Centre (or Center, if you like) of Gravity: Lusitania vs Titanic

[Arun Vajpey]

Looking at the profiles of the ill fated liners Lusitania and Titanic. I was struck by the differences in external design. Obviously, they told only part of the story since there were other factors to consider like position of the engines, boilers, storage holds and perhaps admiralty specifications in case of the subsidized Lusitania, but whereas the Titanic had a more symmetrical design with the superstructure and the funnels more or less equidistant from the bow and stern, The Lusitania's were significantly biased towards the bow with a lot more flat surfaces aft of the 4th funnel.

I wonder how this would have affected their centres of gravity, especially if they had been in each other's position?

 

[Kyle Naber]

I don't think that the funnels would have made any difference as they were made of extremely thin metal frames (the reason why Titanic's funnels collapsed). I'm not sure about the boiler positions, however.

 

[Harland Duzen]

I'm not sure about the boiler positions, however.

Here's the Plans from Google:

 

[Doug Criner]

Looking at the profiles of the ill fated liners Lusitania and Titanic.
I wonder how this would have affected their centres of gravity, especially if they had been in each other's position?

The center of gravity is one thing regarding the stability of a ship. The main thing is the metacentric height, which is the difference in height between the center of gravity and the ship's metacenter (roughly, the center of buoyancy). Practically, it's not possible to precisely determine the metacentric height, or the metacenter, by studying the design of the ship, as you propose. It requires an inclining experiment for each new ship or ship's class, with known weights added to one side of the ship and measuring the list.

There was an inclining experiment performed on Olympic/Titanic and presumably Lusitania. The data probably is available.

 

[Kyle Naber]

I wish we could have detailed research information like we do with Titanic such as persise sinking simulations.

 

[David G. Brown]

Let me assure everyone that the funnels were very functional in a naval architecture way even if they did not carry smoke up from the boiler fires.

In Titanic’s day, only two materials existed at low enough cost for widespread use in ships: wood and steel. Titanic was essentially a steel container for a forest of wooden walls, trim and furnishings. Obviously, funnels were made of non-flammable steel and not the much lighter wood. But, fire danger was not the only reason.

Steel steamships by their nature have a low center of gravity both transverse and longitudinal. Boilers, engines, condensers, and all the associated machinery necessary concentrated a great deal of weight low in the hull. Up to a point this was good. Low CG means better stability meaning the vessel “wants” to roll upright from an inclined angle. But, too much stability causes a distinctly unpleasant “snap” roll to the upright. This kind of ship is called “stiff” by sailors and sea sick city by passengers. Conversely, of course, a ship with a low transverse CG is “tender” and has a most disconcerting slow roll resulting from less stability.

The goal of a naval architect when designing a passenger ship is to create one with a pleasant roll characteristic without compromising safety. With Titanic, there was a need to raise the center of gravity, but there was no practicable way to raise the engines and boilers. The alternative was to put weight up high – above the boat deck. Think of it as negative ballast. Because of a leverage effect, ballast weight aloft has more impact on stability than the same weight at deck level.

What was immediately at hand to create weight aloft? The funnels, smokestacks, or whatever you want to call them. Those big steel cylinders were pretty dang heavy even if made of thin plate. By careful calculation the roll characteristic of the ship could be fine tuned simply by adjusting the height of the funnels. This neat solution became absolutely elegant when coal smoke is added to the equation. High stacks meant less soot and ash would fall on the well-heeled first class passengers in their finery. That’s elegant engineering – comfort and cleanliness.

(See Doug Criner's post above about the inclining experiment needed to prove that the real ship performs up to the calculations.)

So far, we’ve considered only roll. There is another motion which must be accounted for – pitch. This is the up-and-down motion of the bow in a seaway. A traditional method of adjusting pitch is to spread the weight of the cargo evenly fore and aft. Titanic was deprived of this option. Once again, the funnels came to the rescue. They represented considerable weight which could be spread out along the ship’s centerline. Three funnels were needed. But, they would have been concentrated in the forward 2/3rds of the ship and mostly the forward half. Not good for pitching motion. The answer was a fourth funnel which some dummies call a “dummy.” Not so. It served a very important role in giving Titanic a comfortable pitching movement. Hardly a small job.

And, lest we forget, ships are “shes.” They must be what boat builders call “eye sweet” – pleasant to behold. And that four stack appearance certainly looked good when the ship had a bone in ‘er teeth steaming into the sunset. Wouldn’t that make a great movie scene? Naw.

– David G. Brown

 

[Arun Vajpey]

Thanks Mr Brown. The following is an excerpt from the Wikipedia article on the Lusitania. While I am not, repeat NOT claiming that this is gospel, I merely wanted to ask opinion about the highlighted bits as relevant to my original post.

