Evolution of Marine Propulsion

ABSTRACT
The use of a manually-operated paddle to provide motion to a raft or boat, initially required the use of both hands, thereby tending to veer somewhat to one side off course, rather than straight ahead. Eventually, the adoption of "oar-locks" secured to the bulwarks, one on each side of the vessel, serving as pivot points for the oars, allowed for the use of two oars simultaneously that enabled the boat to maintain a reasonably straight course. The harnessing of air pressure, in the form of wind acting upon a hoisted sail, provided an alternate and more effective form of propulsion that was subsequently complemented by use of a "rudder", mounted on the transom (or stern), with which to steer the vessel in any given direction. However, the limiting factors of wind were direction, velocity and fluctuation of intensity. While wind force tends to be quite strong in the open ocean, there are also areas especially within the equatorial regions, known as the doldrums, wherein calm prevails and sail-rigged vessels are at the mercy of prevailing surface currents, until they drift far enough to reach a wind-blown area.

ARTIFICIAL ENERGY
In the year 1698 a patent was awarded to John Savery in England, for a device capable of producing "artificial energy", in the form that ultimately became known as a "Steam Engine". This was followed in 1705, by the efforts of Thomas Newcomen whose engines were used for pumping water out of coal mines. Further steam engine improvements were introduced by James Watt, of Glasgow, Scotland, for which he received a patent in 1769. Another significant historical event was the evolution of mechanically-propelled steam ships that eventually rendered sailing ships obsolete. Among the early pioneers who attempted to adapt steam powered propulsion to boats and ships were two American inventors, namely, John Fitch and James Rumsey. Unfortunately for Rumsey, his first boat broke down during its initial run in 1786. The following year Fitch demonstrated his version of a steam-powered vessel propelled by a series of mechanically-actuated paddles that worked, but was limited to a speed of only three miles per hour. Shortly thereafter, Rumsey showed off his new design based on a steam engine-powered water pump, from which the discharge exited through the transom to provide "water jet propulsion".

The concept of multiple oars or paddles, radiating from a central hub, like spokes in a wagon wheel, installed outboard, on the port and starboard sides, adjacent to the hull, approximately mid ships, and rotated by a steam reciprocating engine, became the prime mover of the "Paddle Steamer", as these vessels were then known, in the early 1800's. The American engineer Robert Fulton and his partner Robert Livingstone, built a steamboat in1803 that they operated successfully on the River Seine, in Paris, France. Upon returning to the United States Fulton and Livingstone built a larger paddle steamer named the CLAREMONT, in 1807. During her first year of service, operating on the Hudson River, between New York and Albany, a distance of some 150 miles, this vessel earned $16,000.00. Yet another version of the paddle wheel, intended mainly for riverine usage, was installed across the stern of a vessel, with paddles (or blade-boards) ) extending horizontally across the beam of the vessel with their outboard ends attached to the circumference of two large diameter paddle wheels, also driven by a reciprocating steam engine The first recorded concept of a "screw propeller" for ship propulsion is attributed to Benjamin D. Beecher in the early 1830's and installed on the S.S. FRANCIS B. OGDEN, in 1837. This was followed in 1841 by the screw-propeller driven freighter S.S. VANDALIA, for operation on the Great Lakes. The basic functional design principle of the contemporary marine screw propeller can be said to be somewhat analogous to that of the threads of a wood screw. Whereas when rotated, the helicoidal pitch of the screw-threads causes the screw to advance axially into the wood, the helicoidal pitch of the propeller blades causes the "prop" to advance axially within the water in which it is submerged. The theoretical linear advance of a rotating helicoidal trajectory within a substance, per rotation, is termed the "pitch". In wood, this would be approximately 100 percent, given that because of the density of the wood, there is no "slip". However, since water is less dense than wood, the effective axial advance (or pitch), of the screw propeller is in the region of 85 percent, meaning a slip of 15 percent. The effective speed of a ship can be determined (in knots), by multiplying the known number of propeller revolutions per minute, times sixty, by the propeller blade pitch (in linear feet), and the result divided by 1.1516, representing the ratio of nautical miles to statute miles. The answer is expressed in knots. (A knot is equal to one nautical mile per hour). In the interest of accuracy, allowances should also be made for the effects of known currents, such as velocity and direction, and when applicable, the condition of submerged hull surface.

