Designing ferries is complex; architects must balance unique hulls and frequent docking with fuel efficiency and noise reduction.
|
Getting your Trinity Audio player ready...
|
Click to view glossary
Designing Ferries
How are ferries different from other commercial ships?
As a core component of Canada’s public transportation system, ferry operations are expected to be safe, reliable, and efficient. Vessel design is shaped by operating requirements, such as frequency of sailings, operating environment, space available for passengers and vehicles, and cost of service. To maximize efficiency, ferries transport as many passengers, vehicles, and cargo as possible in each voyage without compromising safety or passenger amenities.
Unlike cargo vessels, ferries are often double-ended, meaning the bow and stern ends of the vessel are mirrored. Rather than turning the ferry around, the captain turns from one set of controls to another for the return journey. Propellers at each end of the vessel also gives the ferry better stopping power and manoeuvrability than a vessel with propulsion at only one end.1
All vessels have an intended service speed. Deep-sea vessels typically operate at their intended service speed more than 95% of the time, slowing down to below this service speed when approaching a port or manoeuvring in constrained waterways. Durable fixed-pitch propellers are usually a good choice for deep-sea vessels as the propeller pitch can be pre-set at the optimum angle for the desired operating speed to maximize efficiency. However, when operating outside the intended service speed, the vessel is considered to be “off-design” – operating less efficiently than if it were travelling at the intended speed.
Ferries operate at their intended service speed approximately 70% of the time. For the remainder, ferries are either loading or unloading passengers while holding in dock. When a ferry moves into dock, the propeller revolutions typically decrease to slow the vessel for safe docking. The engines reduce power, and the propellers may turn more slowly or be reversed to help stop the ferry. These speed and revolution changes can create more noise than if the ferry were running at faster (optimal) speeds. In addition, noise can be amplified by the sound waves reflecting off the seafloor in the shallow water conditions near shore.
Learn more about how the ocean environment influences noise in the first article in this series, Technology for detecting and analysing underwater radiated noise
Another element is the expected service life for ferries, typically longer than cargo vessels. Some ferry fleets can keep vessels in operation for fifty years or more, with opportunities to retrofit and incorporate new technologies over the vessel’s lifespan. Fleet renewal is a slow process that must consider many factors, such as changing passenger trends and volumes, as well as efficiency, reliability, and safety. Identifying, designing, ordering, constructing, testing, and delivering new vessels – for ferry or cargo service – takes years. For ferry fleets, it is particularly important to maintain sufficient capacity for planned and unplanned (i.e. emergency) vessel maintenance over time so that the required level of service can be maintained when vessels need repairs or upgrades.
What factors do naval architects and marine engineers need to consider for ferry design?
Vessels are designed by naval architects and marine engineers, experts in ships’ structure, systems, and performance. Naval architects focus on the overall design, stability, and hydrodynamics, while marine engineers handle the propulsion, mechanical, and electrical systems.
Learn more about how vessel design influences underwater noise in the second article in the series, Quieter ship design: new-build and retrofit options
Ferry design has been primarily optimized for energy efficiency, to reduce fuel consumption and operating costs as much as possible. With growing concern about underwater noise impacts, marine engineers and naval architects are facing a new design challenge. The propeller, hull design, machinery, noise propagation pathways and other factors work together to influence the amount of underwater noise that a vessel produces. Reducing underwater noise is a multi-discipline problem.
Underwater noise reduction as a co-benefit of lowering greenhouse gas emissions
The Quiet Vessel Initiative funded study, Quiet Vessel Technologies Scan led by Vard Marine Ltd. provides an overview of different technologies that can reduce greenhouse gas emissions and underwater radiated noise. While some options primarily target one of these impacts, many involve trade-offs: certain solutions may reduce emissions but increase noise, while others may have the opposite effect.
In the context of global maritime decarbonization, it will be important to prioritize technologies that both reduce environmental impacts—such as underwater noise—and improve operational efficiency.
