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Author Topic:   Generating 115-VAC Power On-Board
lucas posted 01-10-2006 05:30 PM ET (US)   Profile for lucas   Send Email to lucas  
I just took ownership of a Boston Whaler 27-foot boat. It has a converter that looks very tired and rusty. I am new to this type of system . When I am at anchor will my refrigerator work?

There are outlets in the bathroom for a hairdryer, will that work while i am at anchor as well or will I need to be hooked up to shore power for it to work?

What is an inverter and should I put one in?


Chuck Tribolet posted 01-10-2006 06:17 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
An inverter converts DC (typically 12V) to something
resembling AC (typically 110V or someimes 220V). They come in
three types:

square wave. It's not real sine wave AC, but a square wave.
These inverters are getting rare, but of of that "converter"
is really an inverter it's old enough that it might be
square wave.

modified sine wave. This should really be called modified
square wave. It's square, but as it pass 0V, it stays there
for a bit. Electronics are much happier with this than with
a true square wave. Most contemporary inverters are modified sine

True sine wave. This is real AC. These were quite rare five
years ago, but getting more common, but pricey. I expect the
prices to spiral down over the next few years.

Whether the outlets and fridge will work when not on shore
power will depend on the capacity of the inverter and the


rtk posted 01-10-2006 06:17 PM ET (US)     Profile for rtk  Send Email to rtk     
The refrigerator may work at anchor, if it is a dual voltage unit (12 volt/ 110 volt). If there is a breaker on the DC panel for a refrigerator, it is likely a dual voltage refrigerator. If it is not a dual voltage unit, you will need a 110 volt generator or an inverter to power the unit at anchor.

The outlets in the head are likely on the 110 volt circuit. So you will have to be plugged in to the dock to use it unless you have a 110 volt generator or an inverter.

An inverter will convert 12 volt dc to 110 volt ac. You will need a large battery bank to supply the inverter for high amperage draw accessories like air conditioning, ovens, and even hair dryers.


Jerry Townsend posted 01-10-2006 08:24 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
lucas - be advised that operating the refrigerator off the battery takes a lot of power - hence battery capacity. While on shore power or operating the frig with the engine running does not cause a problem.

Also, the square wave inverters are the easiest and least costly inverters to make - and hence, are the most prevalent inverters found. But they are also pretty hard on electric motors - so beware.

If possible, use a true sine wave inverter - costs a bit more, but is much easier on equipment. --- Jerry/Idaho

Chuck Tribolet posted 01-10-2006 09:13 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Jerry, true square wave inverters are very rare, at least on
the new market today. Magic microchips make it really easy
to make a modified sine wave inverter, and are driving down
the price of sine wave inverters. For example, the
2005 West catalog (mmm, we should get the 2006 catalog RSN),
lists 13 modified sine wave inverters and 11 sine wave
inverters. NO square wave inverters.


jimh posted 01-11-2006 01:34 AM ET (US)     Profile for jimh  Send Email to jimh     
Typically a refrigeration compressor motor and a hair dryer are designed to be operated from 115-volt alternating current.

To power devices which need 115-volt alternating current (115-VAC) when away from the dock, you will have to generate your own 115-VAC power. There are two methods in common use:

--use DC power from your house battery to power an inverter and create AC current.

--use a motor-driven generator to create AC current.

Generating 115-VAC current from a small motor-powered generator is the usual solution when more than 1,000 watts of power is needed.

Generating AC current from a DC battery source is generally avoided for high power loads because:

--it is not very efficient;
--the AC current generated often has a square-wave waveform which can cause damage to AC motors. (A refrigeration compressor is typically run from an AC motor.)

