To complete our ongoing series on exotic hulls, it is now time to turn our attention to a type of vessel that not only straddles the boundary between the ship and the airplane, but arguably crosses it to be even more airplane than ship. Variously known as the Ground Effect Vehicle, Wing-in-Ground Effect or Ekranoplan,¹ these look mostly like aircraft, but take advantage of a quirk of aerodynamics known as "ground effect" to greatly reduce drag, so long as they are operating near the surface.

It's probably worth starting by explaining what ground effect is and how it works. Basically, in the process of producing lift, an airplane's wing also produces a vortex that makes drag.² But if the wing is close to the ground (or water), then the surface will disrupt the vortex, slashing drag dramatically, in a manner determined by the ratio of wingspan to height above the ground. For instance, a wing that is 10% of the wingspan above the ground will only experience about 20% of the induced drag it would at altitude, rising to around 50% at 20% wingspan. This makes taking off in an airplane easier, and landing a bit harder, and in theory could be used to significantly improve the performance of an airplane. Read more...

Tomorrow is the 250th anniversary of the founding of the United States Navy, and as such, the internet is currently flooded with well-wishes. I figured I would add my own, which is also free for anyone to share on social media:

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Given the current concern around drones, it seems worth surveying possible countermeasures in a reasonably systematic way, looking at each category of possible solution and listing the pros and cons of each. Now, this is a big and rapidly-evolving area, and I'm not a specialist in it. But I have done some looking around, and it seems worthwhile to bring this up as a counter to a lot of the triumphalism around drones these days.

First, I'm going to limit the scope here to direct measures aimed at a non-cooperative drone. There are a lot of ways of reducing the drone threat that fall outside this purview, from blowing them up before they launch to compromising their software to simply not being within range, but that gets even more complicated and messy. Second, it's worth pointing out that there are many different scenarios where drones may need to be dealt with, and that some systems may work well in certain situations but not in others. For instance, broad-spectrum RF jamming is absolutely standard on the battlefield, but is not a particularly good first-line solution if you're trying to defend the White House. Read more...

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 essence³ 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 conservative⁴ 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.⁵ 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,⁶ 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,⁷ 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,⁸ 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.