In a d+ game I was playing recently, I had built Leo's and was making a career of trading maps (the frequent contact helps keep the AI friendly), locating anything that floats, and buying it up. With the free upgrades and a small map (40x64), I soon had ~27 NON transports ringing the globe. (Next time, I will re-name my civ FedEx.) Most of what I purchased were triremes, caravels and galleons, all of which upgrade to transports. Although a few ironclads were available, I never bothered with them, as I knew they upgraded to destroyers, and I didn't really have a need for destroyers. I did, however, purchase two or three frigates, out of curiousity, to see how they upgraded. Since both frigates and ironclads become obsolete with the discovery of electricity, when destroyers become available, I assumed they'd upgrade to destroyers. They never did. Maybe this is because frigates have a hold capacity (2 units) and destroyers don't? Maybe there was something wrong with my game? Is this a known phenomenon, and if so, is there a known explanation?