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. Read more...

It is time once again for our regular Open Thread. Talk about whatever you want, so long as it isn't Culture War.

Me with the big guns on that first day

Last Sunday marked the 10th anniversary of my first visit to Iowa, and I figured it was worth noting here, given that Naval Gazing definitely wouldn't have happened if not for that.

I also have the official dates for the 2026 Naval Gazing Meetup. We'll be in Dayton Ohio from May 14th to the 17th. Look for a signup sometime in the next couple months.

Overhauls are Air Attack on Ships Part 3, Understanding Hull Symbols, Nimrod, Battlecruisers Part 3, Secondary Armament Part 2, Spanish-American War Part 7, Riverine Warfare - China Part 1, Standard Part 2 and for 2024 Carrier Design and Organizational Structure, The Flavor of the Military, A Visit to the ADA TSF and Suez Part 1.

In our recounting of exotic hullforms, it is now time to leave things that are clearly ships, no matter how weird, and venture into the realms where air starts to play a major part in the design, possibly to the point that the thing in question isn't really a ship any more, because the thing isn't limited to operations over the sea. Despite this, they still fall broadly in the realm of naval vessels, as various limitations mean that they rarely see service in purely terrestrial roles.

SR.N1, the first true hovercraft

The basic idea is to take advantage of air's much lower density to decrease drag by injecting it underneath the craft to provide lift. Both the simplest and most common version of this is the hovercraft, which at its essence¹ involves using a big fan to pressurize the air under the vessel until the pressure counteracts the weight and lifts it off the ground. The result has effectively no friction and can float over almost any surface, but runs into a simple problem: the amount of air escaping is proportional to the height above the ground, so a low-powered hovercraft will have very little ground clearance, and very limited ability to handle rough terrain or waves, with the first prototype being flummoxed by anything bigger than about 9". While there were some potential uses for this technology, it wasn't really suitable for nautical use, at least until someone suggested adding rubber skirts, which would raise the body of the hovercraft much further and make it possible to cross obstacles. The results were dramatic, with the rebuilt prototype, fitted with 4' skirts, able to handle 3'6" obstacles and operate in 6-7' seas, all while having twice the lifting capacity on the same power. Propulsion was generally provided by aircraft-style propellers, and the lack of any direct contact with the surface meant that speeds of 75 kts or more were fairly easy to achieve, even in moderately rough seas. Read more...

During a recent podcast appearance, I took issue with the phrase "generals are always fighting the last war", and as the conversation moved on pretty quickly, I thought that the idea was worth expanding on.

To be clear, I am not claiming that military thought isn't profoundly shaped by the most recent available combat experience. It obviously is, although it's not uncommon to have the response be "everything involved in that was a horrible mistake and we should never do anything like it again". But the last general who actually thought the next war would be exactly the same as the last one died sometime in the 1880s, and since then the conversation has always been about how different the next war will be and in what ways. And in that environment, the main purpose of "fighting the last war" is to argue against a conservative² take and for making more radical changes. And given that the discussion is happening, it doesn't really add any useful information beyond "unthinking and reflexive conservatism is wrong", which really should be obvious to everyone anyway. Read more...

In my examination of the various exotic forms that naval architects have come up with, so far we've stuck purely to various forms of displacement hull, where the force that keeps the ship from sinking is provided entirely by water being pushed out of the way. But this leaves the entire hull in the water, which in turn means lots of resistance when trying to go fast. So for really fast ships, there needs to be some other method of providing "lift" which keeps more of the hull out of the water.

A US PT Boat planes

The oldest and most common of these methods is known as "planing". To take a simplified model, if a flat plate is towed through the water with the forward end higher than the back end, it will deflect water downward, which in turn will push the plate up, producing lift much like an airplane's wing does.³ This is the basic mechanism behind all planing craft, and has the advantage that once the majority of lift is provided by planing, the resistance grows more or less linearly with speed, a far less onerous scaling than experienced by a displacement hull. But planing as a concept runs into another problem: the amount of lift required to plane varies with the weight of the craft divided by the area of the bottom,⁴ a ratio that rises with vessel size. The result is that while a surfboard can plane while falling down a wave and a jet ski can often plane at 20 kts, bigger craft need a lot more speed to get on the plane, putting it out of reach of anything over 50-100 tons, which means that military use is limited to fast coastal craft like torpedo boats rather than proper warships. Read more...

Last time, we discussed multihulls, the catamarans and trimarans that are make up a small but significant portion of the world's fleet. Most multihulls are designed primarily for high speed, but for high speed to be useful, it needs to be practical if the sea is less than calm. Fortunately, naval architects have worked out ways to solve this problem, some of which can also be applied to conventional ship, and have even come up with a catamaran derivative, known as SWATH, that provides unmatched seakeeping for its size. But we'll start with an illustration of the basic principle at its simplest with the wave-piercing bow.

Axe-type wave-piercing bow and conventional bow, showing the different shapes

The basic concept behind a wave-piercing bow is simple. Imagine a normal ship is sailing along over a flat ocean, then runs into a wave. The bow is going to reach the wave first, and as the ship is horizontal, it will be pushed into the wave. This means that the waterline there will be higher than normal, and on a conventional bow there is quite a bit of flare,⁵ so the bow displaces more water, which means more buoyancy, which in turn pushes the bow up. The end result is that the ship will pitch up and down as it passes through the waves, which is the major cause of seasickness and also impairs decision-making. A wave-piercing bow has less buoyancy than a conventional bow, particularly above the normal waterline, and as such, pitches the ship less when it runs into a wave. The downside to this is that the bow rising keeps the water further away from the deck, so ships with wave-piercing bows tend to be pretty wet forward. This is a serious problem if the designer hasn't taken it into account,⁶ but can work if nobody is expected to be standing in the areas that keep getting wet. Read more...

