Based on my experiences on the Norfolk Broads, a 12/14 ft rowing dinghy travels at about 2-3 knots, if you were actually needing to get somewhere and not idly sculling along. And it really should be sculling if one person is using two oars - strictly speaking, rowing is when you have two, four or more people, each using one oar each with both hands.
Sailing speed is easier - there's a formula, and (conveniently) a page of useful calculators online:
http://www.sailingusa.info/cal__hull_speed.htm
Nauticalia? Bring it on!...
