Violence and the fear of getting caught! There are many skippers out there that understand the levels in their tanks and their burn rates. Or simply the 6 gallon can holds six gallons... When the pump meter reads 7 on a 6 capacity things have a way of going south very quickly. A few will fill their own 1 gallon container first. Two reason 1.) Note the quality! 2.) Score the meter with their own tool.

When it comes to the big ships and what they call bunkering. The new fuel goes in a separate take and the ship does not touch it until an independent lab report comes back. (3) samples are taken. One goes direct to a lab. Seller gets one to hold, Buyer gets one to hold.

On a reasonably busy fuel dock someone with an small 18 gallon tank will catch on before you have to worry about it.