HULL , BOAT SPEED
The calculation yields an approximation, but it should not be that far off. The more interesting of the two measurements you supply is the motoring in calm water example. That one precludes surfing or lengthening the effective LWL through angle of heel and thereby increasing theoretical speed. The first place to look is at the GPS measurement. Was there a favorable current running (tidal or river)? Remember GPS is measuring Speed Over Ground (SOG) not through the water which the calculation approximates. Also is the GPS measurement recent, i.e since GPS selective availability has been turned off (May 2000 I think)? Is your GPS a differential model using beacon or WAAS? The tighter geolocational accuracy of differential also yields more accurate velocity calculations.