The three bladed prop's, boat speed was derived from a GPS and verified by timing between marks. The engine speed was derived by monitoring the prop coupling RPM with a hand held optical tachometer similar to the Cen-Tech unit. The tach numbers were then multiplied by 2.14 to derive the engine RPM.
All work was done in calm waters with no wind. Note that just one foot waves can decrease top speed by 0.3 Knots.