nice pic of a nice boat

warning warning warning

newbie about to espouse half baked knowledge

it is my understanding that all we read about the importance of controlling downhaul instead of mainsheet to depower is only related to modern masts with swept spreaders and diamond wires

like a hobie 16 it looks like your supercat has neither diamond wires or spreaders...

along with swept spreaders came masts designed to be bent fore and aft along their major axis by a combination of diamond tension and downhaul. many of these masts are tapered. and the final addition to the package was a sail designed to be used on a bendy mast with swept spreaders that could be savagely downhauled with powerful blocks

so just adding a modern powerful downhaul to an old cat and whaling? on that may just pull the bottom off your sail....

my nacra 5.2 is also a 1982 boat, it has diamond wires but a straight spreader and a 3 to 1 downhaul

about 1984 the 5.2 got swept spreaders

so i'd guess a simple 3 to 1 downhaul is all you need

apart from this forum experienced hobie 16 sailors should be able to tell you the best way to handle downhaul on a simple mast, as should the supercat forum, (if there is one)