Effects of bulbous bows on ships characteristics

The effects of a bulbous bow can extend to several areas of the ship's design, construction, manufacture and operation, e.g.:

1. Effective drag (total resistance) and characteristics at various draughts.

2. Resistance in a seaway.

3. Seakeeping characteristics.

4. Propulsion characteristics.

5. Course-keeping ability and manoeuvrability.

6. Bow-thruster:

(a) Possibilities for installation.

(b) Efficiency.

(c) Additional resistance.

8. Construction, manufacture and building costs of bow section.

9. Freeboard.

10. Anchor-handling apparatus and operation with respect to danger of anchor striking bulbous bow.

11. Accommodation of sounding devices on fishing and research vessels.

12. Observing length restrictions due to docks and locks.

13. Ice operation.

Of these characteristics, the following have been selected for closer examination:

1. Ice operation with bulbous bow

A certain ice-breaking capability can be achieved if the position of the upper side of the bulb enables it to raise an ice sheet. For operation in medium-thick ice, the bulbous bow has greater advantages than conventional, and even ice-breaking, bows because it turns the broken lumps so that their wet sides slide along the hull, thus causing less wear on the outer shell and less resistance. The maximum thickness which a bulbous bow can break is less than for special ice-breaking bow forms.

2. Seakeeping characteristics with bulbous bow Three characteristics are of interest here:

1. Damping of pitching motion.

Generally speaking, bulbous bows increase pitch motion damping, especially when designed for the purpose. The damping is particularly pronounced in the area of resonance when the wavelength roughly corresponds to the ship's length. There is even some damping for shorter wavelengths. For wavelengths exceeding 1.3-1.5 ship's lengths, ships with bulbous bows will experience an increase in pitch amplitude. However, the pitch amplitude in this range is small in relation to the wave height.

2. The ability to operate without reduction of power even in heavier seas. Sharp-keeled bulbs can withstand slamming effects in more severe seas than normal bulbs. Where the bulbous bow has a flat upper surface, water striking the bow may cause pounding.

3. The increased power requirements in waves.

Bulbous bows increase the added resistance due to waves, despite the smoother operation in heavy seas. This is analogous to the effect of the bilge keel. The energy of damping has to be taken from the propulsive power. For wavelengths shorter than 0.9L the pitching frequency of the ship is subcritical. Then the bulb may reduce the added resistance.

3. Power requirements with bulbous bow

The change in power requirement with the bulbous bow as opposed to the

'normal' bow can be attributed to the following:

1. Change in the pressure drag due to the displacing effect of the bulb and the fin effect.

The bulb has an upper part which acts like a fin (Fig. 2.17). This fin-action is used by the 'stream-flow bulb' to give the sternward flow a downward component, thus diminishing the bow wave. Where the upper side of the bulb rises towards the stem, however, the fin effect decreases this resistance advantage. Since a fin effect can hardly be avoided, care should be taken that the effect works in the right direction. Surprisingly little use is made of this resistance reduction method.

Figure 2.17 Fin bulb

2. Change in wave breaking resistance.

With or without bulb, spray can form at the bow. By shaping the bow suitably (e.g. with sharply tapering waterlines and steep sections), spray can be reduced or completely eliminated.

3. Increase in frictional resistance.

The increased area of the wetted surface increases the frictional resistance. At low speeds, this increase is usually greater than the reduction in resistance caused by other factors.

4. Change in energy of the vortices originating at the bow.

A vortex is created because the lateral acceleration of the water in the CWL area of the forebody is greater than it is below. The separation of vortices is sometimes seen at the bilge in the area of the forward shoulder. The bulbous bow can be used to change these vortices. This may reduce energy losses due to these vortices and affect also the degree of energy recovery by the propeller (Hoekstra, 1975).

Figure 2.17 Fin bulb

5. Change in propulsion efficiency influenced by:

(a) Thrust loading coefficient.

(b) Uniformity of flow velocity.

In comparative experiments on models with and without bulbous bows, those with bulbous bows show usually better propulsion characteristics. The obvious explanation, i.e. that because the resistance is lower, a lower thrust coefficient is also effective, which leads to higher propeller efficiency in cargoships, is correct but not sufficient. Even at speeds where the resistances are equal and the propeller thrust loading coefficients roughly similar, there is usually an improvement of several per cent in the bulbous bow alternative (Fig. 2.18). Kracht (1973) provides one explanation of why the bulb improves propulsion efficiency. In comparative experiments, he determined a greater effective wake in ships with bulbous bows. Tzabiras (1997) comes to the same conclusion in numerical simulations for tanker hull forms.

Figure 2.18 Resistance comparison (ship with and without bulbous bow)

The power savings by a bulbous bow may, depending on the shape of the bulb, increase or decrease with a reduction in draught. The lower sections of modern bulbous bows often taper sharply. The advantage of these bulbous bows is particularly noticeable for the ship in ballast.

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Responses

  • ennio calabrese
    Does bulbous bow reduce frictional resistance?
    2 years ago

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