Exotic Hulls Part 1 - Multihulls

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.¹

No, Dean, the word catamaran comes from Tamil and has nothing to do with cats.²

With a few minor exceptions,³ multihull designs saw little use outside the Austronesian world until the late 19th century, when sailing yacht designers looking for higher performance began adopting the type.⁴ The first broad adoption by people using modern propulsion came in the early 20th century, as several nations, concerned about the loss rate among their new submarine forces, built catamaran vessels intended to provide rescue support to sunken submarines. The catamaran design allowed the rescue equipment to be lowered between the hulls, greatly simplifying the working arrangements versus having to dangle stuff over the side using booms and allowing it to enter the water at the point of minimum motion. One of these, the Russian Kommuna, commissioned in 1915, is the oldest naval vessel still in active service.

Volkhov, now Kommuna, launched in 1913

Eventually, submarines grew more reliable and the catamaran faded out of use for most of the 20th century. But the multihull has seen a resurgence in more recent years, alongside the trimaran, as designers have sought higher speeds. For various and complex hydrodynamic reasons, a catamaran or trimaran will have reduced wavemaking resistance relative to a monohull, although at lower speeds this is entirely offset by the increase in frictional resistance that comes from multiple hulls. But as speed requirements rise well above hull speed and wavemaking resistance comes to dominate, the advantages of the multihull become more pronounced. Starting in the 90s, the type rose to dominate the high-speed ferry market, which had previously been the domain of hydrofoils and hovercraft. Some were catamarans, usually with wave-piercing bows designed to minimize pitching as the ship passed through the sea, while others were trimarans, taking advantage of the fact that the outer hulls, smaller than the main hull, could be positioned to partially cancel out the waves from the central hull, increasing efficiency. In both cases, significant structural concerns had to be overcome, but these have been dealt with, and the multihull has a clear niche in the short-range high-speed low-weight market, primarily from Australian companies Incat (builder of catamarans) and Austal (trimarans).

Spearhead class City of Bismarck

Of course, that brings us to the military use of multihulls. A few slow multihulls exist, like the American Pigeon class submarine rescue ships of the 1960s, but the main use has come as navies have desired higher speeds. The first vessel of this type was HMAS Jervis Bay, a former catamaran ferry chartered by the Australian Navy in the late 90s which proved extremely useful during the Australian intervention in East Timor. The USN saw this and decided that it looked like a good idea, despite the lack of places within a few hundred miles of US bases where intervention might be necessary. They decided to test this out with the Joint High-Speed Vessel Program, which chartered a pair of ferries. For some reason, this wasn't enough to put them off the idea, and starting in 2010, 15 Spearhead class fast transports were procured, each intended to be able to carry a mechanized company or infantry battalion a thousand miles at 45 kts and unload it at a primitive pier. The only problem is that this requires calm seas, with the maximum speed being achievable only if the waves are less than 1.25 m, while rougher seas quickly reduce available performance below that of conventional vessels. It's not clear what problem this is supposed to solve, or why we keep buying them.⁵ And because they're made of aluminum, there are serious concerns about how they'll handle damage if someone shoots at them, as happened to HSV-2 Swift in Emirati service off the coast of Yemen.

Independence in drydock, clearly showing her three hulls

But there are also a few multihull combat warships, by far and away the most famous of which are the Independence class Littoral Combat Ships. Conceived as part of a dubious plan to develop 45-kt corvettes for fighting Iranian speedboats in the Arabian Gulf, they have shown a major advantage of multihull warships even if you don't want to go fast. Unlike warships through WWII, modern warships are generally constrained by volume and deck space, and a multihull offers far more of both for a given displacement. This is best shown by comparing the Independence to the monohull Freedom class, designed to the same specification. Independence has a flight deck that is approximately⁶ 128' long and 80' wide, while Freedom's is only 92' long and 57' wide. This is by far the best flight deck available on any surface combatant, and I expect the trimaran configuration to see wider adoption for small helicopter carriers in the next few decades.

A Type 022 missile boat at speed

The other multihull warships are smaller and designed to operate in the world's most contested seas. The Chinese have built over 80 examples of the Type 022 missile boat, which serve as highly mobile missile batteries in defense of the South China Sea. On the other side of the Taiwan Strait, the larger Tuo Chiang class corvettes, equipped not only with anti-ship missiles but also sporting some air-defense capability and even a 76 mm gun. Both are capable of the same 45 kts or so that the Independence can manage, and are stealthy, at least in theory.

Francisco, the world's fastest passenger ship

But operating at high speed, as both of these vessels do, is often rather rough on the ship and its crew. But further hull tweaks, either to catamarans or to monohulls, are required to achieve this. We'll take a look at these next time.