Exotic Hulls Part 5 - Surface Effect Ships

The hovercraft is an extremely useful configuration for those who need true amphibious performance, but it also has serious limits, most notably the fact that it is inherently an active system, and takes almost as much power waiting as it does at full speed. This prompted designers to think about options for how to use the same basic technology, but in a more ship-compatible way.

A recreation of the "air cushion torpedo boat"

The basic idea of using air underneath a ship to make it go faster isn't new, and the first example predates the hovercraft by decades. Austrian naval officer Dagobert Muller von Thomamuhl developed an "air cushion torpedo boat" that used a separate engine to force air under the hull and could make 32 kts or more, a reasonably impressive performance for WWI, but not particularly spectacular. Other designers continued to produce similar schemes to make use of air to speed up their ships, such as the Hickman sea sled, which used the vessel's own motion to trap an air cushion underneath it and make its planing hull more efficient. A later plan, known as the hydrokeel, was tried on some protoype landing craft in the early 60s, and involved injecting air into the bottom of a W-shaped hull, sealed by a bow flap, in an attempt to make planing more efficient. Test showed that it wasn't really any faster than a conventional planing hull, and the idea was scrapped.

But it was about that time that engineers worked out a better version of the semi-hovercraft, known as the Surface Effect Ship (SES). This was essentially a catamaran with hovercraft-type seals fore and aft. This had several major advantages. First, it takes a lot less power to maintain the air bubble, because the air can only leak out of the skirts fore and aft. Second, the catamaran hulls make the craft a reasonable boat if the air cushion is switched off, able to do normal boat things like stay still and dock without deafening the neighbors. Because there are still hulls in contact with the water even when on the cushion, SESs also rely somewhat on planing for lift, and tend to exhibit the pronounced "hump" of that type, where a great deal of power is required to reach the "hump speed", and then once there, only very little power is required to go significantly faster. Third, unlike a normal hovercraft, the SES can use in-water propulsion mechanisms like propellers or water jets, which are much more efficient than airscrews because water is denser than air.¹ This also allows the SES to dodge a problem that appears for larger hovercraft, namely that as the hovercraft grows, so does the number of propellers needed, and they eventually start interfering with each other.

An SES carrier concept

As a result, the SES was the configuration of choice for those looking to build a much faster fleet in the 1960s. A study reported in a 1966 Proceedings article² describes a study for a 12,500-ton SES, which requires 315,000 hp³ to reach the hump speed of 42 kts, and can then reach 65 kts without any additional power. Getting to 100 kts would take 420,000 hp, while 150 kts would require somehow producing 835,000 hp. But none of these studies came to fruition, as budgets tightened in the 60s and nobody was able to come up with a particularly compelling case for a 60-kt surface ship.

Two of the Coast Guard SESs at speed

Despite this, a tiny number of SESs have seen service. As is probably inevitable for any sort of unusual hull form, the first user was a high-speed ferry. The HM.2 type, developed by the British, was first used across the Solent in 1968, but that service lasted only a few years and reports of further use of SES ferries are sporadic, with a Japanese effort about 20 years ago to build much larger ferries falling victim to rising oil prices. The first military adoption was by the US Coast Guard, which procured three craft originally designed as high-speed delivery boat in the oil industry in the early 80s. These vessels, Sea Hawk, Shearwater and Petrel, were assigned as a group to Key West⁴ where they bolstered the Coast Guard's capabilities for law enforcement (primarily drug interdiction) and search and rescue along the Gulf Coast. Their ability to make 30 kts, at least when everything was working, was a big step up over the 16 kts that conventional cutters were capable of, and they seem to have been at least moderately successful, but they were retired in 1994 and not replaced.

Cardinal

The 70s and 80s also saw the USN make repeated attempts to get into the SES game. In the late 70s, plans were drawn up for a 3000-ton SES frigate capable of 80 kts or more and carrying about the armament as a Perry. The designers were interested in the large amount of topside space and high stability provided by the configuration, but despite a design contract being placed in 1976, the whole program was cancelled by the Carter administration before it got very far. A few years later, they tried again with a pair of smaller, less ambitious programs. The Cardinal class minehunter would be a small, cheap minehunter taking advantage of the air cushion to insulate it from shocks, while the Sea Viking was intended as a high-speed delivery craft for SEALs and the like. Unfortunately, both ran into serious problems, with Cardinal suffering from issues with the special engines⁵ and a test section of fiberglass hull delaminating under explosive shock tests, while Sea Viking was overweight and over budget. As a result, both were cancelled in the late 80s while prototypes were under construction, and the USN continues to have never had an operational SES.

Samum

Instead, the first navy to deploy such a craft was their arch-rival, the Russians. The first combat SES was Bora, a 900-ton missile corvette delivered in 1988. Mechanical problems meant that she wasn't fully commissioned until 1997 and the class was cut off after a single follow-on ship. Both units are currently assigned to the Black Sea, but reports are that they remain unreliable and subject to heavy vibration when running close to their 50 kt maximum speed, and their CODOG propulsion system requires one set of engines to be completely stopped before the second set can take over. The second ship, Samum was attacked in September 2023 by Ukrainian surface torpedoessea drones, although it appears that the strike did only minimal damage.

Skjold

But the most enthusiastic user of the SES has been Norway. They first used the configuration in the mid-90s for a batch of 9 minesweepers/minehunters, taking advantage of the somewhat higher speed to help cover the long coast of the country and the ability of the air cushion to insulate them from mines. These were followed a decade later by the Skjold class "corvettes".⁶ When commissioned, these were the fastest warships afloat, able to make 60 kts in a flat calm, or 45 kts in Sea State 3,⁷ and the Norwegians hoped they would be able to use their speed and stealth⁸ to hide in the fjords of their country's coast before striking with 8 internally-stored NSMs or their 76 mm gun. The Norwegians seem to like the design well enough, and it has achieved a surprising degree of public prominence, but at the end of the day, it remains an FAC, with all of the limitations of the type, and only six were procured.

The only other country to build surface effect FACs has been North Korea. Obviously, details are extremely sparse, and it appears that there are multiple subtypes of what is known to the US as the Nongo class, with a mix of anti-ship missiles and various guns.

But the SES is neither the most exotic nor the least common member of the class of things that are technically ships. That distinction belongs to a vehicle that relies entirely on air for lift and doesn't touch the ground at all, and that looks far more like a flying boat than anything resembling a conventional vessel. We'll take a look at it next time.