TRIPLE EXPANSION STEAM ENGINES
In 1885, prior to the advent of the Diesel internal combustion engine as a marine propulsion power plant, the Cunard Line proudly introduced the S.S. UMBRIA and the S.S. ETRURIA both equipped with more powerful steam reciprocating engines, and both capable of attaining a speed of 20 knots. In due course the compound engine evolved into the Triple Expansion, Double-Acting Steam Reciprocating Engine wherein steam is allowed to expand in three successive stages. Starting with the High Pressure cylinder, which is the smallest in diameter, then to the Intermediate Pressure cylinder, which is somewhat larger, and finally to the Low Pressure cylinder which is even larger. One of the best examples of this type of engine is to be found in the 2,600 brake horse power, Triple-Expansion Steam Reciprocating Engine, originally designed and built in England for marine propulsion. Engines of this type were used extensively for powering the hundreds of "Liberty" ships built in the United States during World War II and supported by twin oil-burning Water-Tube Boilers, designed and built in the United States in accordance with the Rules of Construction of Fired and Unfired Pressure Vessels, contained in the Boiler Construction Code, published by the American Society of Mechanical Engineers (ASME). This design criterion is recognized by the majority of State Governments; the United States Coast Guard; and adopted by the National Board of Boiler and Pressure Vessel Inspectors. This triple-expansion Steam Engine design was successfully adapted for American production by the Hooven, Owens &amp; Rentschler Corp., during World War II and was subsequently built by eighteen different manufacturers in the U.S. The total weight of the engine is 270,000 pounds and it stands 19 feet high by 21 feet long, operating at a normal speed of 65 r.p.m., directly connected to a four-bladed propeller 18 feet in diameter, providing a vessel speed of 11 knots fully loaded.

CURENT TRENDS
Since the end of World War II, there has been a gradual shift from steam to Diesel engines for main propulsion of most merchant ships. This is largely attributable to the relatively higher Thermal Efficiency of Diesel engines compared to that of their steam reciprocating and steam turbine counterparts for both main propulsion and auxiliary engines. Contemporary Ship-owners are also concerned with the requirement to reduce atmospheric pollution produced by the exhaust gas emissions from their main propulsion and auxiliary engines. From the old two-cycle "Crankcase-Scavenged" engines of yesteryear, Diesel engine air intake systems have progressed to engine-driven reciprocating scavenge air pumps, and to engine-driven rotary scavenge air blowers for two-stroke cycle engines, in the course of improving efficiency via mechanical pressure charging of the cylinders. In this regard, a most significant advancement was introduced by Dr. A.J. Buchi in 1911 in the form of an exhaust gas turbine to drive a rotary compressor for pressure-charging the air intake manifolds of two-stroke cycle and four-stroke cycle internal combustion engines.

EXHAUST GAS TURBO-CHARGING
This concept was developed in association with the Sulzer Brothers Diesel Engine Company, in Winterthur, Switzerland, that has since been acquired by the Wartsila Corporation, of Helsinki, Finland. However, unlike the above-mentioned engine-driven blowers and pumps, the turbo-charger (as it became known), does not depend upon any mechanical drive between the engine and the turbo-charger. The only connections being those of structural supports, exhaust gas piping and air ducting. In the Buchi exhaust gas turbo-charger system, air is drawn through an intake filter from atmosphere, pressurized by the centrifugal air compressor unit and delivered through an after-cooler to the engine intake manifold. Exhaust gas is piped to the gas turbine nozzle plate and guided to impinge upon the axial-flow turbine rotor, causing it to rotate at high speed and thereby drive the centrifugal air compressor.

KINETIC ENERGY
Power derived from engine exhaust gas to drive a turbo-charger is in the form of kinetic energy. Turbo-charger design engineering involves complex formulae based on thermo-dynamic principles and gas laws involving such factors as frictionless flow, adiabatic expansion, tangential momentum, etc., beyond the intended scope of this text. However, a brief review of the energy of motion, that is also known as "Kinetic Energy", is warranted to the extent that it applies to this subject. Force produced by a weight W, moving at a velocity V, is capable of potential energy KE1. Some of this energy is expended in overcoming inertia (of the body against which it acts), leaving a balance of KE2. The actual work done during a given time period will be equal to the change in kinetic energy KE3, represented by the product of KE1 minus KE2 = KE3. In this instance, the weight W is stated in pounds and the velocity V, in feet per second.

