Slow-Steaming Sweet Spot: Finding the RPM That Saves the Most
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Slowing down saves fuel, but the cheapest speed isn’t obvious. It shifts with hire/day, fuel price, hull and prop condition, weather, and safe engine-load limits. The tool below scans your speed range, enforces a minimum continuous load, and gives you an “eco band” you can actually steer by. Use it before sailing, then nudge RPM within that band to meet the berth window without burning money.
How to use the Slow-Steaming Sweet Spot tool
What you enter
Tip: keep the speed scan range realistic for the ship and weather.
What you get
- Fuel saved vs your baseline, suggested RPM, and estimated %MCR at the target speed.
- An eco band table for three common states: clean hull, light fouling, head seas.
- A JIT helper that shows how small speed nudges shift ETA without leaving the eco band.
- A Copy Bridge Card button to paste the eco guidance into your orders or chat.
Quick steps
- Enter reference data in “Quick inputs”.
- Select a realistic speed range and press Calculate.
- Note the Suggested RPM and %MCR. Ensure it sits above your minimum continuous band.
- Open “Eco band at a glance” and capture the RPM/kn for the condition that fits today.
- Use the JIT helper to fine-tune arrival while staying in the eco band.
- Hit Copy bridge card and paste the four lines to the bridge log or voyage orders.
Assumptions (kept simple on purpose)
- Main engine fuel scales ~with speed³, fitted from your reference point.
- Hotel/aux is treated as constant per day. Boiler/EGB logic can change the picture at very low load.
- Minimum safe load is enforced so recommendations do not dip below the maker’s continuous band.
Recheck the eco band after hull or prop cleaning, or when weather changes materially.
Want a deeper dive? Open the “Show math” panel under the tool for formulas.
| Condition | Target RPM | Speed kn | kg/nm | Notes |
|---|---|---|---|---|
| Clean hull | — | — | — | Baseline fouling factor 0% |
| Light fouling | — | — | — | +8% shaft power |
| Head seas | — | — | — | +20% shaft power |
- Stay above minimum continuous load band.
- Plan 2–3 h at higher load weekly when running low.
- Follow CPP combinator guidance.
- Watch EGB and boiler logic.
- Chase low RPM in 2 m head seas.
- Ignore rising exhaust temps or TC behavior.
- Lock one RPM without rechecking after cleaning.
| Plan | Speed kn | ETA shift vs ref | Note |
|---|---|---|---|
| Eco target | — | — | Lowest $/nm |
| Eco fast | — | — | Upper eco band |
| Eco slow | — | — | Lower eco band |
Show math
ME fuel/day ≈ k × speed³ (k fitted from your reference). Fuel ton per nm = (ME_day + Hotel_day)/(24×speed). Total $/nm = fuel_ton_per_nm × price + time_value/(24×speed). The scanner excludes speeds that would put load below your minimum continuous band.
SFOC & notes
If you have maker SFOC vs load curves, refine offline or split k by load band. Keep RPM within continuous rating. Schedule periodic high-load if operating at low load for days.
Eco band & checks
- Eco target: — kn, ≈ — RPM, load —% MCR.
- Band: slow — kn to fast — kn. Use JIT to meet berth.
- Do: stay above min load, watch EGB/boiler, plan weekly high-load.
- Recheck after hull or prop clean.
