21 years ago I was in a Korean shipyard where the largest container ships of the day were being built. Their capacity was 6000 TEUs, one-third of the Mary Maersk. The captain of one of the ships (a 27 year old (!) German woman) told me most ports cannot handle the 6000 TEU vessels, so their line will be building mainly 4500 TEU ships.
One interesting thing is that those older, smaller ships had more powerful engines than the much larger current vessels! The engine installed on the 6000 TEU vessel was ~90,000 hp, and the ship was designed for sailing at ~25 knots, the justification being that the value of cargo onboard was so high that the cost of shipping was dominated by the interest charges!
A few years ago I sailed past the port of Felixstowe and we saw the big ships being loaded. The captain said something like "those cranes can handle a ship 19 containers wide. When they built them people said it was madness, that there would never be a ship that big. Well, they're at 17 now, so they're worried they'll have to replace them soon."
Fuel costs have increased hugely since. The present generation of ships are designed for "slow steaming", ~13 kt, at a very significant fuel savings. That's likely to extend for much of the life of the ships (~20 years, possibly more).
Interesting to note - the change in interest rates therefore has an impact on the size of ship engines, with lower interest rates resulting in smaller engines (or perhaps engines being run at more efficient speeds in terms of fuel consumption)
It way more directly influences by the bunker rates. Maybe various for a lot of the cargo it makes most sense to have the container vessel sail quicker. However, that's not taken into account while deciding between shipping lines. Further, a shipping line has loads of cargo. Some high value and time critical, some not at all.
First a note: The useful engine power of ships is logarithmically proportional to the size of the vesel.
A ship that is twice as big will only require and engine that is half as big.
A problematic thing is they way ports generally work, you may generally not book a port for a specific arrival date. You have to rush to get there before other ships, to sit still outside and wait in line.
Not very good for fuel consumption and the environment.
> A problematic thing is they way ports generally work, you may generally not book a port for a specific arrival date. You have to rush to get there before other ships, to sit still outside and wait in line.
For most ports in North Europe that's not the case (though it used to be). You generally agree on a standard arrival date (day of week + time + number of moves), then you should arrive at that time. This differs per region though. For Africa, it's "different". You have terminals owned by customers, etc. More often you still have to rush, then wait for ages.
Maybe things are different if you're a small shipping line and the same terminal also working for a few big ones.
One interesting thing is that those older, smaller ships had more powerful engines than the much larger current vessels! The engine installed on the 6000 TEU vessel was ~90,000 hp, and the ship was designed for sailing at ~25 knots, the justification being that the value of cargo onboard was so high that the cost of shipping was dominated by the interest charges!