Fast Ferry Powering and Propulsors – the Options
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Fast Ferry Powering and Propulsors – The Options By Nigel Gee Managing Director Nigel Gee and Associates Ltd, UK SUMMARY In the 1970’s and 1980’s, fast ferries were used to transport passengers only and most were propelled by a pair of industry standard 16 cylinder diesel engines each driving a waterjet. The size of these vessels was mainly suitable for 300-400 passengers and with speeds of 35-45 knots. Today, passenger ferry sizes have increased and speeds up to 60 knots are now possible. During the 1990’s in excess of 100 fast car/passenger ferries have been introduced into service. The speed of development possibilities for the future are to an extent governed by available prime movers and propulsors. With increasing size and speed, high installed powers are required and this has lead to multiple prime mover and propulsor installations. This paper examines some of the engine and propulsor options open to designers, builders, and operators, and shows how powering and propulsor choices have been made through a number of case studies. AUTHORS BIOGRAPHY Having graduated with an Honours Degree in Naval Architecture from Newcastle University in 1969 and, in the same year, completed a shipyard apprenticeship sandwich course with Swan Hunter Shipbuilders in Newcastle, England, Nigel Gee entered a career in the Naval Architecture of high speed and novel ship and boat forms beginning with Burness Corlett & Partners, Consultants, in Hampshire, England, moved to manufacturing industry with Hovermarine in 1971 being promoted to Engineering Manager in 1976. Left Hovermarine to pursue an academic career in 1979 as Senior Lecturer in Naval Architecture and Fluid Mechanics at the Southampton Institute. Lectured to First Degree level and undertook a number of research projects linked with industry. In 1983 returned to industry with the Vosper Group as Technical General Manager of a department with 60 technical personnel. Left in 1986 to start the design company Nigel Gee and Associates Ltd. Since 1986 the company has undertaken designs for over 120 built fast vessels. These vessel designs range from 10m, 30 knot crew boats, to 200m, 25 knots fast container ships. In the field of fast ferries, the company has produced designs for a number of SES and catamaran designs including two 36 knot ferries introduced into service in New York Harbour in 1997, and a 55 knot vessel which entered service in 1 Argentina in January 1999. A number of designs have been produced for fast car and passenger ferries and fast freight vessels. Design is in progress for a fast car ferry due for delivery in mid 2002 and ten vessels have been constructed to the company’s design for a 25 knot fast feeder container vessel. Further designs for fast freight vessels with speeds from 30-60 knots are in progress. Nigel Gee is a Fellow of the Royal Institution of Naval Architects and a Member of the Society of Naval Architects and Marine Engineers. 1. ENGINE OPTIONS Engine options for powering large fast passenger craft, or fast Ro-Pax craft have been examined. Only engine powers in excess of 2000kW per single engine have been considered. The high speed diesel engines, medium speed diesel engines and gas turbine engines normally considered for fast ferry powering are listed in Tables 1, 2 and 3. These are manufacturers data for dry engines without gearbox. Footprint is calculated from the engine overall length by overall width. Specific Fuel Consumption’s (SFC’s) are manufacturers quotations in ISO conditions. Figure 1 shows the distribution of high speed and medium speed diesels and gas turbines according to power ranges in steps of 2MW. It can be seen that a range of high speed diesels are available to cover powers from 2-10MW with the number of engine choices in each power band falling with increasing power. Similarly, for medium speed diesels there is a wide range of engines available up to 20MW, and then further single engines available up to a maximum of 36MW. Multiple gas turbine choices are concentrated in the range 2-6MW with individual engines covering a number of higher ranges. There is a significant gap in the availability of gas turbines for powers from 8- 14MW which is becoming an increasingly common fast ferry power demand. It is of course possible to fulfil this demand by using multiple engines, albeit with more complexity and possible use of heavy combining gearboxes. Figures 2, 3 and 4 are plots of engine power to weight ratio for a range of powers. Figure 2 shows all the diesel engines and Figure 3 the gas turbines. Power to weight ratios are compared for diesels and gas turbines in Figure 4. It can be seen that the power to weight ratio of the gas turbines is very significantly higher than the diesels, generally ranging between 25 or 40 times as high for gas turbines. Similarly, Figures 5, 6 and 7 show power to footprint ratio, and once again the gas turbine engines are superior, generally having power to footprint ratios three to five times greater than those for diesels. Of course, footprint is not the only consideration when looking at the volume requirements for engine rooms, and the increased volume of intake and exhaust systems and intake air filtration systems required for gas turbines, often means that there is little difference in the volumetric requirements for gas turbine and high speed diesel engines, particularly in smaller high speed passenger ferries. 