Theoretical hull speed of a displacement hull= 1.34 X sqrt LWL
Other factors -
1) planing will take you above theoretical hull speed, usually downwind with a chute of some sort.
2) fouled bottom will slow you down.
You need a knotmeter, not a GPS when dealing with hull speed as it is through the water not over the ground. In addition you want a momentary speed not an average.
ISPA Yachtmaster Offshore Instructor Evaluator
Sail Canada Advanced Cruising Instructor
IYT Yachtmaster Coastal Instructor
As I sail, I praise God, and care not. (Luke Foxe)