March 13, 2019

Weather at Sea

Weather has always been of the first importance at sea. In the age of sail, the reasons for this were obvious, but while officers no longer have to worry about the weather gage,1 they still must consider what wind and wave will do to their ships, and to aircraft they may be attempting to operate. And ships today must fight in conditions that would have had Nelson's captains looking to the survival of their vessels.

A Coast Guard Cutter battles heavy weather in the North Atlantic during WWII

The biggest effect of weather is on the motion of the ship. Besides the obvious motion of rolling, pitch and heave2 are also important to the efficiency of the crew, although in different ways. Any physical activity, such as loading a gun, is made substantially more difficult in cases of high lateral acceleration. This is only loosely related to the actual angle of roll, as a ship with a large metacentric height might roll to angles nearly as great as a less stable ship, but the lateral accelerations will be substantially higher as it snaps back upright. High roll rates also produce serious problems for fire control, although this is difficult to decouple from the effects of acceleration. Even mechanical systems like modern power-worked guns lose efficiency rapidly in these conditions.

The following video, showing clips of escorts in the North Atlantic, will give some idea of just how bad weather at sea can get.3

However, what lateral acceleration does not do is produce seasickness, which has obvious effects on the efficacy of the crew. That is a product of vertical acceleration, which is produced by a combination of heave and pitch,4 and also impairs decision-making among those who aren't physically ill.5 Pitch is obviously irrelevant amidships and worst at the bow and stern, so designers place critical spaces like the bridge and CIC to minimize motion. The best way to minimize both motions is to make the ship longer, with a length of about 250' being considered necessary today in the North Atlantic, the worst piece of ocean that is regularly considered by naval planners.6 Even bigger ships are badly affected. The 360' Leander class was expected to lose 10% of its capability in Sea State 5 (waves 8-12.5', 21% of the time in the North Atlantic), 30% in Sea State 6 (12.5-20', 13%) and 95% in Sea State 7 and over (20+', 7%). Conditions in smaller vessels of the WWII era would have been considerably worse.

Destroyer John Hood, about to suffer from slamming

Rough seas affect the ship as well as the men aboard her. They increase the amount of power required to drive a ship at a given speed, and often make the propellers less efficient as well, further limiting the speed a ship can make. Other effects, ultimately the results of pitching, can force the captain to reduce speed further. If the bow comes clear of the water, as can happen when a ship is driven fast in rough seas, it comes back down hard. This effect, known as slamming, is very good at breaking sonar domes, and can damage the hull, too. Slamming is most likely in long, shallow ships like WWII-era destroyers, while bigger ships with greater draft do not suffer as badly. A fast-moving ship can also take water over the bow. Spray makes it unpleasant to be on deck, while full-fledged green seas can wash away men and break equipment. Wetness is usually caused by insufficient freeboard,7 although some ships, like the Iowas, were wet because of lack of buoyancy in the bow. In really extreme cases, the ship can pitch enough to bring the propellers out of the water. This sends them racing to high speed as they are unloaded, and obviously stops them providing any meaningful thrust.

HMS Inglefield takes water over the bows in the North Atlantic during WWII

Wind and waves can hinder sensors as well. Rain, cloud, and fog obviously make it impossible for lookouts to see enemies, but even when visibility isn't totally blocked, weather can hinder lookout performance. U-boats often attacked with wind and seas astern, forcing lookouts to stare into the spray. Nor are the effects limited to visual detection. Rough seas produce spurious radar returns, which cluttered the simple radar screens of WWII. Today, digital signal processing has mitigated this problem, but it still makes finding small targets difficult. Atmospheric conditions can also cause anomalous propagation, where radar beams travel in unexpected ways, hiding some targets and revealing others at great range. Similar effects are seen with sonar, although the details are outside the scope of this post. Storms produce increased background noise, which hinders passive sonar performance. Active sonar is also affected, both by the background noise and by problems like slamming and bubbles being drawn under the ship into the vicinity of the sonar dome.

Sailors use sledgehammers to break ice on the cruiser Vella Gulf

Arctic operations pose special problems. Spray will freeze on a ship's upperworks, and the resulting ice has to be removed before it becomes a threat to the ship's stability.8 Seas are rougher than in much of the world, and weather is worse. Special care needs to be taken to make sure that everything is properly secured, and lubricants need to be checked to make sure they don't gum up.

