That's good info. BubbleheadMd. The questions that need to be answered are whether or not "spark protected" devices that were designed for gasoline vapor are good for propane as well. I also wonder whether CO2 or Halon would be displaced, diluted, or mixed with a bilge full of concentrated propane. Propane and CO2 molecules are very close in molecular weight, so who would displace whom? Most of the info. on fire suppression substances is based on after-the-fire-starts scenarios. The issue of intervening to eliminate an explosion before it can occur is not usually discussed and is a good topic to explore.
Smurphny, to answer your questions:
1. Yes, a "spark protected" device good for gasoline vapour will also be fine for propane.
2. Commercial Propane is mix of gases (mostly Propane, Propene, Butane and a bit of Ethyl Mercaptan) and, being not all that much heavier than air, will mix with pretty much everything around including lots of oxygen inside a small, rocking, boat and then go off nicely with a little spark.
3. You REALLY don't want to let off either CO2 or Halon extingushers on a small boat and I actually thought it was illegal in many countries to have them on board. Reason being, you might put out a potential fire - but you'll asphxiate yourself first.
That is impressive and eye opening. I suppose a running engine would tend to keep any gasses at bay in the area of the engine b/c it is always drawing in fresh air.
It's already been pointed out that a diesel engine will run happily on propane up the air intake - often to destruction, since there is no (safe) way to shut down the engine. I guess that would make using the engine a BAD way to keep any gases at bay..
The best, most practical, most normal, most-used-everywhere option is, as mentioned earlier, is to turn off all electrical devices and ventilate the boat.