Why Hovercraft Failed as Passenger Ferries, but Not as Icebreakers
When hovercraft were first developed in the 1950s they were viewed as futuristic machines that could traverse everything and anything with an unlimited potential. They were envisioned to be the future of transportation, after all they were “fan”-”sea” looking vehicles. Moreover, hovercraft were hyped to be “seasick” free craft since they could just fly over water.
However, their biggest benefit was their seamless ability to transition between water and land. This made them great for amphibious military operations. Yet, ironically, the military had shelved this idea at first, just to come back to it later with some concepts calling for giant 3,000 ton ships. That never materialized, yet they came close to a 100-knot Navy. While Soviets developed the largest hovercraft in the world, the Zubr Class, Americans went the smaller route, developing Landing Craft Air Cushion or LCAC, to be used on amphibious ships and even on giant hovercraft carrier ships, which is kind of an ingenious idea. Besides their military use, the giant hovercraft were also commercially used as passenger ferries, but the big ones went out of service due to multiple hiccups along the way.
While it may seem that hovercraft failed, the truth is that in certain applications, they are irreplaceable, be it amphibious operations, search and rescue, or as a mode of transportation in difficult terrain. But what happens if the hovercraft engine fails in the middle of the water, and how hovercraft can be used as an icebreaker is not what you think. The simplest way to explain how hovercraft works is with the help of a CD. This is what happens if you take an inflated balloon and let it deflate downwards through the center of the CD.
Newton’s Third Law of Motion says that for every action there is an equal and opposite reaction. In this case, it would create an opposite force upwards which lifts the CD off the surface. And that’s what hovercraft are, they are ground effect machines.
The closer you are to the ground, the less thrust you need to stay above it. This holds true whether it's a helicopter, airplane or hovercraft. But the only difference is that hovercraft, unlike aircraft, cannot hover outside of ground effect. The giant fans pump the air beneath the hovercraft, which makes it hover just like the CD. Additionally the giant propellers on top of the hull provide forward thrust. Steering can be achieved either by rotating the propellers, or the rudders behind it.
Additionally, bow thrusters can be also used to further increase control, especially in tight spaces when a hovercraft is docking or undocking from the well deck of a ship. Modern hovercraft was pioneered by Christopher Cockerell who developed and showcased a model hovercraft to the British military. They loved it so much that they immediately labeled the concept of hovercraft a “secret”, but then quickly lost interest in it. Once hovercraft was declassified, Cockerell presented his invention to National Research Development Corporation which agreed to fund the development of the first full scale prototype, the SR.N1, which was built in 8 months by Saunders-Roe.
During testing, the prototype behaved mostly as expected with the biggest limitation being the relationship between the hover height and weight. During tests, it was found that adding four men aboard the SR.N1 weighing some 700 pounds reduced the height of hover by 1 inch from a maximum hover height of 9 inches. Adding 20 fully loaded Royal marines aboard the hovercraft, the weight of which was 31% of the total weight of the hovercraft, reduced the hover height only by 4 inches.
This demonstration was performed with 20 Royal Marines, as opposed to just 20 people, as a publicity stunt, to get some attention from the military who had previously lost interest in the hovercraft. Another publicity stunt was when the SR.N1 crossed the English Channel in two hours for the very first time between Calais and Dover. It was aimed to spark interest and attract funds for the development of this emerging technology. Note that while “hovercraft” is a generic name for air cushioned vehicles, it's actually a trademark owned by Saunders-Roe, an ex-British aero-marine engineering company.
But besides gaining a lot of publicity, the development of SR.N1 resulted in two important innovations for all future hovercraft. The first one had to do with blowing air from the edges of the craft, instead of the middle. Blowing air from the middle of the hovercraft turned out to be less efficient as the air would easily escape from the sides. But blowing air from the sides toward the middle trapped more air under the craft, which increased efficiency, and that meant that a smaller engine could be used, or that a greater hover height could be achieved. But this gain in performance wasn’t enough.
The SR.N1 still had an impractically low hover height of only 9 inches. Even on a day with relatively low winds, the waves can easily surpass the SR.N1’s 9-inch hover height, so a solution was needed. The second innovation was an invention by Latimer-Needham, a Royal Navy officer, who came up with the idea of a skirt – a flexible fabric around the hovering surface to contain the air.
The addition of the skirt to SR.N1 more than quadrupled the hover height, from 9 inches to 42 inches. While at first it may appear that the skirt was a silver bullet for the hovercraft, and in a way it was, it eventually turned out to be its Achilles’ heel as well. Makes you wonder, what would happen if the skirt inflation failed or the engines stopped? Hovercraft still have a ship’s hull underneath, be it watertight compartments or giant pontoons. So even without an air cushion, the vessel is perfectly at home just like a boat.
