Question

Describe the relationship between frictional resistance and

Ships speed

The wetted area

The surface roughness

The length of the vessel.

As the ship moves through water the water friction between hull and water causes a belt of eddying water adjacent to the hull to be drawn along with a ship, although at a reduced speed.

The belt moves aft and new particles of water are continuously set in motion, the force required to produce this motion being provided by ship and hence the extra power required for this resistance

When a ship is propelled trough water the frictional resistance increases at the rate of an index n of the speed of the ship and follows liner rule of progression as shown in graph

In a slow or medium speed ship, the frictional resistance from the major part of the total resistance and may be as much as 75% of the total resistance.

The amount of frictional resistance offered by water to the hull is depend upon the area of the ship in contact with the water i.e. wetted surface area.

The resistance due to friction will hence be in direct proportion of the ship wetted surface area ‘S’

The frictional resistance increase with the roughness of the hull.

Fouling of hull will cause an increase in drag and hence the resistance to motion will increase with a direct reduction in ship up to 40% in general.

The coefficient of ‘F’ in calculation of frictional resistance depends upon the length and given by

F = 0.417+0.773/L + 2.862

So as the length of eh ship increases the coefficient ‘F’ will reduce, but as there will be also an increase in ships wetted surface area and hence a combined effect will be on the frictional resistance.