Given the LWL and displacement of a Willard 30, it would seem that the boat should cruise comfortably at around 6 knots (still water speed relative to water) on less than an gallon per hour. Seven knots speed would involve a lot more engine rpms and a significant fuel penalty. Without knowing the details of the power train (engine Hp rating at wide open throttle, wide open throttle rpm rating, transmission gear ratio and prop details (style, number of blades, diameter, pitch and disc area ratio), you can't really say more.
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