The most famous example of a hovercraft is arguably the SR.N4 – a class of passenger ferries, 6 of which operated across the English Channel between 1968 and 2000. Under the original configuration, the SR.N4 was able to carry 30 cars and 250 passengers, with the upgraded ferries being able to carry as many as 60 cars and 418 passengers across the channel in about 30 minutes. But you’d be amazed to find out why the giant hovercraft stopped operating as passenger ferries. The passenger cabin was noisy to the point that passengers had to yell at each other to be able to hear.
The ride was also incredibly bumpy during rough seas, resulting in half of the passengers getting seasick. Yes, it turns out you can get seasick on a hovercraft during rough weather. But riding the hovercraft was still a thrilling experience for most passengers! In fact, the SR.N4 operators such as Hoverloyd, Seaspeed and Hoverspeed were booked months in advance! Everyone wanted to travel aboard the SR.N4 which was viewed as a futuristic novelty.
There were two major accidents involving passenger hovercraft. In 1972, a smaller SR.N6 sank during strong winds while operating between Isle of Wight and Southsea.
Another accident occurred in March of 1985 during heavy seas, when the Princess Margaret Hovercraft charged into a pier at the entrance of Dover Harbour. 4 passengers tragically lost their lives. But the cancellation of the service was not due to these accidents either. Here’s the thing – very large hovercraft were somewhat of a technological dead end.
Arguably, the main reason was fuel. For instance the SR.N4 consumed 1000 gallons of fuel per hour. While giant hovercraft might make economic sense when gas prices are low, the oil shocks of the 1970s prevented the hovercraft from being widely adopted. Another issue was reliability.
Hovercraft were able to cross the channel with waves of up to 12 feet, meaning that anytime the weather got bad, the crossings were canceled. And remember the Achilles’ heel of the hovercraft? It turned out that the skirt was getting damaged and needed to be repaired almost after every trip. But even with skirt improvements, it still required weekly repairs which were expensive. The novelty appeal eventually faded, and what used to be booked trips months ahead, turned into empty seats.
But the final nail in the coffin of the fleet of SR.N4 came with the opening of the Channel Tunnel in 1994 and the end of duty free shopping in 1999. The last remaining commercial hovercraft service in the world is the Isle of Wight Hovercraft which runs regularly between Ryde and Southsea in Portsmouth. The reality is that hovercraft are only advantageous for very specific situations.
When it comes to hauling large amounts of cargo during any seastates, using a ship is a much better idea. When traveling on land, nothing is better than a ground vehicle. The question is, how often do you really find yourself in the need to traverse from sea to land, travel over swampy areas or ice? In non-military settings, not that frequently it turns out. Obviously, today hovercraft are used for search and rescue missions in hard to get places or during floods.
They are even used to maintain light bulbs on navigational buoys. But in Canada, where I live, hovercraft are used in a very unique way. Canadian Coast Guard icebreaker hoverboats are operated along the St. Lawrence river. During spring, as ice melts, water moves downstream, and if the mouth of the river gets jammed up, it can result in flooding. So it is very important to prevent the buildup of ice in certain areas. The issue is that conventional icebreakers cannot navigate the river due to shallow waters, so instead, hovercraft are used.
But how hovercraft break the ice is completely different from how icebreakers do it. Many icebreakers feature a distinctive spoon-shaped bow which is built with a toughened hull and internal frame. The curved bow allows the icebreaker to slide and ride up on thick ice, and then, the ship's enormous weight cracks through it. Hovercraft, on the other hand, “wake” up the ice. The cushion of air that leaves the hovercraft generates a wake in the water, which in turn, creates a wave that travels underneath the ice. Since ice is not flexible, a big enough wave can lift up a sheet of ice which then breaks underneath its own weight.
Adding some turns and forward motion by the hovercraft can increase the size of the wave in order to break thicker ice. Besides search and rescue agencies all around the world, the military remains one of the biggest users of hovercraft. Most militaries use hovercraft as a landing craft, used to connect a ship to shore.
The major advantage of hovercraft over traditional amphibious vehicles is that they are much faster and that they can transition from water to land without stopping. However there are two limitations. Military hovercraft cannot carry too much equipment without getting too heavy to hover. They can carry a tank, but in that case, they are only carrying one tank and nothing else.
Unless it’s a Russian one, which can carry 3 tanks. The second limitation is their extremely high fuel consumption compared to other modes of transport, which as mentioned before, was partly responsible for the decommissioning of passenger ferries. This is why hovercraft are used only where absolutely needed. During the 1960s and 1970s the US Navy experimented with Surface Effect Ships which were a crossover between a catamaran and a hovercraft.
