The Zumwalt, America’s newest destroyer, took to the ocean last week and is a geek paradise, which is why I’m writing about it.
At about $US6 billion, the Zumwalt isn’t cheap, and I can think of many better things that $US6 billion could be spent on. Health care, for instance. Food aid. Stuff like that.
However, that’s not what this post is about – it’s about the gob-smacking geekery of the thing, which I can still admire even if I think the money might have had better uses. It’s technically impressive, and ‘destroyer’ might not be the best term for a vessel that displaces over 14,500 long tons. There have been smaller battleships. That ship brings together all America’s latest sea-borne tech in one stealthy package. As I understand it, it’s got a raft of brand new tech from induction-motor electric drive to on-board rail guns. Yah – rail guns. Sci-fi stuff, because rail guns work by scraping two electrically charged conductive objects past each other, which is also how welding works, and the tech challenge is stopping the projectile from welding itself to the rail instead of screaming off towards the enemy at Mach =7. It’s been done, and these suckers were tested by the US Navy’s Dalhlgren Division in 2006.
There are a few down-sides. One is that the Cold War finished nearly a quarter-century ago and big-navy Cold War style warships aren’t where things are at any more. I suspect I’m not the only one to think that, given that the order for this class was slashed from 32 to 3.
Then there’s the radar and air defence tech. The AN/SPY3 system isn’t in the league of Britain’s incredible Sea Viper, deployed since 2010 on their Daring class destroyers and, under a less cool name, on French and Italian vessels. It’s a European system with a lot of British input – I knew one of the physicists who worked on the broad tech foundation leading to the system. During the 1982 Falklands war, the Royal Navy discovered that its radars weren’t able to disentangle sea-skimming missiles from surface sea clutter. My friend got the job of solving the problem – it was his PhD thesis, and was classified until well into the 2000s. After he was allowed to talk, he told me he did it using an experimental rig at infra-red frequencies, which I thought was pure genius (radar doesn’t operate at those frequencies, but I can explain why the science is still valid – ask me in the comments).
But the other Zumwalt issue is more serious. The stealth characteristic comes largely from tumblehome – the hull is narrower at main deck level than on the waterline. That’s great for bouncing radar, but not so good in the water and is something not often seen on big ships at sea since the French and Russians used it in the 1890s to reduce battleship topweight and increase angles of fire.
The plus side to it is that a reverse-angle bow shape cuts through the water very efficiently, improving fuel economy and reducing the power needed to maintain any given speed. But there are down-sides. The main problem is hydrodynamic, which is a good entree for me to get very geeky. Most ships have flared bows and sides. When a ship rolls, the angle of the side increases, relative to the water. That tends to push it back upright again. Bows are flared to stop pitching in the same way, and to push water away from the foredeck where it might otherwise settle. Technically the action involved is leverage along a curve, and the same maths applies. Here it is in (historic) action off Wellington, New Zealand – my city, where we get some of the roughest waters on the planet:
That’s reversed on a ship with tumblehome, where the angle decreases as the ship rolls or pitches. Yah – go figure. A tumblehome-hull ship also rolls harder when turning: in model tests of Zumwalt, to the point where there was risk of the flight deck being swept with water, with possible loss of crew.
Many other factors contribute to ship stability, including metacentric height. The tendency to return to an even keel is known as the righting moment, and among other things is a function of buoyancy and the waterplane, which varies depending on how the ship is immersed. A ship with tumblehome hull has less buoyancy in the crucial areas than one with a flare. To some extent this can be compensated for by adjusting the ship’s centre of gravity. However, because ships also get struck by seas from all angles, and the forces change depending on the ship’s speed, the problem is complex. Naval architects have been concerned about the ability of the Zumwalt to handle itself in quartering seas, particularly at high speeds – something a warship must be capable of doing. Here, a risk of broaching – not holding to course and swinging around in the waves – has been identified. More crucially, there has been concern that the Zumwalt class, which has a substantial metacentric height, might be unstable in certain parametric movements driven by certain sea states and angles of force, leading to abrupt capsize.
Warships have been built before with innate stability problems – including HMS Captain, an early ironclad ‘turret ship’ which sank in 1870 during a storm, with very heavy loss of life. Various Japanese warships of the inter-war period were also problematic because of a policy of trying to out-spec any foreign vessel on limited displacement.
I guess time will tell as far as Zumwalt is concerned, but meanwhile we can go oooh and aaah at the technology. And at the fact that her first captain is James Kirk (no, not that James Kirk. Another one.)
Copyright © Matthew Wright 2015