resistance to the displacement of ships is generally predicted and studied with tests on basin models. To apply the results obtained from model to prototype ,we must taken into account the parameters of similarity

the observed and desired results are in general resistance to the the displacement of ships

In this case the resistance ( Rh ) Is reduced for easier operation to a coefficient of resistance called specific resistance. This coefficient ( Ch) is the ratio of resistance ( Rh ) On the displacement ( D ) (weight of water equivalent to the immersed volume X density of water): C h = Rh / D

Considering that the geometric shapes and weight distribution of the model are identical to the actual ship (prototype), the parameters determining Rh are :

• the characteristic of environnement
  • g garvity
  • r density of water
  • n kinematic viscosity of water
  • ra density of air
  • na kinematic viscosity of air
  • t Surface tension water air
  • Pa atmospheric pressure
  • Pv vapor pressure of water

• characteristics of the hull
  • L Characteristic length (LWL)
  • V Feed rate
  • K hull roughness (represented by a coefficient of roughness average)

resistance to the march Rh is based on the number of Froude (as part of the wave resistance) , the number of Re ynolds and the relative roughness K / L (as component of friction).

the transition from model to prototype scale, g garvity , r density of water, n kinematic viscosity of water, ra density of air, na kinematic viscosity of air, t Surface tension water air Pa atmospheric pressure Pv vapor pressure of water will remain fixed.

This implies that the values L Characteristic length ( waterline length ) and V (velocity) should evolve from model to reality by keeping the Froude number and Reynolds number constant without changing settings g Severity and n kinematic viscosity of water,... which is impossible to realise.

As we can not preserve together the similarity of Reynolds (Re = VL / n ) and Froude ( Fr = V / w (gL), The similarity of Froude : (Fr model) = (Fr real) is solved by adjusting the model speed.

with Ch coefficient resistance to displacement and Cv component of viscous resistance on (see Froude ):

So we have: Ch real = Ch model - ( Cv model - Cv real )

-( Cv pattern - Cv real ) = Correction of friction , from the fact that we neglected the similarity of Reynolds

correction of friction is a negative value is applied according to the Froude number: it can vary from -0.15 Ch model for fast vessels Fr high (0.5 _ 0.6) to -0.25 Ch model for slow ships(Fr low) (0.15 _ 0.2)