PS Advisor: 04/15/05

Installing DC Inverters
I'm considering the possibility of installing an inverter on board my boat. What would be your counsel to an owner regarding a DIY installation of a DC inverter?

Sam Humphrey
Miami, FL

We'll caution you right from the start that installing a DC inverter isn't for the faint-hearted, but certainly it can be done. Remember that you're dealing with the potential for a deadly electrical shock.

Most units come with clear instructions, and of course each will vary somewhat, so we won't address specific installations, but rather the general errors that are made in this process.

To find out where most boat owners go wrong in this process, we knocked on the doors of several well-respected boat yards in the New England area and asked their electrical installers what common installation faults they encounter with DIY DC inverter installations. We also asked them to render any useful tips about this delicate topic.

Almost everyone we spoke with told us that it is essential to install that little safety sticker that comes with every inverter: "Warning! Electrical Shock Hazard. This electrical system is equipped with a DC to AC power inverter. Disconnect all electrical sources including the inverter's AC and DC inputs before servicing the vessel's electrical system." The safety sticker should be adhered directly to the AC breaker panel. We heard plenty of stories of how service techs have gone down to a vessel to solve some DC wiring problem, unplugged the AC dock cord, and next thing they know they're being bit by an AC voltage as their hand tool inadvertently shorts out a presumably un-energized line. This can be very dangerous.

One knowledgeable marine technician told us to make sure that we use a digital voltmeter that measures AC voltage in RMS values if we wanted to get an accurate reading of the inverter's AC voltage output. Apparently if you measure a modified sine wave inverter's output with a low-cost digital voltmeter (non-RMS capable), the reading that you will observe will be 2 to 5% lower than actual.

Another topic that marine electrical pros feel passionate about is the DC grounding of the inverter's case. It appears that there are a lot of self-installed inverters out there that have not been properly grounded. The chassis ground point on the case of the inverter must be connected to your vessel's DC grounding system as recommended by the ABYC, and as outlined clearly in your inverter's installation manual. Take the ground seriously and connect it with 1/0 or larger marine-grade, green-jacketed battery cable.

A couple of these sources also mentioned that most inverters will not survive a momentary reversal of the input polarity. Therefore, our experts strongly suggest that the battery terminal lugs be clearly marked, in the event the battery bank is removed for storage or maintenance, and then inadvertently lowered back into the battery box 180 degrees backwards with the battery posts facing the wrong way.

And speaking of batteries, the rule of thumb is to buy quality batteries for your inverter bank, and don’t undersize the bank. AGM batteries hold up best under the severe draw and re-charge conditions that an inverter will impose. Remember that inverters are DC-hungry and will under perform if you don't feed them properly.

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Calculating Paint Needs
Over the years, I've wasted a lot of very expensive bottom paint. It seems like just when I get used to how much I need to to buy, I get a new boat (we've had six different ones), and out the window goes my empirical information. There's always some left over in the can, but it doesn't last until the next spring. Is there a way to calculate just how much bottom paint a given boat will need?

Sam R. Thompson
St. Clair Shores, MI

For a sailboat with a short fin keel and spade rudder, use the following formula: Bottom Area = LWL x (B + D) x 0.5. In this case, "B" denotes waterline beam, which usually is less than deck level beam. "D" represents draft. For a full-keeled sailboat: Bottom Area = LWL x (B + D) x 0.75. For a shallow-draft sailboat (or a full-bodied powerboat): Bottom Area = LWL x (B + D).

Using these square foot figures, read the paint can to see what coverage the manufacturer claims. Or, you can divide the square footage by 9 to get square yards, but be advised that the coverage of bottom paint, depending on its thickness, ranges from 35 yards to 55 yards per gallon. This last method isn't really all that precise.

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