The primary purpose of a ship's funnel(s) is to lift the exhaust gases clear of the deck, in order not to foul the ship's structure or decks, and to avoid impairing the ability of the crew to carry out their duties.
In steam ships the funnels also served to help induce a convection draught through the boilers.
The required funnel cross-sectional area is determined by the volume of exhaust gases produced by the propulsion plant. Often this area is too great for a single funnel. Early steam vessels needed multiple funnels (SS Great Eastern had 5 when launched), but as efficiency increased new machinery needed fewer funnels.
Since the introduction of steam-power to ships in the 19th century, the funnel has been a distinctive feature of the silhouette of a vessel, and used for recognition purposes.
A key part of the deception practiced by ships carrying out commerce raiding during both the First World War and Second World War was to disguise their ship's outline, and this included using false funnels or by changing the height or diameter of the actual funnel(s).
Merchant shipping companies (and particularly liner companies such as Cunard Line and ferries such as Red Funnel) were quick to recognise the publicity value of distinctive funnels, both in terms of shape, number of funnels, and the colours they were painted. Each company would have their own "house colours", which were often used in publicity material as well as for recognition.
For example, the RMS Queen Elizabeth 2 was built with very distinctive wind-scoops at the base of her funnel. When fitted with new diesel engines in 1987, which had a different exhaust requirement to the old boilers, the new funnel was built to the same silhouette as the old one, in order to retain this distinctive recognition feature.
In the late 19th and the first half of the 20th century the number of funnels became associated with speed and reliability. For this reason a number of the great liners carried false funnels that they did not need. Examples included the White Star lines Olympic, Titanic and Britannic; Hamburg America Line's Imperator (which became Berengaria under Cunard) and later the Cunard's Queen Mary and its rival the French Line's Normandie. In all cases the false funnel was the aftermost of the funnels. The false funnels did have their uses however - a stoker who survived the sinking of the Titanic escaped the boiler room by ascending the false funnel and the aft funnel of the Normandie was equipped with dog kennels.
A mack is a combined stack and mast, as fitted to some classes of 20th century warships. Although they can reduce top-weight, they have not gained universal popularity due to the problem of exhaust smoke fouling of electrical aerials and equipment.
- John Asmussen. "Admiral Graf Spee in Disguise". Retrieved 10 February 2011.
- TheShipsList. "Funnels, Flags, and Night Signals of the Transatlantic Lines". Retrieved 10 February 2011.
- Rob Lightbody. "QE2's Major 1986 - 1987 re-engining refit". Retrieved 10 February 2011.