TURBO-CHARGER PERFORMANCE
Therefore, if the turbine of an exhaust turbo-charger receives exhaust gas at the rate of 2.0 pounds per second, entering the turbine nozzle plate at a velocity of 1,250 feet per second, and leaving the turbine at a velocity of 800 feet per second, the resultant change in Kinetic energy as absorbed by the turbine will be as follows:

Kinetic energy KE = Weight (W) x Velocity x Velocity (V squared) 2 x 32.2 feet per second per second

Example 1: KE1 = 2.0 lbs./sec. x 1,250 ft./sec. x 1,250 ft./sec.

2 x 32.2 feet per second per second = 1,562,500 ft.-lbs./sec.

32.2 feet per second per second = 48,524.84 ft.-lbs./sec.

KE2 = 2.0 lbs./sec. x 800 ft. sec. x 800 ft. sec.

2 x 32.2 ft./sec./sec. = 640,000 ft.-lbs./sec.

32.2 ft./sec./sec. = 19,875.77 ft.-lbs./sec.

KE3 = 48,524.84 - 19,875.77 = 28,649 ft.-lbs./sec.

Therefore: KE3 = 28,649 ft.-lbs./sec 550 ft.-lbs./sec. = 52.08 b. h. p.

From the previous example showing how to compute available horsepower from conversion of kinetic energy, the next step is to find how this power can be applied to drive a blower. Determination of required b.h.p. is based on the volume and pressure of air to be delivered, plus blower efficiency which usually ranges between 78% and 85%, for an average of 83.5%. For a given air delivery rate at a specific pressure, the required bh.p. may be found as follows:

Example 2: B.h.p. 62.383 x P x Q = 62.383 x 138.5 x 2.386.98 x 0.835 12 x 33,000 = 396,000

Where: 62.383 = constant P = air pressure, in inches of water, = 138.5 inches of H2o. = cubic feet of air per minute = 2,386.98 cfm x 0.835 = 1,993.12 cfm 12 = constant 33,000 = ft.-lbs./min equivalent of 1.0 h.p.

AIR INTAKE SYSTEMS
For a given air delivery rate at a specific pressure, the required b.h.p. may be found as follows:

Example 2: B.h.p. = 62.383 x P x Q = 62.383 x 138.5 x 2,386.98 x 0.835 12 x 33,000 = 396,000

where: 62.383 = constant P = air pressure, in inches of water, = 138.5 inches of H2O. Q = cubic feet of air per minute, = 2,386.98 cfm x 0.835* = 1,993.1283 cfm 12 = constant. 33,000 = ft.-lbs./min equivalent of 1.0 h.p.

By transposition of the formula any one of the five factors may be computed providing the other four are known. For this example the delivery air pressure will be taken as 5.0 p.s.i, and since 1.0 p.s.i. is the equivalent of 27.7 inches of water, the factor P will be 5 x 27.7 = 138.5 inches of H2O. Based on a b.h.p. of 52.08, as computed in the previous example, and by appending an assumed blower efficiency of 83.5%, the formula may be transposed to find the air delivery rate in cubic feet per minute: 52.08 x 33,000 x 12 206,236.80

Example 3: Q = 62.383 x 138.5 x (0.835) = 8,640.04 x (0.835)

Therefore: Q = 2,386.987 c.f.m. x 0.835 efficiency = 1,993.134 c.f.m.

On the basis of an assumed air consumption rate of 3.2 c.f.m. per b.h.p., the nominal horsepower of an engine for which this blower is suitable will be: 1,993.134 c.f.m

Example 4: Nominal B.h.p. 3.2 c.f.m. = 622.854 brake horsepower.

Turbo-chargers in the low pressure range develop delivery pressures between 5.0 p.s.i. and 10.0 p.s.i., offering the following advantages:

1) Higher thermal efficiency through waste heat recovery.

2) Higher ratio of b.h.p. per pound weight of engine.

3) Generally, small turbo-charged engines are less expensive that their larger naturally-aspirated counterparts of the same rated brake horse power.

ENVIRONMENTAL PRESERVATION
The U.S. Environmental Protection Agency has expressed concern over the adverse contribution of nitrogen-oxide (NOx) emissions from marine engines to atmospheric pollution. Elaborating upon existing rules applicable to railroad locomotives and off-highway heavy equipment engines, the EPA has extended its jurisdiction to include marine engines by publication of an Advance Notice of Rulemaking, under which, marine engines would be grouped into three categories, as follows:

Category I — Engines of 2,000 bhp or less.

Category II — Engines classed as "locomotive—derivative", meaning typical medium-speed main propulsion engines found on coastal freighters; ferries; offshore supply boats and tugboats.

