What is the cap on trade ships increasing Harbour resource capacity?

//ads -- adsterra.com -- native banner
?>
Cargo Ships upgrade increases Harbour capacity with more trade ships, and the Wiki says it does this with diminishing returns, but I noticed that the trade ship effect suddenly stopped completely, not diminished gradually.
Could someone explain how the behavior really works?
Pictures about "What is the cap on trade ships increasing Harbour resource capacity?"



Naval Action - Guide to Trade Ships
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Julius Silver, Pixabay, Tom Fisk, Tom Fisk