ROB (Remain On Board): What It Is and How to Calculate It

ROB, short for Remain On Board, is the quantity of fuel left in a vessel's tanks at a given moment. It is one of the most used numbers in ship operations: the master reports it, the office plans voyages around it, and whoever orders bunkers needs it to decide how much to buy. Knowing what ROB is and how to calculate it properly is the difference between planning on a solid figure and planning on a guess.

What ROB means in daily operations

The term applies to any liquid carried on board, such as lube oil or fresh water, but in most conversations ROB means fuel. It usually appears in the noon report, in departure and arrival reports, and in the paperwork around every bunkering.

ROB answers three practical questions:

• Is there enough fuel on board for the next voyage, with a margin for weather and delays?
• How much space is left in the tanks before the next bunker?
• Does the fuel that was used match what the engines should have burned?

The first two are about planning. The third is about control, and it is where a careless ROB figure causes the most trouble.

How to calculate ROB

The basic fuel balance is simple. For any period, a day, a voyage or a month:

ROB at the end = ROB at the start + bunker received − consumption

In practice, a fourth term is needed, because the calculated figure will never match the tanks exactly:

ROB = start ROB + bunker − consumption ± adjustment

The adjustment is the correction made after the tanks are sounded, and every adjustment should come with a reason.

Here is a worked example. The numbers are purely illustrative:

| Item | Litres (example) | |---|---| | ROB at the start of the week | 40,000 | | Bunker received | 25,000 | | Consumption during the week | 18,500 | | Calculated ROB at the end of the week | 46,500 |

If the sounding at the end of the week shows 46,100 litres, there is a 400 litre gap between the book figure and the tanks. That gap is not automatically a problem, but it has to be explained before it is written off as an adjustment.

Book ROB versus measured ROB

There are really two ROB figures on most vessels.

The book ROB is the calculated balance above. It is only as good as the numbers that feed it: the start balance, the bunker quantity and the consumption.

The measured ROB comes from sounding the tanks and converting the readings to volume with the tank tables, corrected for trim, list and temperature. It reflects what is physically in the tanks, but it depends on careful readings and on tanks that have had time to settle.

Comparing the two regularly is the whole point. A small, stable difference is normal. A difference that keeps growing in one direction is a signal worth investigating.

Why the two figures drift apart

The usual causes are ordinary rather than dramatic:

• Consumption is estimated, not measured. Running hours multiplied by an assumed rate will not follow real engine load, so the book ROB drifts as soon as the vessel works harder or lighter than assumed.
• The bunker figure is the supplier's number. If the book adds the quantity on the delivery note rather than what actually reached the tanks, any short delivery ends up hidden in the ROB.
• Volume versus temperature. Fuel expands as it warms. Mixing volumes measured at different temperatures without correcting them to a common reference creates gaps that are not real losses.
• Sounding conditions. Trim, list, movement and hurried readings all affect the result.
• Unrecorded movements. Transfers between tanks, drains, sludge, leaks and fuel leaving the vessel without a record all show up as an unexplained difference.

Keeping a ROB record you can trust

A few habits make ROB far more useful:

• Record it at the same time every day, in the same unit and at the same reference temperature.
• Keep bunker received as a separate line from the supplier's figure.
• Keep measured and estimated consumption apart, so you know which part of the balance is solid.
• Record every adjustment with a reason, who made it and when.
• Look at the trend of adjustments, not only the latest number.

How MANTRAZ keeps a running ROB

MANTRAZ keeps a running fuel balance for each vessel using the same formula: start balance, plus every bunker, minus what the engines burn, with adjustments so the balance matches the tanks. The balance is logged daily, weekly or monthly, so you can see how each figure was reached.

The consumption side comes from the engines. Main engine fuel is measured by flowmeters on the supply and return lines, while auxiliary engines are usually estimated from running hours, and reports keep measured and estimated fuel apart. Bunkers are measured as they happen, with flow rate, density, temperature, mass and volume recorded through every delivery. That gives you a ROB figure you can plan the next voyage or the next bunker order around.

To see a running ROB balance on data similar to your fleet's, request a MANTRAZ demo.