Just finished my "Mercury Switch Unit" for the 15 sec timer Bernard helped me with awhile back, (that will drive the siren) that all, will become, my "Boat Burglar Alarm.
Is kinda lacking in quality, but, will work.
The objective is to incorporate two mercury switches (SW 2/SW 3), adjustable, for setting sensitivity.
The principle is to click the switch (SW 4) to "set" and adjust one side until the LED goes out, meaning the mercury is not touching the contacts, then, adjust the opposite side till LED comes on, then back it off till the LED goes out, then, it is set and one clicks the switch back to "on" and it's ready.
Still got to figure out the installation, etc.......

 

Is this to keep people from grabbing stuff out of the boat, or from taking the entire boat?

If it is for boat theft, have the circuit keep a Normally Open relay closed across the low side of the ignition coil.

Hack up a cheap IR Remote for instant arm/disarm if you point the remote, TV remote, any kind of pulsed IR source, at the right inconspicuous spot in a shaded area.

 

Both.
Hopefully, if they start getting in the boat (to steal the batteries) it will rock enough to set the alarm off, and, of course if they're trying to steal it, etc....
I have thought of also incorporating a proximity unit, such as found in bathrooms too. Will evaluate this one to see just how effective it will be.
Then again, it's very easy to reach into my boat and grab the rods without having to actually get in it.
It's just an effort in that direction, plus, a neat little project to do.
Will consider your suggestions.......

 

Passive IR sensors are getting cheap ($15).

Useful if most of the scumbags, err, I mean "unknown borrowers" visited in the evening/dark hours.

Put a few bare wires along the top/bottom/outside of the fishing rod holders, so a quick grope would hit at least one, but open enough for easy access. Use a capacitance contact switch (basically a JFET) would set off the alarm as well.

Same concept for battery holders.

Use a Very Loud siren and maybe an LED based Strobe Light so people would look in the direction of the sound, and see the "shopper".

I don't suppose you have any flashbangs laying around... (j/k)

 

Yep, she sits out by the side of the house, and that's where my main concern lies. Got some crack-heads living across the street, etc.
I do have a light that comes on automatically, when a cat/wind/rain and anything else comes by.
Your suggestion of the wires will have to be passed on.
I'll know better how to further proceed once I get this one installed and see how it's going to perform.
Big thing is, neighbor's house is ten feet away, right where their three little girl's bedroom is, so, absolutely can't have siren going off at 3am unless is a real deal, etc.
Here's a pic of my baby, the fellow sitting in her is a friend of mine.......

 

Thanks for the pictures and follow-up, neat board.

 

Nice boat!

A beam/sensor probably wouldn't work due to saltwater spray fuzzing everything up.

Maybe for fun, take the guts out of a disposable camera and have the flash go off when the alarm goes off, so they think their picture was taken.