01Executive summary
Four milestones in which marine propulsion moved from consuming most of the ship to leaving room for the cargo that paid for it.
The screw propeller displaced the paddle wheel on ocean-going vessels. Compound, triple and quadruple expansion at higher pressures cut fuel consumption by as much as half. Diesel propulsion went to sea in 1903 and deep-sea in 1912. And oil firing removed a labour requirement whose scale is difficult to credit today.
02Screw against paddle
The screw propeller was suggested by Watt as early as 1770, and engineers were still arguing the relative merits of paddles, mechanical oars and similar forms long after propellers had been tried. The argument took decades to settle, and it was settled by seagoing conditions rather than by efficiency in still water.
It stays immersed
A paddle wheel's immersion changes with loading and with every roll and pitch, so thrust varies and in a seaway one wheel can leave the water entirely. A propeller aft and deep runs at consistent immersion.
It is out of the way
Paddle boxes consume beam, obstruct berthing and are vulnerable to damage and to gunfire. The Great Eastern's beam was 25 metres inside the paddle boxes and 37 outside — twelve metres of width devoted to propulsion.
It suits higher engine speed
A paddle wheel must turn slowly because its tip speed is limited. A propeller runs faster, which suits the compact, higher-speed engines that followed and reduces gearing.
Shallow and inland water
A paddle steamer draws less and can work water a propeller cannot. The screw did not win everywhere; it won at sea, which is the case that mattered for trade.
03Multiple expansion: using the steam more than once
A simple engine admits steam, expands it through the stroke, and exhausts it still carrying a substantial fraction of its available energy. Multiple expansion passes that exhaust into a second, larger cylinder where it expands further against a lower back pressure, and then in triple and quadruple arrangements into a third and fourth.
Expanding steam through a very large ratio in a single cylinder causes a large temperature swing in that cylinder over each cycle, so the walls alternately condense incoming steam and re-evaporate it — exactly the loss James Watt attacked with the separate condenser in the first series of this set. Splitting the expansion across cylinders means each one works over a modest temperature range, so cylinder-wall losses fall sharply. The gain comes from limiting the swing in any one place, not from the number of cylinders as such. Staging a process to keep each stage within a narrow operating range is a general and widely applicable move.
The economics were decisive rather than marginal. Fuel consumption fell by as much as half, which means the bunker space and the coal weight required for a given voyage roughly halved, and that space and weight became available for paying cargo. Steam moved into profitable competition with sail on the transatlantic run, which it had not been in the first days of ocean steam.
Note that this is the same argument as multiple-effect evaporation, staged compression with intercooling, and cascaded refrigeration. A process that must span a wide range of some variable is usually better split into stages each spanning part of it.
04Oil firing: an engineering change that was mostly a labour change
The figure worth sitting with is this: one of the last large hand-fired steamers required 324 firemen and trimmers to cart and shovel about 1,000 tonnes of coal a day into her boilers. That is not a footnote about working conditions; it is the operating cost, the crew accommodation, the victualling and a substantial part of the ship's internal arrangement.
By the mid-1950s oil firing accounted for 79 per cent of steam tonnage and had greatly reduced costs. Marine diesels — installed in two Caspian tankers in 1903, five years after commercial production began, and in the 7,500-tonne Selandia of 1912 as the first important ocean-going motor ship — went further, with higher thermal efficiency than steam plant. The trade is that diesel oil is a highly refined product and more expensive than residual fuel oil or coal, so the comparison is efficiency against fuel price, which is a genuine engineering-economic judgement rather than a technical ranking.
05Takeaways for current practice
- Stage a process to keep each stage in a narrow range. Multiple expansion, intercooled compression and cascaded refrigeration are all the same move.
- Count what a change does to crew, not just to efficiency. Oil firing was mostly a labour change and that is where the money was.
- Judge on conditions of service, not bench performance. The screw beat the paddle because it stays immersed in a seaway.
- Efficiency against fuel price is the real comparison. A more efficient engine burning a more expensive fuel is not automatically cheaper.
- Fuel saved is payload gained. On any vehicle, bunker weight and volume come directly out of what earns.