The Five Components of Calm-Water Resistance

5 min read

What you'll take away Name the five components of calm-water resistance and say roughly how much of the total each one is responsible for.

Take the calmest day a vessel will ever see: no wind, no waves, no current, deep water. Hold her at service speed and read the shaft power. That number — before weather, before fouling, before anything a voyage adds — is the vessel's resistance floor, and everything in fleet performance work is measured against it.

The floor is not one force. It is five, and they behave so differently that lumping them together hides exactly the information an operator needs.

Frictional resistance is the water itself gripping the wetted surface of the hull. It is the dominant share — up to 70–90% of total drag for a typical commercial vessel — and it is also the component an operator can actually influence in service, because its size depends on the condition of the hull surface. It gets a full lesson next.

Wave-making resistance is the price of pushing a pressure field through a free surface: the ship spends propulsive power building the wave pattern it drags along with it. At low speed it is nearly free; push the speed up and it grows steeply. It behaves like a tax on going faster, which is why it shapes every speed decision.

Viscous pressure resistance — form drag — comes from the pressure at the stern never quite recovering to match the pressure at the bow. It is a design inheritance: full-bodied hulls carry more of it than slender ones.

Air resistance is the one most people forget exists in calm weather. A ship doing 15 knots through still air is meeting a 15-knot headwind of its own making, and the superstructure pays for it continuously.

Appendage and steering resistance covers everything bolted to the hull — bilge keels, rudder, shaft lines, thruster tunnels — plus the quiet cost of holding a straight course.

Two things are worth fixing in mind before the detailed lessons. First, the proportions: friction is the giant, and the rest are corrections — important ones, but corrections. Second, the split between what design fixed the day the ship was launched (form drag, appendages) and what operations still controls every day (surface condition, speed, trim). This course walks through the five in that order of leverage.

Go deeper: the original article The Five Components of Calm-Water Resistance.

Check yourself

1. Which component typically accounts for the largest share of calm-water resistance?
2. Wave-making resistance is described as the 'tax' on what?
3. Which of these belongs to calm-water resistance?
4. What is a vessel's resistance floor?
5. Which hulls carry more viscous pressure (form) resistance?
6. Which of these fall under appendage and steering resistance?

Select all that apply.

7. Hull surface condition was fixed at the design stage and cannot be influenced once the ship is in service.
8. A vessel steams at 15 knots through still air. How strong is the headwind of her own making?
knots

Type a number in knots.