I've read conflicting information on how often SEL airplanes have to
land 'right now' because of engine problems. In 3000 hours, most
behind an IO 360, we never have had that kind of emergency, but have
landed because of alternater failure, having a bank of spark plugs
fail, vacuum pump failure, those sorts of things. Someone wrote, and I
don't remember if it was supported by hard data, that on average an
unavoidable unplanned landing might be as often as every 1500 hours or
so. Do you know? Information like that would allow a more reasonable
estimate of the risks associated with long overwater (or night IMC IRF
-- we do that a lot) flights.