[laminar]

I had an idea for a simple and effective safety system based on a single buoy idea last year but never had the time to test it out. Perhaps it might be a solution:

The concept is that you have a wheel or drum with a circumference of one metre with a crank handle on each side. The rope spools around the drum and the drum is fixed just above the water with buoys (more like boat bumpers) on either side. I envision a thin light aluminum frame to hold it all together. It wouldn't very large at all, no larger than a large buoy. In fact, you could also use an inner tube. The drum itself could be hollow as much as possible so that it is easy to turn. No more than 2-3 kg of bottom weight would be used. It's not for competition, just for training.

Anyway, clutches or carabiners could be used to stop and release the rope. But the concept would be simple. One (or in a real emergency, two) attendants could turn the crank(s) to lift the line. Given the circumference of 1m, the line would likely be able to be lifted at more than 1 m/s. You could choose to make the drum have a larger circumference.

An additional safety protocol is to decide on a timing for the dive (ie. the freediver's expected dive time +10 seconds) and then start pulling up the line at 1 m/s. Then you are sure that if a retrieval is necessary, the freediver won't sink deeper before getting pulled up. The line could be reeled in on every single dive. It would be some work (although I'm sure you could figure out some mechanical advantage with the crank) and it would be best to have a dedicated attendant and safety freediver to make it safer.

I'm sorry I don't have a drawing of it. I'll try to get one up.

Pete