RMS Britannic:Engines

Britannic's engines

By Mark Chirnside

 Following construction of her two elder sisters, Britannic incorporated a number of improvements and modifications; not only was her passenger accommodation markedly improved, but also the ship’s structure, machinery and systems. In this article the focus will be on the changes made to her propelling machinery – the boilers, her reciprocating and turbine engines – and a brief technical outline.

 When completed for commercial service, it was planned that Britannic would be slightly larger than her sisters. Although she was the same length, contrary to popular myth, as Olympic and Titanic, Britannic’s moulded breadth was increased from 92 feet to 93 feet six inches. (The maximum, or ‘extreme breadth’ was 94 feet, compared to 92 feet six inches on her elder sisters.) Her estimated displacement at thirty-four feet seven inches was ‘over 53,000 tons’ – compared to some 52,060 tons for Olympic at the same measure. It was intended that Britannic would have ‘sufficient power to enable her to maintain an average sea speed of 21 knots in any weather’ – the same service speed as Olympic had been designed for, rather than attempting to improve her speed.[1] Naturally in better weather conditions the ship would be able to travel even faster with ease. However, even to maintain the same speed as her sister, Britannic would require additional engine power; not only was she heavier, but her slightly wider hull would indicate a slight disadvantage, as longer, thinner hulls traditionally proved superior for speed and fuel consumption. Harland & Wolff retained essentially the same engine design for Britannic, but incorporated modifications compared to Olympic which would raise the power developed and ensure the ship’s speed performance met or even exceeded expectations.

 Before modifying the engines, the boilers needed changes to enable them to increase their steam production. Britannic was equipped with twenty-four main double ended boilers and five single ended reserve boilers, as her sisters had been. However, the double ended boilers were enlarged so that their length was one foot greater than those on Olympic; Britannic thus also had 159 furnaces, but she had some 151,000 square feet of heating surface, compared to Olympic’s figure of 144,000 square feet.


Britannic's boilers (Ulster Folk & Transport Museum)


 Britannic’s twin reciprocating engines did not differ radically from those on Olympic, but nonetheless there were several noteworthy changes. Four cylinders, with diameters of 54, 84, 97 and 97 inches, were provided as on Olympic, while the two low-pressure cylinders were situated at opposite ends of the engines for the purpose of allowing optimum balance, a primary consideration for mechanical operation and the reduction of vibration with such engines. However the cylinders were all fitted with piston valves, whereas Olympic’s low pressure cylinders had been equipped with slide valves with relief rings at the back; the change was made owing to the intended slightincrease of steam pressure in the low pressure cylinders, before the steam exhausted to the turbine engine. Each engine had nine bearings, two for each crank pin, but an additional bearing was fitted in the centre of the engine, which had not been the case on Olympic, for the purpose of purposely ‘steadying the shaft.’ General changes included structural reinforcements, including the engine bedplates, to ensure maximum strength and quality of service over the ship’s predicted length lifespan. Interestingly, strengthening plates were fitted in June 1911 on Olympic’s bedplates, although these had not been fitted originally and to my knowledge had not been included in the original design. Presumably these were later fitted on Titanic.[2]


Britannic's reciprocating engines under construction (Ulster Folk & Transport Museum)

 Britannic’s wing propellers were three bladed manganese bronze blades on a cast steel boss, with a diameter of 23 feet nine inches which was slightly larger than Olympic’s original propellers and even larger than those installed during her 1913 refitting. Generally propeller diameters are larger when the propeller pitches are smaller; and so presumably the pitch of Britannic’s wing screws was lower than that of Olympic, whose pitches had been increased to thirty-six feet during the 1913 refit to compensate for the smaller propeller diameter. It was reported that Britannic’s two screws, running at seventy-seven revolutions per minute, would receive thirty-two thousand horsepower from the ship’s reciprocating engines.


 As mentioned earlier, it was intended that the steam pressure in Britannic’s low pressure cylinders would be slightly higher than the equivalent on Olympic, for the steam exhausted into the low pressure turbine was to be at a higher pressure, increasing the turbine’s power output. Due to the greater size of the components from which it was constructed, the weight of Britannic’s turbine had increased from Olympic’s 420 tons to 490 tons although the design was essentially similar. The rotor itself weighed 150 tons, its drum being constructed of forged steel. Designed to indicate 18,000 horsepower while running at 170 revolutions per minute, it does not seem unlikely that the turbine was able to produce even more power; in 1911, Olympic’s central turbine ran at a maximum rate of 190 revolutions, while it had been designed to indicate 16,000 horsepower at 165 revolutions. ‘Guestimating’ Britannic’s turbine’s potential, over twenty thousand horsepower actually seems realistic, although it would be interesting to find specific official figures on the topic. Prime importance is attached to a ship’s propellers since these actually drive the ship forward from the power of the engines; Britannic’s central propeller retained the manganese bronze four bladed sixteen feet six-inch diameter design of Olympic’s original propeller, despite indications that Olympic’s propeller had been changed in 1913. Her propellers are visible underwater today and photographs taken from the wreck site indicate that these specifications had been followed. Britannic’s turbine casing was ‘produced in the foundry at Messrs. Harland & Wolff’s works – one of the largest in the country, and now equipped with the important accessory of a well-equipped laboratory.


The turbine's rotor (Ulster Folk & Transport Museum)

 Although such information is interesting from a mechanical viewpoint, it is also important to record the changes to Britannic’s propelling machinery as they have often been overlooked. One well known source merely stated that she used ‘the same’ arrangement of twin reciprocating engines and a low pressure turbine as her sisters. Even if you are not mechanically minded, such information adds to our knowledge of Britannic, a ship that has all too often been overlooked, for nearly ninety years. All we can hope for is that that state of affairs will change, as more people become interested in a ship which in its day was to be the White Star Line’s answer to the finest liners from Germany and Britain’s Cunard Line, the crowning glory of the ‘Olympic’ class, and which was once described as ‘as perfect a specimen of man’s creative power as it is possible to conceive.’



[1] Interestingly Olympic’s engines received some changes with a similar aim to those modifications on Britannic, but not until her overhaul and refitting of late 1932 and early 1933. During this refit, balance weights were fitted with new crankshafts, the intention being to improve the balance of the engines, although in the original design such weights had not been used. It was considered whether or not to install additional column ties for the engines, but as the performance on sea trials had been so satisfactory, it was decided not to install them.

[2]It has been speculated upon that Britannic’s intended service speed was 23 knots. At least two sources record this figure, but in the light of many considerations such as the maintenance of her crossing schedule with Olympic, not to mention the additional fuel consumption involved, the speed of 21 knots seems more accurate. The German Imperator, launched in summer 1912, was designed for a speed of 22½ knots, which may explain Britannic’s higher figure; nonetheless, Imperator was not notably faster than her British rivals.


·     Engineering magazine, February 27th 1914.

·     The Shipbuilder magazine, spring 1914.

·     Belfast News Letter, 1914.

·     The London Times 1914.

·     Olympic statement of Chief Engineer Fleming and Engineer Therle.

·     Olympic letter, Messrs. Ismay, Imrie & Co., 1911.


My thanks to Scott Andrews and Bruce Beveridge for helping me to proof the article for technical accuracy. If any errors have occurred, they are most definitely not responsible.