Choosing and Stacking: Savings That Survive Addition
Seven lessons of technology end at a spreadsheet. Somebody has to turn the brochures into a shortlist, the shortlist into a budget, and the budget into savings the ship can prove it made. This lesson is that somebody's method.
Savings multiply: they never add
Stack two measures that each claim 10% and the naive answer is 20%. The arithmetic disagrees: the second measure acts on the 90% of consumption the first one left, so the stack delivers 1 − (0.9 × 0.9): 19%. One point lost to honesty with two measures; the gap widens with every addition, and percentages summed across a long wish-list drift into fiction.
That is the mild interaction, pure arithmetic. The physics can cut deeper, because measures share loss mechanisms. Lesson 3's rig unloads the propeller; lesson 5's pre-swirl duct earns its keep from heavily loaded propellers, so wind assistance shrinks the very loss the duct was bought to recover. Air lubrication changes the boundary layer and wake that both hull coating claims and ESD claims were measured in. None of this makes combinations wrong; well-chosen stacks are how deep cuts happen. It means each claim was measured alone, and the portfolio must be priced on interaction-adjusted numbers, the reduced remainder, the reduced loss pots, not on the brochures' sum. DNV's catalogue of measures is candid about which families overlap, which is one reason it is worth the registration (the report is free after registration).
Payback, with carbon in the fuel price
An efficiency measure is a bet that fuel will keep costing money. The bet improved: every tonne of marine fuel burned releases roughly three tonnes of CO2, and where emissions carry a price, those tonnes are a cost riding on top of the bunker invoice. A measure saving 300 tonnes of fuel a year is also avoiding roughly 900 tonnes of CO2, at meaningful allowance prices, a second revenue line that shortens every payback. Rank candidates by cost per tonne of fuel-plus-carbon avoided over the ship's remaining trading life, and the order rarely matches the order the brochures arrived in: operational measures first, energy-consumer measures close behind, the big steel last, and a five-year-old ship justifies steel a fifteen-year-old ship cannot.
Regulation adds its own sorting logic. EEXI is a one-time technical threshold: a retrofit that clears it stays cleared. CII is an annual operational rating that tightens year on year: it rewards the measures that keep working and keep being verified. The Korean Register's selection guide organises the whole toolbox precisely as candidate EEXI and CII responses (the guide is open); our Maritime Regulations Explained course covers the frameworks in depth.
Verify, or you bought a story
The last discipline decides whether any of the above was real. A measure without a pre-installation baseline cannot be verified afterwards, no matter how good the intentions: there is no before for the after to beat. The routine is unglamorous and non-negotiable: establish the baseline while the ship still runs without the measure; install; then measure performance normalised for speed, draught and weather over defined evaluation windows, and compare. A lucky first voyage in following seas proves nothing; a normalised quarter proves a great deal. Sequencing serves the same end: install one measure per evaluation window where the schedule allows, because a stack fitted all at once in a single docking can never be attributed: the owner learns what the package did, and nothing about which components to repeat across the fleet.
That routine is a course of its own in this academy: Vessel Performance in Practice teaches the baselines, filters and indicators, and Ship Resistance Fundamentals the physics underneath. Here it is enough to state the standard: a vendor's percentage is a hypothesis. Your monitoring is the experiment. Owners who run the experiment stop buying measures that do not work, and, just as valuable, keep buying the ones that do, with numbers no budget meeting can wave away.