All I know is back in the day,those 18 foot Marhans used to race with a tandem outfit and a nice straight six Mercury.A 150hp with power trim and a fourblade Rekord cleaver would do about 60mph.A great classic.
When the V6 came out a few raced with them but the boat got stretched to 19 foot and then 21 foot.
In Guernsey (70s)we had four 18 footers including the Black streak,the Blue Streak and Solitair(Orange)Chris Flack raced a Red One"Gitanes"?
Then phantoms and Rings started to come out.
I loved those old Sixes though !