[swdw]

I think you're misinterpreting what they mean by economical.

Could be wrong, but here's how I interperet this.

Economical does not mean 80-90 isthe most fuel efficient of all the running combinations possible. What it means is this is the what the navy considered the best load/speed for normal full load/speed (flank operation with full electrical load). It's economical because it gives longer life for parts, better MBT (meantime between failure), requires less maintenance costs, fuel costs at "full load", and for cost vs performance increase it's not worth it to run the diesels at 100-100 as the full load/speed conditions.

So to me, this doesn't mean it burns less fuel at 80-90 than running at a standard bell, but that the normal full load should be 80-90 of the emergency rating for longest life.