You are captain and navigator for five crossings, picked at random from the Ferry Lab map. Choose your speed, then work out when you will arrive and how much fuel you will burn. Arrive on time, use as little fuel as you can, and get your sums right.
Start at the target arrival time and work back to the slowest speed that still gets you there.
Planning backwards. You leave at 09:15 and must arrive by 11:45. That gives you 2 h 30 min, which is 2.5 hours. The crossing is 20 nautical miles, so over the sea bed you need 20 ÷ 2.5 = 8 knots. A knot is one nautical mile an hour.
The current is 2 knots against you, so the engines must make up for it: 8 + 2 = 10 knots. If the current were with you, you would take it away instead: 8 − 2 = 6 knots.
Working forwards. Say you choose 12 knots. Against a 2-knot current you make 12 − 2 = 10 knots over the sea bed. 20 ÷ 10 = 2 hours, so you arrive at 11:15. If the engines burn 300 litres an hour, the trip uses 300 × 2 = 600 litres.
Water pushes back on a moving ship. Go twice as fast and the water pushes back about four times as hard. You also cover the distance twice as quickly, so the engines need about eight times the power.
You get there in half the time, so for the whole trip a ship going twice as fast burns about four times the fuel.
But going very slowly is not free either. The lights, heating, kitchens and pumps use fuel every hour you are at sea. Somewhere in between is the best speed. Big cargo ships save fuel by sailing slowly on purpose. Sailors call this slow steaming.
Tides matter too. A current pushing you along is free speed. A current against you slows you down. The Pentland Firth, between Caithness and Orkney, has some of the strongest tides in Britain.