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SR.N4: The Largest Passenger Hovercraft Ever Built

Written by Laura Davies


Ever wish you could cross the English Channel as quickly as it takes to use the channel tunnel, with the noise of a jet engine and the sickness-inducing motion of a catamaran? Well, I hate to break it to you, but you’re too late. The glorious, adrenaline-fuelled, and nauseating voyages of the SR.N4 are tragically over. But, they are fondly remembered by whatever percentage of their 80 million passengers managed to hold on to their lunch and still stand as an incredible feat of human engineering.



The Saunders-Roe Nautical 4, or SR.N4 for short, still holds the record as the largest passenger hovercraft ever built. At its biggest, it measured 56m (185ft) long and weighed 320 tonnes. On a calm day, it could reach speeds of 70 knots (130 km/hr) and carry 60 cars and 418 passengers.


The power needed to get this enormous beast off the ground, or sea, came from four Rolls-Royce Proteus marine turboshaft engines. Each one was responsible for a lift fan and a 5.8m (19 ft) steerable propulsion propeller. At the time the SR.N4 was introduced, these were the largest propellers in the world. Each engine would consume a climate-changing 1000 gallons of fuel per hour at cruise speed, and the craft was able to store 8068 gallons. This gave it a range of around 150 miles.

Inside, the passenger cabin was very similar to that of an aeroplane, with forward-facing seats and narrow aisles. It was built to rival air travel during its glamourous heyday, so there was a tasteful smattering of leather, wood panelling, and even legroom for anyone over 5ft. Of course, it was the 60s, so there were also no seatbelts, but they made up for it with cigarettes and alcohol.

Less luxury was afforded to the cabin crew as the hovercraft was designed by aircraft engineers, and it showed. The control cabin was tiny, unlike the spacious bridges of regular ships, and it was crammed with pedals, joysticks, yokes, and engine speed controls. Three crew members, the captain, first officer, and navigator, were squeezed into a space so small that there wasn’t even room for the navigator up front. He’d have to sit behind the captain and sing out his observations from the Decca 629 radar, the only thing that stood between them and a fatal collision. This meant that in foggy conditions, the captain would be flying blind and relying totally on the eyes and judgement of the navigator.


At first, this might not seem like a wise choice. However, it’s likely one of the design features that led to the hovercraft being one of the least deadly modes of transport ever. When a captain has a radar up front, their attention has to be split between that, the controls, instrumentation, and actually looking out of the window. Plus, radars weren’t easy things to interpret. There could be hundreds of blips on the screen, all travelling in different directions and at different speeds, which would have to be calculated by the length of their tails and speed of fading. Having more than one crew member reading the screen would result in discussion, rather than action. This is fine on a vessel travelling at 30 knots, but when you’re hurtling over the surface of the sea at 60 knots, there’s no time to chat about whether or not that blip might be coming right at you.

The whole vessel was held aloft by a 12-tonne rubber skirt, which trapped a bubble of air beneath it. This was the engineering breakthrough that took the hovercraft from a four-man ship to the enormous passenger craft it turned out to be.



The first seed of the idea that would eventually become the SR.N4 was planted by Sir John Isaac Thornycroft in 1877. He was a British boat designer who was wrestling with the fact that friction between the hull and the water was preventing boats from reaching high speeds. He suggested that a cushion of air under the boat could solve the problem, and he built a model with clockwork bellows that continuously pumped air between the hull and the water. It worked, and the boat’s speed significantly increased. Unfortunately for Thornycroft, he was ahead of his time and engine technology was not yet available to scale up his invention.

Over 70 years later, British inventor Christopher Cockerell turned his thoughts to the same problem. He didn’t pursue Thornycroft’s original design due to issues containing the air bubble. Instead, he devised a way of expelling the air through a narrow slot around the circumference of the ship in a system now known as a peripheral jet. This had two advantages. Firstly, the airflow would act as a curtain, trapping a bubble beneath the ship; and secondly, it would require less air to be pumped, and therefore less power from the engines.

He tested his theory, quite Britishly, in his kitchen by fixing a cat food tin inside a coffee tin and attaching an air blower to the base. When he turned it on, the air would be forced out through the narrow opening around the cat food tin and, when directed at a set of kitchen scales, could lift substantially more weight than when blowing through the coffee tin alone. The only thing that could make it more British is if his tins were tomato soup and baked beans. Shockingly, despite the high calibre of his demonstration, he couldn’t get any officials to listen.

Thankfully, in 1955, he enlisted the help of a boat builder friend, built a scale prototype, and got ready to dazzle the world with his incredible craft that could travel over both land and water. Unfortunately, the officials he approached with his contraption were still unimpressed, and he had the added hurdle that he’d invented something that couldn’t really be classified. He later recalled, “The navy said it was a plane, not a boat; the air force said it was a boat, not a plane; and the army was plain not interested.” Of course, his cause wasn’t helped by his demonstration technique, and he loved to recount a meeting in Whitehall where his hovercraft catapulted around the office, spewing fumes and forcing the Admiralty and Ministry of Supply officials to jump onto their chairs.

