Yacht Design Explained:
A Boat Owner's Guide to the Principles and Practice of Design ~ by Steve Killing
offers a short explanation of the bulbous bow on page 37.
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A bulbous bow (or “bow wave depressor" - a protruding bulb below the waterline at the bow of the ship) reduces drag, and affords an increase in speed, range, and fuel
efficiency generally in the range of 10 to 15% (over a formerly “conventional” bow).
The purpose of the bulb is to reduce the bow wave system, by creating a counter-wave that interferes and dampens otherwise larger bow wave.
At higher speeds (generally above 6 knots), wave making resistance accounts for the greater portion of the drag.
At lower speeds (generally below 6 kts), proportionally more of the resistance is taken up by wetted surface area frictional drag.
Accordingly, at slower speed, the bulbous bow actual increase drag.
A bulbous bow also increases directional stability - which may be advantageous (or disadvantagous).
Extensive tank testing has determined that the benefits of a bulbous bow are most noticeable on boats 50 feet and longer. Because there is a relationship between the ideal diameter of the bulb and the area of the mid-ship section, a number of issues become inherently impractical with smaller boats. If your craft is over 50 feet in length and is a displacement
vessel,, and you plan on doing some very long passages at a constant speed ~ then you might consider a bulbous bow.