|From Ocean Navigator #54 |
The bow bulb has become a common feature of the modern commercial ship. Now, some long-range power yacht builders are also turning to bulbs.
Several builders have tank tested bulb-equipped models and a few power yachts have been retrofitted with bulbs. The results of the tank tests and the retrofits suggest bulbs can improve the performance of smaller vessels.
Those performance gains for smaller vessels, however, have not come in the area that naval designers originally expected. In fact, even though for many years just about every large merchant ship has carried a bulb, and the benefits are known, naval architects and marine engineers don’t agree on exactly how these protuberances work.
For large ships, the main benefit of a bulb rests with its ability to reduce wave-making resistance. Reducing wave-making resistance is important for all displacement hulls because a displacement hull has to move water out of the way in order to move forward. As it does this, the displaced water piles up to form a bow wave. As the boat moves faster, this bow wave gets higher and also deepens and extends aft. As the boat’s speed continues to increase, the wavelength also grows until there is a wave crest at the bow, a crest at the stern and a trough extending between them. The hull appears to be suspended between two wave crests.
When a vessel reaches this speed, the wavelength of the displaced water equals the waterline length of the vessel. The vessel is said to have reached its “hull” the vessel can’t go much faster because it is “stuck” in a hole of its own making. It is difficult for the vessel to push any more water out of its way since the wave-making resistance has become nearly insurmountable.
We can determine hull speed with this equation:
So, for a vessel with a 60-foot waterline, the "formula" hull speed is 10.38 knots. On the other end of the spectrum, for a vessel of 900 feet, formula hull speed is 40.2 knots.
While hull speed can be calculated using the above formula, not every vessel will conform exactly to this standard. Vessels with small beam to length ratios will be capable of moving somewhat faster than the formula suggests. For example, a destroyer has a narrow hull for its length and can exceed its formula hull speed. To do this, however, plenty of power is required. This is no problem for destroyers, which are built with brawny steam turbine engines. However, there is a price to be paid: all that extra power means burning a great deal of fuel. For a destroyer in combat, the price of fuel is not much of a consideration, but for recreational power yachts, it doesn’t make sense to burn all that extra fuel to steam one knot faster. So, formula hull speed represents a point at which it becomes inefficient and expensive to drive the boat faster.
The other way a vessel could increase its speed would be to lift itself up and “climb” over its own bow wave. Once this happens, though, it ceases to be a displacement hull,the vessel is now on a plane and is riding on the surface of water. Some large power vessels are designed to make the transition from displacement to planing. Big sport fishermen, for example, are often designed with flat sections in the aft end of the hull. These flat sections allow the boat, once enough power is applied, to rise up and plane. This type of boat is said to have a semi-displacement hull.
It’s also possible to get a full displacement hull to plane for short periods. Just ask any sailor who has been in big winds and big seas. Even a fairly heavy boat can develop enough energy from the wind and from gravity to “surf” in big seas.Reducing wave making
If at any point in the process of displacing water discussed above, the wave-making resistance of a vessel can be reduced, energy will be saved. This is what a bow bulb does: With less resistance, less fuel is required to move the boat at a given speed, thus saving money for ship owners. Since merchant ships generally steam many thousands of miles a year, even a small reduction in resistance can add up to significant savings.
For example, the Matson Navigation Company, which operates containerships between the West Coast and Hawaii, recently launched R.J. Pfeiffer, a 714-foot containership equipped with a long, elliptical bow bulb. “We run this ship at 22 1/2 knots” says Ron Briggs, project manager for R.J. Pfeiffer. “At that speed, a five per cent decrease in resistance is substantial.”
As a displacement vessel moves through the water at higher speeds, wave making is the major type of resistance a hull must overcome. However, at lower speeds, skin friction assumes a larger role. A bulb must designed for a specific speed or a relatively small range of speeds. It cannot work throughout a vessel’s speed range. In fact, even a well-designed bulb that reduces wave-making resistance at, say, 15 knots, will increase skin friction at five knots. A bulb is designed, however, so that the gain in high-speed efficiency balances the low-speed loss.
While it is known that bow bulbs can reduce wave-making resistance, the exact mechanism has yet to be fully explained. One explanation is simple enough. We saw above that waterline length and wave-making resistance were related. Vessels with short waterlines will start to produce serious wave-making resistance at low speeds. Longer vessels will require higher speeds for this to happen. By placing a bulbous bow at the waterline, the effective waterline length of a vessel has increased it “appears” to be a longer boat. And a longer vessel will have a higher hull speed and less wave making resistance at a given speed than a shorter vessel.
Another explanation for how a bow bulb works involves the phase angle of waves. Imagine a sphere moving just below the surface of the water. In order to move forward, the sphere will have to move water out of its way. Displaced water will go around the sides of the sphere and over its top. After the sphere has passed, the water will “rebound” and fill the space vacated by the sphere. This will create a momentary “hollow” behind the sphere. This slight depression in the water surface is the opposite of a wave, it’s a “negative” wave, or trough. We know that a ship’s bow produces a “positive” bow wave. If we bring together the negative wave with the positive wave, the result (if their amplitudes are equal) is flat water. Flat water is another way of saying no bow wave to push out of the way—we are able to move forward with less resistance.
Bulbs are placed on the bow of a ship such that the negative wave will coincide with the positive bow wave, canceling out the bow wave as much as possible at a given speed. Even with a bow bulb, a ship will still produce bow waves. However, a bulb will lessen the extent of the bow wave and thus promote fuel efficiency.
