If supplying fuel for a labor-saving machine (for instance a sawmill) takes just as much or more slave labor as is saved by the machine (sawing), it is prefereable to just put the slaves directly to sawing instead of gathering fuel because it requires less oversight over the slaves.

In roman times the most efficient to move a large amount of fuel would have been a trireme with slaves at the oars and a very basic sail. In the 18th century the same would have been possible with a ship manned with just a handful of sailors. From this perspective the wind power revolution in the age of sail contributes to the preconditions for the industrial revolution.