2 In general, it can be stated that the weight of gas turbine installations will be very significantly less than for diesel installations, and volumes may be less particularly in larger power installations. On the basis of weights and volume alone, gas turbines would be favoured. Figure 8 shows a comparison between specific fuel consumption for the range of gas turbines and diesels, and it can be seen that at any given power level gas turbine SFC’s are higher than diesel SFC’s. Of course it is also true that gas turbine installations require less power for a given vessel speed, because of their lower weight contributing to a lower displacement for the vessel. If less power is installed then there is an effective saving in SFC and this is shown in Figure 8. Nevertheless, in general even this reduction in fuel consumption is insufficient to offset the increased SFC of the gas turbine in most cases. Table 4 shows some typical vessel displacements and machinery weights for a range of vessels designed by Nigel Gee and Associates. In each case, the percentage reduction in displacement which could be achieved by substituting gas turbines for diesels is shown together with the percentage SFC increase. In four of the five cases, the SFC increase more than offsets the displacement reduction. In the case of the very large 40 knot ro-pax, there is a larger displacement reduction than SFC increase. This particular vessel has an installed power in excess of 100MW, which accounts for the large reduction in weight and at this power, gas turbines fuel consumption is approaching that of large diesel engines. These figures must be viewed with some caution since the weight of the engine and gearbox only has been considered and not the associated inlet and exhaust. Nevertheless, there is a clear indication that for larger vessels the installation of large lightweight efficient gas turbines could have advantages in fuel consumption as well as weight and volume. A criterion often advanced for assisting in the choice between diesel and gas turbine installation is that of range. Figure 9 shows the variation of speed with range for a vessel fitted with alternative diesel or gas turbine installations of the same power. Clearly, the gas turbine vessel will have a lower empty weight and, therefore, a higher speed at low range. However, since the fuel consumption is higher, then as range increases, the amount of fuel carried increases to the point where at a certain range the gas turbine vessel is actually heavier than the diesel vessel and its speed lower. The preliminary conclusion would be that at up to the critical range the gas turbine installation would be selected and above the critical range the diesel selected. However, if the power of the diesels are increased to give the same empty speed, then because the percent weight increase is normally less than percent SFC saving the diesel vessel will show benefit throughout the range. Weight, volume and SFC are of course only part of the story. Table 5 lists other considerations which owners and operators will need to look at before deciding on a particular engine installation. The figure is self-explanatory and perhaps the main features are the purchase and maintenance costs and the reliability, availability and maintainability of the chosen units. The case studies in Section 3 of this paper show how these considerations often become dominant in engine choice. 3 2. PROPULSORS Table 6 shows the propulsor options of fast ferries, together with an indication of the maximum efficiency that might be expected, and some qualitative descriptions of the potential advantages and disadvantages of a particular propulsive device. For most small vessels, the choice is between propeller or waterjet and the selection will normally be made on the basis of speed. Vessels having a speed capability over 35 knots are most likely to have waterjet propulsion because high efficiencies are still possible. Propeller efficiencies at these higher speeds are diminishing and propeller sizes may become unmanageable on small lightweight high speed vessels. For very large vessels electric podded propulsion is becoming a significant option. 3. CASE STUDIES Table 7 below lists six vessels designed by Nigel Gee and Associates which are used to illustrate some of the criteria used in selecting prime movers and propulsors for these vessels.