On a saildrive the ropecutter gets it drive from the prop. I have fitted the Ambassador rope cutter (I think its one of the best). It comes with raised allen screws that mate into the back of the propellor. From memory I also removed the spacer. Be careful if you have a feathering prop as the screws must recess into the prop boss and not bridge between the moving parts of the feathering propeller.
I had a feathering prop and had the screws in the ropecutter repositioned.
if its a standard Volvo non folding prop you will have to cut/ grind off a small lip at the back of the prop and then drill two holes to accept the drive screw heads of the cutter. Ambasador marine website has full instructions.