Our Drive - Part II: Passenger Vessels

While various alternative fuels are already being tested on cargo vessels, passenger ships are increasingly coming into focus. Awareness of sustainable drives is also growing significantly among passengers.

From hourly sightseeing tours and multi-day river cruises to weeks-long ocean voyages—passenger ships serve a wide variety of purposes. Unlike other modes of transportation, the primary function of these vessels is not simply to carry passengers from point A to point B. Instead, the ship itself becomes a platform for delivering memorable experiences.

The rapid advancement of technology enabling extraordinary onboard activities continues to raise passenger expectations. Entertainment options, the number of cabins, and environmentally friendly propulsion systems compete fiercely for the limited space available on board.

In our design process, we determine the optimal combination of these competing elements based on the vessel’s intended purpose and operating region. Since 1994, we have been a trusted name in the cruise ship industry, with a proven track record across numerous projects. Our partners have experienced the full range of the industry’s highs and lows—fluctuations that, from our perspective, are more pronounced than those in other maritime sectors such as special-purpose or cargo shipping. In this article, we aim to offer insights into the current state of the industry and reflect on its evolution.

The demands are increasing – and with them, the size of the ships

Interest in cruising continues to grow steadily. According to CLIA, the number of cruise tourists worldwide rose from 15.87 million in 2007 to approximately 30 million in 2019. Following the decline in passenger numbers during the COVID-19 pandemic, the industry has seen a steady recovery since 2021. Projections for 2025 estimate around 35 million cruise passengers. The most popular destinations remain North America and Europe.

To accommodate the increasing demand, cruise ships are becoming larger and more elaborate. To stay competitive, cruise lines are expanding their onboard entertainment offerings. Go-kart tracks, simulators, waterslides, surf pools, and multifunctional spaces are designed to provide a diverse and engaging experience. Expansive pool decks are being protected from environmental elements with increasingly sophisticated glass roofs. Additionally, large open atriums are being incorporated more frequently, blurring the lines between indoor and outdoor spaces.

The main attraction is no longer the journey — it's the ship itself.

Thanks to the wide range of offerings, cruises are increasingly attractive to people of all ages. High passenger capacities help reduce individual ticket prices, making these trips more affordable for families. Most of these large ships operate on HFO (Heavy Fuel Oil) or MGO (Marine Gas Oil). However, since December 2018, an increasing number of vessels have been equipped with LNG propulsion systems.

In recent years, we have observed a shift in the design of new inland vessels from traditional day-tour ships to event ships. These vessels are tailored for multi-hour functions such as live performances, parties, product launches, business meetings, or full-day training sessions and conferences. Such offerings are attracting a younger demographic on board. The trend of retractable glass roofs has also made its way onto event ships, further blurring the boundaries between interior and exterior spaces. With modern stages and multifunctional layouts, a wide range of entertainment formats can be accommodated. This development calls for increasingly close coordination between naval architects, interior designers, and outfitters.

River cruise ships are typically constrained in size by the dimensions of locks and bridge clearances in their operating regions. A common size for these vessels is 135 meters in length and 11.45 meters in width. Europe’s largest river cruise ship expands this width to 17.70 meters, sacrificing geographical flexibility in favor of higher capacity. Unlike ocean-going cruise ships, river cruise vessels cannot rely on size-based superlatives due to such limitations; even the largest river cruise ships must balance capacity with strict dimensional constraints.

We anticipate that river cruising will soon expand into coastal waters, merging the benefits of both ocean and river cruises. This shift would allow for more extensive use of shoreside infrastructure for energy supply, while also enabling the development of new routes—for example, Cologne to London, the Po Valley in Italy, or the river deltas of mainland Europe. In contrast to the increasingly entertainment-driven ocean cruise experience, these voyages would place a stronger emphasis on the destination rather than the ship itself. However, the development of this new vessel type requires a comprehensive analysis of structural and longitudinal strength to meet the greater demands of coastal operation.

Climate neutrality poses a significant challenge for ship operators.

