Is the stern drive unit a reduction gearbox ?
IE If the motor revs at 2000rpm will the prop on the stern drive rev at 2000 rpm?
regards from South Africa, 24 Fly Bridge

Yes it is.
Common reductions on older single prop units (250,270,280) are :
2.15 : 1 often used with 4-cyl 2.0 litre engines.
1.89 : 1 often used with inline 6 cyl 3.0 litre engines.
1.61 : 1 often used with V8 and diesels.

/Christer Cederqvist

It is a reduction gearbox.

http://hem.passagen.se/coronet75/