Reason 1:
Promotions are tied to UnitCombatTypes not UnitTypes and not UnitClassTypes (This is technically a huge difference.)
Thus I can and will make the Promotions absolutely independent of specific Units.
Units may change throughout a game by Techs, but UnitCombatTypes will always stay same throughout the game.
Sorry for not using correct tech babble, but in an earlier post I'd already pointed out which roles ships had to fit during the colonial era:
The ship types I'd like to see in the mod eventually would be (in decreasing size): flagship - battleship - cruiser - coast guard (large merchantman - medium merchantman - colonial "coastal" merchantman). This roster would be entirely filled during the whole game length, with new designs occasionally popping up and replacing an outdated one. During the WOI this would like like : 100-gun three-decker - 74-gun two-decker - 18-pounder frigate - cutter ("merchantman" - brig - brigantine), while a bunch of ships of the respective earlier design might still be floating.
Reason 2:
.When Techs lead to a new generation of Units of the same general kind usually the following applies:
It can simply be upgraded and the Promotions are not lost. (e.g. an old Type of Caravel is replaced by a new Type of Caravel)
An upgrade of individual ships (sort of "click this button and spend 50 lumber to rebuild (=replace) your carrack into a wargalleon") will hopefully never be implemented. For good reasons ships are units and don't have crews. If you want to prevent the loss of promotions it sounds a bit like you'd like to introduce crews. IMHO ships should only be a commodity and not become iconic units. What did stay over time was only the (rising) professionalism of the navies. TBH having the ships from 1492 around in the 1770s is one of the worst design decisions of COL. The "tech" tree should be the cure for this.
And I really hope that we don't end up with a system where I disarm my infantry and then re-arm it with the same weapons again and by doing this my Arquebusiers magically transform into Musketeers.
[btw, if we introduce different types of guns and cannons for the evolving army units these can no longer be part of the double-row of commodities of the city and Europe screen which funnily allows us to introduce a few more yields]
Reason 3:
The timeline in Civ4Col is not long enough to have as drastic differences in Units as in e.g Civ4BTS.
The differences in new Units being unlocked will be noticable but not big enough to have existing old Units become totally worthless.
Thus even existing Units that will not be upgraded will not become useless and their already given Promotions will still be valuable.
The timeline in Civ4Col is long enough to make your caravel look severely outdated once the caravela redonda and the galleon show up. That happened within 40-50 years. And you don't want to compare them with a three-decker, e.g. Victory, or even the standard ship of the line, the 74. For a short time you might want to keep your old ships around but usually the more modern ships will be significantly better. Overall size (=strength), cargo capacity/slots, speed, better withdrawal chances and higher bonuses in combat come to mind. Re prices of ships I'd even suggest to make these dirt cheap so that there is no point in keeping sluggish and low capacity ships. (re buyable ships I'm in favour of not being able to buy major warships like the ship of the line) If we introduce transport capacity for ships, a concept which is also dearly needed by our land transports to eventually fix/alleviate the auto-transport issues, we'll probably need to increase the overall number of ships if we take the historical transport capabilities of sailing ships into account. More ships is with respect to the naval side of the game an advantage.
[Automated trade with Europe and/or colonies will mostly remove the burden of its organisation from the player. Hopefully one day we'll have naval missions which work automatically...]
Everything else would completely ruin the game because Civ4Col does not have as many different Combat Units as Civ4BTS.
(We can and will maybe unlock a handful of high tier Units, but that is it since we do not have that many Units to chose from and there need to also be all necessary Units available at the start.)
It won't ruin the game if we have only limited numbers of different naval units as long all UnitCombatTypes are filled (with historically correct ships) at any time.
Individual promotions for ships are no longer needed if we 'constantly' rebuild our navy. What will be left is something which might technically be 'promotions' but don't have any resemblence to the earlier system. Well, the status of the training level of your ships crew (untrained - trained - capable - experienced - veteran) could be shown that way as "Crew training". "Officer training" could be used to represent the effects of naval doctrines and the professionalism of your naval officer corps (=naval tradition). [Keep in mind, that I see xp in a strict sense as combat experience.]
Well, then you have a very different Tech concept in mind than myself ...
Sort of...
In my concepts Techs may:
1. Unlock a new Unit. (But never ever a full UnitCombatType.)
2. Unlock a new Promotion. (Still not conflicting with my Promotion Overhaul, because the Promotion will simply be available later in game.
3. Even unlock a completely new feature concept (which would then be totally indendent of Promotions).
In my understanding a "tech" tree will allow that
#1 The accumulation of sailing experience leads to the invention of new hull forms, sail types and improved rigging, which, combined, eventually unlock a new unit of each of the already existing UnitCombatTypes
#2 Fighting battles in each of the unfamiliar New World terrains changes over time the specific terrain modifiers of your army units from a negative to a positive value reflecting the ability of your colonists to adapt to new terrain; these may also be learned from friendly Native tribes/allies
#3 Tactical innovations lead to the introduction of new tactics
#4 Technical inventions introduce new guns and cannons (since we won't exchange cannons on existing ships only the newly invented units will benefit)
#5 Extensive naval exploration leads to the discovery and charting of tradewinds, ocean currents and eventually ports in Africa
#6 The discovery of tradewinds and ocean currents leads to shorter travel times to Europe and Africa (certain prominent FFs can give you this knowledge ahead of time)
#7 Extensive land exploration leads to the discovery of new yields and new professions; these may also be learned from friendly Native tribes/allies
#8 Technical inventions, fueled by a sizeable production in the colonies, lead to the introduction of new production techniques and also to enlarged buildings
#9 The location of your country in Europe, the location of your colonies in the Americas and their relative position to tradewinds and ocean currents will give specific modifiers to strength, speed and capacity of ships which will further individualize and differentiate ships from your European rivals (compare Spanish galleons to English/Dutch galleons) ["Shipbuilding traits"]
A Tech may unlock a new Unit or give its UnitCombatType some new Promotions.
But it will never ever change the general functionality or role of a specific Unit.
(It may make it faster, it may make it stronger, it may give it other appearance ... but generally it is still the same role and funtionality it fills in the game.)
By unlocking inventions the next ship of a UnitCombatType will be faster, stronger or has additional capabilities. The role and funtionality of a UnitCombatType won't change but the ships will.
An outdated unit loses its function once a newly invented unit of the same UnitCombatType can be produced which fits its role better. It should definitely no longer benefit from inventions which might make it faster or stronger. For a few turns it could be kept around but its underperformance compared to newly build ships of the new unit will lead to its breaking up eventually. (The term "relegated to secondary tasks" comes to mind.) There was a reason that no more caravels or carracks were build past a certain date.
Please trust me, I am no beginner in Civ4 / Civ4Col modding.

I know when there are dependencies between Systems and when there are none.
Trust you? Well, that depends. You're responsible for some remarkable features but IIRC the thinly disguised railroad is still part of the mod.
I know that the complete Combat System is one of the weakest aspects in the game.
The same for the Exploration of the Ocean and many other aspects considering Ships that are unrelated to trade and economy.
Yes, there is still a lot to be done. And that is precisely why I posted here. Not to criticize, but to show up avenues which haven't been yet pursued.
You
really simply do not know how bad things were before we e.g. even had Winds and Storms or Goodies on Water or at least a little movement by Wild Animals or Fishing Boats or ...
Compare the mod now with the situation 12 years ago with Vanilla and then we talk again how much is possible when modders are given enough time.
Well, I do remember the disaster the unfinished release of Civ4Col was. I've ditched Vanilla after a very short test game and continued playing the original.
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