Electrical Basics: Solenoids

6 Dec

Solenoid, as an electrical term, covers a lot of ground and can get very complicated, but for boats solenoids are pretty simple. A solenoid, or solenoid relay, is a magnetic switch used for remotely switching power. Anywhere you’ve got a big electrical load that needs to be switched is ripe for a solenoid, especially if the load is in a remote location.

Every engine has a solenoid on the electric starter. This is because the starter requires a lot of amperage, so to actually switch the load to the starter would mean running cables as big as your thumb up to the helm (or the dashboard of your car) where you would have to switch the load with some gigantic knife switch out of a Frankenstein movie. Not very practical.

From the helm or dashboard we actuate a wee switch, the key or starter button, which connects to the starter solenoid with some small wires. The solenoid then does all the heavy connecting for us. Most starter solenoids also engage and disengage the pinion gear, so they do two things at once.

Engine starters. The solenoids are the cylindrical gizmos on the right sides, with the electrical terminals. Incidentally, the solenoid is usually the first thing to give out on a starter, so a faulty starter can often be repaired just by replacing the solenoid. In other words, a starter will usually live through several solenoids. Sometimes you can even replace a solenoid without removing the starter from the engine, but not on typical sailboat installations, where the engine is wedged into a file cabinet.

Another common place for a solenoid is the windlass, which is another big electric motor. We can run all the current for the windlass through the foot switch, but most installations use a solenoid. This is especially true on boats that have remote windlass controls from the cockpit. When we raise the anchor using the windlass we’re not switching the power with our little switch in the cockpit: we’re actuating a solenoid, which is switching the high current.

In this photo of a typical solenoid we see four terminals, the two switching terminals and the two high current terminals. On the two switching terminals we connect positive and negative of ship’s voltage: It doesn’t matter which is which, but one of them, usually the positive, would go to a smaller switch. The two high amperage terminals are our feed and our load. When the solenoid is switched on, the high amperage connection is made. Switch it off and the connection is broken. Or you could connect nothing at all to the high amperage terminals, and just have a little device that goes click-clack-click when you flip the switch.

Starter solenoids and windlass solenoids are probably the two most common uses on boats, but there are many others, and once you know how solenoids work they can solve a lot of problems and save a lot of money by not having to run large gauge wire long distances.

In a recent job on a power boat, the owner had installed the largest stereo system I’ve ever seen on a boat. This system had eight or ten woofers that were each a foot in diameter, plus a whole battery of smaller tweeters and mid-range speakers – in the cockpit, in the cabin, on the bridge – you get the idea. This system was installed by a car stereo guy, not a marine electrician. That’s where I came in.

The installer had wired the whole system directly to one of the ship’s batteries, which was promptly drained because there was no way to turn this system completely off: the gigantic amplifiers drew over three amps, even in sleep mode.

The seamanlike solution would be to power the stereo system from the main distribution panel, where it would be turned on and off with a breaker. A little exploration showed that the entire distribution panel was fed with a couple of ten gauge wires, and the stereo system was fed with four gauge wires (4 gauge is WAY bigger than 10 gauge). I assumed the feed wires to the stereo system actually had to be as big as 4 gauge, and the installer didn’t just use them because they looked cool and blue. To feed the stereo, with its 4 gauge wires, from the main distribution panel, with its 10 gauge wires, would be like trying to supply a water main from a garden hose.

To make it completely right would mean running new, big feeds to the distribution panel, connectors, buses, breakers…much too involved and expensive for the simple problem of turning off a stereo system. Enter the solenoid.

I kept things as the car installer left them, more or less, with the key installation of a solenoid near the battery. The solenoid is now activated by a switch on the main distribution panel, fed by small wires, rather than running all that heavy cabling all over creation.

IMG 0476
The stereo solenoid: The big blue wires are the feeds to the stereo system. The lower one goes directly to the positive terminal on the battery, which is in the foreground. The upper one runs off to the stereo system. The switching terminals have a black wire, which runs directly to the negative terminal on the battery (it was close) and a red wire that runs to a breaker on the main distribution panel that says “Stereo/TV.” The stereo installer put an in-line fuse along the big blue cable. The switching circuit to the solenoid also needs circuit protection, which was provided by the breaker on the panel, in this case.

This article was syndicated from The Adventures of the Vessel Condesa


  1. Steven Gondron

    I change my solenoid my 1994 60hp mercury. Does it matter how the wires are put back i have no trim unless its the wrong one?

  2. Clark Beek

    Hi Bob, If it’s got a control box like one of those pictured this would indicate that either the foot switch/remote switch isn’t getting juice to the tab on the control box for up, or the power lead to the windlass itself isn’t getting juice. When you try to power up can you hear a click, or nothing at all? If you hear the click that means the solenoid is being actuated, but no juice is getting to the windlass. If no click, then the juice isn’t getting to the switching terminals on the control box. On a brand new system we’ll assume nothing is corroded yet, and this is just a connection problem. Not all windlasses work like that: some, like Simpson-Lawrence, I think, reverse the polarity on the two leads to the windlass motor to change the direction, so the control box is doing something different. Other windlasses, like mine, have a positive lead for up, a positive lead for down, and a negative lead, all going to the windlass motor.

  3. Bob Coulter

    Hey!Just installed new Windlass. Think all is correct with installation. However, can only lower the anchor. Hoisting function doesn’t respond. Any suggestions?

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