Bunker Fuel for Operators: ULSD vs MGO, the BDN Trap, and Why Hawaii Pays 30% More

A friend running two Sub T cats out of Maalaea told me he was paying $5.42/gal for ULSD in May 2025. The boat next to his — same fleet size, same route, same fuel type — was paying $4.78. Same supplier. Same week.

The difference came down to one thing: the second guy's engineer would walk the dock and ask three other operators what they paid before signing the BDN. The first guy's engineer would sign whatever the truck driver handed him.

Multiply 64 cents per gallon by 1,200 gallons per voyage by 200 voyages a year. $153,000 a year. From paperwork.

This is the post I wish I'd had when I started running boats commercially. It's not exhaustive — it's what actually matters when you're standing on a dock at 6am with a pumper waiting on you.

ULSD vs MGO vs HFO — when each makes sense

Three fuels you'll see in commercial marine. In rough order of how often Hawaii Sub T operators use them:

ULSD (Ultra-Low Sulfur Diesel, ≤15 ppm sulfur). What 90% of small commercial boats run. Same spec as on-road truck diesel. Cetane around 40-50. Burns clean in modern Tier 3 / Tier 4 diesels. Cost: highest per gallon. Available at every fuel dock in the US.

MGO (Marine Gas Oil, sometimes called "marine diesel"). Up to 1.0% sulfur in non-ECA waters, ≤0.1% inside ECAs. Slightly heavier than ULSD, cheaper at the dock, but you'll need to verify your engine's manufacturer-approved fuel list. Older Cummins QSM11s for example are happy with MGO. Newer Tier 4 engines with DPFs (diesel particulate filters) generally are not — the higher sulfur fouls the filter.

HFO (Heavy Fuel Oil / Bunker C / Residual). Up to 3.5% sulfur outside ECAs, 0.5% inside post-IMO 2020. Tank vessels, tugs >1,500 GT, ferries. Won't see this on a 79-GT charter cat. If you're on this stuff you already know it.

The mistake: assuming MGO is always cheaper than ULSD. Spot price fluctuates. In 2025 there were stretches where Hawaii MGO ran higher than ULSD because of refinery contracting. Your engineer should know both numbers every week.

BDN — Bunker Delivery Note — what to verify before you sign

The BDN is the receipt the truck driver hands your engineer. It's also a legal document — once signed, it's the basis for any future dispute about quality, quantity, or sulfur content.

Five things your engineer should check before pen touches paper:

1. Vessel name + delivery date + port. Sounds dumb. We've seen BDNs with the wrong vessel name because the driver was running three boats that morning.
2. Quantity in MT (metric tons) AND volume in gallons. Both should be present. Most disputes are about quantity. Ask for the dipstick reading or the truck's flow meter ticket. Photograph it.
3. Sulfur content in % m/m. ULSD should say ≤0.0015% (15 ppm). If it says 0.05% or higher, you're getting MGO labeled as ULSD. Refuse to sign until corrected.
4. Density at 15°C (kg/m³). ULSD is around 832-850. MGO is heavier (860-890). HFO is heavier still. If the density doesn't match the product on the BDN, something's wrong.
5. Sample tag. Federal regs (33 CFR 156.118) require the supplier to draw a sealed sample at the delivery hose. Get the sample and store it on the boat for at least 12 months. If you have to dispute quality later — engine damage, contamination — that sample is your only evidence.

I've seen captains sign BDNs without the sample, then six months later the main starts running rough and there's no way to prove the fuel was off-spec. The supplier's lab sample magically goes missing. Always take the sample.

IMO 2020 — what changed and what it means for tankers

January 1, 2020, IMO global sulfur cap dropped from 3.5% to 0.5% outside ECAs. ECAs (Emissions Control Areas) — North American coast, Baltic, North Sea — went to 0.1% in 2015 already.

For Sub T charter operators in Hawaii: nothing changed. You were already on ULSD or MGO. Move on.

For tank vessels and any vessel that was running HFO outside ECAs: huge. You're now on either:

• VLSFO (Very Low Sulfur Fuel Oil, 0.5% sulfur) — cheaper than MGO but quality varies
• ULSFO (Ultra-Low Sulfur Fuel Oil, 0.1%) — equivalent to ECA-compliant
• MGO with a scrubber (open-loop or closed-loop SOx scrubber lets you keep burning higher-sulfur fuel)

The gotcha: VLSFO from different refineries doesn't blend well. If you bunker VLSFO from supplier A in Honolulu and then top off from supplier B in San Pedro, you can get sludging in the day tank. We've seen blocked filters within 200 hours from incompatible blends. Most operators now insist on a single VLSFO supplier per voyage and bunker fully empty rather than topping off.

