Viscous Pressure (Form) Resistance

8 min read

What you'll take away Describe why pressure never fully recovers at the stern, and what happens when the aft geometry is too abrupt.

In a perfect fluid, a ship would get a refund. The high pressure the bow builds by pushing water aside would be repaid at the stern, where the flow closes back in and pushes the vessel forward. Bow push and stern push would cancel, and pressure would cost nothing.

Water is viscous, and the refund never arrives in full.

The boundary layer spoils the recovery

As water runs the length of the hull it builds the same boundary layer we met in the friction lesson — thin at the bow, thicker with every metre aft. At the bow the layer is so thin that the high pressure bears directly on the plating. By the stern, the thickened layer sits between the flow and the hull like a cushion, and the pressure recovery it was supposed to deliver arrives muffled.

The result is permanent: pressure at the stern is always lower than at the bow. That standing difference acts like a gentle vacuum applied to the aft end of the ship, pulling backwards for every mile of the voyage. This is viscous pressure resistance — form drag — and unlike friction, which is spread over the whole wetted surface, it is concentrated at the stern.

When the flow lets go entirely

The gentle version becomes expensive when the aft geometry asks too much of the flow. If the hull curves inward too abruptly, the water cannot follow the surface. It separates — detaches from the hull entirely — leaving a region of chaotic, turbulent eddies and a broad low-pressure wake dragging behind the ship. The mild pressure imbalance becomes a severe suction, and propulsive energy is spent stirring the sea.

This is why full-block vessels — bulk carriers, tankers — are the most susceptible: their generous cargo blocks must be closed out over a short aft body, and the closing angles flirt with separation by design necessity. Slender, high-speed hulls taper gently and largely escape the problem.

What design fixed, and what operations can still touch

Most of a ship's form drag was decided at the design stage, when naval architects shaped the aft body — today with CFD specifically to keep the flow attached. But the operational levers are not zero: draught and trim change the pressure distribution along the hull, and with it where (and whether) separation occurs. That is one reason trim optimisation is a real performance topic and not a fad: on a hull that operates near the edge of separation, a different trim can move the flow to the right side of that edge.

Go deeper: the original article Viscous Pressure (Form) Resistance.

Check yourself

1. Where is form drag mainly generated?
2. Why can't pressure recover completely at the stern?
3. Which vessels are particularly susceptible to form drag?
4. In a perfect fluid with no viscosity, what would the hull's pressure field cost the ship?
5. If the aft body curves inward too abruptly, what happens to the flow?
6. Where along the hull is the boundary layer thinnest?
7. Which of these can still change a hull's form drag once she is in service?

Select all that apply.

8. Naval architects shape the aft body with CFD specifically to keep the flow attached.