I'm similarly skeptical about a propeller gap's quenching. The only thing that I can see superior quenching-wise is that the air flow over the gap may be better than in a cylinder gap. But if that was all you need for superior quenching, then an air-blast gap should be the best solution of all.
I would think that a mult-segment cylinder gap is the best at actual quenching, due to the fact that being divided into multiple small arcs, they would be easier to cool and extinguish. But I also believe that multi-segment gaps exhibit higher losses (each gap represents a fixed voltage drop, and the more gaps in series, the greater the total gap voltage drop, and loss).
The benefit of a propeller gap comes about in that it's a rotary gap. If it's a sync gap, it's superior because the bangs can be engineered to be consistent in size and timing, rather than the chaotic mode inherent in static gaps. If it's an async gap, it may be better than a static gap if the power supply is larger than what can be effectively handled with a static gap.
Regards, Gary Lau
MA, USA