Why do we require preserve the model? It may be necessary for us to study the model characteristics later on, or also do further tests which were not required at the beginning such as a propulsion test by fitting a propeller, or doing maneuvering test, or sea keeping test if need arises later on. Nowadays, if some tanks are using PVC foam- polyvinyl chloride foam- for manufacturing a model, that also is light, but it does not give the inner surface as much as a FRP model can give for housing other equipments and instruments. The model finish must be smooth- this we have discussed- model finish must be absolutely smooth, less than 25 microns roughness should be there- a micron is 10 to the power of minus 6 meters or 10 to the power minus 3 millimeters- the roughness of the model scale should be less than 25 millimeters, which can be considered as zero, so there is no allowance on model for roughness.