We used to be able to override the river problem manually, it went away with the 1.1 patch. Now only if you are mayor, and you have a cog or holk capture, can you have one sail upriver. You can repair it, but can't use it as an autotrader or as outrigger (You could before the patch). Once loaded it sails downriver, but that is it, it will not return (unless you auctioned it and recapture it). The +/- bug is gone, heaven knows just what caused that!
Another difference, in PIII I could affect just where my ship landed in North America on the first voyage. So, it was possible always to hit an Indian village dead on (using S/L of course). Doesnt' work in the loaded game, I can hit the Chesapeake bay region, or around Jacksonville, that is it.
But the critical number idea needs testing in P3. The game was hurried to market with a number of bugs, no question. Perhaps this section is the same programming, perhaps not.
@Bizpro: Humbling Rookie? Are you the guy who beats up on kindergartners, too?