For 20 years, we have been working on integrating alternative propulsion systems on ships. In this context, the fuel for future river cruise ships is becoming increasingly important. Over the past few years, we have developed several concepts for this purpose.

Hybrid solutions, typically based on a DC link, are often the preferred option. These systems consist of a base of battery storage, which is supplemented with additional power from another energy source. The potential alternative fuels we are considering at this stage include, in addition to a fully electric option, green-produced methanol, ammonia, and hydrogen.

The design often starts with a diesel-hybrid variant. Due to its high energy density, the space requirement for the fuel is minimal. Additionally, diesel tanks on inland vessels can be accommodated in the hull and double bottom, meaning they compete little with passenger capacities. In an engine room with traditional generators, electrical energy is produced and fed into the DC link.

The methanol hybrid system is closest to the diesel hybrid. Methanol can be used both in hydrogen fuel cells and methanol combustion generators. The fuel is stored like diesel in structural tanks in the hull, but these must be equipped with an additional surrounding cofferdam to comply with regulations, which reduces the available space in other below-deck areas. In various combustion engines, methanol can be injected directly from the tank. When using a fuel cell, the fuel undergoes a preparation process in which pressure and temperature are adjusted. It is then reformed to release hydrogen, which is fed into the fuel cell. Methanol is not locally emission-free, but it requires significantly less exhaust treatment compared to diesel.

The ammonia hybrid system undergoes a similar fuel preparation process. The difference from methanol is that ammonia cannot be stored in structural tanks on the ship but must be stored in special pressurized tanks. In addition to pressure and temperature adjustments, it undergoes a "cracking" process, where it is split into hydrogen and nitrogen. The hydrogen is then fed into fuel cells.

The pure hydrogen hybrid variant does not require any conversion processes. The fuel can be directly fed into the fuel cells, which is an advantage over the other variants. However, the key advantage of methanol and ammonia is that the fuel can be stored in a highly compressed form below deck. The diffusion of hydrogen atoms through the tanks makes storage in enclosed spaces possible only with high safety measures. Often, it is more practical to locate hydrogen tanks on the upper deck, outside the ship. For hotel ships, this would mean sacrificing expensive, sought-after cabin space.

The safety of passengers is the highest priority

The greatest challenge in using alternative fuels in the passenger sector is the associated safety risks. Any area where the fuel system interfaces with the ship is considered a potentially hazardous zone and must be explosion-proof and separated from other spaces. Additionally, there are strict requirements for ventilation, which define the exit point of air from these areas well away from passenger spaces. For example, this is typically achieved on ocean-going freighters through tall, rigid exhaust masts.

However, in river environments, the fixed point heights and lock sizes must also be observed. In our designs, we often implement ventilation using a large telescoping mast on the upper deck, which extends about 20 meters aft and 10 meters upward. Scenarios such as passing under bridges or operating in locks impose special requirements on energy systems, which must function without using the primary fuel source.

The sizing is derived from sea freight regulations, as the standards for such systems have not yet been defined for inland passenger vessels. On the high seas, safety measures ensure the protection of the crew and the ship itself, while in inland waters, the safety of nearby residents and large cities must also be considered. In docking situations, people must be located outside the ventilation mast radius, both on the ship and on land. In the event of fuel leakage, passenger and crew evacuation can occur quickly in inland areas, but environmental damage could still occur in the vicinity of the incident.

High costs, limited range, limited support

To establish a benchmark for the space efficiency of alternative fuels, we assumed a constant passenger capacity across all concepts. On average, under this assumption, we were able to accommodate only about a quarter of the energy capacity for methanol, one-sixth for ammonia, and one-eighth for hydrogen compared to a diesel-powered system on the ship concepts. This is due to the volumetric energy density of the fuels, as well as the space required for additional operational components such as fuel preparation. These values are averaged across several projects and different ship sizes.