Category III —Heavy duty, slow speed main propulsion engines found on ocean-going oil tankers; bulk carriers and container ships.

In keeping with world-wide concern for environmental preservation, the Marine Environment Protection Committee (MEPC) of the International Maritime Organization (IMO) has recommended quantitative limitations to restrict atmospheric pollution by harmful exhaust emissions from marine propulsion plants of all types. The objective was to reduce nitrogen oxide emissions to 70% of current levels and that of sulfur dioxide to 50% of current levels, with even more restrictive levels during the next few years. The importance of minimizing current levels of such toxic effluent and its adverse impact upon us should be evident from Table 1 which shows typical toxic exhaust gas emissions and their respective harmful effects. Effective limitation of exhaust gas toxic emissions will require a concerted research effort to devise means that are capable of achieving significant reduction of harmful emissions while minimizing adverse impact upon fuel consumption economy.

EXHAUST GAS COMPOSITION
Prior to combustion the elemental constituents are air and fuel. Volumetric composition of air at sea level is approximately 78% nitrogen and 21% oxygen, at a ratio of: 78 divided by 21 = 3.714:1, meaning that combustion air includes 3.714 mols of nitrogen for each mol of oxygen. Since nitrogen is an inert gas it does not participate in the combustion process except for a small portion that reacts chemically to form oxides of nitrogen (N0x), the production of which increases in proportion to temperature rise. A typical grade of Diesel fuel may comprise 13.51% hydrogen and 84% carbon (by weight), plus minor amounts of sulfur, oxygen and ash.

Given 1.0 lb. of fuel, nominal weight factors are 0.1351 lbs. of hydrogen plus 0.84 lbs. of carbon. During the combustion process a portion of the oxygen in the charge air reacts with the carbon, hydrogen and sulfur of the fuel to produce carbon dioxide (002), water vapor (H20) and sulfur dioxide (S02) which become the products of combustion. Approximately 3% of the sulfur dioxide oxidizes in the atmosphere to sulfur trioxide (S03). In its original form nitric oxide (NO) is not considered hazardous, however, as it cools upon discharge into the atmosphere much of it will convert to nitrogen dioxide which is toxic.

TABLE 1 — EXHAUST GAS EMISSIONS AND EFFECTS Product Harmful Effects NOx: Nitrogen oxides cause acid rain, are toxic and contribute to smog. SOX: Sulfur Oxides contribute to acid rain, cause agricultural problems and are corrosive. HC: A wide variety of Hydrocarbons exist, some of which are toxic. Some are carcinogenic, some contribute to the greenhouse effect. CO: Carbon Monoxide is toxic and reduces the ability of blood to transport Oxygen. Particulates: Soot and particulates contain hydrocarbons and are considered Carcinogenic.

SCR SYSTEMS FOR SLOW SPEED DIESEL ENGINES
Tests conducted in Europe during the 1988-89 time frame disclosed that Diesel engine exhaust emissions are conducive to Selective Catalytic Reaction. Subsequently, a project was launched by the German Diesel engine builder, MAN-B&amp;W Group in cooperation with Haldor Topsoe (HTAS), a Danish chemical engineering company. The objective was to determine basic design parameters of a catalytic reactor, estimated volume of gas flow, in terms of cubic meters per hour, estimated concentration level of nitrogen-oxide (NOx) therein, plus the resultant or target level of NOx concentration, in terms of parts per million, after catalytic reduction. Next factor to be established was the required injection rate of ammonia (NH3) since the effectiveness of NOx removal is contingent upon the volume of ammonia injected into the gas stream, expressed as the NH3/NOx ratio. The importance of this factor is based on the need to avoid an excessively high ratio beyond what is normally required for catalytic reaction, such unused ammonia is termed NH3 slip. In the cooling process of the flue gas passing through the tubes of an exhaust boiler or heat exchanger such as found on many motor ships, reaction of the NH3 with sulfur trioxide (S03) in the exhaust gas, is conducive to the formation of ammonium sulfates that over a period of time would result in fouling of the tube surfaces. Upon satisfactory conclusion of the above-mentioned testing program, the Hyundai Heavy Industries shipyard in Ulsan, Korea, was awarded a contract to build three new bulk carriers of 30,000 deadweight tons, to transport steel products from the Korean Steel Mill of POSCO, in Pohang, Korea, to the United States Steel facility in Pittsburg, California. A MAN-B&amp;W type 6S50MC slow speed Diesel engine of 10,680 bhp @ 127 rpm was chosen as the prime mover for these vessels that in 1989 became the first three ships in the world equipped with Selective Catalytic Reduction Systems. During test bed trials of the main engine for the first vessel, at the Hyundai Diesel Engine Factory, prior to routing exhaust gas via the S.C.R. system, the concentration of uncontrolled NOx was measured at approximately 1,200 parts per million. Under actual operating conditions within California waters, where the S.C.R. system is in continuous use, the nitrogen oxide concentration has been reduced to the level of 130 parts per million.

