The main component of a ship's 'trail' was smoke, not steam. The amount of smoke produced was a result of several factors, some of which could be controlled. One key factor was the choice of fuel. Coal produced a lot more smoke than oil fuel. By WWII, most warships used oil, but older ships, or cheaply built escorts, could burn coal. The location of origin of the oil was also important. The Royal Navy preferred to use oil from the Persian Gulf, which it felt burned more cleanly, and created less smoke. However, the pressures of wartime forced it to switch to oil from the Carribbean and the USA, which was perceived as being dirtier.

On a more technical level, the amount of smoke produced depended on the proportion of the fuel that burned. A ship that wanted to create a smoke screen would reduce the amount of air going to the boiler fires, and inject in extra fuel oil. This meant that a much smaller proportion of fuel would burn, creating large clouds of black smoke. The reverse was also possible to some extent; restricting the fuel input and increasing the would lead to more complete combustion and reduce the amount of smoke. However, doing so might also reduce the amount of heat produced by the fire, reducing the amount of steam produced, and thus slowing the ship down.