Delivery at an exact time is of course a constraint, but I’ve never seen any sort of constraint on the exact fuel usage. This isn’t anything like aviation where

Delivery at an exact time is of course a constraint, but I’ve never seen any sort of constraint on the exact fuel usage. This isn’t anything like aviation where excess fuel has to be dumped in order to land safely. If you don’t use the fuel for your voyage, you just fill up less for your next voyage. Weight effects are countered via water ballast, as they always have been. And sails for power augmentation do not complicate timelines at all. If there’s not enough wind, you use more fuel. Nobody has meaningfully proposed relying 100% on wind propulsion.