In one of my beasts (31ft mono this one) I have a 9.9 high trust long shaft in a well near the aft end of the cockpit. Works very well, no fumes bit a little noisy.
The OB is mounted onto a wooden plate which is in a alloy U shaped section each side so it slides up and down. The whole alloy slide/frame is hinged in the bottom so I just lift
the OB up and then it tilts forward just enough to get clearance of about 3" under the prop. Then I have a little door/flap that drops into the hole which is flush underneth. So then I have zero drag from prop, legs or hole. The flap has a small venturie in it so any water that does sneek in, which it does at speed, just drains out.
Judging by my boat a 9.9hp high thrust 4 stroke
would push a 28ft Cat very very nicely. Flat water with a clean bum I cruise
at about 6.5 knots at 1.5 lts/hr. Opened right up 8kts and about 3.5lts an hour. Distance crusing about 4.5-5kts at about 2/3rd of a lt/hr. On a 28ft cat it wouldn't have to work at all so be very very cheap
to run and still leave pleanty of power to punch into yucky stuff.
The key to it all Kelldog is getting and keeping the prop in the water so whatever you do try to get that prop deep. Further aft you put it the harder that will be I'd think.