The White Star Line's Olympic-class vessels were almost 100 ft (30 m) longer and slightly wider than Lusitania and Mauretania. This made the White Star vessels about 15,000 tons heavier than the Cunard vessels. Both Lusitania and Mauretania were launched and had been in service for several years before Olympic, Titanic and Britannic were ready for the North Atlantic run. Although significantly faster than the Olympic class would be, the speed of Cunard's vessels was not sufficient to allow the line to run a weekly two-ship transatlantic service from each side of the Atlantic. A third ship was needed for a weekly service, and in response to White Star's announced plan to build the three Olympic-class ships, Cunard ordered a third ship: Aquitania. Like Olympic, Cunard's Aquitania had a lower service speed, but was a larger and more luxurious vessel.

Because of their increased size the Olympic-class liners could offer many more amenities than Lusitania and Mauretania. Both Olympic and Titanic offered swimming pools, Turkish baths, a gymnasium, a squash court, large reception rooms, À la Carte restaurants separate from the dining saloons, and many more staterooms with private bathroom facilities than their two Cunard rivals.

Heavy vibrations as a by-product of the four steam turbines on Lusitania and Mauretania would plague both ships throughout their careers. When Lusitania sailed at top speed the resultant vibrations were so severe that Second and Third Class sections of the ship could become uninhabitable.[42] In contrast, the Olympic-class liners utilized four traditional reciprocating engines and only one turbine for the central propeller, which greatly reduced vibration. Because of their greater tonnage and wider beam, the Olympic-class liners were also more stable at sea and less prone to rolling. Lusitania and Mauretania both featured straight prows in contrast to the angled prows of the Olympic liners. Designed so that the ships could plunge through a wave rather than crest it, the unforeseen consequence was that the Cunard liners would pitch forward alarmingly, even in calm weather, allowing huge waves to splash the bow and forward part of the superstructure.[43]
The vessels of the Olympic class also differed from Lusitania and Mauretania in the way in which they were compartmented below the waterline. The White Star vessels were divided by transverse watertight bulkheads. While Lusitania also had transverse bulkheads, it also had longitudinal bulkheads running along the ship on each side, between the boiler and engine rooms and the coal bunkers on the outside of the vessel. The British commission that had investigated the sinking of Titanic in 1912 heard testimony on the flooding of coal bunkers lying outside longitudinal bulkheads. Being of considerable length, when flooded, these could increase the ship's list and "make the lowering of the boats on the other side impracticable"[44] — and this was precisely what later happened with Lusitania. Furthermore, the ship's stability was insufficient for the bulkhead arrangement used: flooding of only three coal bunkers on one side could result in negative metacentric height.[45] On the other hand, Titanic was given ample stability and sank with only a few degrees list, the design being such that there was very little risk of unequal flooding and possible capsize.[46]

Lusitania did not carry enough lifeboats for all her passengers, officers and crew on board at the time of her maiden voyage (actually carrying four lifeboats fewer than Titanic would carry in 1912). This was a common practice for large passenger ships at the time, since the belief was that in busy shipping lanes help would always be nearby and the few boats available would be adequate to ferry all aboard to rescue ships before a sinking. Interestingly after the Titanic sank, Lusitania and Mauretania would only be equipped with an additional six more clinker-built wooden boats under davits, making for a total of 22 boats rigged in davits. The rest of their lifeboat accommodations were supplemented with 26 collapsible lifeboats, 18 stored directly beneath the regular lifeboats and eight on the after deck. The collapsibles were built with hollow wooden bottoms and canvas sides, and needed assembly in the event they had to be used.[47]

This contrasted with Olympic and Britannic which received a full complement of lifeboats all rigged under davits. This difference would have been a major contributor to the high loss of life involved with Lusitania's sinking, since there was not sufficient time to assemble collapsible boats or life-rafts, had it not been for the fact that the ship's severe listing made it impossible for lifeboats on the port side of the vessel to be lowered, and the rapidity of the sinking did not allow the remaining lifeboats that could be directly lowered (as these were rigged under davits) to be filled and launched with passengers. When Britannic, working as a hospital ship during World War I, sank in 1916 after hitting a mine in the Kea channel the already davited boats were swiftly lowered saving nearly all on board, but the ship took nearly three times as long to sink as Lusitania and thus the crew had more time to evacuate passengers.

Slightly outside the scope of my question but related nevertheless, is it right to assume that the lack of a longitudinal bulkhead on the Titanic permitted comparatively even flooding so that neither the initial starboard list nor the later port list affected the dynamics of lifeboat launching too much? Or putting it another way, how much did the Lusitania's longitudinal bulkhead contribute to the heavy list and speed of sinking?