A shaft-driven boat having a separate rudder positioned aft of a marine propulsion system, the marine propulsion system comprising: a recess formed in the hull of said boat and including a drive aperture formed through the hull; and a drive cassette, said drive cassette comprising: a housing; a static tube extending rearwardly from said housing and fixed relative thereto; at least one shaft bearing; thrust bearings (32, 32a) located within the housing; and at least one straight drive shaft extending through said static tube and also through said housing; the straight drive shaft being supported for rotation within the static tube by said at least one shaft bearing, and being rotatably supported within the housing by said thrust bearings (32, 32a); said static tube, said straight drive shaft, the or each said shaft bearing, and said thrust bearings (32, 32a) all being coaxially aligned with one another; wherein said housing and said recess are mutually configured such that said housing is engaged within said recess to install said drive cassette within the drivetrain of the boat; and wherein said recess is an external recess provided below a waterline of the boat and such that said housing is engaged within the recess from outside the hull of the boat and with a forward end of the straight drive shaft being passed through the drive aperture. a recess formed in the hull of said boat and including a drive aperture formed through the hull; and a drive cassette, said drive cassette comprising: a housing; a static tube extending rearwardly from said housing and fixed relative thereto; at least one shaft bearing; thrust bearings (32, 32a) located within the housing; and at least one straight drive shaft extending through said static tube and also through said housing; the straight drive shaft being supported for rotation within the static tube by said at least one shaft bearing, and being rotatably supported within the housing by said thrust bearings (32, 32a); said static tube, said straight drive shaft, the or each said shaft bearing, and said thrust bearings (32, 32a) all being coaxially aligned with one another; a housing; a static tube extending rearwardly from said housing and fixed relative thereto; at least one shaft bearing; thrust bearings (32, 32a) located within the housing; and at least one straight drive shaft extending through said static tube and also through said housing; the straight drive shaft being supported for rotation within the static tube by said at least one shaft bearing, and being rotatably supported within the housing by said thrust bearings (32, 32a); said static tube, said straight drive shaft, the or each said shaft bearing, and said thrust bearings (32, 32a) all being coaxially aligned with one another; wherein said housing and said recess are mutually configured such that said housing is engaged within said recess to install said drive cassette within the drivetrain of the boat; and wherein said recess is an external recess provided below a waterline of the boat and such that said housing is engaged within the recess from outside the hull of the boat and with a forward end of the straight drive shaft being passed through the drive aperture.