The general formula for hull speed is: 1.34 X the square root of the water line length.
Assume yours is 15' than your "theoretical" hull speed would be 5.19 knots.
In reality your semi planing hull speed would be the maximum speed that you can maintain stability(and sails) in a blow. You should expect to see some lift around 12 knots. My guess, based on observations of other boats like yours is about 7-8 knots.