Editing Boundary layer (section)

==Naval architecture==
Many of the principles that apply to aircraft also apply to ships, submarines, and offshore platforms, with water as the primary fluid of concern rather than air. As water is not an ideal fluid, ships moving in water experience resistance. The fluid particles cling to the hull of the ship due to the [[Adhesion|adhesive force]] between water and the ship, creating a boundary layer where the speed of flow of the fluid forms a small but steep speed [[gradient]], with the fluid in contact with the ship ideally has a relative velocity of 0, and the fluid at the border of the boundary layer being the [[Free streaming|free-stream]] speed, or the relative speed of the fluid around the ship.<ref name=":0">{{Cite web |title=Resistance and Powering of Ships |url=https://www.usna.edu/NAOE/_files/documents/Courses/EN400/0207_Chapter_7_Jun20.pdf |access-date=14 February 2024 |website=usna.edu}}</ref>

While the front of the ship faces normal pressure forces due to the fluid surrounding it, the aft portion sees a lower acting component of [[pressure]] due to the boundary layer. This leads to higher resistance due to pressure known as 'viscous pressure drag' or '[[Parasitic drag|form drag]]'.<ref name=":0" />

For ships, unlike aircraft, one deals with incompressible flows, where change in water density is negligible (a pressure rise close to 1000kPa leads to a change of only 2–3&nbsp;kg/m<sup>3</sup>). This field of fluid dynamics is called hydrodynamics. A ship engineer designs for hydrodynamics first, and for strength only later. The boundary layer development, breakdown, and separation become critical because the high viscosity of water produces high shear stresses.