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,1 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,2 but can work if nobody is expected to be standing in the areas that keep getting wet.
Wave-piercing bows on HSV-2 Swift
Wave-piercing bows were adopted on high-speed catamarans because they encounter more waves more frequently, and because the catamaran configuration allows the deck to be a ways above the bows so it doesn't get wet, but the concept has recently been adapted for other uses. One case is offshore support ships, which routinely operate in extremely rough seas,3 and which have found that the reduction in pitching makes things better for their crews.4 Another is for stealth warships, most prominently the Zumwalts, which use a wave-piercing bow to try to minimize the ship's motion in the hopes of reducing its radar signature.
SWATH USNS Effective in drydock
But this principle can be taken further in the form of the SWATH (Small Waterplane Area Twin Hull) configuration. Instead of merely trying to minimize how much buoyancy changes with the waterline at the bow, the SWATH does so for the entire waterline, by concentrating almost all of its buoyancy in a pair of pods deep under the water that are connected to the above-water portions of the ship by thin stalks. This way, if, say, a wave hits the ship crossways, as it climbs up one stalk, it produces only a very small amount of extra buoyancy, minimizing how much rolling force is produced.5 As a result, SWATH is the absolute best option if you are building a ship that is going to need to operate in very rough seas, and it has become popular for research vessels and the US Navy's SURTRSS vessels, ships designed to tow extremely large towed array sonars in the North Atlantic.
Sea Slice, a SWATH derivative with four underwater hulls
Of course, the SWATH is not without its drawbacks. Most prominently, as a ship gets heavier, it must displace more water, so it sinks down by an amount determined by the change in weight and the size of the waterplane area, which the SWATH configuration is designed to minimize. As a result, SWATH ships have to operate within a pretty narrow displacement band, to the point that the underwater hulls will need ballast tanks to compensate as fuel is burned off, as well as active control systems and the like. This makes them extremely unsuited to carrying cargo. There are other issues ranging from structural concerns to the issues of figuring out where the engines go so that they can have good access to air and to the propellers.6 In some ways, it's easiest to think of the SWATH as an above-water hull balanced on top of two submarines, and as such, it will always remain a niche type, invaluable in cases where a ship needs to operate in the worst possible seas.
German SWATH pilot boat Duhnen
In the late Cold War, the British, who for some reason were very concerned about protecting the North Atlantic shipping lanes, seriously investigated SWATH frigates for operations in that notoriously difficult stretch of water. They found that while a SWATH design was bigger and more expensive than a conventional ship of the same payload (6950 tons vs 5330 tons) it was far smaller and cheaper than a conventional monohull designed to provide the same seakeeping capabilities (9030 tons).7 The effect is even more pronounced for smaller ships, with Coast Guard trials of a 200 ton SWATH showing seakeeping equivalent to a 2500 ton monohull. Sadly, the SWATH frigate program ended up being a victim of the end of the Cold War, and as the probable center of naval conflict has swung to the Pacific, it seems unlikely to return.
1 The technical term for the bow sloping outward above the waterline, increasing the displacement added as the waterline climbs. ⇑
2 The Iowas have very fine bows with little buoyancy, and were quite wet forward, something I will now ascribe to premature adoption of the wave-piercing bow. ⇑
3 It's worth noting that the ability to tolerate rough seas scales with size, so a supertanker's crew might barely notice weather that makes things very uncomfortable for a platform supply ship with 1% of the displacement. ⇑
4 The most common form for this, the X-bow, is also touted as reducing drag due to the longer waterline it creates. I find this slightly silly, as you could just make a conventional hull longer, but at some point that might run into issues with dock sizes and the like. ⇑
5 "But bean," I hear the engineers reading this say, "doesn't that mean that it will have to roll a very long way to reach equilibrium again?" The answer to that is yes, it would, if it was going to reach equilibrium. But the sea moves, so there's no way for such a situation to persist very long, and the ship is big enough that the actual movement coming from the unbalanced force is pretty small. ⇑
6 This is much easier now than it was in the 80s, thanks to developments in electrical propulsion. ⇑
7 All numbers are from D.K. Brown's The Future British Surface Fleet. ⇑
Comments
How well do SWATH ships handle damage? If a missile, or a torpedo, or a suicide boat, or a drone, or something, hits the stalk, how much at risk is the ship? (I'm almost sure the naval architects have taken this into account, but for most navies it's been a few s/years/decades since they had to for-real worry about damage control...)
Seems like SWATH would do much better with nuclear propulsion.
Tony Zbaraschuk:
Probably about as well as a couple of submarines with a surface ship on top.
But I guess you can just add more hulls and the fact that they won't be as deep might help compartmentalization.
I'm not sure I've ever seen anything specific on that. I'd expect it to not be great, given that there's basically no reserve buoyancy in the conventional sense. You could compartment the underwater hull and hope to pump out ballast tanks and the like, but I suspect that's one of the more serious drawbacks for most warship use.