Does this mean that late game light ships will give you 50% more trade power?
They get 150% additional trade power by modernising... Barques have 2 trade power... Great Frigates have 5 by the end of the game.
Simply put, a galley fleet was able to bear more cannon towards a singular direction with much faster speed and agility. When you consider that roundships had 8 to 12 cannons in their broadside in 16th century, on much longer platform, several galleys next to each other had more firepower on their bows.
You seem to be neglecting the oars.
Oars require a lot of width. They'd triple the width of the galley - or galley like craft (see below). These wouldn't be kayaks with little paddles - oarsmen need lengthy oars to get a longer power stroke; a long power stroke means the craft accelerates faster, and has a higher top speed.
So the width of the oars isn't negotiable on a galley. Stowing oars is not feasible in a rowing competition, and it wouldn't be in galley combat either. The term 'dead-in-the-water' would apply. Stopping invites being rammed, boarded, or shot - or all three.
Note the oar length, compared the width of the boat (scull). Competitive rowers use a paired-down scull/shell not unlike a tiny galley. (Thanks https://rowing.at/)
So it's highly doubtful galleys would mass together. They'd get in each others way. The naval term for the result then would be 'fouling the oars', leading to the whole 'dead-in-the-water' scenario again.
I agree, though, that galleys would be comparatively agile in the earlier days of EU4's timeframe.
The downfall of the galley was much more linked to rigging improvements than that of the ease of production of guns,
The scaleless sage is offering a very plausible interpretation here.
The predominant winds in the Mediterranean are northerlies - north-to-south. The seasonable variation doesn't change the direction much. Going back a thousand years or so, a journey from Athens to Alexandria could be something like a week going south, but over a month in going north. People seemed to rely on a convenient tail wind.
This implies that sailing ships weren't good at tacking i.e. they could not get much speed from it. And in combat, the a sailing ship might need to tack to-and-fro
quickly - particularly against Galleys. They might have been using lanteens which have a 'bad tack' where the sail has to press against the mast when going in one direction.
Or they could have been just using square sails, with no tacking ability at all.
Either way, an oared ship could capitalise on it's comparative manoeuvrability; getting south of a sailing ship if the wind was a northerly, and cutting off the best line of retreat.
However, once a sailing ship's ability to tack quickly reaches anything like modern-day yachts, galleys become less advantageous.
"Heavies" were not used as support ships, or in strictly defensive roles, while there was a period of galleons and later designs being used to support fleets these were in no way warships, they were very clearly simple sailing cargo ships, something the galleon and other round ship styles was originally developed to be as part of the Italian merchant shipping. The "heavy" style ship was an offshoot of these rounded hulled cargo ships.
I believe the large 'rounded hull' designs were encouraged because they could be refitted for warfare. Commerce and war were pretty closely linked for the Italian maritime powers (if remember rightly).
The 'heavy ship', 'light ship', 'transport' designations in EU4 are fun from a gameplay standpoint, but I don't think they're historical until we get to our 'two decker' type ships which sound more like pure military vessels.
But yes, your fluffy fishiness, please continue.
Later on development of roundships due their more open-ended nature and dependence on sails rather than oarsmen made them big enough to never be troubled by smaller vessels and all naval development became an arms race in amount of cannons.
You're not concerned about propulsion? Seaworthiness in different conditions? Manoeuvrability? Navigation? Not even a little bit?
As I said, if you are interested in the topic I recommend the book I mentioned, "Gunpowder and Galleys" by John Guilmartin. You won't find any other source as thorough and detailed on the topic.
Or you should watch some sailing races? He who cannot move cannot fight. Get a feel for the speed of cutting-edge superyachts! The America's Cup competition begins in March next year - don't miss it!