Has anybody ever heard of the Microscream?
It’s a little gizmo that the airplane guys use to notify themselves that the LIPo battery in the model is at the cut-off level. See RCGroups Link for more information.
I think this would be nice to have in a model yacht because it would stay protected inside the hull and it emits a loud sound when the battery gets to the cutoff level, with enough power to get back to the dock.
It’s very light, only a few grams( 3 grams) total, so you could even put it inside your Footy for endurance sailing 
The circuit is designed for LiPo cells, but it can be modified for NiCd/NiMH cells.
The photos below show the circuit, some built on a PCB, one on a scale, and one with an L.E.D. on it.
thx
Ummm - ever been around a group of people when one of the group’s cell phone begins to ring?
Suddenly everyone is looking around, or at their phones, to see if it’s theirs that is ringing. Somehow I see the same scenario on the water. The LED makes a lot of sense 
Somebody had one that chirps like a bird.
The original design just buzzes, so it would be LOUD and obvious.
Not if used by a lot of sailors with 14 boats rounding the far, windward mark !
" Is that yours, Frank?"
“Naw - it must be Harry’s”
“Nope - not mine - maybe Jerry’s”
“Crap, it was mine all along ! Who is in charge of the crash/rescue boat today?”
You have the option of adding an L.E.D. for visibility too.
Most people use theose annoying ‘ringtones’ or music things, so it should be fairly obvious which phone was ringing.
Ah, a very handy tool when on port tack and not giving way to the starboard boat!
I bet you could activate the alarm with a radio function!
You know, the Corinthian spirit!!
Just kidding, we have these on our RC sailplanes and combat foamies.
The most fun is when several combat foamies are screaming back and forth with alarms going during the heat of battle, should be a hard hat area as these are going in excess of 70 MPH at times!
If anyone lands, they wussed out, last one flying!
Clouds of dust after a thud and you are still flying…priceless!
Now one of our big bucks glass ships is flying, time to nurse her down safely and everyone gets outa the way!
neat concept, but i’ve never even seen my lipo’s get close to being tired…even after several hours on a windy day…
look easy enough to build though…
Marc
This might help. I haven’t been able to check it yet.
The new version of the RMG winch has the ability to sense battery voltage and stop the winch (at a broad reach position) while enabling voltage to the rudder servo at a preset voltage designed to protect LiPo’s from over discharging.
That’s fine if you have an RMG winch, but will it notify you with a sound or light so you can get back to the dock to change batteries? You will notice that the winch stops functioning, which is fine as long as you know it was supposed to do that instead of something else like the radio getting wet.
For the rest of us with opaque deck patches or hatch covers, an audible voltage monitor with sound is a good thing, specially if someone can design a circuit board for something like a microscream with a chart for the resistor values for different cutoff voltages. The best thing about the device is that it can be used with any kind of battery. It just monitors the voltage. The first use was of the microscream was for the LiPo batteries, tho, but the circuit can be tailored for other voltages and output or notification styles, like L.E.D. and/or sound, and even custom sounds, like pulse or chirp. That way you shouldn’t have a problem with everyone thinking it was their boat making the sound, like Dick’s telephone example.