An Old Salt Trick

Experienced skippers don't need a knot log to know how fast the boat is going--they merely observe the location of the bow wave.  

The pictures below we're taken while sitting atop Knock Off 37VT66's wheelhouse.  At 4.5 knots the bow wave is about even with the front of the wheelhouse.  At 7.3 knots it's even with the midship hawse cleat.

By the way, it's a displacement hull's inability to get over the bow wave that results in maximum hull speed.