To supply current to run refrigeration from DC power will probably be beyond the capability of any battery and inverter combination whose cost and complexity would be reasonable. At that level of power consumption it is more effective to have a separate AC generator. Even if you were to invest in an excellent quality high-power inverter, you would also need to invest in a very expensive bank of batteries to be able to operate for long periods of time.

lucas posted 01-11-2006 08:44 AM ET (US)     Profile for lucas  Send Email to lucas     
Jim and others,
Thanks for the information. So I have learned that with the setup I have now, consisting of simply a converter, I can only run my refrigerator when I am connected to shore power. If that is the case and I have the refrigerator full of perishable goods, then when I leave the shorepower to go for a cruise, the refrigerator will not work. Is that correct?

In addition, is the original converter on the boat also a battery charger?

Chuck Tribolet posted 01-11-2006 09:22 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
I'm still not sure what you mean by a "converter". It would
be helpful if you would post the manufacturer and model number.


jimh posted 01-11-2006 09:52 AM ET (US)     Profile for jimh  Send Email to jimh     
Lucas--It really is not possible for me to know precisely how your boat is wired and configured. Marine refrigeration units are available in several designs:

--115 VAC powered, just like a home refrigerator
--12 VDC powered
--mechanically powered from a belt-driven compressor on an engine

Arrangements of engine driven battery chargers, 115 VAC driven battery chargers, and DC to AC inverters can be quite varied. There is no way to know for sure what is on your boat.

Moe posted 01-14-2006 11:31 AM ET (US)     Profile for Moe  Send Email to Moe     
A CONverter inputs 120VAC and produces a DC voltage capable of powering so-called "12 volt" appliances. These usually include lights, ventilation fans, drinking water and bilge water pumps, as well as "12 volt" electronics. This DC voltage also CHARGES the batteries. On a single-stage converter, it is usually in the range of 13.6-13.8 volts DC, with a current output capability of 40-80 amps DC. There are also converters that can provide 3-stage charging.

An INverter inputs 12 volts DC and produces 120VAC, either a modified or "true" (smooth) sine wave. Because they are less than 100% efficient, the inverter takes more than 10 times the current of 12VDC to provide a particular current of 120VAC. A short rule of thumb is to divide the wattage on the output by 10 to find the 12 volt input current. For example, an inverter putting out only 500W of 120VAC (a bit over 4 amps), is typically drawing 50 amps from the batteries. I use the 500 watt/50 amp example, because it would equal 25 amps being drawn from each of two house batteries in parallel.

Inverters are hard on battery capacity, which goes down exponentially as the current goes up. You can see this with the Reserve Capacity Rating, which are the number of minutes from fully charged to fully discharged (defined as 10.5 volts) at a current of 25 amps. A typical Group 27 100 amp-hour battery (20 hours at a 5 amp draw) has a reserve capacity of 180 minutes (3 hours), not 100 amp-hours/25 amps = 4 hours. So its capacity at 25 amps is 75 amp-hours, not the 100 amp-hours achieved at a 5 amp draw. The published amp-hour rating is usually the capacity at a current draw that will fully discharge a battery over 20 hours. A typical Group 24 80 amp-hour battery only has a Reserve Capacity of around 150 minutes or 2.5 hours x 25 amps = 62.5 amp-hours. This reduction in capacity is due to something called the Peukert Effect, and you can calculate the Peukert Capacity and Exponent for any battery using its amp-hour and reserve capacity ratings. The end result is that amps drawn at a high rate take more out of the battery than those drawn at a low rate.

If the refrigerator is an absorbtion type, it cools by using heat, which isn't a significant current at 120VAC. If an absorbtion refrigerator does have a 12VDC heater, it is usually a much smaller one, that often makes the refrigerator marginal, barely able to hold a cool temperature. An absorbtion refrigertator with 120VAC power can cool down a hot refrigertator and its contents, but typically uses 300 watts/120 volts=2.5A. Powering that with an inverter would draw about 30 amps from the batteries, a higher than practical continuous load.

If the refrigerator is a small one with one of the super-efficient compressors designed for 12VDC use, it draws about 3.5 amps from the batteries, and cycles on and off, using anywhere from 20-30 amp-hours every 24 hours. These often come with a small converter for use with 120VAC, and many of these auto-switch.