When the air cushion was used, only a small portion of the vessel remained underwater – either propellers or waterjets. The top secret specifications of the Surface Effect Ship called for the ability to travel at speeds of over 80 knots and deliver a tank and 100 marines onto a beach. As part of this project, two 1/10th scale versions of the 3,000-ton concept were built, the SES 100A and the SES 100B, weighing about 100 tons each. The SES-100B was able to operate in waves of up to 6 feet high, and during one of the tests, it achieved speeds in excess of 96 knots.
The US Navy was really planning to be a 100 knot navy. But that never happened. The development of the 3,000-ton hovercraft concept stopped in 1980. Meanwhile, on the other side of the globe, yeah I said it flat earthers, a super hovercraft was being designed. This hovercraft is literally a beast, the Zubr-class, which in Russian means bison.
It can handle waves up to 8 feet. This is the largest hovercraft in the world, with a full load displacement of 555 tons. Zubr is big, really big, as you can see here. The hovercraft is causally stopping at a Russian beach to pick up supplies, most likely vodka, and the beach goers appear to not be impressed at all. Designed by the Soviets in the 1980s, during testing, Zubr was able to reach a speed of 74 knots.
That said, normally it operates at a speed of 60 knots, because when it goes too fast, there are some stability issues. But what limits the top speed of a hovercraft, is quite unique to this type of craft. As the hovercraft speeds up and moves through the air, there is a point at which the air resistance encountered by the craft collapses the front of the skirt. This results in loss of the air bubble that keeps the craft above the water.
The skirt’s ability to withstand the force of the air is the limiting factor for the maximum speed of the hovercraft. Zubr can go over barriers of up to 6.5 feet tall, carry 3 tanks or 10 armored vehicles with up to 140 troops. It is also the only military hovercraft that is armed. Zubr is equipped with 2 gatling guns and 2 retractable rocket launchers that are used for suppression of the beach defenses.
The 3 engines at the top provide the thrust necessary to move forward, and the 2 engines inside the ship generate the air required for the air cushion. The giant hovercraft is also very thirsty, consuming 9.5 tons of fuel per hour from its 56 ton fuel tank. The propellers on Zubr are actually from Tu-95 bombers. Currently, Russia operates only 2 Zubr-class hovercraft, while Greece and China purchased and operated 4 each. In contrast to the 2 mighty beasts operated by Russians, Americans operate several dozens of much smaller LCACs which have a displacement of 182 tons each.
LCACs are unarmed and can only transfer one tank at a time or up to 60 tons of cargo. In total, 97 LCACs were built and they are currently being replaced by up to 73 Ship-to-Shore Connector air cushioned vehicles that would have a maximum payload of 74 tons or 145 marines. The new hovercraft would be broadly similar to LCAC, with major changes including an upgraded skirt design, more powerful engines and fly-by-wire cockpit.
For the most part, LCACs are stationed aboard amphibious assault ships, but they can also be deployed aboard alternative, much more exotic vessels. There is one more gap that needs to be addressed. Large cargo ships cannot interface with LCACs. They have no means of transporting cargo between the two.
This is where the Expeditionary Transfer Dock ships, or ESD, come into the picture. The US Navy has 2 ESDs: USNS Montford Point and USNS John Glenn. Their primary mission is to act as a self propelled pier.
Large-draft ships would be able to unload vehicles and equipment onto the ESDs with the help of a vehicle transfer ramp. From then on, LCACs would transfer the vehicles and equipment ashore. This is especially useful where port facilities do not exist or otherwise have been destroyed. The weak point of these ships is that they are completely defenseless against missile attacks.
This is why the US Navy was initially looking into the possibility of arming these two ESD ships. But in 2022, the Navy requested for the retirement of both ships, even though they still had decades of service life ahead of them, and could prove invaluable during a potential conflict with China in the Pacific. Even though ESBs can do as many as two dozen trips to shore while transporting over 70 pieces of equipment, that’s only possible in fair weather conditions. ESBs cannot operate in waves that are over three feet in height, since they cannot safely connect to the sealift vessels via the ramp.
This is a big limitation compared to the newer Lewis B Puller class, which can use up to four CH-53 helicopters. This is why the US Navy wanted to retire the two ESD ships, but Congress rejected that motion in December of 2022. The thing is, ESDs work great for low intensity conflicts. They can help free up larger amphibious assault ships equipped with well decks and LCACs to operate in more dangerous waters. The large amphibious assault ships cost $2.5 billion dollars, but for $500 million, you
can take an Alaska oil tanker, remove the centerpiece, give it the ability to ballast up and down … and you got yourself an Expeditionary Transfer Dock. All that said, hovercraft and its variants still remain a crucial part of any military for amphibious operations that can break the enemy, and as for Canada, to break the ice!