The marine-hull programme's last step called a small survey boat "energy-neutral" in a month when the monthly mean sunshine covers its load. That is a mean. A battery only bridges so many dull days, so this step ran the check on real daily data: NASA POWER all-sky shortwave for five ocean sites, 3,653 days each (2015 to 2024), against a 5 m and a 4 m boat holding 1 m/s with a 10 kWh pack.
- The 5 m boat inside its neutral months never emptied the pack at four of five sites. The worst run drew down 7% of the pack at the Equator and 29 to 70% at 42 N and above.
- At 25 N the boat counted as neutral all year, yet it emptied the pack on nine days, all in January (eight in a row in January 2024). A monthly flag overstated it; the pack, about 11 kWh at worst there, is the number to read.
- The 4 m boat survives its neutral windows but uses 37 to 54% of the pack doing it.
- Year-round, 42 N needs a median 41 kWh a year and 60 N over 100 kWh, so panels carry a summer, not a winter.
Limits: one grid cell per site, one decade, the boat holds speed instead of slowing down when low, panel mass and battery losses ignored. Log, code and data: research/marine-hull/LOG.md (milestone 5f). Written with AI assistance; the numbers come from the committed script, not from memory.