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Any cheap alarm system is not ruggedised for marine use. That means hatch sensors are not really appropriate (but see below). That indicated that the PIR (motion) detector which can be mounted well into the cabin would be the more appropriate. But would it be set off by the changing sun coming in through the windows and differentially heating the cabin as the boat swings on the mooring? To reduce this risk, the PIR sensor was upgraded to a pet-
It may also be possible to position both parts of the door sensor on the main hatch with Velcro just before locking up. In this way it will not normally be exposed to weather. This is probably not necessary if the PIR (above) works.
The control panel can be located in any locker (so long as it can send/receive gsm signals). If it is located in a locker, the burglar will not see it, but the microphone and speaker will not work too well. But if the control panel is visible, then the burglar might smash it before it can dial a number. One possible compromise would be to make it visible, but locate it among other instruments and radios, but also suppress the audio alarm (remember, I have the advantage that there is already another independent audio alarm in a different location). Then the burglar will not be directed to which is the alarm instrument, but the speaker and microphone will work, so it will be easy to hear what is going on in the cabin.
The advice is that the PIR should look across the line of advance of the intruder (i.e. Across the boat). But it may be easier to look along the cabin. This is where the Velcro attachment will score well. We can try both
We are discussing the possibility of using wire loops to sensors in waterproof boxes attached to the deck (or below deck) to secure valuable deck items such as wind vane self steering and liferafts. Further development of these issues is subject to successfully proving the main system. But, in principle, such frills are easy and relatively cheap to add.