by B.J. Porter (Contributing Editor)

Or “why every boat owner should understand a little about electricity.”

Unless your only boat has no lights and no battery, electricity will be a part of your boating experience. Like every part of the boat, the more you understand, the better your experience will be. Knowing a little about how to troubleshoot an outboard or some plumbing fundamentals will improve your boating and save you a money, just like understanding how the electrons flow around your boat and keep your systems running will.

Key to understanding how any electrical power works is Ohm’s Law. We addressed some of this in the article Why Are My Lights Dim a couple of years ago, but we’re going to get into a little more detail now. That older article will be a helpful read if you missed it.

We’re planning to re-visit boat electricity several times this winter, and if you stick us until spring there’s a good chance you’ll have a better idea why your boat sometimes does what it does, and maybe how you can make it do a few things more efficiently.

Fair warning…there will be math. But it’s pretty simple math – nothing you can’t do with a piece of scratch paper or a calculator app.

What is Ohm’s Law?

Ohm’s Law describes the relationship between voltage, current, and resistance. The main formula is very simple:

Voltage = Current X Resistance (or V = IR)

And the corollary power formula is equally simple:

Power = Voltage X Current (W = VI)

That’s it. But…what does it mean and how do you use it?

A Quick Review – Volts, Amps, Ohms, and Watts

If you re-read Why are My Lights Dim, you can skip this. But we can’t talk about electricity without understanding these four important terms and how they’re used.

Volts (abbreviated V) are the measure of electrical potential. The common voltage for boat batteries is 12 Volts, though other voltages are used. 12V is the nominal voltage. The actual voltage of a 12V battery varies with the charge state. But nominal voltage is accurate enough for calculation and planning.

Amperes (A), or Amps, measure the current of electricity through a circuit, like gallons of water through a hose. You may find the amp consumption listed on a device’s information sticker. This is the number of amps consumed by one hour of device use at the stated voltage. Current is abbreviated (I) in formulas, but measured in amps.

Ohms (R) measure resistance to current flow, similar to resistance to water through a hose. A small hose has more resistance to high water flow than a large pipe, and running a lot of amperes through a small wire creates lots of resistance. Any electrical work by a device, whether a bilge pump or a light bulb, creates resistance.

Watts (W) are units of power, independent of current or voltage. So Watts are equivalent anywhere. Manufacturers almost always include Watts used per hour on electrical stickers. A 90 Watt laptop power brick will consume 90 Watts every hour, whether you plug it into 110V or 220V. But the current draw will be different.

Variations on a theme

Reaching way, way back to algebra class and beyond, we can rearrange Ohm’s Law to help us get other important applications.

Since Voltage = Current X Resistance, you can change the formula to:

Current = Voltage / Resistance (I = V/R)

Resistance = Voltage / Current (R = V/I)

And for the power function we can derive:

Voltage = Power / Current (V = W/I)

Current = Power/ Voltage (I = W/V)

But why is this important? Because it allows you to figure out how electrical devices in your boat will affect your battery and wires, and lets you plan around their use and your available power.

And if you are adding or changing devices, these formulas can show you if your wires are safe and if you’ll have enough power on board to run them.

A Practical Example

All the formulas are nice, but they’re abstract. If you are planning any changes in your electrical system or having problems, practical application of these formulas can save you a lot of headaches. Without understanding a problem, it’s easy to fall down an expensive hole, spending money on upgrades and “fixes” that don’t always address the real problem.

To show this, we’ll walk through a boating scenario and step through how, with a little figuring, a less obvious solution presents itself. Spoiler Alert – the solution costs a lot less than the most obvious answer and takes very little work.

A Dark Night at Anchor

A sailor spends his first night at anchor on a new-to-him but older 24-foot boat. He uses a couple of cabin lights for a few hours, turns the anchor light on at dusk, and charges his phone and tablet. He also plugs in a portable 12V cooler for the night, since there is no refrigeration.

The Problem

When he wakes up, the cooler is off, and the lights won’t come on. What went wrong?

To find the answer, we can use a little math to figure out what each electrical device is using, find the big power drains, and see if he needs a bigger battery, a second battery, or if he just can’t bring that cooler along.

