Typically it takes around 60% more fuel for a ship to travel the last 30% of it's speed range.
A huge engine running a large pitch prop on a cargo ship cruising at 12 knots uses less fuel than a high revving high power engine running higher revving props (albeit through gearboxes in both cases).
On SY Gondola we use around 350-400kg of fuel per day cruising at 6 knots, to do 10 knots we use around 500kg, and to sustain a maximum cruising speed of 12 knots we would use 500kg in around 2.5 hours.
The reason for the extra fuel is that all ships with displacement hulls, even a highly specialised hull design such as a destroyer, have a maximum hull speed, that as it is approached becomes harder to acheive as the ship simultaneously trys to overreach it's bow wave and drive it's stern under.
This doesn't occur with planing hulls, as once the ship is on the plane (until it's planing it's classed as a displacement hull) it has overcome it's bow wave and is skimming the surface.
Greg