THE COST OF COMPLIANCE
While the enforcement of restricted exhaust emissions is of prime concern to ship-owners, so too is the difference in cost between conventional heavy fuel (380 centistokes) averaging 2.5% Sulfur Oxide (SOx), currently available for about $480.00 per ton, versus Marine Diesel Oil (MDO) rated at less than 0.1% SOx, reportedly, around $695.00 per ton, depending upon the location of the bunkering port. Hence, compliance with the law becomes a rather expensive proposition. In addition to which, the contemporary solution to such outlawed exhaust emissions, involving switching from conventional heavy fuel (380 centistokes), or from No. 2 Diesel fuel, to a distillate fuel with very low sulfur content, known as Ultra-Low Sulfur Fuel (ULSF), has caused problems of its own. Typically, related incidents involving ULSF have adversely affected cylinder liner lubrication of large-bore, slow-speed main propulsion Diesel engines, and the diminished lubricity of ULSF has also been determined to be the cause of Fuel Injection Pump binding of generator engines, resulting in loss of power. In one such incident, the generator failure resulted in a loss of steering power aboard a vessel heading into port. Furthermore, it has been found that bio-fuel blends (also used as substitutes for conventional heavy fuel), are detrimental to elastomer sealing materials (a form of polymerized compounds), used in certain fuel transfer pump oil seals, apparently due to acidity of the biofuel due to oxidation.

DUAL FUEL
Yet another possible alternative, lies in the feasibility of converting contemporary main propulsion Diesel engines, whereby they become capable of burning "Dual Fuel". This would involve retaining the conventional Marine Diesel Oil (MDO) capability and modifying the engines to be capable of also burning Liquefied Natural Gas (LNG). Several recently built European tankers, designed for the LNG trade with dual fuel (DF) capability plus a fleet of Norwegian ferries currently in operation are also burning LNG, from which the exhaust emissions are reportedly extremely low. While conventional wisdom appears to fault marine Diesel engines for their exhaust emissions far more so than that of their highway or railroad counterparts, the truth is that for a riverine tugboat pushing a "Fleet" of twelve of fifteen loaded barges upstream, the Diesel engine exhaust emissions are estimated to be approximately 0.470 grams per ton/mile; which is about 27.70% lower than that of railroad trains and 35.60% lower than that of highway trucks, and are expected to become even lower if and when they eventually switch to burning LNG, given the current trend toward the adoption of LNG as a preferred marine engine fuel.

READINESS
Since most ships are now powered by oil-burning Diesel engines, either direct-drive, geared-drive or Diesel-electric drive, the number of steam ships still in operation has diminished considerably. However, in the case of heavy oil tankers, such as VLCCs, (Very Large Crude Carriers), despite their large main propulsion Diesel engines, of 50,000 bhp or more, steam boilers play a prominent role in providing steam to sustain the cargo tank heating coils. This is due to the fact that in cold climates such as found in Northern European seaports, the relatively low sea water temperature has chilled the heavy black crude oil to a such a high viscosity that the cargo can not be pumped out without being pre-heated by the steam-heated tank coils. In such instances, the critical factors become (a) the oil seals of the fuel oil transfer pumps and boiler service pumps and (b) the steam boiler burner tips. Are they all currently designed for use with ULSF? Or do they also need to be replaced with ULSF-compatible components? The inevitable conclusion to all this is that solutions to certain problems that in turn, create problems of their own, are unacceptable and must be carefully evaluated before allowing one problem to be replaced by another potentially worse. Hence, responsible ship-owners and ship-managers can not afford to ignore the above-mentioned factors and have a moral obligation to alert their crews to such potential problems and take appropriate instructive and/or corrective measures to ensure that the required level of "readiness" is capable of sustaining the appropriate level of sea-worthiness. LNG - Likewise, in view of the growing trend towards Dual-Fuel Diesel engines, despite their several years of experience with various grades of Diesel fuel, an appropriate level of readiness will be required of all those crew members involved with bunkering and monitoring of LNG fuel aboard ship. While traditional bunkering procedures are required to be followed to ensure maximum security and safety, the requirements for LNG bunkering are even more rigid. For instance, given that each flange or coupling connection is a potential spillage hazard, requiring absolute caution in connecting/disconnecting, the number of such connections should be held to a minimum. Furthermore, the LNG Bunkering Rules and Procedures shall include a mandatory "Emergency Shut-Down Procedure" (ESD), drill to be practiced periodically. In this regard, the eventual enforcement of ECA and SECA legislation is fast approaching, and the new Emission Control Area (E.C.A.) for North America takes effect August 1, 2012, requiring ships to burn fuel oil with a maximum of 1.0% sulfur content. This in turn will be reduced to 0.1% as of January 1, 2015, hence, conversion to LNG appears to be the most logical solution, and in this light, Ship-Owners are advised to consult with the major fuel refineries and Classification Societies for training programs that cover Formal LNG Bunkering Procedures.

