The ship Hydrodynamics is studying the hydrodynamic resistance force on ships. This resistance is composed of forces from several phenomena that are distinguished in the study but whose interactions are intimately intertwined.

The results of calculations or measurements are most often given as hydrodynamic coefficients Ch, by relating the hydrodynamic resistance Rh, the weight and volume of water displaced (Ch = Rh / displacement)

* the displacement of a vessel is the weight of water it displaces, so the immersed volume X density of water. .

 

We assume that the environment is calm (no waves or current or wind) and we limit our discussion by neglecting the resistance of superstructure (about 3% of Rh).

The resistance of ship is study into two parts: resistance viscous resistance to the wave. (see Froude)

1. viscous resistance (Rv), the viscous resistance coefficient (Cv) is related to Reynolds number (Re) and roughness (K) on the hull length (L)

• Rv= Cv . (Re . K/L)

If the object is near the surface, the viscous resistance or drag is accompanied by a resistance due to the formation of surface waves:

 

1. the wave resistance (Rw), whose coefficient of resistance to the wave (Cw) is related to Froude number (Fr)

   There are different types and causes of waves ( resistanceof waves more..)

 

• Rw= Cw.(Fr)

• Cw =Rw/(1/2)r.s.v²(r density, S surface in contact with water, V velocity.

 

Total coefficient of hydrodynamic resistance (Ch=Rh/Displacement) for a ship hull moving near the surface is defined as:

• Rv= Cv . (Re . K/L)

• Rw= Cw.(Fr)

• Ch=  Cw.(Fr) + Cv.(Re.K/L)

Different types of approaches to assess the hydrodynamic coefficient (Ch):

• Method of testing models in basin :

Despite advances in computer modeling, testing models are still essential. The effects of friction measured experimentally in basin are themselves subject to problems of scaling resolved through laws of similarity . (see Froude )

Method of testing models in basin

• Method series: Another technique for evaluating hydrodynamic behavior is to compare data measured on a series of standard forms and extrapolate the consequences of changing forms. The specific hydrodynamic resistance (Rh / Move *) is given as a function of Froude number in series:

• the series of tailor, by Admiral Taylor Model Basin Experimental in Ashington

The series of tailor is still required for the most comprehensive research in the effects on power efficient

.

• In naval hydrodynamics, to evaluate independently the viscous resistance of the surface resistance , the method of dual models is sometimes used . It involves deep immersion in tow a geometric shape composed of model hull of a ship and its symmetry to the waterline . The measured drag force is then divided by two . The experiment at sea is costly and complex , so it is achieved in wind tunnel. The profiles that drag (Cd) is already set in Mecaflux thus serve to evaluate the hydrodynamic viscous resistance. .

This resistance is composed of viscous frictional resistance and resistance form. The frictional resistance can be calculated with the detailed study of flat plate in Mecaflux

• Wave resistance (Rw), whose coefficient of resistance to the wave (Cw) is linked to the Froude number (Fr)

  • Rw= Cw.(Fr)

  • Cw =Rw/(1/2)r.s.v²(r density, S surface in contact with water, V velocity.

A body that moves on the calm surface of water produces a wave systèm. This system is produced by the pressure field around the body and wave energy is given by the body . This transfer of energy from the body to the surrounding system, product our wave resistance . ( the boat made pretty waves for water skiing but it cost him in energy and that's why water skiing is expensive ...)

There are two types of wave systems produced by the hull:

•   a system of diverging waves inclined from the axis of the boat

•   a system of transverse waves perpendicular to the axis of the boat .

This system of waves, divergent and transverse, is produced by the stern (stern) and stem (bow). The interference between these systems of waves, and generates peak along the wall of the hull. .

Simplifying, one can say that resistance of the wave is generated by the difference between the pressure on the bow and stern , if the surface of bow generating low pressure decreases because of the bottoming , resistance to the wave decreases ... In this curve wave resistance based on speed and length of the hull ( Froude) shows that the wave resistance evolves by highlighting the peak or trough at the stern .

Graphs of the evolution of resistance coefficient Cw wave, depending on the ratio Speed / length

In total, the coefficient of hydrodynamic resistance (Ch) is defined as :

Ch=  Cw.(Fr) + Cv.(Re.K/L)

(Ch=Rh/Displacement)