Complex sentence: Despite the urgency of the situation and their maximum speed of around 25 knots, the ships travelled at a steady 10 knots--running at high speed used significantly more coal, so to complete the long journey it was necessary to travel at the most economic speed.
Split into simpler sentences: Despite the urgency of the situation and their maximum speed of around 25 knots, the ships travelled at a steady 10 knots. Running at high speed used a disproportionately greater quantity of coal, so to complete the long journey it was necessary to travel at the most economic speed.