Hydrodynamic level, the destructive interference of the primary and secondary wave trains causes an overall reduction in drag which is beneficial to the vessels resistance characteristics would be true. Or on a more physical level, that the water coursing over the top of the bulb is exerting a downward pressure that is keeping the stern from squatting, thereby allowing flatter trim, causing the vessel to run with less resistance. It is a fact that bulbs do work and in some cases reduce resistance as much as 25%.
Their proportions are derived from the features and dimensions of the vessel itself. The diameter (volume) is a direct result of the hull midship area. The length is determined by the stem profile, as the farther forward the bulb extends the more leverage it has but is generally kept shorter than the bow overhang. The section shape may be a modified ellipse to reduce pounding in head seas. The vertical placement is calculated so the bulb is just below the surface where it will create a wave in front of the ship interfering with the natural wave train of the vessel, creating a wave hollow where a crest should be. In this way the vessel will run flatter and the overall wave height will be reduced.
The vessel will pitch less which will cause fewer disturbances in the water and the passage of the vessel through the water will be achieved with less fuss and muss. Any time a vessel can be moved through the water with less waves and overall disturbance to the surface less power has been transmitted to the water to create those waves. To be able to move a vessel through the water with no perceptible notice of its passage is the ultimate goal of all Naval Architects and the ultimate in power savings.
Anil
