I get that mono sailors can't get their heads around using outboards on
boats up to 12 metres. In
Australia we have outboards powering cats like the Oram 44 very well.
What is common amongst all the
outboard powered
boats is their light weight (less than about 6000kg) and good sailing characteristics. As has been stated by others, heavy
charter cats need diesels. They are heavy and don't sail as well, especially to windward. Most monos need diesels even when they do sail well - monos are much heavier than the relevant cats and have nowhere to mount the outboards or store
fuel safely. But that doesn't mean that light weight cats need diesels too. They don't.
As for cost - the cost of a
diesel is not limited to the cost of the
engine. as I have stated elsewhere, the cost of a Gori folding prop is roughly $1500 US. Stuffing boxes, bearings, prop shafts,
engine controls, sound
insulation and couplings can push the accessories beyond the cost of an
outboard - before you even buy the
motor.