Luckily, Lord Louis Mountbatten saw promise in Cockerell’s invention and eventually managed to convince the military to take second look. This time they were interested but dealt another frustrating blow by putting the idea on a secret list, preventing Cockerell from discussing it with anyone else. It makes finding funding pretty hard if you can’t tell anyone what you need it for.

Finally, he managed to secure $1000 from the National Research and Development Corporation, and with it, his life savings and what he managed to scrape together from selling his belongings, he was finally able to have a full-sized craft built by Saunders-Roe. This would turn out to be the SR.NR1, or Flying Saucer, capable of speeds of 30mph and crossing the English Channel in two hours.

Cockerell was on board during the maiden voyage as ‘dynamic ballast’. Essentially, he’d throw himself from one side of the ship to the other to keep it upright. A few months later, the Duke of Edinburgh visited to try it out. Not content with being hovered around, he took control and floored it. Examination of the craft afterwards revealed damage to the bow, but it was never allowed to be repaired and instead given the nickname ‘the Royal Dent’.




While the SR.N1 was undoubtedly fun, it would need to be able to cross the channel much faster and carry significantly more than 4 people to be financially viable. It also needed to reach heights greater than 15 inches, as waves are higher than that and you want to go over them, not through them. The breakthrough came from Cecil Latimer-Needham, with the design of a flexible skirt. It could extend the air curtain far below the base of the craft and provide the lift needed. It also gave engineers the opportunity for endless skirt jokes: “She’s lost her skirt.”, “I’m just gonna take a peek under her skirt.”, “Frank’s been under her skirt for two hours, I think we’ve lost him.” You get the idea.

The next step was to build the SR.N2, which was much larger and capable of carrying 48 passengers. Unfortunately, this still wasn’t enough for the cross channel craft envisaged but it had a happy life as a ferry between Portsmouth and the Isle of Wight. The SR.N3 was a military version of the N2 but it never entered full service and instead had an undignified end as target practice.

If you can count, you might think this is where the SR.N4 comes in. You’d be wrong. Plans for the large passenger version of the hovercraft were abandoned in 1962 when priority shifted to the smaller SR.N5 and SR.N6. Don’t feel too bad about it though. The time on the shelf meant any issues and improvements were able to be figured out on the N5 and 6 before the N4 was constructed. Not only did this result in a more successful vessel, but it was far cheaper due to the developments made, which were vital for a craft designed for purely commercial purposes.

Of the improvements, the most significant was to the skirt. Pilots of the N5 and 6 had been frequently finding themselves skirtless and exposed, so tweaks were needed to keep it attached. Adjustments also needed to be made to the overall design as parts of the skirt were losing airflow and therefore height. After much experimentation, engineer Denys Bliss at Hovercraft Development Ltd. designed individual extensions of the skirt that could be fitted to the bottom. These became known as “fingers” and were a huge part of the hovercraft’s ongoing success. Now, the SR.N4 would be able to handle gale force 8 winds and 3.5m swells without risking its dignity.

In October 1967, the N4 was completed and named the Mountbatten class after its early supporter. In February 1968, it made its first flight on water, and work began to find an appropriate cross-channel route and landing sites. There wasn’t any funding for a pre-service passenger trial, so it entered commercial service in August 1968 as the “Princess Margaret”, with its guinea pigs, sorry, paying customers, onboard. Six SR.N4s were built in total, and some were later updated to the MkII and then the MkIII, increasing their capacity and, therefore, profits each time.



Commercial Use

Aside from the magic of a boat that could essentially fly, speed was the biggest attraction for passengers. The SR.N4 halved the journey across the channel from Dover to Boulogne to roughly 30 minutes, and Margaret’s sister ship, the “Princess Anne”, still holds the record for fastest channel crossing by hovercraft at 22 minutes. Of course, this is only the official record due to the 70mph speed limit in the channel, preventing recognition of the time Anne made it across, without passengers, in 15 minutes and 23 seconds.

As it would be another 26 years until the completion of the Channel tunnel, there were two options for commercial passengers to cross the channel with their cars at the time. Standard, boring, slow conventional ferries or new, exciting, frighteningly fast hovercrafts. So it’s not surprising they were a success and the two princesses managed to ferry 80 million passengers and 11 million cars between England and France in their 33 year lifetimes.