Most of the underwater noise produced by a vessel comes from the propeller. The amount of noise increases when the vessel is above its cavitation inception speed, which is influenced by the vessel’s design and the operating conditions. Cavitation is more likely with a higher amount of load or resistance through the water. When choosing a propeller, ferry designers must consider both “design” and “off-design” operating conditions. Typically, a controllable pitch propeller is a good choice for a ferry, as the operator can change the pitch or angle of the propeller blades for different operating conditions. This allows the propeller to continue to spin at a constant rate (revolutions per minute or RPM) while the vessel goes faster or slower by changing the angle of the blades. However, changing the propeller pitch to reduce speed can increase underwater noise since the propeller continues turning at the same RPM.
For more information on propeller types, watch this video by Casual Navigation on YouTube
How do ship designers know what will be quieter?
Before building vessels—or vessel components—naval architects will test different design options. Today, simulations and computer models are widely used to predict how vessels and vessel components will behave in different conditions. For example, for the Quiet Vessel Initiative project, Computational Fluid Dynamics Tools for Prediction of Cavitation-Induced Underwater Radiated Noise, the University of Victoria developed a cost-efficient model that can predict the noise produced by different vessel designs, without requiring vast amounts of data. Nevertheless, like all models, this model does require data. The most critical data for vessel noise models are the hull form and propeller designs. However, these designs are often proprietary and even ship owners may not have all the design details for their vessels as these are often retained by ship builders. To support the development of this computational fluid dynamics tool, BC Ferry Services Ltd. (BC Ferries) is sharing both past and recent design data for their ferries with University of Victoria researchers.
Alongside computer models and simulations, physical models are also widely used to test design elements. In the Lloyd’s Register and Defence Research and Development Canada project Composite Propeller Design for Noise Reduction, the team built a scale model of the hull of an Orca Class patrol vessel to test a new propeller design in a tank. Innovation Maritime’s project Plateforme de tests en milieu contrôlé pour l’évaluation de mesures de mitigation du bruit et vibrations générés par les navires (Controlled Environment Test Platform for Evaluating Noise and Vibration Mitigation Measures Generated by Ships) also used a physical model in a tank to assess different ways of reducing noise generated by ship machinery.
Learn more about these projects in the second article in the series, Quieter ship design: new-build and retrofit options.
This article was prepared by Clear Seas on behalf of Transport Canada as part of the Quiet Vessel Initiative and is part of a four-article series on ferries and underwater vessel noise.
Continue learning about the new discoveries and challenges in making vessels quieter with the other topics in this series here
The Quiet Vessel Initiative is a federally funded program through Transport Canada. Industry partners and researchers interested in potential research and development collaborations to advance innovative solutions in marine technology are invited to contact the Quiet Vessel Initiative team at Marine-RDD-maritime@tc.gc.ca.
References
Cavitation: Propeller cavitation is created by rapid changes in water pressure around the propeller. When a propeller turns, it creates a low-pressure area on one side of the blade and a high-pressure area on the other. When the propeller turns quickly, or the vessel and propeller are under a heavy load, the rapid pressure drop causes the water to evaporate and form vapour bubbles that move over the blades. As the bubbles reach the high-pressure area, they collapse, making noise.
Measurement efforts have shown that propeller cavitation is more common at higher speeds due to greater loads on propeller blades or when propeller blades are misaligned or damaged. Other vessel factors, such as the hull design, also influence cavitation. Propeller cavitation isn’t just a noise issue. It also erodes the propeller surfaces, reducing their performance and efficiency.
Computational fluid dynamics: Computational fluid dynamics is a method used to study how fluids, like water and air, move. It involves using computers to solve mathematical equations that describe fluid motion. With computational fluid dynamics, engineers and researchers can create virtual simulations to see how fluids flow around objects, such as ships, or specific components, like the hull or the propeller. Understanding how water moves around and interacts with vessels allows designers to see where drag or turbulence might occur and focus on optimizing vessel design. We can also use computational fluid dynamics to create simulations of different design concepts. This way, we can test and refine components without needing to physically build them, allowing for a more efficient design process.
