Following Commander Pete's suggestion, I went to www.geocities.com/jvandry and clicked on Sailboat Math. This takes you to a neat calculator that gives you Hull Speed, minimum horsepower to achieve Hull Speed and the speed that your particular horsepower engine will drive the boat.

Using the following data for the Ariel / Commander

Displacement 5200 lbs
Waterline 18.5 feet
Beam 8.0 feet
Sail Area 305 square feet
Horsepower 5.0

Here are the results

Hull Speed 5.8 knots
Min Horsepower needed 7.7 hp
Speed with 5hp engine 4.6 knots

Displacement / Length Ratio 367
Sail Area / Displacement Ratio 16.3
Capsize Screening Ratio 1.85
(Below 2.0 recommended for off shore sailing)