Sustainable Water Travel: Could hydrofoiling boats revolutionise the waterborne transport industry?
In 2022, 20.4 million people travelled on cruise ships worldwide, and this is just one statistic out of the many forms of waterborne transport. According to the ICCT’s global shipping emissions inventory, the world’s largest and most efficient cruise ships emit around 250 gCO2/pax-km (carbon dioxide per passenger kilometre), meaning that a lot of water travel is actually less sustainable and eco-friendly than aeroplanes, and other forms of air transport. To combat this, in the last few years, some companies have taken a different approach, such as a company called Navier.
Navier was started in 2020 by Sampriti Bhattacharyya (CEO), and Reo Baird (CTO). They have formed a company designing boats that use electric hydrofoiling to reduce the CO2 emissions and decrease the negative effect on our environment. Their aim is to make waterways as accessible as highways, and enable a new era of clean waterborne transportation.
What is hydrofoiling and how does it work?
A hydrofoil is a foil or wing under water used to lift the boat’s body until it is completely outside of the water. Tina Rosado, a lecturer at the Massachusetts Institute of Technology, explains that at low speeds, the body of the ship sits in the water, and the hydrofoils remain entirely submerged in the water. As the speed increases, the hydrofoils create lift, and eventually the sum of the boat and cargos’s weight equals the lift created by the hydrofoils, therefore lifting the boat above the water. The use of hydrofoils also decreases the drag, as the boat is out of the water, which results in more efficient travel.
There are three main explanations for how the hydrofoil actually creates lift. The first explanation is called the Equal Time Theory. While this explanation is incorrect, it is the most widely circulated and well known.
NASA’s Glenn Research Centre explains that the Equal Time Theory states that on any airfoil (in this case hydrofoil), there is a longer upper surface and shorter bottom surface, meaning that the air molecules on the upper surface have farther to travel than those on the bottom surface in order for them to meet. This means that the air particles on the upper surface must travel faster than the others below. Since the upper flow is faster, using Bernoulli’s equation we know that the pressure is lower. This difference in pressure causes the lift. To see this effect in real life, you can actually take a strip of paper and blow on it. The air particles on the top will travel faster, there will be a difference in pressure, and the paper will lift.
This theory is incorrect for a number of reasons, one of them being that not every hydrofoil is created with a shorter and longer surface, some symmetrical hydrofoils do exist! This shows us that the difference in speed (leading to the difference in pressure) cannot be the cause for the lift. It also doesn’t explain why aeroplanes are able to fly upside down.
The next theory explaining the lift is the Particle Kinetics Theory. This theory is based on Newton’s Third Law, which states that for every action, there is an equal and opposite reaction. According to the American Institute of Physics, in simple terms, incoming molecules collide with the lower side of the hydrofoil, and deflect downwards. This causes an upwards force, lifting up the hydrofoil. However, unfortunately, this theory isn’t completely correct. This is because it fails to account for interacting molecules, and it also does not explain pressure differences on both sides of the hydrofoil.
The third and final theory is the Venturi Theory. According to NASA, the Venturi Theory is based on the idea of a Venturi nozzle, where there is a constricted section reducing fluid flow, and therefore reducing pressure. The idea is that the upper surface of the hydrofoil acts as a Venturi nozzle and constricts the flow. Due to this constriction, the flow speeds up, and so the pressure decreases, which creates a difference in pressure above the hydrofoil and beneath the hydrofoil. The difference in pressure is then what causes the hydrofoil to lift. Unfortunately this theory also does not explain how aeroplanes are able to fly upside down, or how a flat wing is able to generate lift.
As you can see, there is a problem that all of these theories can’t explain, and so scientists are still working on nailing the specifics and the details as to how the hydrofoil generates lift.
Why this is important:
It is important to continue research on how hydrofoils work so that scientists have a better understanding of them, and so that they can therefore be more widely used. Hydrofoiling boats are more efficient and sustainable than most current forms of waterborne transport, so if they could replace the current water transport, then it could help to protect and preserve the Earth’s environment.
Currently, waterborne transport isn’t sustainable, and its carbon dioxide emissions are even worse than aeroplanes and other airborne travel. To combat this issue, companies such as Navier are developing electric hydrofoiling boats that are more sustainable and efficient, with aims to hopefully replace big boats and cruise ships. Although scientists are currently divided on the way that hydrofoils work, with further research they can deepen their understanding, so that it can be used to implement hydrofoils even more, until waterways are as accessible as highways. Ultimately, global warming’s effects will soon be irreversible, and so in order to prevent that, there must be change, and this is definitely a step in the right direction.
Naya Olmer, Bryan Comer, Biswajoy Roy, Xiaoli Mao, and Dan Rutherford, Greenhouse Gas Emissions From Global Shipping, 2013–2015, October 17, 2017 https://theicct.org/publication/greenhouse-gas-emissions-from-global-shipping-2013-2015/
Tina Rosado, Reports on How Things Work, MIT Reports, 1999 https://web.mit.edu/2.972/www/reports/hydrofoil/hydrofoil.html
Tom Benson, Glenn Research Centre, NASA, Incorrect Theory #1, date not mentioned https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/wrong1.html
Shailesh S. Ozarkar, Ashok S. Sagani, Volodymyr I. Kushch, and Donald L. Koch, A Kinetic Theory For Particulate Systems With Bimodal and Anisotropic Velocity Fluctuations, 2008 https://surface.syr.edu/cgi/viewcontent.cgi?article=1020&context=bce
Tom Benson, Glenn Research Centre, NASA, Incorrect Theory #3, date not mentioned https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/wrong3.html