: It exhibits fairly strong weathercocking; it sits
: level on the water so the hull on its own is balanced and my
: theory is the area of the paddlers body aft of midships
: unbalances it.

Weathercocking (turning into the wind) indicates that your weight is too far forward. Adding weight to the stern reduces weathercocking and it doesn't take much to make a significant difference. For a quick test, just chuck a full water bottle as far as possible into the stern of the boat and see the difference.