It's time for our regular Open Thread. Talk about whatever you want, so long as it isn't Culture War.

This past week, I was a guest on Patrick McKenzie's Complex Systems podcast, talking about ways that the defense industry differs from others, why discourse about "drones" is often wrong, and posing him puzzles from the history of Operations Research. I was delighted to be able to talk to him, and I think it turned out well. (Oh, and for those who don't do audio, there's a detailed transcript at the link as well.)

For those who got here from Complex Systems, hello, and thank you for coming. Naval Gazing started off as a blog about battleships, and has since spread out to cover a lot of stuff. If you're new to defense issues, I would recommend looking through the Intro tag, where I've tried to collect the writing that would be of most use to someone who has no background in the field. And feel free to ask questions, which either I or one of the regulars here should be able to answer.

Overhauls are A Brief Overview of the US Fleet, Military Spaceflight Part 4 and for 2024, Early Underwater Weapons, The Problem of Defense Economics, and reviews of military museums in Boise and the Danish steam frigate Jylland.

Over the years, naval architects have come up with a variety of unusual hull types for specialized roles, and it's worth taking a look at some of them. There are benefits and drawbacks to any departure from the conventional monohull, and while for most roles, the tradeoffs aren't worth it, a few have seen reasonably wide use. We'll start with the oldest of these by far, the multihull. Typically, these can be divided into catamarans, with two fairly similar hulls and trimarans, with a single big hull in the middle and two smaller hulls out on the sides.

A replica Polynesian catamaran

The basic idea of making a ship with more than one fairly conventional hull dates back to the outrigger boats used by the Austronesians in their expansion across the Pacific. At first, these were simply two dugout canoes with a platform between them, a design probably adopted in search of the great advantage of the multihull, excellent stability. The reason for this is extremely straightforward. Stability is the resistance of a ship to rolling over, and it comes from hull on the downward side being pushed deeper and displacing more water while the hull on the upward side displaces less. On a catamaran, with the hulls split apart, this effect is significantly stronger, a useful feature if you are planning to carry a large sail on a small boat. But any case where there are two or more separate points of contact with the water can produce the same effect, and the Austronesian catamaran eventually evolved into the more typical outrigger designs used in the astonishing expansion across the Pacific Ocean.⁷ Read more...

On May 17th, 1987, USS Stark was on patrol in the Arabian Gulf. Iraq and Iran had been at war for most of a decade, and things had escalated from a simple battle for territory into both sides trying to interdict the oil that was funding the other's war machine. Iran was faced with a trickier situation. They had cut off direct Iraqi access to the Arabian Gulf early in the war, so Iraqi oil was transshipped via neutral powers, such as Kuwait and Saudi Arabia, and those same powers were funding Iraq's war effort. Kuwait had finally become concerned enough to request American involvement in protecting its tankers, and while the actual program had yet to begin, the USN was increasingly active in the region. Iraq had the easier job of things, as Iran's main oil facilities were quite close to its territory and the tankers that moved its oil out of the gulf were relatively easy to attack with French-made Exocet missiles. In the previous 10 months, they had flown 340 sorties and launched 90 Exocets, which had reportedly damaged about 40 tankers. And while this was annoying to the insurance companies (because the Exocet was too small to be a serious threat to a tanker) bystanders were generally safe.

Stark earlier in her career

On this specific day, Iraq was deploying a new plane. The Dassault Mirage F1 had been the backbone of Iraq's naval strike force for several years, but its range was pretty limited, so the Iraqis had requested that Dassault modify a Falcon 50 business jet into a "trainer". A Mirage F1 cockpit was installed on the right side, and an Exocet hardpoint was fitted under each wing. As the plane, codenamed Suzanna,⁸ headed out over the Gulf, it was picked up by a USAF E-3 Sentry operating out of Saudi Arabia, which thought it was a Mirage F1, and classified it as a friendly fighter on the Link 11 network operating in the Gulf. Read more...

This is the second half of John Schilling's two-part effortpost on SCUBA diving, originally written for Data Secrets Lox. The first part covered basic recreational diving, while this one looks at more advanced topics.

This time we’ll be focusing on technical and professional diving. Note that these aren’t strictly the same thing. “Technical diving” refers to anything beyond the limits of no-decompression open-cycle scuba diving. And there are amateurs who learn those skills for recreational purposes, e.g. visiting interesting or historically significant shipwrecks at 40-80 meters. There are also professionals who can do their jobs using nothing but recreational-grade scuba gear and techniques. But for our purposes, I’m just going to fold them both together into the broad category of stuff beyond ordinary recreational diving.

Most of this is going to be stuff that’s more difficult, more expensive, and more dangerous than ordinary recreational diving, but also more rewarding. Let’s start with the risks. I already listed all the things that could kill you in the course of a brief recreational dive to no more than forty meters, if you aren’t careful. Here’s a few more to worry about if you’re going really deep. Read more...