How long is a piece of string? For example, who knows. Also tragic things happen in a boat's life that might not mean anything at the time. For example, suppose previously in the boat's life they were running at cruise and the engine ate an impellor. Hot exhaust with no water flow could've got the muffler much hotter than it normally gets, yet the alarm went off, they stopped or slowed the boat and shut down the motor before any damage, yet the fiberglass muffler was compromised or weakened. Only to show up later. Or suppose the boat wasn't winterized properly and water froze in the muffler cracking or weakening it. My theory is excessive backpressure. If the exhaust was weakened, it would've cracked the muffler and broken maybe some small pieces off or the main piece would've still been attached at one side. For a hole that size to be blown out completely, I think something was blocking the exhaust exit and a lot of pressure built up. I've started up SF with 8-12" exhaust and watched a 3 gallon buckets worth of leaves and floating crap come out of it, if it's sitting in a corner slip in the marina where the debris settles. A floating basketball, beachball, dead animal, any of that could've floated in there.......or maybe Rcrapps forget his fender wedged up in there! hehehe Fuel is a bit far fetched IMO, the muffler is pretty far back from the engines riser and has water sitting in it all of the time.