Fast modern passenger and car ferries are often twin hull boats, propelled using water jets instead of conventional propellers. A pumping system throws water out backwards at high speed. One such ferry is the Stena Lynx II, which sails between Fishguard in Wales and Rosslare in Ireland.
Some data about the Stena Lynx II:
Resistive forces at cruising speed = 8.0 x 10^5 N
Mass of water per second ejected by the jets = 60000kgs 1
Show that water must be thrown backwards at about 13 ms-1 while the boat is maintaining cruising speed.
The pumping system converts energy which is expended in two main ways: working against resistive forces and giving kinetic energy to the water thrown backwards.
Calculate the power expended against the resistive forces at the cruising speed of 20 m s 1.
Power = ……………………………………
Calculate the rate at which the water in the jets gains kinetic energy.
Rate=………...............................
Hence show that the overall efficiency of the pumping system at cruising speed is about 40%.
Fuel consumption of pumping system at cruising speed= 1.6 litres/second
1 litre of fuel provides 3.4 x 107 J of energy when burnt
