With regards to Riverine Transports.
- The US, as far as I know, did not build any riverboats other than the armored variety.
- The US did lease and hire riverboats extensively on all shippable rivers of the US, where they needed supplies and troops transported.
- Riverboats carried far more supplies for the US than the rail did during the war.
- The Riverine Transport Pool (RivTP) is a pool of generic transports, not on map, representing riverboats leased, or constantly hired, by the US for transporting supplies and troops.
When a player uses the RivTP to move troops on a river, he simply gives them the Use RivTP orders during his plotting phase and moves them off along a river, without any other planning. What bothers players about this is that the player does not have to plan ahead to do this. He simply does it and it immediately works. Also, the only limit to the size of the force to be transported is only the size of the players RivTP for the entire map. The concept that riverboats are spread out over all sections of all rivers is not considered.
One thing the devs did to balance this somewhat was to have it take 5 days for a stack using the RivTP to debark; ie moving from a river region onto a land region. Unfortunately this also carried over to troops debarking from on-map Riverine Transport Units; only the gods know why.
What a good house rule might be to work around these shortcomings is difficult to say. Completely forbidding the use of the RivTP would be unhistorical and too restrictive, but to allow unfettered use of the RivTP would be unhistorical as well. Something in between would probably be reasonable, but also probably not greatly affect the game. Why? Because it should be possible to move a very large force on fairly short notice with the RivTP.
It is not not practicably posable to historically support a framework for how much of the RivTP should be usable for moving land stacks, because reality does not differentiate between the games on-map Riverine Transports and those in the RivTP. There are also not a lot of cases of large forces being ferried on the rivers. Sherman use riverine transports to attack at Chickasaw Bayou, Grant transported some of his force at the outset of the Fort Donelson campaign, and probably the largest troop movements by water I can think of would be McClellan's Peninsula campaign, which used a lot of oceanic transports to move from northern Virginia to Fort Monroe.
Although the comparison is very limited, it illustrate what organization and determination can achieve. On the 23rd of September 1863 the XI Corps and the 2nd Division of the XII Corps, 3 division in all, began transportation by rail from northern Virginia to Bridgeport, AL, arriving on the 30th of September. The orders were given by Stanton directly after having received the news of Rosecrans' defeat at Chickamauga on the 20th. This is not even nearly possible in the game, as it would take about 4-5 to move anything that distance, and was an extremely difficult task at the time considering that in the era of the war railroads were not a network of continuous rail lines. Rail lines when from one city to then next and actually only went
through smaller cities. To transport goods or troops
through a large city, they would have to be debarked from the train on the rail line bringing them into the city, carried or marched to the next rail line out of the city on the other side, and re-embarked on to the next train on that line out of the city on the other side. This had to be done in every large city along the way, which meant that trains had to be ready and waiting for the troops and/or goods to be moved along on their journey.
Riverboats had no such restrictions. They could move from one harbor to the next unhindered, and bypass cities and harbors, were they had no interest. They were slower in linear speed and had to follow the bed of winding rivers, but they could easily carry hundreds of tons of goods at a time while trains could carry only 1/10th of that. But I've digressed enough.
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One house rule for the RivTP might be to restrict it's use to transport troops to, at the most, be 50% of the RivTP (or what ever the players agree upon).