What you have IS a downhaul. A cunningham is connected to the sail independent of the boom. Many racing boats <Lightnings for instance> have both a downhaul connected to the boom and a cunningham connected to the sail. For a cruising boat such as a Potter, a downhaul is sufficient. I haven't thought about it much but think it would cause extra wear and tear on the sail if you had only a cunningham as it would have to pull down not only the cloth of the sail but the boom as well.
Bill de Ment P-15 #1044 'Sukoshi'
Eagle, Idaho