The case for multiple engines
There is a case developing where the use of multiple engines – usually several high speed units - can be justified on deep-sea ships on the grounds of both improved safety and improved economy, writes Dag Pike.
The image of merchant shipping is one of single-engined ships steaming across the oceans. Historically single engines on a ship offer the lowest construction costs and the highest efficiency and with optimised hulls, engines and propulsion systems this remains a valid case for most ships.
The safety case for multiple engines is strong, because engine failures can and do occur and with a single engine the ship is then helpless. It is the same with a single steering system and the two systems go hand in hand. Any failure on a vessel with single engine and single steering system is more likely to occur in rough seas when all systems are under greater stress. So in the interests of reducing the risk of pollution, if not in saving lives, it is surprising that a twin engine and steering system is not demanded by IMO for new construction.
Safety and reducing pollution may not be at the top of every ship owner’s priorities but economy of operation certainly is. This is where the case for multiple engines has become stronger in recent years. We are seeing a trend here, where multiple engine installations are growing in popularity. Multiple engines are not new, most of the famous liners of old and many warships featured triple or quadruple engine installations, but this was done largely in order to accommodate the high power required for high speeds.
Today’s cruise ships are not so focused on speed but multiple engines are used on virtually all passenger vessels. The introduction of azimuthing drives and diesel electric systems has made it much easier to accommodate multiple engines and drive systems. Even with the more conventional propulsion systems found on most ro-pax ferries, multiple engine installations are the norm; with most having four engines coupled to two shafts. The logic of such a system is simple. These ferries operate to very tight schedules and even with one engine out of action, the ferries can still maintain an adequate service. By using CP propellers, each shaft can operate at its optimum using one or two engines.
The new ‘Safe Return to Port’ requirements for passenger vessels that have recently come into operation demand that not only should passenger ships have two or more engines but that they should also be in separate compartments so the flooding of one engine compartment will not immobilise the ship. This sounds like a sensible requirement for all shipping but the focus on single engines and a single steering system for cargo ships still remains, with perhaps the only exceptions being the new generation of large, fast container ships and with ships carrying LPG. In these ships multiple engines are installed for practical or economic reasons rather than for safety considerations
Another non-safety related reason for a twin engine installation on these ships is to reduce the draught because a single engine demands a large propeller at the limits of current technology. Using multiple engine installations as a means of reducing draught is a tried and tested formula and is used to great advantage on the crew boats that serve the Gulf of Mexico oil sector. These crew boats, which can range up to 200ft in length, and which carry cargo and personal out to rigs sometime 200 miles offshore, have up to five engines installed, each with its own propulsion system. The beam of the hull allows three or four engines to be installed across the stern and with five engines they tend to be staggered in a two/three configuration. The engines are installed mainly on vee-drives, but more recently water jets have been the propulsion of choice, which gives greater operational flexibility and lower draught.
The engines are usually Cummins or Caterpillar diesels of between 1,500hp and 2,000hp each. Compared with larger diesels these are relatively low cost units for the same total power because they are based on production engines rather than the one-off assembly used for larger engines. For the power installed, the cost of these multiple engine installations is lower than a twin engine installation and there are the added advantages of a shallow draught and the ability to maintain operations with one or two engines out of action.
A similar but larger scale installation is used on many fast ferries. Large catamaran ferries employ two engines in each hull, installed one forward of the other within the narrow hulls. The most usual types of engines used are closer in concept to medium speed diesels rather than the high speed diesels found on smaller craft, and they are coupled to water jets again to offer a reduced draught and the ability to operate with one engine out of action.
A similar type of installation was used on a catamaran tug built in the US where four 1,000hp Caterpillar diesels were used, two in each hull. The main objective here was to achieve a shallow draught when pushing a 15,000 dwt sea going barge.
The leisure sector is moving towards multiple engine installations and here the incentive has been lower costs, greater flexibility and reduced space requirements. These multiple engine installations have been made easier by the introduction of pod drives that are now available to handle power outputs of over 1,000hp. One problem that has been found with these pod drives is that because they are not retractable, there can be a considerable drag penalty when trying to operate at anything over displacement speeds.
There could be considerable attraction in using multiple engine installations in coastal shipping but the challenge here is to find a propulsion system to match the requirements of a series of small engines. Small high speed diesels that are based on mass produced truck engines are a cheap option compared with the hand-assembled larger engines and with modern common rail injection systems they can offer a matching economy. Three or four of these engines could provide the 3-4,000 hp required for a coastal ship but at present there is not a propulsion system that can transmit this flexible power to the water. Pod drives might provide the answer but ideally these should be capable of being retracted when not in use so that the ship can be operated effectively with only two or three engines operating. Another advantage of using a series of small engines is that one or more can be used to power the generator when in harbour for further cost savings.
The focus on engine choice for shipping has been based on economy of operation, simplicity and the ability to operate on heavy fuel oil. The latter requirement rules out the use of smaller mass produced diesels but with increasing demands to use only low sulphur fuel in coastal areas, the case for multiple small engines gets ever stronger. Improved gearboxes make them more feasible, and propulsion systems are waking up to the new requirement. Such engine installations offer greater redundancy and thus great safety and the ability for the ship to maintain operations even with one or more engines out of action.
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