Bunker pricing 101 — spot vs index vs hedged

Three ways operators buy fuel:

Spot. Whatever the dock charges that day. 90% of charter operators. Easiest. Most expensive on average. You're at the mercy of crude swings — when WTI moved $15 in two weeks in 2025 you ate the entire spread.

Index pricing. You sign a contract that says "we pay Argus West Coast ULSD spot price + $0.18/gal markup." You're insulated from the supplier's margin games but you're still exposed to the underlying. Used by mid-size operators (5+ vessels) with stable bunker volume.

Hedged. You buy a forward contract or a swap that locks in the price for the next 6-12 months. Real money — you need a CTA (commodity trading advisor) and a bank that does derivatives. Worth it for operators >15 vessels or any tank vessel where fuel is 35%+ of OPEX.

If you're a Sub T charter operator with 1-5 vessels: stay on spot. The complexity and credit cost of index/hedged isn't worth it until you're moving 100k+ gallons/month.

Hawaii-specific — why we pay 25-40% over mainland

Two stacked premiums:

1. Jones Act. All fuel arriving in Hawaii on a US-flag vessel costs more because the supply chain is constrained. Foreign-flagged tanker can't unload here. So Par Hawaii's Kapolei refinery plus a small amount imported on US-flag is the entire supply.
2. Distance from refinery. Even on the mainland, fuel at a Hawaii dock has crossed 2,400 nautical miles. Logistics premium.

Net: Hawaii ULSD at the dock runs $0.80-$1.20 over LA/Long Beach in any given week. There's no fix. Plan on it.

What you can do:

• Bunker as full as your trim allows on every visit so you maximize purchase efficiency
• Avoid topping off small amounts (the per-gallon cost is the same as a full delivery, but you eat the truck dispatch fee twice)
• Track your $/nm by route and surface the worst routes — sometimes it's a slow speed problem, sometimes it's a routing problem

Common gotchas

• Short delivery. Truck driver's flow meter says 1,200 gal but the boat only took 1,165 gal. Difference goes back to the truck. Always reconcile against the boat's tank gauge or sounding pipe before signing the BDN.
• Water in fuel. Fresh-water contamination shows up as a hazy interface in the sample bottle within a few hours. If you see a waterline in the sample, refuse the delivery and call the dispatcher. Don't pump it through to the boat.
• Cetane swap. ULSD spec is cetane number 40+ in the US. Some bargain suppliers blend in heating oil that drops cetane to high 30s. Engine starts harder, runs rougher. The lab sample is your only way to prove it after the fact.
• No additive on long passages. ULSD has poor lubricity vs old high-sulfur diesel. If you're going to sit in tankage for 30+ days, add a fuel stabilizer + lubricity additive. ~$0.02/gal extra, saves the lift pump.

What Binnacle does

We built the Bunker module and the Voyage P&L so the questions above stop being "did anyone write it down" and start being "what does the data say."

• BDN capture. Snap a photo, we OCR the quantity / sulfur / density / sample tag fields and store the BDN with the bunker operation row. Twelve-month retention so the sample-tag problem becomes a non-problem.
• $/nm tracking. Voyage P&L pulls bunker cost per voyage, divides by distance, and surfaces the trend. Outlier voyages get flagged. You'll see exactly which routes are bleeding fuel margin.
• Supplier comparison. Multi-supplier operators see a leaderboard ranked by $/MT delivered. Same fuel grade, same week, different prices — the contrast is the value.
• MARPOL Oil Record Book Part I. Every bunker operation auto-logs a MARPOL entry per 33 CFR 151. The CFR compliance falls out of the operational logging — no double entry.

Run the ROI calculator — pick "fleet of 3-5 boats" and "missed a renewal in the last 12 months." It's calibrated against the bunker savings most of our pilot operators see in the first 90 days.

The point

Bunker is the single largest variable cost on most charter and tug operations. Most operators run it on instinct. A spreadsheet saved by an engineer who left two boats ago. A paper BDN folder that gets thrown out at the end of the year.

You don't need a CTA and a swap desk to fix this. You need:

1. Train your engineer to check the five BDN fields every time.
2. Always take the sample.
3. Track $/nm by route. Surface the outliers.
4. Compare suppliers monthly. Move volume to whoever's cheapest.

Items 1-2 cost you nothing. Items 3-4 are what we built Binnacle for.

If your bunker costs feel high but you're not sure how much, start with the calculator — it's free and takes two minutes.

— Capt J · founder · binnacleai.com · hello@binnacleai.com

Binnacle AI is not affiliated with any fuel supplier, OEM, or class society. We're a maritime compliance + ops SaaS, not a bunker broker.