[safasfasf]

"It's a roughly accurate measure though"

It's the ships speed relative to the water what causes any fuel consumption. For the purposes of saving fuel, if you don't know the water surface speed, you know nothing.

[andy_ppp]

Precisely! You could be going slowly against the tide or current or weather and using loads more fuel - you ask anyone who has been at sea about weather and localised conditions that don't show up on satellites are common enough for me to believe the on ship reports.