Most people in medieval times couldn't swim at all, armor or no. I simply stated that it was possible to swim in plate armor, a point i have proven. If you wish to start arguing over semantics in a desperate attempt to dodge away from the argument, that's fine, but don't expect me to join you.
I just now joined the conversation and have no real stake in it to be "desperate" about. Not sure what you're ranting about there.
The real point here is that you said it was hard for a fit man, such as your friend, to swim in armour under ideal circumstances, so that you reckoned he might last only ten minutes in those circumstances. I was a fairly avid swimmer and used to swim two hours non-stop.
Let's explore those ideal circumstances related by your friend, btw. He probably did not actually endanger himself by swimming
- in a place with a current, such as most rivers and tidal waters
- in a place with seaweed or other entanglements
- when whe was feeling tired already
- when he was panicked from men trying to kill him or in fear of actual drowning
All these conditions constitute how a knight might normally have experienced having to swim in armour. Some of them I as an experienced swimmer would not hazard naked. You need a shitload of strength to swim
unencumbered through seaweed and can drown in a river current easily. If you add weight and extra surfaces for seaweed to ensnare upon or currents to catch, it raises the peril that much more. Most natural bodies of water have these conditions in spades--still ponds nearly always have seaweed throughout, rivers have currents and eddies, and oceans have kelp and tidal currents.
Despite this, swimming was, by the way, not an uncommon activity in the Middle Ages, particularly among the nobility that comprises most of the armour-wearers. Einhard writes in his biography of Charlemagne, the
Vita Caroli, that Charlemagne
loved to swim and no one in his realm could better him at it; but Charlemagne lived inland for all his life not on campaign. Yet his swimming is grounds for a boast compared to others around him? Charlemagne, btw, stands astride the Middle Ages proper and an even darker era of ignorance and superstition known as the Migration Age. And he swam brilliantly.
I have not noticed any noticable restriction in mobility in properly built plate and mail, tournament pieces most certainly do restrict movement, but combat pieces are specifically designed to not do so, as restricting one's movement in combat is a really, really bad idea. Certainly it's harder to move with plate than without, as regardless of how well it is made it does weigh more than not wearing plate, but there is no stiffness or restriction of movement that classical thought would have you believe.
Mobility is impaired if you follow by saying "It's harder to move." Mobility is the ease of movement. And in a situation involving swimming in anything more than a swimming pool or carefully designated and maintained area of a natural shoreline, you do not want to be swimming with, say, rondels snagging on every passing strand of seaweed. What made swimming much more dificult for your friend in ideal conditions makes normal natural conditions deadly.