Homemade Wireless MOB (Man Over Board) Device

What happens if you fall overboard while sailing alone? In this episode, we put several traditional Man Overboard (MOB) safety techniques to the test and build a simple DIY wireless MOB safety system for around €25 that can automatically stop the boat’s engine if you fall into the water.

We test different methods for preventing a man overboard situation, look at ways to improve your chances of survival when single-handed sailing, compare commercial MOB systems, and finally build our own low-cost wireless engine kill switch. All of this with plenty of real-world testing, falls into the water and, of course, a good dose of humour.

Preventing a Man Overboard When Sailing Alone

The first and most important objective when sailing solo is to avoid falling overboard in the first place. We look at several prevention techniques, especially lifelines, safety harnesses and elastic safety tethers with carabiners.

I personally use the following:

✅ Decathlon life jacket
✅ Elastic safety tethers made from a hollow climbing strap with a shorter elastic cord inside and climbing carabiners sewn onto both ends
✅ Flat webbing lifelines
🔵 Ready-made lifelines: https://amzn.to/3HF6K0Q
🔵 Or you can make your own by doubling and sewing this webbing: https://amzn.to/3zJVzSl

Commercial Wireless Man Overboard Systems

Another very effective solution for solo sailing safety is a wireless Man Overboard device that can automatically stop the engine if the skipper falls into the water.

If you prefer a ready-made commercial solution, there are several good options:

🔵 ACR OLAS Guardian — €389: https://amzn.to/4AahaAo
🔵 Fell Marine MOB — €260: https://amzn.to/4gLSrL7

DIY Wireless MOB System for Around €25

But you can also build a simple DIY Man Overboard system yourself for around €25. The basic concept is very similar to commercial wireless MOB systems: if the wireless transmitter is activated when you fall overboard, the system triggers a relay and stops the boat’s engine.

You can see the complete build and testing process in the video. These are the main components used:

🔧 PARTS USED TO BUILD THE DIY MOB SYSTEM

✅ Wireless windlass remote control:
🔵 AliExpress: https://s.click.aliexpress.com/e/_c3b9c5nj
🔵 Amazon: https://amzn.to/3UAUCbN

✅ Relay controlled by the remote:
🔵 AliExpress: https://s.click.aliexpress.com/e/_c4CHVH7j
🔵 Amazon: https://amzn.to/4xLMfZk

✅ A couple of time-delay relays:
🔵 AliExpress: https://s.click.aliexpress.com/e/_c40wwS3X
🔵 Amazon: https://amzn.to/4A6lRee

✅ 12V buzzer:
🔵 AliExpress: https://s.click.aliexpress.com/e/_ANMmep
🔵 Amazon: https://amzn.to/3VlQRXO

✅ Power switch:
🔵 AliExpress: https://s.click.aliexpress.com/e/_98T9FX
🔵 Amazon: https://amzn.to/4igrznz

With these components, you can build a compact wireless MOB safety unit inside a small plastic enclosure or waterproof box.

How the DIY MOB Engine Stop Works

The output from the final relay can be connected in different ways depending on how your engine is stopped.

If your engine is stopped by pressing an electrical stop button, you can use the COM and NO contacts of the relay and connect them in parallel with the existing stop button.

If the engine stops when the ignition circuit is interrupted, you can instead use the COM and NC contacts and route the ignition wire through those relay terminals.

Engines With a Mechanical Stop Lever

If your engine, like mine, does not have an electrical stop system and must be shut down by pulling a mechanical stop lever, you can still automate the process using a 12V linear actuator.

The actuator can be installed so that it pulls the engine stop lever when the MOB system is triggered. The electrical diagram would look something like this:

⚙️ ADDITIONAL PARTS

If your engine cannot be stopped simply by using a switch or cutting the ignition, you may need a few additional components, as I did:

✅ Linear actuator:
🔵 AliExpress: https://s.click.aliexpress.com/e/_c4WBh8QN
🔵 Amazon: https://amzn.to/4imimu2

✅ Automotive relay:
Useful if the actuator is installed away from the rest of the electronics, so you don’t have to run heavy-gauge wiring over a long distance.
🔵 AliExpress: https://s.click.aliexpress.com/e/_AtXW1r
🔵 Amazon: https://amzn.to/4yPg6Az

✅ Automotive fuse holder — I use a 5A fuse:
🔵 AliExpress: https://s.click.aliexpress.com/e/_c2xPogfX
🔵 Amazon: https://amzn.to/4gIgVF0

Installation

The final installation can be adapted to your own engine and electrical system. In my case, the actuator was installed next to the mechanical engine stop lever.

Battery Life and Possible Improvements

With the standard wireless controller, you can expect around 10 hours of battery life. If you plan to use the system regularly, it is a good idea to keep spare batteries available:

https://s.click.aliexpress.com/e/_AMGpXb

Another option is to modify the remote and replace its small 12V battery with an external holder for eight AA batteries:

https://s.click.aliexpress.com/e/_A34fDR

This modification can dramatically increase the operating time and makes the system more practical for long passages and extended solo sailing.

Is a DIY Man Overboard System Worth It?

When sailing alone, falling overboard is one of the most dangerous situations you can face. A harness and lifelines should always be your first line of defence, but a wireless MOB engine kill system adds another important layer of safety.

Whether you choose a commercial solution such as the ACR OLAS Guardian or Fell Marine MOB, or decide to build your own low-cost DIY version, the important thing is to think about what would happen if you suddenly ended up in the water while the boat continued sailing away.

You can see the complete build, installation and real-world tests in the video above.