Small-Boat Electricity Primer

If you’ve been hesitant to deal with electrical stuff on your boat, hopefully this column will get you off the pier

by Jerry Culik

Some time ago, when life was a bit simpler and battery voltages were either 6 or 12 volts, the editor suggested a column on basic electricity for small boaters. Now, with the ever-increasing sales of electric outboards and advanced-technology batteries, voltages and currents are getting seriously dangerous. If you’ve been hesitant to deal with electrical stuff on your boat, hopefully this column will get you off the pier. And if you’ve already messed around with wiring and things that can spark, maybe you’ll still get something out of it.

Before getting involved with any high power issues, let’s define a few concepts and terms and then focus on “traditional” 12-volt electrical systems. Voltage (V) is what makes current (I) flow. Think of water running downhill…the steeper the hill—the higher the voltage—the faster the water and the higher the electrical current will flow. On many small and large boats, the usual voltage source might be a 12-volt sealed lead-acid (SLA) battery. And we talk about direct current (DC), in contrast to alternating current (AC) – what we get out of the wall outlets. A DC voltage source has both a positive and a negative side—current goes out one terminal (by convention, the positive one) and into the other. The negative side of the battery is usually connected to the “common” or grounded side of the boat wiring (and ABYC’s E-11 wiring standard – “AC and DC Electrical Systems on Boats”—actually requires it). The battery’s positive terminal is typically connected to the “hot” or electrified side of a boat’s circuits.

Energy (E) stored in the battery is used to do something useful (“work”) at a load, like turning a motor or lighting a light. Current, measured in amps (A), is drawn from our voltage source, the battery. The amount of power (P), measured in watts (W), is simply the voltage at the load (e.g., a light) times the current into it: volts times amps (or P = V x I). Just to be clear, though, the current goes through, not just into, the loads. To do useful work, that current must return to the battery, which leads to the concept of a circuit. The amount of energy used, or work done, is proportional to the power and to the time (that is, E = P x time). More power equals more light. And the longer the light is on, the more energy from the battery is consumed.

There are a couple more concepts that we’ll run across. One is resistance ® to current flow, which is measured in ohms (ꭥ). Ohm’s Law, drilled into me back in physics class, states simply that the voltage across a resistance is equal to the current through the resistance times the resistance (V = I x R). Or conversely, that the resistance is equal to the voltage divided by the current (R = V / I). Therefore, for a given voltage, if the resistance should double, the current will drop by half (I = V / R). This last statement explains why a chafed wire or that corroded connection on the battery terminal – increasing resistance – causes my lights to flicker, dim, and eventually go out.

Now while voltage is equal to current time resistance (V = I x R), the power consumed by a motor or a light is equal to the current times the voltage (P = I x V). And to help us remember how Ohm’s Law works, there are a couple of useful voltage and power “pyramids.” You put your finger on one spot, and you get the relationship between the other two. For example, I = V / R and P = V x I.

By using a little math, we can figure out that the power consumed by a resistive load is the current, squared, times the value of its resistance (that is, P = I x I x R). Alternatively, power is equal to the voltage across the resistor, squared, divided by the resistance (P = V x V / R).

A battery’s storage capacity is typically shown as amp-hours (Ahr) – the length of time, in hours, that a battery will provide a given number of amps of current. For example, a battery that is rated for 50 Ahr might (theoretically) supply 10 amps of current for five hours, or 2 amps of current for 25 hours. The energy capacity of a battery is simply its amp-hour rating times its rated voltage, in watt-hours (W-hr) or kilowatt-hours (kW-hr). And just to wrap things up, the energy used by a load, also given in watt-hours, is proportional to the power into it (in watts) and the time (E = P x t). OK, that’s enough electrical theory…now moving on to practical stuff.

The materials and tools needed to complete the electrical tasks on a small boat are pretty basic. Wire of the appropriate size—and color (see Ancor’s Marine Color Code); a tool to cut the wire; a tool to strip the insulation off the wire ends; and a tool to “crimp” connectors onto the stripped wire ends. Add a cheap meter to measure voltage, current, and resistance, and you’ve got the basic toolkit.