Hull Speed

In my posting of 16 October I reported the measurements in the RPM and SOG columns in the table below, taken during our sea trial:

RPM .. SOG ... Ratio

1000 ... 2.9  ...  .0029
1500 ... 4.4  ...  .0029
1800 ... 5.3  ...  .0029
2000 ... 5.7  ...  .0029
2200 ... 6.4  ...  .0029
2400 ... 6.9  ...  .0029
2600 ... 7.4  ...  .0028
2800 ... 7.7  ...  .0028
3000 ... 8.0  ...  .0027
3100 ... 8.1  ...  .0026

Bob commented thought that we had exceeded Pachuca's hull speed.  Unfortunately at the time I wasn't sure of Pachuca's LWL (length of waterline) from which to calculate the boat's theoretical hull speed.

Last night the adjacent boat was taken out of her slip for the weekend, giving me a good side view of Pachuca, and I took the opportunity to use a plumb bob to mark the ends of the waterline on deck then measured the distance between the marks.

I measured Pachuca's LWL to be 33.75 ft (10.29 meters).  Using the equation HS = 1.34 sqrt(LWL) I calculated Pachuca's hull speed to be 7.78 knots which means that we exceeded hull speed at somewhere above 2800 rpm.

The literature states that "Going faster than the theoretical hull speed is possible, but the wave moves under the transom so the boat has to plow through the water at an angle, bow up which then requires exponentially more power to move the vessel once theoretical hull speed is reached." 

The "Ratio" column above contains the ratio of SOG (speed over the ground) to engine RPM.  The relationship of SOG to RPM is remarkably linear through 2400 rpm.  Thereafter it degenerates in the sense that there is there is a comparatively lower gain in speed for a given increase in RPM.

I know now not to cruise at above 2400 rpm.