LIQUEFIED NATURAL GAS, also known as LNG, is natural gas in its liquid form. When natural gas is cooled to minus 259 degrees Fahrenheit (-161 degrees Celsius), it becomes a clear, colorless, odorless liquid and is not corrosive. Natural gas is primarily methane, with low concentrations of other hydrocarbons, water, carbon dioxide, nitrogen, oxygen and some sulfur compounds. During the process known as liquefaction, natural gas is cooled below its boiling point, removing most of these compounds. The remaining natural gas is primarily methane with only small amounts of other hydrocarbons. LNG weighs less than half the weight of water so it will float if spilled on water. The choice of LNG is evidently encouraged by proven claims of lower exhaust emissions resulting from higher efficiency, cleaner burning characteristics, reduction in NOx and particulate levels averaging 75 percent, with almost zero levels of SOx. However, LNG is not without its limitations, one of which is that LNG storage tanks have to be heavily insulated to maintain a constant temperature of minus 165 degrees Celsius. However, the insulation is required not only to preserve the ultra-low temperature of the LNG tanks, but also to protect the ship structures from the effects of the cryogenic temperatures of the LNG. Additionally, aboard ship the required storage space for such tanks is estimated to be as much as 250 percent larger than that required for conventional Diesel fuel tanks of corresponding fuel capacity.

DIESEL-ELECTRIC PROPULSION
It is generally agreed that Diesel-Electric drive offers many advantages over steam turbine plants, medium speed Diesel geared drive, or slow speed Diesel direct drive. Although initial cost is somewhat higher, actual operating expenses are lower due to reduced engine room manning, reduced maintenance work load and greater fuel economy. This in turn ensures maximum efficiency and reduced air pollution by nitrogen oxides in the exhaust emissions, as a result of constant engine speed, optimized combustion and more stable engine loading. Furthermore, changing from the ahead to the astern direction is accomplished simply by reversing the rotation of the propulsion motors. For special purpose vessels such as missile, satellite and antisubmarine tracking ships; hydrographic, oceanographic, seismographic and fisheries research ships, the feasibility of cruising at slow speed for prolonged periods of time without imposing a corresponding penalty on the prime mover(s), is yet another area in which the flexibility of Diesel-Electric drive has proven itself to be a worthy alternative to competitive conventional drive arrangements. In addition to the improved fuel economy afforded by Diesel-Electric propulsion, in the case of vessels fitted with electrically-powered Azimuthing propulsion pods, there are several other benefits worthy of note such as the following: 1) The installation of azimuth propulsion pods (A.P.P.), completely dispenses with the need for conventional long drive shafts, shaft tunnels, steering gear and rudders. 2) The space thus saved can be used for revenue earning purposes. 3) If required, the A.P.P. system can also be used to maneuver a vessel during berthing or un-docking. 4) The A.P.P. system produces much less vibration and is much quieter than line-shaft propulsion. Except for maneuvering during docking and un-docking, sea-going ships spend most of their time operating at a speed of "full ahead" in the open sea. Harbor tug boats, on the other hand, invariably operate in close quarters; with due regard to the relative proximity of other ships moored at adjacent piers, and the traffic of passing vessels, and are required to exert pushing or pulling power in short bursts of high energy, in order to safely maneuver their ship into its designated mooring space. These short intermittent bursts of high energy, of brief duration but repetitive occurrence, tend to detract from the ideal cycle of constant speed, steady load and stable combustion conditions, thereby resulting in rapidly fluctuating cylinder temperatures, incomplete combustion, and consequent abnormal exhaust emissions. periods at partial loads, while the main propulsion Diesel-generators are operating at constant speed.

