Historical development

Today the bulbous bow is a normal part of modern seagoing cargo ships. Comparative model experiments show that a ship fitted with a bulbous bow can require far less propulsive power and have considerably better resistance characteristics than the same ship without a bulbous bow.

The bulbous bow was discovered rather than invented. Before 1900, towing tests with warships in the USA established that the ram stem projecting below the water decreased resistance. A torpedo boat model showed that an underwater torpedo discharge pipe ending in the forward stem also reduced the resistance. A bulbous bow was first used in 1912 by the US navy, based on a design by David Taylor. It was not until 1929 that the first civil ships were fitted with them. These were the passenger ships Bremen and Europa belonging to the Norddeutscher Lloyd of Bremen. A more widespread application in cargo shipping did not happen until the 1950s. The first bulb for tankers, invented by Schneekluth, was installed in 1957.

Bulbous bows are defined using the following form characteristics:

1. Shape of section.

3. Length of projection beyond perpendicular.

4. Position of axis.

5. Area ratio.

6. Transition to hull.

Some of these characteristics can be expressed by numbers. Bulb forms

Today bulbous forms tapering sharply underneath are preferred, since these reduce slamming. The lower waterplanes also taper sharply, so that for the vessel in ballast the bulb has the same effect as a normal bow lengthened (Fig. 2.12). This avoids additional resistance and spray formation created by the partially submerged bulb. Bulbs with circular cross-sections are preferred where a simple building procedure is required and the potential danger of slamming effects can be avoided. The optimum relation of the forward section shape to the bulb is usually determined by trial and error in computer simulations, see Section 2.11 and, for example, Hoyle et al. (1986).

Modern bulbous forms, wedge shaped below and projecting in front of the perpendicular, are geometrically particularly well suited to V section forms.

Figure 2.12 Modern bulb form

Cylindrical bulbs, projecting forward of the perpendicular, and Taylor non-projecting bulbs can easily be faired into U forward sections. Whether these combinations, suitable in form, lead also to minimum power requirements has yet to be discovered.

Bulbous bow projecting above CWL

It is often necessary to reduce the resistance caused by the upper side of bulbous bows which project above the CWL creating strong turbulence. The aim should be a fin effect where the upper surface of the bulb runs downwards towards the perpendicular. A bulbous bow projecting above the waterline usually has considerably greater influence on propulsion power requirements than a submerged bulb. Where a bulbous bow projects above the CWL, the authorities may stipulate that the forward perpendicular be taken as the point of intersection of the bulb contour with the CWL. Unlike well-submerged bulbs, this type of bulb form can thus increase the calculation length for freeboard and classification (Fig. 2.13). Regarding the bulb height, in applying the freeboard regulations, the length is measured at 85% of the depth to the freeboard deck. Consequently, even a bulb that only approaches the CWL can still cause an increase in the calculation length of ships with low freeboard decks, e.g. shelter-deckers (Fig. 2.14).

Figure 2.13 Position of forward perpendicular with high bulbous bows

Figure 2.13 Position of forward perpendicular with high bulbous bows

Length to comply with freeboard regulation

Figure 2.14 Length of freeboard calculation with low freeboard deck

Projecting length

The length projecting beyond the forward perpendicular depends on the bulb form and the Froude number. For safety reasons, the bulbous bow is never allowed to project longitudinally beyond the upper end of the stem: 20% B is a favourable size for the projection length. Enlarging this size improves the resistance only negligibly. Today, bulbs are rarely constructed without a projecting length. If the recess in the CWL is filled in, possibly by designing a straight stem line running from the forward edge of the bulb to the upper edge of the stem, the resistance can usually be greatly reduced. This method is hardly ever used, however.

Bulb axis

The bulb axis is not precisely defined. It should slope downwards toward the stern so as to lie in the flowlines. This criterion is also valid for the line of the maximum bulb breadth and for any concave parts which may be incorporated in the bulb. The inclination of the flowlines directly behind the stem is more pronounced in full than fine vessels. Hence on full ships, the concave part between bulb and hull should incline more steeply towards the stern.

Area ratio

The area ratio ABT/AM is the ratio of the bulb area at the forward perpendicular to the midship section area. If the bulb just reaches the forward perpendicular, or the forward edge of the bulb is situated behind the forward perpendicular the lines are faired by plotting against the curvature of the section area curve to the perpendicular (Fig. 2.15). At the design draught, the resistance of the ship with deeply submerged bulb decreases with increasing area ratio. A reduction of the area ratio (well below the resistance optimum) can, however, be advocated in the light of the following aspects:

1. Low resistance at ballast draught.

2. Avoidance of excessive slamming effects.

3. The ability to perform anchoring operations without the anchor touching the bulb.

4. Too great a width may increase the resistance of high bulbs, since these are particularly exposed to turbulence in the upper area.

Figure 2.15 Bulb with projecting length. Theoretical bulb section area of the forward perpendicular

The effective area ratio can be further increased if the bulb is allowed to project above the CWL. Although the section above the CWL is not included in the normal evaluation of the area ratio, it increases the effective area ratio and can considerably reduce resistance, provided that the bulb is of suitable shape.

Transition

The transition from a bulbous bow to the hull can be either faired or be discontinuous (superimposed bulb). The faired-in form usually has lower resistance. The more the hollow surface lies in the flowlines, the less it increases resistance. In general, concave surfaces increase resistance less.

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