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If you want an external GPS alarm, usually the circuit out of the GPS limits the current to 100ma max.

This basically means small piezo horns which pretty much suck in db especially if your anchored and sleeping.

The way around that is use a relay with a coil loading of the GPS max current load on the alarm circuit (usually the yellow wire) or less to switch a much louder and higher current draw horn.

Those relays can be had at Radio Shack and if you have an alarm system for the bilges and engines, it can be tied into that also with the use of some diodes.

I like those 8" round fire bells for effect.

For about 40 to 50 bucks in parts at RS plus the bell you can make a real nice alarm system to handle just about anything.

Trust me, a old fashion door bell is plenty sufficient to get your attention, if it doesn't give you a heart attack first. My boat has one rigged up in the overhead with a button on the flybridge to get the attention of the people warming themselves in the cabin. The label under it says "Beer Bell".