The letter writer who delivered a tirade of abuse regarding adding weight to the ferries in order to improve efficiency ought to be sure of his facts. (“Greater weight can’t save fuel”). He is correct in saying that adding weight requires more energy to move it but that may not be the entire story. There’s hydro and aerodynamic drag to consider and possibly other factors as well. I’m not a marine engineer, so cannot be specific. I do know that these guys are not idiots and to assume they are is very ungracious.
I am an aerospace engineer and can give you an example of an increase in weight improving efficiency. That is the addition of winglets to an airliner. These add considerable weight from the winglets and the wing reinforcement needed to carry their load. But these winglets increase aerodynamic efficiency and decrease fuel burn by up to 7 percent, which is a huge savings in the cost of operation. Possibly adding the extra weight to the ferries has a similar effect. I don’t know but I would be careful about criticism before knowing the facts.