to an AC load panel that provides circuit protection and distributes power to the various AC circuits. A stand alone inverter has only an "AC Out" connection which is wired to the AC load panel, whereas a combi unit has both an AC Out and an AC In connection. The latter supplies AC power to the unit in its battery charging mode. Combi units also have an internal automatic transfer switch which allows incoming AC power to not only power the charger but also to power all of the circuits in the AC load panel directly.

AC Source Selector Switch as Inverter By-Pass

It's always a good idea to install an AC Source Selector Switch on the AC Out side of a hard-wired inverter. For stand alone inverters the AC Source Selector Switch allows the operator to direct shorepower or genset power, when available, to the AC Load Panel. For combi units this switch serves as an "inverter by-pass", allowing shorepower or genset power to by-pass the inverter if it requires service. AC Source Selector Switches with three positions are available for systems with both inverter, genset and shorepower AC power sources.

Part III

If you have a large AC electrical load and spend time away from dockside shorepower,  you'll probably need to look beyond the capabilities of DC-to-AC inverters and start investigating gen-sets. Even if a properly sized inverter is capable of handling your load, the charging system needed to replace battery drain may well be impractical.

A good example of an AC load too large for most inverter installations is air conditioning. The instantaneous AC current draw from air conditioning ranges from fairly modest to high, depending on the BTU rating of the unit. But it's not the current draw that creates the problem for inverters, which can comfortably handle large loads such as microwave ovens, toasters, and coffee makers. The problem lies in the length of time and the time of day air conditioning is normally operating. Air conditioning runs for long periods of time and is used primarily at night, when charging sources have a tough time replenishing the battery drain. Generally speaking, large AC loads that run for extended periods of time require an engine-driven AC power source, the most common option being a diesel--fueled gen-set.

Gen-sets use two primary pieces of equipment, a rotary AC generator and a matched diesel- or gas-fueled engine. For the sake of efficiency, a gen-set's engine is properly sized to do the job required, usually with little or no wasted capacity, but you can increase the overall system efficiency dramatically by using an inverter along with the gen-set. Use the gen-set to power large or constant AC loads, then use an inverter to power smaller or intermittent AC loads.

Choosing a Gen-Set

Equipment weight and size are important considerations for boaters, especially those with light displacement craft, and cost is a universal concern. Gen-sets can be big, heavy and expensive to purchase and to install, but don't let that deter you from considering one. There are compact units on the market that are reasonably priced and relatively lightweight, ideal for those with modest size boats, and installation can be simplified if you know what you're doing. Once you've accepted that a gen-set is right for your situation, take some time to research the various units on the market.

An ideal gen-set would be lightweight, smooth, quiet, fuel-efficient, powerful, low cost, reliable, and fit in a spare locker. Since it's difficult satisfying all of these requirements in a single gen-set, you'll need to establish an order of importance for your situation. If high power and low cost are your priorities, you may end up looking at models that are heavy, bulky and only moderately quiet. If silent operation, light weight and compact size are the top requirements on your list, you'll undoubtedly have to pay a premium price in terms of $/kW. It's best to determine what characteristics are most important for your situation before you shop around.

Fuel & Engine Type

The first thing to determine is fuel type. It's logical to choose the fuel that your auxiliary engine runs on. If you are choosing between the two, diesels are the marine engine of choice as they tend to last longer and be more fuel efficient, yet they are typically more costly to purchase and repair. As with an auxiliary, proper venting and exhaust is crucial.

Gen-set engines vary in how many cylinders they have. Many of the small, high-speed models use Farymann or Yanmar single-cylinder engines, which are surprisingly balanced and relatively quiet. Yanmar, Kubota and Mitsubishi 2- and 3-cylinder engines are also popular for modest sized gen-sets. In general, three and six cylinder engines tend to be the most quiet and have the least vibration.

RPM Rating

Industry standard gen-sets such as those from Kilo-Pak, Kohler, Northern Lights and Onan are made to run at 1800 rpm (for 60HZ output; 1500 rpm for 50HZ output). Large gen-sets (over 20 kW) using six pole sets in the generator run at 1200 rpm (60HZ) or 1000 rpm (50HZ), while the compact gen-sets from Entec West, Fischer Panda, HFL and Mase are designed for high-speed operation at 3600 rpm (60HZ) or 3000 rpm (50 HZ). Relative newcomers to the field are the compact models from Next Generation Power, which operate at a mid-range speed of 2800 rpm. The 1800 rpm proponents claim lower noise and longer engine life, while the manufacturers of high-speed models claim they've designed their units accordingly and can produce more electrical power for a given weight and volume by using high-speed diesels. Next Generation claims their mid-speed models are as quiet as the 1800 rpm units, without the need for sound enclosures as with the high-speed units. While it's true that gen-set noise varies widely, I feel all engines on board need to be soundproofed if you're going to enjoy your time on the water. Most of the high-speed models use water cooling in both the engine and the electrical generator itself, allowing the gen-set to be completely enclosed in a well insulated, soundproof box.

Power Rating & Efficiency

Power ratings for gen-sets begin in the 2.5 to 3 kW range, and continue up to 20 kW or more. The most common gen-sets for pleasure craft are in the 4 to 12 kW range. Gen-sets achieve their rated power at a given engine speed necessary to produce the required AC frequency; engine speed remains fairly constant, regardless of electrical load. Even though the actual load on the engine is related to how much electricity is being used, running a gen-set to satisfy a small AC load is inherently inefficient. Some gen-set manufacturers (notably Balmar) have embraced what is known as VST (variable speed technology). Gen-sets with VST adjust their speed according to electrical demand, while maintaining the correct AC frequency and waveform. These models cost more, so you'll have to decide if you can afford the extra efficiency.

Independent Energy GuideKevin 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. Sailors Multihull Guide


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