You're making the assumption these ships saving fuel and emitting less pollution, have to be slower than conventional freighters. The shipping companies are all about the bottom line, balancing costs and and earnings. If the ships are slower they will balance fuel savings with time-is-money. If slightly slower the fuel savings may be worth the trade. If they are not slower is the savings worth the expense of retrofitting or new ships.
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The descent of man ~ Nixon, Friedman, Reagan, Trump.
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