Propulsion

Learning objectives

Be able to calculate work done during an energy transfer

Be able to calculate power and efficiency

Annotate the image of the Mersey class ‘in action’ on worksheet 1. Label as many types of energy as you can see in the image.

Classify each type of energy as ‘input’, ‘useful’ or ‘wasted’.

Input energy Chemical energy Input energy Chemical energy

Useful energy OUT Kinetic energy, Electrical Light

Useful energy OUT Kinetic energy, Electrical Light

Wasted energy out Sound Heat

Wasted energy out Sound Heat

Input Useful out Wasted

Chemical Kinetic Sound

Electrical Heat

Light

The Mersey lifeboat from Lytham St. Annes, Lancashire was launched to rescue a sick yachtsman whose boat was in danger of capsizing.

In a moderate breeze his boat had run aground on a sandbank just off the coast 6.9 miles from the lifeboat station.

The whole rescue took just over one hour.

Lifeboat crews rescue man off Fylde coast

1 litre of diesel in the lifeboat’s engines supplies 36Mj (36 million joules) of chemical energy.

The Mersey lifeboat uses a traditional propeller design and 14.4Mj are transferred to kinetic energy.

How efficient is this engine?

Energy Transfer

1 litre of diesel in the lifeboat’s engines supplies 36Mj (36 million joules) of chemical energy.

The Shannon lifeboat uses a water jet propulsion system and 21.6Mj are transferred to kinetic energy.

How efficient is this engine?

Energy Transfer

The Mersey lifeboat from Lytham St. Annes travelled for 6.9 miles (1 mile = 1600m). Its engine gives out a thrust of 33kn .

What is the work done?

Work Done = Force x Distance(J) (N) (m)

The jet propulsion system on the Shannon class uses 2 engines that each give out a thrust force of 32kN.

If Lytham St. Annes had one of the new Shannon class lifeboats, how much work would the engines have done in the sick yachtsman rescue?

If 1 litre of diesel supplies 36MJ, how many litres will be used to travel to the sick yachtsman?

Work Done = Force x Distance(J) (N) (m)

Power = Work done / Time (W) (J) (s)

If the lifeboat takes 14 minutes to travel 6.9 miles what is the power output of the jet propulsion system?

What is the average speed of the lifeboat as it travels to the scene of the incident?

The RNLI are deciding whether Lytham St. Annes should replace the ageing Mersey lifeboat with one of the new Shannon class boats.

Put yourself in the place of a propulsion engineer and outline the key advantages and disadvantages of changing lifeboats.

The RNLI have issued their coxswains with the guideline “twenty is plenty”. This guideline says that when they are not answering emergencies they should drive at 20 knots instead of 25 to save fuel.

Calculate the difference in work done, power, speed and fuel usage if a Shannon coxswain was to follow the same route as the sick yachtsman rescue in a non-emergency situation and to follow the “20 is plenty” guideline.