A cunnin old bastard up the river one day went flyin past us with a 5 hp motor on a tinny at probably 15-18 kts and when i asked him what the go was he said he pulled the number 1 of the 15 because 5hp didnt need to be registed down here. You would maybe get away with it unless something went wrong or you were unlucky enough to cop a diligent waterways bloke. 80 od bucks is a lot for a rarely used tinny especially if it its a second boat