USS Jason Dunham in rough seas

The convoy routes to North Russia were particularly brutal. Navigation was difficult in the high latitudes, and German forces in Norway battered the ships. During the summer, constant daylight meant that attacks continued around the clock, denying the crews much-needed rest. The following clip shows some of the weather these ships encountered:9 Foul-weather operations have been an area of considerable focus since the end of WWII. In the aftermath of the war, submarines capable of high underwater speeds became common, and escorts needed to be able to hunt them in all weathers, as the submarines themselves were obviously unaffected by conditions on the surface. This usually meant bigger ships, and different hull forms. Before WWII, ships tended to be designed for the highest possible speed in smooth water, which greatly compromised performance in rough seas. Today, speed requirements include sea state. In practice, a modern Arleigh Burke, capable of 31 kts, is as fast as a WWII-era Fletcher, theoretically rated for 38 kts. The growth in size of warships has helped, but modern naval architects have a wide range of techniques for analyzing behavior in bad conditions that were unavailable to previous generations, and deploy them to keep ships operational in rough weather. One of the main advantages of Vertical Launch Systems (VLS) used in modern warships is that they have much larger motion limits than conventional rail launchers.

USS Langley during Typhoon Cobra

But occasionally, weather at sea can get extremely bad, so that even modern warships are essentially unable to fight. Rarely, they have to fight merely to survive. The most famous incident of this type was during WWII, when Admiral Halsey led the Third Fleet directly into the path of Typhoon Cobra. Many ships took extensive damage from the rough seas, and the carrier Monterey suffered a fire after aircraft broke loose in the hangar. Throughout the fleet, more than 100 planes were lost overboard, and nine warships had to be detached for repairs. But the worst casualties were the destroyers Hull, Monaghan and Spence, all three of which capsized. Spence was low on fuel, and had her tanks empty in preparation for refueling, which ultimately compromised her stability. Several of her sister ships ballasted their tanks with seawater and rode out the storm with relatively little trouble. The other two ships were older destroyers that had received a great deal of extra equipment, driving topweight up to unsafe levels. Astonishingly, 93 men were recovered from these three ships, but approximately 790 perished in the greatest non-combat loss of life in the history of the US Navy.

Battleship Duke of York battles waves while escorting a convoy to Russia

I've only been able to scratch the surface of what is a complex topic. For instance, wind and ship motion are both critical in aviation operations, but they fall outside the scope of this post.10 And there are certainly many details of seamanship that don't get written down where I can find them. But weather remains an important part of naval warfare today, and will undoubtedly remain so for centuries to come.

1 Nitpick: The statement in that article that being upwind was not an advantage to the Germans because the shells were falling less steeply is essentially bunk. I don't have a range table for any of the guns involved (or at least I don't know where one lives offhand) but taking the 16"/50 AP shell as a baseline, a 10-kt wind along the line of sight would have changed the range of the shell by a maximum of 80 yards (at 26,000 yrds, about the maximum range of the engagement), reducing the angle of fall by 6.5' (about .1°) at that range. The actual guns might have been affected slightly more, but not enough to materially change penetration. As the range closed, wind deviation and resulting changes in angle of fall would have decreased even more. Having clear rangefinders is a much bigger advantage than a tiny decrease in angle of impact.

2 Vertical motion of the entire ship.

3 The clip is from about 18:45 onward in Episode 3 of the series Victory at Sea. The flashing lights you see from some ships are signal lamps in use.

4 The Greek root of the word nausea comes from their word for ship.

5 A very wide ship can also see fairly high vertical accelerations near the edges as it rolls.

6 Compare this to the Flower class corvettes of WWII, which were 205' long, and had a reputation for being terrible in the North Atlantic. It didn't help that the crew quarters were in the forecastle, where pitching was the worst.

7 The rule of thumb is that a ship should have a freeboard of at least 1.1*sqrt(length in feet). Almost all modern warships meet this criteria, but it was occasionally breached by ships of the WWII era, and egregiously violated by many of the ironclads. Needless to say, they were very, very wet.

8 The Truman strike group deployed to the arctic during Trident Juncture, the first time a USN carrier had been that far north since the early 90s. They consulted an old publication on Artic operations, and the first item for snow/ice removal was baseball bats. So Truman deployed with 48 Louisville Sluggers. I'm not entirely sure what they were used for, but I'd assume it was hitting things to break ice.

9 Like the previous clip, this is from Victory at Sea, specifically Episode 11.

10 This will probably be covered when I finally get around to discussing naval aviation operations, but that's not going to be soon.


  1. March 14, 2019Neal Schier said...

    If the props do get out of the water, is there any type of governor that keeps them and the shaft from overspeeding? Or is the time when they have no load on them so short that nothing can really be done?

    I am thinking about all that mass and torque to suddenly go no-load. Any stories of powerplants being damaged this way?

  2. March 14, 2019bean said...

    I'm not aware of any governor, probably because it wouldn't work. The basic problem is that steam turbines in particular aren't that fast on the throttle. Gas turbines are better, but even they have measurable spool-up and spool-down time. There's just too much energy/mass flowing through to make stopping the engines quickly practical. And screws tend to come out for only short periods anyway.

    It's certainly not a good thing for the plant, but I don't have any stories offhand of specific damage.

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