The analysis

Looking at each cabin light, we discover they have a 15W incandescent bulb. Small lights, but there are two fittings. The anchor light has a 25W incandescent bulb. And the sticker on the cooler says it consumes 50W at 12V. Research turns up that his iPad Pro 9.7″ consumes 12 Watts to charge, and most cell phones are around 5 Watts.

Calculating the Amp draws for all equipment (Current = Power / Voltage)

For each cabin light: Current (I) = 15W/12V = 1.25 Amps (per hour)
For the anchor light: I = 25W/12V = 2.08 Amps (per hour)
For the cooler: I = 50W/12V = 4.08 Amps (per hour)
Phone & Tablet combined: I = 17W/12V = 1.42 Amps (per hour)

Estimating Amp-hours used overnight (Amp-hours = Amp draw x hours used)

If the cabin lights are on for four hours, and the anchor light is on for 10, that’s a total of 30.8 Amps of power consumed by the lighting in one night.

House Lights: 1.25 Amp x 2 lights x 4 hours = 10.0 Amp-Hours (Ah)
Anchor Light: 2.08 Amps x 10 hours = 20.8 Ah

Total lighting: 20.8 Ah + 10.0 Ah = 30.8 Ah

The cooler does not run nonstop. It cycles on a thermostat as the contents warm up. If it only runs 25% of the time, it’s still drawing about 1 Amp every hour, though it may run more than that.

Cooler 4.08 Amp x 10 hours = 40.8 Ah
But it only cycles on 25% of the time, so estimated consumption is 40.8 x 25% = 10.2 Ah.

Plugged in for 10 hours, that’s at least 10 Ah. With another 4-6 Ah for the phone and tablet charging for a few hours, we’ve used at least 45 Amp-hours of power that we can account for.

Lights + Cooler + Device Charging = 30.8 + 10.2 + 5.0 = 46Ah

A look at the battery

This boat has a typical installation for its size – a single group 24 deep cycle battery with about 40 Amp-hours of usable capacity. This rating is for a brand new and completely charged battery. If it’s older and worn, it may have less.

It’s clear the boat owner is consuming all the usable battery power before morning and leaves the battery discharged below 50%. With only one battery, it may not start the engine the next day.

The Solution

He could install a new battery to get more power, or a second battery for house power. “More power” is the obvious solution, but is it the best one? A new battery with more capacity is bigger, and expensive. It may require changing the battery storage area to fit, and will require new wiring, switches, and other components for a second battery.

Our analysis suggests a better option, because the cooler really isn’t the problem. It’s those incandescent bulbs that are still installed on many older boats.

Replace the incandescent lights with LED bulbs. Replacing 15W incandescent bulbs with LEDs costs about $15 each. Their power draw is 1.5W, or 1/10th the original bulbs, cutting their amp draw to 1 for the night total, from 10. The anchor light bulb is a little more expensive at $25.00, but it draws 2.5W, and uses 2 amps over 10 hours instead of 20.

For each cabin light: I = 1.5W/12V = .125 Amps (for four hours = .5 Ah consumed)
For the anchor light: I = 2.5W/12V = .208 Amps (for ten hours = 2.08Ah)

Investing about $50 in new light bulbs cuts the light power consumption by 90%, leaving 27 more amp-hours for the cooler and phone charging.

No bigger battery upgrade is needed, saving a much more expensive and complicated project. By comparison, a new, slightly larger deep cycle battery could cost close to $300 just for the battery. And a larger capacity Group 27 or Group 31 battery may still not have been enough, though a second battery might.

Where we’re going…

This example (with that bit of math) highlights that understanding your electrical system gives you options to solve problems and expand your system sensibly.

It’s not simple, but it’s not rocket science, either. I’ve heard too many boaters say “Oh, I just can’t understand electricity” and turn the problem over to someone else to solve for them. This is often an expensive and less effective approach.

Our goal is to help boat owners overcome this “too hard” hurdle, and understand enough to get to the best solutions. You don’t need to learn to solder, crimp wires, or write an electrical schematic. But if you’re paying someone to fix your boat, understand what you are paying for and why.

We will cover topics such as

• Developing and understanding your boat’s “power budget.”
• Planning and adding new electrical equipment.
• Basic circuit concepts.
• Elementary troubleshooting.
• Battery and wire sizing.

The topics will develop as we move forward, and I’d love to hear in the comments from anyone who wants any particular topic covered. Be forewarned…I kind of like to talk about boat electricity!