For at least 20 years, I have kept a little black book on my desk: Pocket Ref by Thomas J. Glover. This pocket-size book contains a wealth of useful and useless data and formulas. Need the airport code for Kalamazoo, Michigan? It’s AZO (elevation 874 feet, by the way). Want to calculate pound-feet of torque from a known horsepower and rpm? This book can help you. Need the area for a polygram? No problem, Glover has you covered. Over the years a few formulas have stood out as particularly useful when reasoning my way through a boat-related question.
WATTS = VOLTS x AMPS
Electricity seems particularly baffling to most new cruisers. This simple equation can answer many of the questions that come up. To understand this formula, let’s use the analogy of a pipe full of water. The voltage can be thought of as the water pressure, while the amps can be likened to the amount of water flowing through the pipe. Watts are the energy output from the pipe. While a physicist would rebel at this analogy, it does provide a helpful description. This simple calculation can help unravel a number of onboard mysteries.
Let’s begin with a basic application. In your house, when you turn on a lamp with a 60-watt bulb, how many amps are used? We fill in the known information as follows:
60 watts = 120 volts x ? amps
Answer: 60/120 = .5 amps
The answer in this example is .5 amps. In each case, if we know two of the three variables, the unknown can be determined. Now let’s imagine that for the same situation we know the house voltage and the amps drawn and want to know the watts:
? watts = 120 volts x .5 amps
Answer: 120 x .5 = 60 watts
Now let’s turn the example into a meaningful question. You are at anchor and would like to use the microwave, which draws 1,500 watts. You have a choice: start the generator or use the inverter. When running the generator, the formula works out this way:
1,500 watts = 120 volts x ? amps
Answer: 1,500/120 = 12.5 amps
By converting DC battery-bank power for 120-volt AC appliances, inverters greatly improve life aboard. Inverters make it possible to use AC appliances like coffeemakers, computers and cellphone chargers, and much more, without the need for the generator or shorepower. Using the inverter comes at a price, however—the loss of amp hours from the house batteries. Now you want to know, if I use my inverter to power the microwave, how many amps will be drawn from the batteries?
1,500 watts = 12 volts x ? amps
Answer: 1,500/12 = 125 amps
Keep in mind that the amperage draw is referenced per hour of use. Even so, 30 minutes of use would consume 62.5 amps—a significant sum for the house bank on a modest-size cruising boat. In the interest of clarity, we are leaving out more complicated considerations, such as resistance in the wiring and efficiency lost by converting from one voltage to another. Sometimes it is better to see the forest than the trees. This formula can also help you work your way through questions about generators, which are usually referenced by their output in watts. A 5kW (kilowatts) generator produces about 5,000 watts. If the generator is set up for 120-volt power, then we can easily calculate how many amps it can handle:
5,000 watts = 120 volts x ? amps
Answer: 5,000/120 = 41 amps
The reality is that most generators are rated for about 80 percent of the maximum output, which reduces the available watts to 4,000 and the amperage to just over 30. In other words, if a 30-amp shorepower cord can handle your system demands, then a 5kW generator can do the same.
Boat owners often ask about running an air conditioner off an inverter. Let’s look at the math. Air-conditioning units are rated in BTU (British thermal units) and a quick catalog check for a 16,000-Btu unit shows a current draw of 12 amps at 120 volts. Knowing two of the three variables, we can convert the energy draw to watts:
Watts = 120 volts x 12 amps
Answer: 120 x 12 = 1,440 watts
Now we can answer our question: What would happen if I ran the air conditioner through an inverter, drawing power from my 12-volt DC battery bank?
1,440 watts = 12 volts x ? amps
Answer: 1,440/12 = 120 amps
Unlike the microwave, the air conditioner will be running for hours. Ignoring, for this discussion, issues such as higher start-up loads when the compressor kicks in, and lower loads when it cycles off, we can estimate that running the unit for eight hours will consume almost 1,000 amp hours of battery capacity, and that is typically a deal killer.
Occasionally we want to determine the capacity of a tank. While modern fuel tanks are stamped with useful information such as material, thickness and capacity, older tanks are sometimes a mystery. Perhaps there is a holding tank and no one knows the capacity for sure. If the tank is rectangular, the problem is easily solved. Measure the length x width x height in inches to determine cubic inches. From there it is simple division if you know this formula:
Cubic inches/231 = gallons
If the tank is 24 x 20 x 32 inches, the tank has 15,360 cubic inches of volume. Divide by 231 and you will find that the tank will hold 66.5 gallons of liquid. If the tank happens to be cylindrical, Glover will provide the formula for that calculation as well:
π x r2 x length
If you are off cruising and find yourself needing a new exhaust system, knowing this formula can be critical. You might find yourself depending upon a mechanic you don’t know and who lacks the experience and knowledge that inspire confidence. Matching the water lift muffler to the hose volume is critical and must be calculated accurately. Here’s why: When you shut down the engine, water in the exhaust hose runs downhill from the high point back to the engine. The muffler must be sized so that it can hold the volume of water in the hose without allowing it to back up into the engine. You can ensure that you or your mechanic are installing the correct muffler by applying the formula. Let’s assume that you have an 8-foot run of 4-inch diameter (2-inch radius) exhaust hose. The volume can be calculated this way:
3.14 x 22 x (8 x 12 inches) = 1,206 cubic inches
The mechanic has selected a lift muffler with a 12-inch diameter (6-inch radius) and 12 inches tall. Is it sized properly? We know that we need at least 1,206 cubic inches.
3.14 x 62 x 12 inches = 1,356 cubic inches
The calculations tell us that the muffler can handle the volume of water contained by the exhaust hose. Of course you will want to check the engine installation guidelines for other parameters.
AND MUCH MORE
Pocket Ref can answer countless other questions. Are you wondering about the wind speed? Page 209 tells you that “fairly frequent whitecaps” appear with wind in the 11- to 16-knot range. Not sure about the proper word for the letter J on the VHF? Check page 158, it’s Juliet. Having a hard time drilling into aluminum? Page 390 tells us that kerosene or lard oil will work in a pinch. All of this information, of course, can be found on your smartphone, but with the Pocket Ref, the connection is more reliable, the information more concise, and for some unknown reason, the feeling of self-reliance much more satisfying.