Viscous Pressure (Form) Resistance

Continuing our series on the breakdown of calm-water resistance, today we examine the hidden drag at the stern: Viscous Pressure (Form) Resistance.

While frictional resistance affects the entire wetted surface, form drag is highly localized. It is driven by how fluid pressure changes as water travels the length of the hull.

As a vessel moves forward, it pushes through the water, creating an area of high pressure at the bow. Ideally, if the water could flow perfectly around the hull and close perfectly behind the stern, the pressure at the aft would recover completely, pushing the ship forward and cancelling out the bow resistance.

However, because water is viscous, this perfect recovery is impossible.

As water travels along the hull, it forms a boundary layer due to friction. At the bow, this boundary layer is extremely thin, so the high pressure acts directly against the hull. But as the flow continues toward the stern, the boundary layer grows thicker.

This thickened boundary layer at the aft acts like a cushion, preventing the water pressure from fully recovering against the hull. Consequently, the pressure at the stern is always lower than the pressure at the bow.

This permanent pressure difference acts like a vacuum, continuously pulling the vessel backward.

The problem worsens dramatically if the hull geometry curves inward too abruptly at the stern. When the angle is too steep, the water can no longer follow the shape of the hull. The flow separates entirely, generating chaotic, turbulent eddies and a massive low-pressure wake directly behind the ship. This flow separation turns a moderate pressure difference into a severe suction force, wasting significant propulsive energy.

Full-block vessels, like bulk carriers and tankers, are particularly susceptible to this form drag compared to slender, high-speed designs. While naval architects use advanced CFD to design aft geometries that minimize separation, operational factors like draft and trim can also influence where this separation occurs.

At Blue Autonomy, we help you understand complex hydrodynamic realities (as the above), enabling physics-backed decisions that minimize hidden drag and maximize fuel efficiency across the fleet.