When operating off batteries, you'll get much longer life out of them if you don't discharge them below 50% charge. Further, their charge rate isn't linear, and the last 20% of charge from 80% to 100% takes a VERY long generator or alternator run time, as long or longer as it would take to charge from 20% to 80%. While you might have near 100% charge after leaving the dock from an overnight of charging off shorepower, you might not run an outboard long enough per day, and definitely wouldn't want to run a portable generator, long enough to reach 100% charge. Those of us who use the latter when "boondocking" with RVs typically shut them off at about 80-85% charge, so we have at best 1/3 of the batteries' rated amp-hour capacity as usable, at a low discharge rate.

Because of the extremely high cost in amp-hours, we don't use battery power to make heat, when propane, butane, CNG, kerosene, or alcohol is available, to boil water for coffee on a stove, rather than use a 120VAC coffeemaker with an inverter, for example. Our ideal setup is to equip as much as possible with 12VDC-native versions of things, and let the CONverter power them while it's charging the batteries.

In summary, CONverters CHARGE batteries, INverters heavily DISCHARGE batteries.


Moe posted 01-14-2006 11:57 AM ET (US)     Profile for Moe  Send Email to Moe     
Addition to the 1st paragraph above.

A CONverter is electrically just a battery charger with very good filtering. Earlier converters had an unfiltered output for the battery and a filtered one for the loads. Current models typically have one filtered output for both.


andygere posted 01-16-2006 08:17 PM ET (US)     Profile for andygere  Send Email to andygere     
Gas absorption refrigerators are popular in campers and RV's, and I have seen a few larger sailing yachts equipped with them. These are typically 3-way units that run on AC, DC or propane. Norcold is one of the more popular manufacturers. Since a propane burner is used to create the heat needed for the gas absorption process to work, these typically are not operated unless they are relatively level (say up to 6 degrees or so), and I'd guess the gas mode would only be used while on a sheltered mooring. Using flammable, heavier than air gas in any boat requires extreme caution, and may not be a good application in a 27 foot Whaler.

A better idea may be to put some block ice in the refrigerator, and while under power use the refrigerator system to keep things cold and extend the life of the ice. 1 gallon milk jugs make ideal block ice containers that won't melt all over your food.

davej14 posted 01-16-2006 09:12 PM ET (US)     Profile for davej14  Send Email to davej14     

Excellent review, thank you.

Moe posted 01-17-2006 11:46 AM ET (US)     Profile for Moe  Send Email to Moe     
I agree with Andy on the ice, but also don't overlook the 1/2 gallon milk jugs. We found that with the dimensions of our coolers, they allow us to get more ice in, while leaving more room for food and drinks. Of course, this depends on your cooler dimensions.


handn posted 01-18-2006 10:36 AM ET (US)     Profile for handn  Send Email to handn     
Moe has all his facts right.
My 305 Conquest has a small dual voltage compressor type refrigerator. I will take Moe's word for it when he says that this refrigerator draws only 20-30 amp hours per day to run. When cruising, I have run the refrigerator 48 hours straight on 2 house batteries and had enough left to start the starboard engine.
The boat will overnight easily running the refrigerator, lights, stereo, pump and the other 12 volt appliances and start easily the next morning.
I have a generator, but if I were buying the boat over again, I would consider doing without. The generator with air conditioning and an electric hot water heater is expensive, adds weight, maintenance costs and mechanical complexity and isn't necessary unless you want to run the air, have hot water, use the microwave or dry your hair.
No matter how hot it is, we prefer to sit on the deck and let the wind blow dry our hair. We shower during the hot part of the day, usually after a swim, with the fresh water washdown. The water from the washdown is at least as warm as the water in the sea.
Without the generator, the boat is equiped with a single propane fueled stove burner. We usually cook with a stainless steel kettle grill that sticks in one of the rod holders.

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