Where Performance Begins: The Resistance Floor

7 min read

What you'll take away Explain why vessel performance is measured as change against a calm-water baseline, and why percentage speed loss beats fuel-per-day as the measuring stick.

Ask a technical department how a vessel is performing and you will usually be shown a fuel number: tonnes per day, this month against last month. It feels concrete. It is also, on its own, close to meaningless, because fuel per day moves when the ship slows down, when the weather turns, when she loads deeper, when a current helps or hurts. The hull could be pristine while the number looks terrible, or growing a beard of barnacles while the number looks fine.

Performance work begins by choosing a better question. Not how much fuel did we burn but: how far above her floor is this ship running?

The floor

Every hull has a calm-water resistance floor: the resistance the clean ship meets at a given speed and draught in still, deep water, and with it the power it takes to push her. That floor is set by the physics of the hull form and was fixed the day the ship left the yard. (Our course Ship Resistance Fundamentals walks through the five components that make it up.)

Everything a voyage adds, wind, waves, current, shallow water, and everything time adds, hull fouling, coating breakdown, propeller roughness, stacks on top of that floor. The art of performance analysis is separating the two stacks: the weather washes in and out of the record; degradation accumulates. One is noise, the other is the signal.

Calm-water floor Degradation: the signal Weather: the noise power at one speed time in service since dry-docking → same speed and draught throughout: indicative shapes, not measurements
The performance question in one picture: the floor is fixed, the weather term washes in and out of the record, and the degradation term only accumulates. Reading the ship means separating those two stacks. Shapes are indicative.

Why speed loss is the measuring stick

The industry's standard signal is percentage speed loss: take the power the ship is actually delivering and the draught she is actually at, ask the baseline what speed a clean ship would make under those conditions, and compare it with the speed through water she is actually making. The shortfall, expressed as a percentage, is the performance value. Its sign is the standard's: negative when the ship is slower than the baseline, which is the bad direction, positive when she is faster. Where a course states the same thing as added power or excess consumption, the sign flips: positive is worse. Lesson 3 works the arithmetic.

Speed loss has three properties that fuel-per-day lacks:

  • It is condition-corrected by construction. Power and draught go into the expected-speed lookup, so the comparison is like-for-like.
  • It isolates the hull-propeller system. Engine efficiency drift shows up in fuel figures; it does not change how much thrust a given delivered power produces at the propeller.
  • It compounds honestly. Because power scales with roughly the cube of speed, a "small" speed loss is a large fuel penalty: holding the baseline speed against an 8% speed shortfall costs (1/0.92)³ ≈ 1.28 times the power, close to 30% more. The rule for any shortfall s is (1/(1 − s))³. A percentage that sounds minor is money that is not.

What this course does with it

The chain we will build over the next nine lessons runs: baseline → measured change → decision. Lesson 2 asks where baselines actually come from, and why a sea-trial curve deserves scrutiny before you hang a year of conclusions on it. Lessons 3 to 5 build the measurement machinery: the ISO 19030 speed-loss method, the four performance indicators, and the data quality battles that decide whether any of it means anything. Lessons 6 to 9 spend the knowledge: fouling economics, weather routing, just-in-time arrival, and heavy-weather engine operation. Lesson 10 assembles it into a management system that survives crew changes and budget seasons.

One habit to adopt from this lesson onward: whenever someone shows you a performance number, ask what it was compared against and what was corrected for. A number without a baseline and a correction method is an anecdote.

Check yourself

1. A vessel's performance baseline describes the ship in which condition?
2. Why is fuel consumption per day a poor performance indicator on its own?
3. Percentage speed loss compares which two speeds?
4. A ship shows 8% speed loss. Roughly what does that mean for fuel at constant speed?
5. Added resistance from wind and waves belongs in the performance baseline.
6. Which of these genuinely degrade a vessel's performance relative to her baseline?

Select all that apply.

7. The gap between a ship's in-service power demand and her baseline power demand at the same speed and draught is best described as what?
8. A sister vessel makes 0.4 knots more at the same power. Fleet staff conclude the slower ship is fouled. What is the weakest point in that conclusion?

That was the free sample of Vessel Performance in Practice

The other 9 lessons, the Excel workbook, the final examination and the certificate come with the full course: €179 + VAT, paid once.

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