by Chris Collins
(USA)
Many people are lost when electrical repairs are needed. Experimentation, finding a friend to help, and hiring a professional are usually their only options. Believe it or not, neither rocket science nor magic is needed. With a basic understanding and a few simple tools, we can use logic to find and fix most electrical problems on a boat.
This will be the beginning of a multi-part series that will begin with basic electricity then detail steps to diagnose and repair many issues on a sailboat. With a little learning, you should be able to test your battery, determine how well your charging system is functioning, repair lights, know whether your starter needs replaced, and reduce the fire hazard caused by most shore power wiring. You will not, for instance, be ready to open and repair your VHF radio or your chart plotter. Topic suggestions and questions are invited.
For the next electrical article, I will talk about batteries and charging systems. In this article, I will start with very basic electricity. All subsequent electrical articles will assume you have this basic knowledge! If after reading, things are still fuzzy, do an internet search on “basic electricity”. You will find many articles. There are many more fun things to learn than I will present here.
For our purposes, electricity will be defined as the “flow of electrons.” Electrons flow through a wire just like water flows through a hose. You just can’t see the electrons like you can see the water. We measure the flow of electrons with an ammeter and call it “current.” The unit of measure is the Ampere. When a certain number of electrons flow past a point in one second, the current is said to be one ampere. So an ampere is a rate of flow just like gallons per minute would be a rate flow through a garden hose. To measure amperes, the ammeter must be inserted in the circuit so all the current flows through the meter. Some ammeters are designed to clip over the wire and the meter is activated by the magnetic field created by the current flow.
Voltage is the pressure that pushes the electrons through the wire. Voltage is measured with a volt meter. The unit of measure is the Volt. The test leads are connected across the source or across a load. Loads are usually the devices we wish to power with electricity. A light bulb, motor, and radio are examples of a load. In most cases the boat battery is the source of electricity and a charger keeps it from running down.
All wire and devices that we power have resistance to the flow of electrons or current. Using our water hose example, resistance would be like squeezing the hose or covering some of the holes in the sprinkler. Anytime electrical current is forced through a resistance, heat is generated. Resistance is measured with an Ohmmeter or calculated with Ohm’s Law. The unit of measure is the Ohm. It is doubtful that either of us have a meter sensitive enough to measure the tiny resistance of a large battery cable, but that resistance does exist. By the way, never connect an Ohmmeter when voltage is present as it will likely damage the meter. To measure resistance, take the device out of the circuit before connecting the Ohmmeter.
Since I mentioned heat, I will briefly touch on that. Heat is measured in watts. Watts = Volts x Amps. If you know the voltage and the watts, you can quickly calculate the amps with math.
Speaking of calculating, there is a mathematical relationship between volts, amps, and ohms. It is called Ohm’s law. Volts = Amps x Ohms. If you know any two values and a little math, you can calculate the third value. Everyone should know Ohm’s Law but in truth, you don’t need it to make electrical repairs on your boat.
In review, the things you should understand and remember are as follows. Voltage is the pressure. Amperage is the current or flow of electrons. There will always be some resistance to the flow. As resistance increases the flow will decrease. As resistance lessens, the flow will increase. Bigger wire has less resistance than smaller wire. When current is forced through a resistance, heat is generated.
Chris Collins owns, sails, and maintains a Catalina 30 on El Dorado Lake in Kansas and a 45 foot ketch in Rockport, Texas.
Aug 15, 18 11:59 AM
This simple, if rather tedious, exercise in evaluating your boat electrics is necessary for battery bank sizing but the spreadsheet on this site will make the whole process a great deal easier.
Aug 15, 18 11:35 AM
Calculate your Amperage Requirement with this Daily Current Draw Calculator and use this information in selecting the Appropriate Battery for your Boat
Jul 29, 18 03:29 AM
Wharf: Fiskars Finland Material: GRP Year: 1977 Lead Fin Keel Wheel steering 6 Berths Length: 10,24 m (33ft) Beam: 3,12 m Draft: 1,62 m Weight: 4,5t Displacement: