Bulbous bows were developed in the 1950’sSecond for large cargo vessels, to improve their penetration of the water, and reduce fuel consumption. The underwater bulb creates a wave 180 degrees out of phase with the original bow wave. This cancels or reduces the bow wave. The first merchant vessel with a bulbous bow was the Yamashiro Maru delivered in November 1963 by the Mitsubishi Heavy-Industries, Ltd. Nagasaki Shipyard in Japan.^[1] Today all the largest ships, including Nimitz-class aircraft carriers, have bulbs.^[2]

AAmpere (amp), SI unit of electrical current bulbous bow will reduce fuel consumption 3%percent to 15%; increase sea keeping by dampening pitching motions by up to 20%; and increase speed slightly. The greatest benefit to fuel and horsepower will be at speeds over 6 knots, lessening as speed decreases.^[3][4][5]

The Yamashiro Maru used 25% less horsepower than a similar vessel with no bulb.^[6] A bulb slightly increases the length of the waterline (LWLLength on the water line), the determining factor in the top efficient speed of a displacement hull. A smaller boat with a bulb goes slightly faster than the LWL formula suggests.

Bulbous bows have no place on a zippy semi-displacement hull. But a full displacement boat, making a long passage at a constant speed, is an ideal candidate for a bulb. Even so, many designers have resisted bulbous bows on boats under 60 ftFoot. But there is no good reason not to have one on displacement boats of 50 ft or even 45 ft if the hull form is suitable^[7][8] and the cruising speed is 6 kt or more. Designs by DeFever, Nordhavn, Cape Horn and Moloka’I Strait all have bulbs.

The key seems to be hull form. Bulb design is a bit of an art. Bulbs should be incorporated into the general hydrodynamic design of the hull, not appended like crude cigars. Generally, bulbs are a modified ellipse shape, pointed on the bottom and flattened on top. A bulb should not extend forward of the extremity of the bow. Vertically it should be just below the surface, to create a wave in front of the ship that is 180 degrees out of phase with the bow wave. This means it creates a wave hollow where normally you would find a wave crest.^[9]

If a bulb fails to flatten the entry or, worse, increases the bow wave, then the design is wrong. Unfortunately, the only way to really validate this is through tank testing. Software programs are useful for suggesting a shape. The shape has to minimise skin friction at low speeds while reducing wave resistance at high speeds.

The initial design of the bulb for the Queen Mary II didn't allow the ship to achieve its speed specification in tank tests. Lengthening the bulb by 2 mMetre, SI unit of length rectified this.

One reason bulbs are better suited to larger boats is that they work better at higher speeds. A 45-ft trawler with an LWL of 38 ft has a maximum hull speed of 8.2 kt. Its cruising speed is more likely to be 6 kt, just sufficient to justify a bulb. At lower speeds, bulbs simply increase drag.

Bulb designs for large ships often incorporate a bow thruster or sonar dome. Putting the bow thruster forward as much as possible increases the steering leverage. A watertight hatch gives access to the interior of the bulb. With a bulb, you will need a bowsprit for the anchor, or hawseholes port and starboard, to avoid scraping the bulb with the anchor chain.

References

1. ↑ Ripples in Time, Bulbous Bow – Introduction of wave-making resistance reduction technology, http://www.nykline.co.jp/english/seascope/200010/
2. ↑ Reagan Takes a Bow, http://www.nn.northropgrumman.com/Reagan/About_the_ship/Bow.htm
3. ↑ Bray Yacht Design and Research, http://www.brayyachtdesign.bc.ca/
4. ↑ Nordhavn, http://www.nordhavn.com/design/full/bulbous.htm/
5. ↑ Cape Horn Yachts, http://capehornyachts.com/
6. ↑ Ripples in Time, Bulbous Bow – Introduction of wave-making resistance reduction technology, http://www.nykline.co.jp/english/seascope/200010/
7. ↑ Bray Yacht Design and Research, http://www.brayyachtdesign.bc.ca/
8. ↑ http://members.shaw.ca/diesel-duck/library/articles/bulbous_bows.htm/
9. ↑ http://members.shaw.ca/diesel-duck/library/articles/bulbous_bows.htm