Having read some ideas on sub safety, this is what I came up with, after nearly loosing my perry submersible. The down side is: you have to experiment to meet your requirments, (time required for system to release) but it's worth it, because the system works very well.
Paint some paper gum glue on to some plastic sheet, with out air bubbles if you can! When the glue has curred, remove the membrane from the plastic sheet, try not to touch the membrane with your fingers; grease from your fingers will stop it working.
Cut out some square section of double sided sticky tape, the size controlls the time, for example: how long it takes for the drop weight to relese. Clean the membrane with alchol, then place the sticky pads to the membrane
Once one side of the membrane is coved, turn it over and repeat the process, making shure that the sticky pads are oposite one another.
next; cut all the pads out, but leaving one mm around all the edges. If one cuts to the edge, the system may fail.
Removing the protective layer from the pads, stick thes between the sub and a drop weight. In my case, the drop weight releases after two hours.
naturally, drop weights are no good, when the model is stuck in weeds, this is what happened with me, but threre is a back-up. The back-up being a bouy, which is made from yellow styrene. Wrapped around this small styrene drum, is 40 feet of fine fishing line; the bouy is released after four hours.
It is recommended, not to make the bouy too large, you do not want poeple walking past, knowing what is on the end.
Finally, fit a small L.E.D light, then you can see, where the model is at night.
From Roger Suitters