amazing modification spotted in a boat yard by Gravy Graham of YBW fame
The Wiki Answer to how it works:
In a conventionally shaped bow, a bow wave forms immediately before the bow. When a bulb is placed below the water ahead of this wave, water is forced to flow up over the bulb. If the trough formed by water flowing off the bulb coincides with the bow wave, the two partially cancel out and reduce the vessel’s wake. While inducing another wave stream saps energy from the ship, canceling out the second wave stream at the bow changes the pressure distribution along the hull, thereby reducing wave resistance. The effect that pressure distribution has on a surface is known as the form effect.
Some explanations note that water flowing over the bulb depresses the ship’s bow and keeps it trimmed better. Since many of the bulbous bows are symmetrical or even angled upwards which would tend to raise the bow further, the improved trim is likely a by-product of the reduced wave action as the vessel approaches hull speed, rather than direct action of water flow over the bulb.
A bulbous bow with a complex shape. The through tunnels in the side are bow thrusters (July 2006).
A sharp bow on a conventional hull form would produce waves and low drag like a bulbous bow, but waves coming from the side would strike it harder. Also, in heavy seas, water flowing around the bulb dampens pitching movements like a squiggle keel. The blunt bulbous bow also produces higher pressure in a large region in front, making the bow wave start earlier.
The addition of a bulb to a ship’s hull increases its overall wetted area. As wetted area increases, so does drag. At greater speeds and in larger vessels it is the bow wave that is the greatest force impeding the vessel’s forward motion through the water. For a vessel that is small or spends a great deal of its time at a slow speed, the increase in drag will not be offset by the benefit in dampening bow wave generation. As the wave counter effects are only significant at the vessel’s higher range of speed, bulbous bows are not energy efficient when the vessel cruises outside of these ranges, specifically at lower speeds.