A bulbous bow is a hydrodynamic appendage used to decrease resistance of a vessel. Bulbs are designed to operate at a set speed. They work both to increase the effective waterline length, as well as to develop a small wave which will destructively interfere with the bow wave.
The largest constituent of a hulls resistance as it approaches hull speed is the formation of waves. The bulbous bow creates its own wave, whose trough at the design speed occurs in line with the peak of the unaided bow wave of the parent hull. When superimposed, the wave produced by the bulbous bow will destructively interfere with the bow wave, reducing its' size.
The chief benefit is a reduction in resistance, which can lower fuel consumption or alternatively increase speed. If fitted to a vessel with particularly fine entry, they will restore some bouyancy to the bow, and can aid trim issues arising from such (alternatively, the entry can be made 'finer' for the same sectional area curve). Alternatively, bulbs can be designed to generate some lift, raising or lowering the bow (dependant upon design).
The production of bulbs is considered somewhat akin to a black art (though, certainly not to the degree of anti-roll tanks). Bulbs add complexity and weight to the bow. They are prone to damage from slamming, ice, and dock strikes. If ballasted, they may draw the bow down into waves (notwithstanding the aforementioned trim benefits when buoyant). As the destructive effects only work within narrow ranges of speed, they can be detrimental when the vessel cruises outside of these ranges; particularly at lower speeds, where they just induce drag. Similarly, if the vessel cannot maintain bulb immersion in light loads, it will not provide a clean entry and increase drag.
In naval operations, sonar packages can be fitted within the bow using acoustically transparent coverings, the placement being as far as possible from the ships own propulsion noise. Conversely, bulbs tend to be placed closer to the water surface than traditional sonar blisters, which can induce flow noise. When pitching, such as in high seas, the extreme location of a sonar bulb may subject the sensitive electronics to slamming. For some of these reasons, the US Navy DDG-51 report notes a combined sonar/bulbous bow would be best used for auxiliary systems .
The vessels operational profile and hullform dictate the feasibility of fitting a bulbous bow. The vessel requires extended operations near its maximum design speed. The bulb must be considered for its operational impact (i.e., interference while working over the bow or in ice). The increased wetted area & resistance must be lower than the efficiency gain. The shape of the bulb must be suitably faired into the hull lines. The additional weight and bouyancy must be considered in the vessels arrangement. The bulbs structure must be suitably interfaced in the ships bow. Finally, an economic assessment should be made; it may be more beneficial simply to increase the waterline length.
- based on the work of Havelock (1928, flow around submerged sphere, 1931, ellipsoid), Taylor (experimentation, derived from ram prows which lead to the development of first purpose-designed bulb, USS Delaware), & Mitchell (wavemaking resistance - 'Mitchell Integral' 1898).