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When checking our blog’s statistics the other day, I came across a website that had linked to a couple of my security-related posts (specifically this one and this one). While looking through their site, I noted that they also had a post discussing alarm systems for boats. You may recall that, once or twice in the past, I have alluded to our super-secret boat alarm system. Up to this point though, I have yet to post any details on it. Working under the assumption that no would-be thieves will be following our blog, I decided to share the details on our KISS companionway alarm.

Note: I can take no credit for this alarm. It was on the boat when we purchased it. We do like it though, and use it every night.

Parts required:

  • Fishing line
  • Tiny on/off toggle switch
  • 12V relay
  • 12V siren

The fishing line is strung across the companionway door, and is all but invisible if you don’t know that it is there. One end is fixed and the other end is attached to the toggle switch. The switch is left in the off position (open), and is aligned such that when someone presses against the fishing line, the toggle is pulled, turning the switch on (closing the switch).

When the switch is closed, the relay is energized, providing power to the siren. In our case, the relay and siren are located inside a cabinet, adjacent to the companionway.

12V relay

12V siren

Does the alarm work? Believe it! Even though no one has attempted to break into our boat, I can say that it works with confidence because both Rebecca and I, having forgotten that the alarm was armed, have set it off. The siren in our system is definitely loud enough to alert anyone on board that someone is at the companionway, and hopefully, also loud enough to scare away any tiefs. The only thing that would make this better is if instead of triggering a siren, the alarm set off a recording of a bunch of dogs aggressively barking!


  1. I do like your alarm, but still believe that keeping intruders OUT of your boat is much better than letting you know that they are now IN your boat. The intruder now has the choice of confronting you versus fleeing. If you “come out swinging”, there will be a nasty confrontation.

    • So what is your alternative? Locking the companionway door? Tacks on the deck? 😉

      • We keep the companionway door closed as soon as we leave the cockpit for the night and go down below.. The door is ventilated with grills. We sleep in the aft cabin anyway, so we wouldn’t benefit from an open companionway door. We do have open dorades and port lights in the aft cabin – none of which would be of any use to an intruder. If we want a hatch open above our head, we restrict its opening with SS wire, so it can only be lifted up about 6 inches. My whole philosophy is to prevent a confrontation…..

        • That’s a valid objective, and I understand your reasoning. Given that we already have bars on all the hatches, I’m not too keen to close the companionway door too. Plus, I haven’t had a good confrontation in a while. I’m getting rusty. 😉

  2. At this point in time we have 3 Aussies on board that are very protective of their boat! We leave today to go get our new to us boat, no their new boat!!!
    Love your alarm system!!

    • By Aussies I am assuming you mean dogs, not people. If so, we have friends who cruise with two dogs that were ALWAYS very noisy whenever people came close to the boat. Always, except for the time that they were boarded in Columbia and robbed. The dogs reportedly sat there and watched the entire thing happen. I think they elaborate on the story on their site:

      That aside, congratulations on the new boat!

  3. Mike,
    Have you ever been boarded?

  4. id rig it to a recording of gunshots………automatic gun…fire……..l;maooooo……{home alone} the movie!!

  5. On a catamaran, I’ve toyed with how to do this across the back of the sugar scoops. You’ve given me some good ideas!

    • Stopping someone from getting onboard a cat is definitely tougher. Just getting on our boat with it’s high freeboard would be a challenge when the ladder is raised (as it is always at night).

  6. An alarm? Barking dogs? Gun shot sounds? The name says it all! You’d better have a loud recording of Kiss Shout it out Loud playing or you ain’t got a hair!

  7. The relay is used so all the current doesn’t go thru the tiny toggle switch.

  8. I love the simplicity of this! You could easily add functionality like triggering a camera or turning on the spreader lights. Thanks for sharing.

  9. Well done Mike!

    I’ve been considering another type of ‘alarm’ for when I move onto my boat. That would be a single small wire run “thru” all the components that could get up and walk away (outboard, kayak, dinghy, etc.) that is a single “hot” circuit. As soon as the circuit is broken the alarm horn starts making noise. The idea being that the ‘tief’ could cut the wire or just rip it out and the noise would commence. Then I realize I am probably making the simple life too complicated…. 🙂

    • Yeah, a bit, but you’d figure that out soon enough once you got out here.

      If your dinghy is raised out of the water at night, it’s 99% safe. If you lock it too, it’s 99.99% safe!

      Thieves don’t want kayaks, they want 9.9-15-25HP outboards.

  10. Thanks for the link. Sometimes I wonder if anyone else out there (out here?) actually looks at the site 🙂
    Any other ideas you would like covered?

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