COMPARATIVE EXHAUST EMISSIONS
With a variable speed engine operating at approximately 30% of maximum continuous rating (mcr), such as may occur when a cargo ship is entering or leaving port; or the slow speed operational routine of special purpose vessels mentioned above, the exhaust emission of nitrogen oxides peaks around 2,230 ppm, which is 280% higher than the NOx emissions from an engine operating at 100% of mcr. Conventional wisdom appears to fault marine Diesel engines for their exhaust emissions far more than their highway or railroad counterparts. What is apparently ignored, is the fact that far less power is required to move a given tonnage of cargo for a given distance by river barge than either a truck or train requires to haul the same total load over the same distance. Given that river barges are usually "Fleeted" in groups of four, twelve or fifteen barges per tow, pushed by a single river push boat, it requires considerably less engine horsepower to move a ton of cargo by river barge for a given distance, than that required by either a conventional forty foot truck trailer or railroad box car to cover the same distance. Consequently, since the volume of fuel consumed by a river boat per ton/mile is considerably less than that of its highway or railroad counterparts, it follows that the Diesel engine exhaust emissions are commensurably less. Recent studies have shown that, as opposed to exhaust emissions from ship-docking tugboats, the nitrogen-oxide emissions from riverine Tugboat Diesel engine exhaust, estimated at 0.470 grams per ton/mile, are about 27.70% lower than that of railroad trains and 35.60% lower than that of highway trucks. However, just as automobiles and trucks in somewhat modified configuration will inevitably constitute the primary form of highway transportation for the foreseeable future, internal combustion engines, also in modified format, are likely to be in use for several years to come, both for land-based vehicles and sea-going ships. Initially, one may expect the major changes to be from spark-ignited gasoline engines and compression-ignition Diesel engines, to spark-ignited gas engines, burning either Liquefied Natural Gas (LNG), Compressed Natural Gas (CNG), Propane or derivatives thereof, for which the current Diesel engine basic configuration is most adaptable, until it is replaced by either a fuel cell or alternative form of prime mover of comparable power range. Currently there is a fleet of car/passenger ferries operating off the coast of Norway powered by Rolls-Royce Bergen Diesel engines burning LNG exclusively from which the exhaust is practically free of toxic emissions. It is worthy of note that conventional naval submarines originally employed Diesel-Electric propulsion, using the main engines for propulsion while simultaneously using the Diesel-driven electric generators to recharge their batteries.For submerged operation the Diesel engines were shut down and declutched from the generators, which then became propulsion motors powered from the batteries. Contemporary nuclear submarines however, while equipped with an auxiliary diesel generator, rely upon the heat generated by the nuclear reactor to generate steam that is then fed to a steam turbine for propulsion purposes. Their ability to remain submerged for up to ninety days at a time is attributable to their capability of drawing in seawater, extracting oxygen from the seawater to replenish their own atmosphere, and disposing of the hydrogen by pumping it back overboard. It is foreseeable that this capability may eventually be extended to surface vessels powered by Hydrogen Fuel Cell engines. In this case they may be capable of drawing in seawater and extracting the hydrogen to feed the fuel cell, using a form of hydrolysis. However, the current process for doing so is considered to be energy-intensive and it may require more in-depth research to find a less expensive method.

CONTEMPORARY FUEL CELLS
The basic requirements for a Hydrogen Fuel Cell are 1) fuel, 2) oxidant, and 3) an electrolyte plus a negative anode and a positive anode. Currently there are several different kinds of electrolyte available in which the corresponding operating temperatures and electrochemical reactions may vary according to the chemical constituency of the electrolyte involved. In a typical Polymer Electrolyte Membrane Fuel Cell (PEMPC), such as that specifically developed for Space Missions of the 1960's, the system operates at 80 degrees Centigrade as follows:

• Hydrogen is fed into the anode, which is the electrically negative post of the fuel cell.

• In the center of the fuel cell the electrolyte absorbs an electron from the hydrogen atom, using it to make electricity.

• The cathode, as the electrically positive post of the fuel cell, is where the electrons recombine with the hydrogen and oxygen to make water, which is the exhaust effluent.

This transition of protons and electrons is known as the ionic conduction mechanism that characterizes a Fuel Cell. The protons travel through the electrolyte toward the cathode, while the electrons move through an external circuit before re-combining with protons to form water.

