[wilf leek]

Denny - I'm not an electronics expert, but I think you have to size the diode to allow for the full running current the fan draws, plus some for the start up surge?? That's assuming the diode is to be connected in series with the feed.
you don't need a particularly high inverse voltage rating - after all it will only see 12V, but it is going to have to be rated at 25 Amps minimum, unless I'm missing something??

Maybe the thing should actually be connected across the +ve and -ve terminals on the fan itself (obviously not so it shows up as a "short" to the feed, then it would only "short circuit" the fan motor when it acts as a generator?
Hmm - not sure I'm helping here, think I'll crawl back under my stone.