Like most good ideas, this one is not new. For example, in C.A. Marchaj’s encyclopedic work Aero-Hydrodynamics of Sailing, he writes of the English designer G.L. Watson who built a cutter for himself that was fitted with a bow bulb—in 1871. Marchaj also cites an instance of a 6-meter sailboat equipped with a bulb in 1976. The main effect of the bulb seemed to be a significant increase the boat’s directional stability: it just didn’t want to tack. This, of course, is not the best attribute for a racing sailboat.Bulbs for power yachts
Currently, there are at least two power yacht firms, Pacific Asian Enterprises (PAE) and Delta Marine, that are exploring the use of bow bulbs on their vessels. PAE, manufacturers of the Nordhavn line of long-range power yachts, has incorporated a bulb into the molds for their Nordhavn 62.
Both companies have also retrofitted bow bulbs on some of their boats. Delta has retrofitted bulbs on two 70 footers, a 100 footer, and a 120 footer. PAE has put a bulb on a Nordhavn 46.
Even though the general shape and dimension of a workable bow bulb is known by naval architects, the exact dimensions must be custom designed for the particular hull on which the bulb will be fitted. The best method for doing this is to tank testing scale model of the hull. Various dimensions of bulbs can be mocked up and tested.
Both PAE and Delta decided to undertake tank testing for optimizing their bulb designs. And both West Coast firms went to the University of British Columbia and their BC Research facility in Vancouver.
For their tests, PAE used a nine-foot-long model of their Nordhavn 62 and fitted it with two different bulbs: a cylindrical design and a parabolic design. The tests, supervised by Gerald Stensgaard of BC Research, were run in a 300-foot testing tank at scale speeds of six to eleven knots.
“We were looking for any reduction in wave-making resistance” , says Jim Leishman, a designer at PAE,”but we also had some indications that a bulb would have a damping effect on pitching motion.” As far as reducing wave-making resistance, the tank tests showed that at high cruising speeds, resistance was reduced by about 12% in calm water. At low speeds, though, the bulb did increase overall resistance due to the greater wetted surface area.
The other area of investigation, pitching motion, also yielded encouraging results: According to Leishman, the bulbs reduced pitch amplitude 20%, and reduced pitch acceleration 18%. It seems that in addition to the effect of the bulbs in reducing resistance, the horizontal surface area of the bulb acts on the water to dampen pitch motion. Additionally, a bulb adds buoyancy forward, which also helps to reduce pitching.
PAE got a chance to check on their tank results with a real boat when a Nordhavn 46 owner, David Hamilton, offered his boat Arcturus as a test platform for bulb tests. It turned out to be cheaper to retrofit a bulb onto his boat than pay for a round of tank testing. The design of the bulb was based on the bulb for the 62 footer. However, since tanks tests weren’t done, it was necessary for Leishman to make some educated guesses on the exact dimensions of the bulb. After a few months the bulb was completed and Arcturus was taken out on trials.
The results of the trials indicated that the bulb did work quite well in dampening pitch. But it was less successful in reducing wave-making resistance. At top cruising speed, the bulb only contributed a 4% savings in resistance. These results point to the tricky nature of designing bulbs. According to Gerry Stensgaard at BC Research, the influence of a bulb can vary from actually increasing resistance to showing a 15% reduction in resistance, depending on the placement, draft, length, size, and shape of the bulb. And the best method for getting a bulb design correct is to do trial and error tank testing. A bulb must be tuned for a hull based on a fairly specific range of depth and speed.
To get the proper dimensions for R.J. Pfeiffer, Matson contracted with the Netherlands Boat Basin in Vagenagen, Holland to do model tests. The Dutch company built a 20-foot model of the Matson ship and conducted tests for the optimum bulb. “We asked the people at Vagenagen to optimize the bulb for a speed range of between 22 and a half knots and 19 knots”, said Ron Briggs of Matson, “…and for a draft variation of about four to five feet.”Reduced pitching
In their separate tests conducted at BC Research, Delta Marine also found reductions in pitching motion as the result of fitting a bulb. “We tested several models at BC Research”, said Jay Miner, naval architect at Delta. “We found that the bulb was more valuable for reducing pitching and for overall seakeeping than for cutting down on resistance. After our tests, we retrofitted bulbs to one of our 100-foot boats and one of our 120 footers.”Delta is currently building a 54-foot power yacht that will be equipped with a bulb.
And the large power vessel design company Fryco, located in Houston, has recently drawn plans for a 63-foot power yacht with a bulb. "The primary use of a bow bulb on a vessel this size is to reduce pitching”, said Gregory Marshall, vice president at Fryco.
Another possible advantage of a bulb is as a “shock absorber” in a collision. A collision bulkhead can be built where the bulb intersects with the normal stem line. Then, should the boat hit something substantial, the bulb could absorb much of the impact, thus maintaining the vessel’s watertight integrity.
However, there are some downsides to bulbs, too. If the bulb is not designed correctly, it could actually cause a net increase in resistance. The bulb might catch floating debris, nets, and other materials. While a bulb might prove a good shock absorber, it also might prevent a vessel from riding up on and driving under an object the boat might strike in the water. Also, PAE found that the bulb tended to throw more spray on deck than a standard bow. And, under specific sea conditions, a bulb can cause a vessel to pound into the seas.
Perhaps in light of this, not all large power yacht builders find bulbs a worthwhile addition to their vessels. While the Dutch builder Feadship did recently launch a 200-foot yacht, My Lynn, that was fitted with a bulb, Feadship is not moving toward bulbs for its designs. “We have not found them to be that advantageous”, said a spokesman for Feadship North America.
While not without their problems, bow bulbs do have advantages for both big ships and perhaps for smaller power yachts. In the shipping world, bulbs have become a standard design feature. They may eventually catch on for smaller power vessels, too.