Furthermore, the capital expenditure (CAPEX) for alternative propulsion systems is significantly higher than that of a diesel-hybrid system, and it is still unclear which alternative fuel will become the dominant choice across the industry. As a result, infrastructure development has stagnated, driving up operating costs (OPEX). Ship owners who take the step toward adopting an alternative propulsion system are taking on significant risk due to the uncertainty surrounding future developments. At the same time, large shipping companies have the opportunity to shape the direction of the industry through a "First Mover Advantage." Smaller shipping companies, who would like to retrofit their vessels with an alternative propulsion system, are reliant on government subsidies and must keep pace with the ongoing developments.

Refits are often a challenge due to limited space

The German inland fleet alone consists of approximately 1,000 passenger ships. Due to the limited capacities of shipyards, it is not feasible to replace all ships promptly with new builds featuring alternative propulsion systems. Many existing fleets on European waterways are still in good condition and have several decades of operational life ahead, making the immediate replacement with new builds economically and ecologically unfeasible.

Through hybridizing the existing fleet using modern battery systems, efficient generator sets, and active exhaust treatment, emissions can be significantly reduced. In line with this, there is a program to promote emission-free and low-emission drives, as well as the sustainable modernization of inland vessels. We have already successfully supported this process in several projects. The limited space conditions require precise placement and sizing of the components, which we ensure through accurate 3D scans of the existing ship spaces.

Many passenger ship operators are partially publicly owned, as they are affiliated with cities and municipalities. So far, these operators have often been excluded from subsidies. The involvement in daily operations makes it particularly difficult for small companies to thoroughly address the technical issues surrounding the energy transition, as well as the available funding programs. Without these subsidies, retrofitting for emission reduction is hardly economically viable.

A common issue is the unclear regulatory framework from authorities. Existing diesel-mechanical propulsion systems often do not meet current emission standards. Retrofitting to a new, more environmentally friendly propulsion system may require re-certification of the ship according to current regulations, depending on the extent of the changes. This could lead to regulatory issues and possibly the decommissioning of the vessel, as, for example, current stability criteria may no longer be met. So far, each case has required an individual review of which regulations must be followed due to the retrofit. For planning and budgeting a retrofit, a unified decision matrix would be helpful. This matrix should clearly outline which regulations apply in which cases, and which components are protected under the grandfathering clauses.

Ship owners could use such a matrix to develop a long-term modernization strategy. This would help assess which ships are worth retrofitting and which should be replaced by new builds in the future. It is currently not entirely clear which propulsion systems for passenger vessels will dominate in the respective markets. Predicting developments over the next 15 years is not a trivial task.

Engine manufacturers are heavily investing in the development of methanol engines. Their product portfolios are being expanded in terms of performance from the top down. Large methanol engines are already in operation. For passenger shipping in coastal and inland areas, the performance range of around 150-700 kW is crucial. More engines in this performance range are becoming available. It is expected that authorities will soon tighten regulations for methanol, allowing ships to be planned and built to consistent standards.

The use of fuel cells is also showing new developments: Recently, Viking Cruises, together with Fincantieri, presented their strategy for the use of containerized fuel cell tank modules powered by liquified hydrogen.

The future is characterized by uncertainty

The audience for inland and coastal cruises is becoming younger. This brings new demands for the cruising areas, layout, design, and equipment of the ships. Among this target group, a heightened awareness of sustainability is expected, which will particularly influence the requirements for propulsion systems.

While the pressure for emission reduction increases, operators must still act economically. Striking a balance between passenger capacity, entertainment offerings, and the space needed for environmentally friendly technologies is especially challenging.

As already seen in the cargo shipping sector, there is a lack of a unified strategy for the transition to climate neutrality in the passenger cruise industry. The infrastructure for alternative fuels is insufficiently developed due to the absence of clear political guidelines. As a result, shipping companies that choose sustainable technologies face significant entrepreneurial risks. Europe-wide uniform regulations, along with coordinated infrastructure development and targeted funding programs, could significantly increase the attractiveness of transitioning to a climate-friendly propulsion system.