New Tricks from the Old Towing Tank

As kids, we enjoyed making paper boats, watching them float along a water channel, and during the rainy season, puddles of water would be the water basin for our boats to float on. The make or the design of the paper boat determined how long or how much of the natural forces it could withstand. Similarly, by towing a ship model into a humongous basin of water, naval architects can identify and understand the different factors that contribute to the seakeeping, maneuvering, and icebreaking capabilities of the ship.

Towing Tank or Experimentation Pool

Traditional towing tanks are large pools of water several hundred meters long or experimental facilities that take the shape of a swimming pool. These towing tanks have propelled the science of ship design, providing insights into ship hydrodynamics in different seawater conditions. The rudimentary practice of testing a ship design is to tow the model into a water tank using a towing mechanism and measure the force on the model during the process. William Froude, back in 1870, felt the need for a towing tank to study and predict the ship resistance; this opened doors to advanced towing tanks that naval architects in the marine industry widely use now.

For accurate measurements, it is essential for the towing tank to be built inside a climate-controlled building because a slight temperature change can tamper with the results. The towing carriage used for towing the ship is a large movable platform that stretches across the tank, has sensors equipped on them, and can hold a few scientists, those working closely on the ship design results. The model to be towed is often placed in the middle of the carriage. Towing tanks used for studying ship hydrodynamics are like wind tunnels used for aerodynamic testing of vehicles.

There are three tests carried out for the power prediction of a ship, and they are:

• Calm Water Resistance Test: This test helps in determining the resistance required for the ship's design speed. In this test, the ship is towed into the tank at different speeds (at least five different speeds), and respective measurements of force on the ship are recorded. It is often required to scale up the measurements for the full-scale model. This test is central in ship construction contracts where the contractors will be penalized if they do not meet the specified speed requirements.
• Open Water Propeller Analysis: This test is used for testing the propeller performance. Here, the propeller is tested at 20 RPS with the dynamometer running at different speeds. A towing tank is not essential for propeller testing. It can also be tested in smaller basins of water. For a propeller towed into the tank with a dynamometer set at different speeds, accurate measurements of torque and RPM are recorded. At different propeller speeds, the corresponding speeds of the dynamometer can be graphed to study the performance of the propeller.

• Self-Propulsion Test: Unlike the open water propeller analysis, here, the efficiency of the propeller is dependent on the speed of the ship. There will be slight changes in propeller behavior when it is placed behind the ship (in comparison to the results obtained from the open water propeller test). During the test, a dynamometer and a motor is installed inside the model ship and is towed into the tank at a fixed speed. After a point, there will be no drag on the towing carriage, and this no-drag neutral point helps calculate the propeller efficiency.

Advanced Towing Tank

Although towing tanks were initially designed for single testing purposes, i.e., ship resistance tests, as the design of ships progressed over time, the need for advanced towing tanks was realized. Today the towing tanks can perform a meticulous list of tests providing measurements close to real-life sea scenarios with expensive and advanced data-acquisition systems installed on the carriage. The three advanced towing tank tests include:

• Seakeeping Test: One of the advancements is to add waves to the towing tank (using wave generators). Not the regular waves but a combination of different wave types to create a realistic pattern. The ship's response to these waves can be measured. These measurements help analyze the required engine power, the ship design for passenger comfort (to prevent seasickness), and other measures for effective sea keeping. One limitation is that the towing tanks only work for bow waves.
• Maneuvering Coefficients Test: This test provides precise maneuvering coefficients for different hull types providing an answer to what rudder design perfectly matches the hull model for best maneuvering. Planar Motion Mechanism (PMM) on the carriage sways the hull side to side while moving it down to the tank and consequently records all the forces exerted while this is done. This side-to-side motion helps in deriving accurate maneuvering coefficients.
• Icebreaking Test: In this test, a block of ice of a specific thickness is fabricated, and a specialized carriage tows it into the tank. This testing can be quite expensive as new ice needs to be created after the previous one is destroyed. This test helps record the power required to break through the ice and the speed of the ice as the ship cuts through it.

Virtual Towing Tank

Cadence Fidelity Marine CFD simulation comprises dedicated, virtual naval architecture and marine design tools that function as a virtual towing tank, offering easy-to-use, scalable, highly automated optimization processes and unparalleled free surface modeling. Solve and optimize propulsion, resistance, seakeeping, wind studies, and maneuvering in your designs through our dedicated workflows and team. Equip yourself with the utmost accuracy and efficiency with Cadence CFD solutions.

With constantly changing sea, transport, loading, and racing conditions impacting the ship design performance, hundreds or even thousands of simulation runs may be required to finalize a design and to feel confident about the accuracy of the results. Hence, automation is crucial for minimizing turnaround time without compromising the accuracy of the results. Our solutions have automation for resistance at low and high Froude numbers, seakeeping, (self-) propulsion, trim optimization, open water propellers, and many other facets of ship CFD analyses. Our team of naval engineers has worked closely with our customers to create dedicated automation workflows. Whether you are deep in an America’s Cup or Vendee Globe campaign or are working to reduce drag and improve the propulsion efficiency of ships, you can trust our automated capabilities to provide optimal results with accuracy and speed.

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