New rules for the safe operation of Fuel Cells for marine propulsion purposes have recently been proposed by the world's largest Classification Society, Bureau Veritas, according to BV Product Manager Gijsbert de Jong. The intent being to establish a regulatory framework within which, building and testing of prototype Fuel Cell systems can be safely conducted while ensuring that the technology is developed and applied in accordance with safe performance-criteria. In his comments Mr. de Jong stated that "BV's guidelines for the safe application of fuel cells on ships take into account all relevant existing IMO conventions and guidelines, together with a wide range of international non-marine standards. They reflect BV's in-house knowledge and expertise, and could have important commercial - as well as environmental -implications for shipowners and operators." He further explained "The object of the BV guidelines is to provide criteria for the arrangement and installation of machinery for propulsion and auxiliary purposes, using fuel cell installations, which have an equivalent level of integrity in terms of safety, reliability and dependability as that which can be achieved with new and comparable conventional oil fuelled main and auxiliary machinery. The guidelines currently have preliminary status and are subject to internal and external review. After taking into account all relevant feed-back, they will be published as a Bureau Veritas Guidance Note entitled "Guidelines for Fuel Cell systems on board commercial ships." Of the various fuels available for use in Fuel Cell systems, hydrogen appears to be the most logical, given that (a), it is non-toxic, (b), yields a higher ratio of chemical energy per unit mass than that available from natural gas, and (c), it is abundant as an unlimited resource in atomic form. In addition to which, hydrogen is nonpolluting. Liquefied Natural Gas (LNG) which is now widely used in many Dual-Fuel marine Diesel engines, is also a strong contender for Fuel Cell use. Currently, there is an on¬going initiative in Holland known as the Green Tug project, piloted by the Offshore Ship Designers' Group including participation by Bureau Veritas, featuring a hydrogen-powered Fuel Cell tugboat designed for near-zero exhaust emissions level, and estimated to increase propulsion efficiency by almost seventy percent compared to that of a conventional Diesel-direct-drive plant.

THE TURN OF THE SCREW
After the naval architects have contoured the hull for best hydro-dynamic performance, further enhanced by the smoothest, long-lasting anti¬fouling surface coating possible, and powered by the most efficient marine prime mover power plant available, it finally becomes the "turn of the screw" to bring all these high-tech efforts to fruition, by ensuring the most effective propulsion system known to mariners. In this regard, it can be said that much of the credit for the successful development of contemporary marine propellers can be attributed to the tireless efforts and expertise of the Karlstads Mekaniska Werkstad company, located in the town of Kristinehamn, on the northern shoreline of Lake Vanern, in Sweden, that subsequently became known as Kamewa, before becoming a part of the Rolls-Royce Group. Rather than relying solely upon several years of manufacturing water turbines, Kamewa founded a turbine testing station in 1906 followed by a laboratory for testing model turbines in 1914 and a facility specially equipped for cavitation research in 1923. This experience coupled with the knowledge of controllable pitch water turbines, led to the introduction of Controllable Pitch (C.P.) propellers in the mid 1930's that has since become and continues to be primary product of this company. In addition thereto, since 1970, Kamewa has conducted extensive research resulting in the successful production of water-jet propulsion units on a large scale.

With the advent of the marine azimuthing propulsion pod during the late 1980's, Rolls-Royce figured prominently with their version known as the Rolls-Royce Mermaid. The most recent units are available in five frame sizes, from 1850 mm to 2770 mm motor stator diameter, and ranging from 5 Mw to 27 MW. Taking the marine propulsion task a step further, Rolls Royce has recently introduced the PROMAS LITE integrated propeller and rudder system that in most cases can be installed on vessels already in service to improve their propulsion efficiency. According to Rolls-Royce, behind a normal propeller hub, there is a strong low pressure vortex that acts on the propeller hub, increasing drag and reducing propeller thrust. With the PROMAS LITE system, a special hubcap is fitted to the propeller, which streamlines the flow onto a bulb that is welded to the existing rudder, effectively reducing flow separation immediately after the propeller. The result is an increase in propeller thrust as previously wasted energy is recovered from the flow. Roll-Royce also states that the installation of a PROMA LITE system on a European cruise ship increased the vessel's propulsive efficiency by more than 10 per cent at the cruising speed of 17-21 knots, with a corresponding savings in NOx. SOx and CO2 emissions.

Contributor
Louis John Lemos