Flights for passengers began with check-in at an exclusive hoverport, followed by drinks, a meal, and a browse of the duty-free shops. They’d then be able to drive directly onto the car deck of the craft, find their seats and enjoy 30 minutes of intense noise, frightened fellow passengers, and a fair amount of vomit. As glamourous as hovercraft travel looked, the reality was often a bit much. The stewardesses, or “budgies” as they were known, had the, most of the time pleasant, job of looking after those on board. On bad days, they were tasked with selling as much duty-free as possible as the ship shot across the channel to a cabin full of screaming, fainting, puking, and occasionally praying passengers.



Fortunately, most of this terror was completely misplaced. In their 33 years of service, there was only ever one fatal accident. Although incredibly tragic, as four people lost their lives, this makes it one of the safest modes of transport ever. How it achieved this is incredible once you learn what actually went on. Most cite the buoyancy of the vessel as the reason for its great safety record. Others lean more towards incredible piloting and a fair amount of luck.

The first indication that hovercrafts might not have been as safe as their passengers were led to believe starts with the Coregs. These are the rules and regulations that apply to everything that floats, with the aim of keeping everyone safe. Unfortunately, as they were designed for vessels travelling at 15 knots, they were difficult to apply to the 70 knot SR.N4.

For example, rule 7 requires all vessels with radar to use some means to plot other vessels on their screen. This often involved a wax pencil on Perspex. The N4 was travelling so fast that this was completely unworkable and so rule 7 had to be ignored. Instead, the navigator would simply call out their observations as fast as they could.

Another rule discarded was that all vessels should travel at a moderate speed in poor visibility, usually agreed as around half. As this was designed as a way to reduce speeds by about 7 knots, it made no sense for the hovercraft to drop by 30. So this rule was thrown out too, and a hovercraft captain would frequently be found zooming across the channel at 60 knots, unable to see anything but fog out of the window.

The only thing that made this less stupid is that a hovercraft, unlike other vessels, can stop within a length and a half. This manoeuvre was called a “drop” and involved cutting the engines, causing the skirt to lose air and the craft to plough into the water. It wasn’t comfortable, but it was life-saving. Hovercraft have to be light and therefore flimsy, so a 70-knot collision would be fatal for all on board.

The channel was, and is, a crossroads for all sorts of vessels and tricky enough for those in normal boats. At that time, there were 400 transits and 150 crossings a day, resulting in at least one collision a month. To make things worse for hovercraft pilots, not all of the crossing vessels could be picked up by radar, as, in addition to large boats and other hovercraft, you’d also find yachts, pleasure boats, channel swimmers, and even people attempting to row across in tin baths.

Extra issues occurred in Pegwell Bay as the hovercraft would draw worms to the surface. This, in turn, attracted local people who’d collect them for profit, often directly in the path of the N4s. Most of the time, pilots would just have to steer around them, but one night, in thick fog, the pilot didn’t spot a dinghy in time. His only option was to keep going and fly straight over them. They all survived and got a peek up the skirt.

In another instance, now known as the Bentley Club Fiasco, a hovercraft carrying the Bentley club and their cars on an outing to Europe identified a small pleasure craft on the radar. They made normal adjustments to avoid it, but at the last minute, the smaller craft panicked, turned 180o and forced the hovercraft pilot to drop. Every Bentley, vintage Rolls, Bugatti, and Maserati were shunted nose to tail down the car deck.

Fortunately for the crew, it was accepted that if there was ever any risk of a collision, they should drop first and think later. If at any point the navigator shouted “Down!”, the captain had to drop the ship without hesitation, and it’s this that led to an extraordinarily low death rate. Yes, the princesses lost their skirts and suffered considerable damage from the plough-ins on multiple occasions, but it was worth it to save lives and reputation.




Despite the noise, drops, and risk to life, those who had the chance to cross the channel on an SR.N4 often speak positively of it. So what happened? Why don’t we cross the Channel in hovercraft anymore?

You might assume the opening of the Channel Tunnel was responsible, but the truth is that hovercraft were no longer financially viable. The gas-guzzling engines were incredibly expensive to run, maintenance costs were high, and with fuel prices rising, the only thing keeping the crafts profitable was the sale of cigarettes and booze. So, the end of duty-free sales in the EU in 1999 was the final nail in the coffin for the hovercraft, and the last N4s were retired in 2000.

Once decommissioned, both Princess Margaret and Princess Anne were bought by Wensley Haydon Baillie for £500,000 and stripped for engine parts for the superyacht, Brave Challenger. In a cruel twist of fate, the site of their storage and desecration was right next door to the hovercraft museum, whose managers had to watch, helpless, for years as the craft slowly rotted. Their end almost came in 2016 when the land was taken on by the Homes and Communities agency, which proposed redevelopment of the site and scrapping the NR4s. Fortunately, following a petition, Princess Anne was saved and given to the museum for renovation. Tragically, Margaret, who was in a worse state, was stripped of anything of use to Anne and finally broken up in 2018.

By Laura Davies



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