DieselShip

Hull resistance

For a vessel to move through the water certain resistance have to be overcome.

These can be classed into three groups;

• Frictional resistance
• Residual resistance
• Air resistance

When a vessel moves the water contact the hull exerts a pressure dependent on the vessels speed, the density of the water and the wetted surface area of the hull

K =1/2. V ².AS
Where
K is Force if the vessels was being stopped completely by force of water
V is the speed of hull through water
AS is the wetted surface area of the Hull including rudder

(From Bernoulis)

The total resistance R = C x K
Where
C is a dimensionless coeeficient

C can be calculated using known factors such as the wetted surface area, block coefficients etc and can be confirmed using towing tests in tanks.

Frictional Resistance R_F

This is related to the wetted surface area. It can make up the main part of the total resistance reaching 70 to 90% for large slow moving vessels such as tankers reducing to 40% for high speed craft.

Hull resistance increases as the hull fouls due to marine growth the effect of which is to reduce vessel speed and increase fuel consumption. The use of antifoulant paints attempts to restrict this. A heavily fouled hull can have increased frictional resistance by 25 to 40%.

Residual Resistance R_R

This consists of two main factors

• Wave resistance - causing energy loss due to wave formation by the vessel
• Eddy Resistance - causing energy loss due to flow seperation and eddy formation

Wave resistance- At low speeds the wave resistance is proportional to the square of the velocity but increases much faster as the speed increases. This effect means that there is an effective maximum sped of the vessel were increaseing power does not rsult in effective increases in speed instead the excess energy simply goes into creating large waves.

Residual resistance can greatly increase in shallower water as the displaced underhull water has more dificulty moving aft

Air resistance R_A

Is proportional to transverse sectional area above the waterine and the square of the vessels speed It normally adds up to 2% of the total resistance however for arge high seed craft such as Container ships it can be as much as 10%.

Effects of sea and weather state

The vessels trading pattern can be seriously effected by the prevailing weather and sea conditions. For a medium sized tanker of bulk carrier the effective resistance can increase by 100 to 200% due to weather and sea effects only.

Towing resistance R_T

This the summation of all the resistances. The power required to move vessel is said to be the vessels speed multiplied by the total resistance.

P_E = R_T x V
R_T= R_F + R_R + R_A
This power is somewhat less than the propulsion power due to propeller losses due to flow around the propeller and the propeller efficiency it self