A geometry / seasonal-β / forecast-α decomposition of optimizer fuel-ROI across ocean regimes
— current-aware (ERA5 + GLORYS), calibrated MR tanker, 12 monthly departures (2025), hindcast. Confirmatory vs. a pre-registered
protocol; nothing deployed.
Read the baseline and the foresight. Savings are reported against a
constraint-respecting geodesic (shortest legal path), not the ship's planned route, so route-shortening
is not miscredited as weather skill. Every number is
hindcast (the optimiser sees the true weather) —
an
upper bound on operational, forecast-limited value; the tactical term α especially.
The decomposition
The total routing gain (planned route → spot-optimal), flown through the same realized weather + currents,
splits additively into three components with very different robustness and commercial honesty:
$$ \underbrace{F_{\text{plan}}-F_{\text{spot}}}_{\text{total gain}}
= \underbrace{(F_{\text{plan}}-F_{\text{geo}})}_{G\ \text{geometry}}
+ \underbrace{(F_{\text{geo}}-F_{\text{prior}})}_{\beta\ \text{seasonal+current}}
+ \underbrace{(F_{\text{prior}}-F_{\text{spot}})}_{\alpha\ \text{forecast (hindcast UB)}} $$
- Geometry $G$ — route-design / shortening. A shortest-path tool delivers it; not weather routing.
- $\beta$ (structural-seasonal + current) — the climatologically better corridor + riding the
quasi-stationary boundary current. Knowable a priori from routeing/current charts; forecast-free.
- $\alpha$ (tactical) — reacting to this departure's synoptic weather over the seasonal prior.
The only forecast-dependent part — what real-time services sell. Reported in hindcast = an upper bound.
Routes
Honesty controls
Negative control (trade-wind, no dominant current): the method must show ≈0 weather gain — it does
(weather term ≤ 0, no manufactured saving). In-sample β: the season-prior is the medoid of this
year's optima ⇒ β slightly optimistic, α slightly conservative; the clean estimator uses a multi-year
climatology (specced). Operational α: the realisable forecast-error number is lower than the
hindcast α here and needs an archived forecast aligned to historical dates (specced, not run — forecasts are
not fabricated). Currents via CMEMS GLORYS; an earlier wind/wave-only run understated value on the
current routes — currents are quasi-stationary (they enter β), so including them strengthens the
structural>tactical conclusion.