Frictional Resistance: The Dominant Component

9 min read

What you'll take away Explain where frictional resistance comes from, what the ITTC-57 line estimates, and why hull roughness turns into fuel.

Run a hand through water and you feel it: water does not let a surface slide through for free. Scale that up to several thousand square metres of wetted hull and you have frictional resistance — the largest single component of a ship's calm-water drag. For a new, slow-speed ship it typically accounts for 70–90% of the total; even for fast vessels, where wave-making takes a bigger share, friction still claims around half.

Where the force actually lives

The drag is generated in the boundary layer: a thin sheet of water travelling with the hull, sheared between the plating (where water sticks to the surface) and the free stream (where it does not). Inside that layer, viscous shear stresses act on every square metre of wetted surface — which is why wetted area appears directly in the resistance formula, alongside water density and the square of speed.

Whether the layer flows smoothly (laminar) or churns (turbulent) matters enormously, and the deciding parameter is the Reynolds number. Beyond roughly 1×10⁶ the flow trips into turbulence — and a full-scale ship at sea is so far beyond that threshold that its boundary layer is turbulent over practically the whole hull. Turbulent friction is the friction that matters commercially.

The standard estimate is the ITTC-57 model–ship correlation line:

C_F = 0.075 / (log₁₀ Re − 2)²

A curve fit, not a law of nature — but it is the shared reference the industry prices friction with, and it makes the dependencies explicit: Reynolds number, wetted surface, density, speed.

The part operations controls: the surface itself

Everything above assumes a smooth hull. Real hulls age. Average Hull Roughness (AHR) puts a number on it: around 65 µm is a hull in very good condition; drift well past 200 µm and the surface is measurably costing power. Roughness thickens the turbulent boundary layer and raises shear — the physics is invisible from deck, but the fuel invoice sees it clearly.

Biofouling is the aggressive end of the same story. Severe growth can double the power needed to hold a speed compared to a clean hull, and the penalty compounds quietly between dry dockings as coatings age and organisms accumulate.

Why small losses become large bills

Required power scales roughly with the cube of speed — doubling speed can demand eight times the power. The same non-linearity works against a fouled hull: holding schedule with a rough hull means pushing more power through the same cube law, so fuel consumption and CO₂ emissions climb out of proportion to the roughness that caused them. That is the commercial logic for treating hull condition as a performance variable to monitor, not a maintenance chore to remember — and it is where this course connects directly to compliance: every extra tonne of fuel is now also a ratings and carbon-cost problem.

Go deeper: the original article Frictional Resistance: The Dominant Component of Drag.

Check yourself

1. The ITTC-57 model–ship correlation line is used to estimate which coefficient?
2. Which Average Hull Roughness generally indicates very good operational efficiency?
3. The power needed to overcome resistance is approximately proportional to:
4. On full-scale ships, boundary-layer flow commonly turns from laminar to turbulent above a Reynolds number of about:
5. Frictional drag is generated in which part of the flow?
6. Which of these does the ITTC-57 line make explicit as dependencies of frictional resistance?

Select all that apply.

7. Severe biofouling can double the power needed to hold a given speed compared with a clean hull.
8. Above roughly what Average Hull Roughness does the surface start measurably costing power?
µm

Type a number in µm.