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.
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