resource trading question

[Brundlefly]

How can I trade with a distant civ whom I'm connected to by a harbor without fear that the sea trade route will be interrupted?

What is not clear to me is how the sea route interruption occurs. If an enemy civ or barbarian ship appears anywhere along the route is that enough to disrupt the trade or does that bad ship have to blockade a port for the trade to be dissolved? If the first part is true, then that means I need to blockade every enemy port along the sea route.. If the second part is true then I only need to blockade the port of the civ I am trading with.

I really think Civ3 is unrealistic for giving a penalty (no other civs will trade resources with you) to the human player if a sea (or land) trade route is disrupted by a third party civ.

[Harovan]

To trade safely, you need a sea route through water tiles you can travel at (coast+sea+ocean after Navigation or Magnetism, coast+sea after Astronomy or with the Lighthouse, coast only before Astronomy and without the Lighthouse). This route shall not go through territorial waters (i.e. inside cultural borders) of a civ you or your trade partner are at war with. You need to "see" (i.e. have explored) the whole route. Note, that you also can use a harbor of your neighbor civ to trade, given you're at peace and connected with a road.

[stonewall]

For ships to block trade routes, enemy ships would have to occupy a "wall" around all of your ports that could be used for trade OR your trading partner's ports. The "wall" is what would be needed so that you could not move a ship out of your city with the harbor.

Also, like Sir Ralph said, you can "use" the harbors of other civs that you are not at war at, so those would have to be blocked too.

[Nym]

After the discovery of astronomy, and furthermore magnetism, the only practical cases where you can not trade with a civ are wars and trade embargos.
In fact I never saw a sea trade route interrupted.

[bongo]

Before astronomy, trade lines can only go over coastal tiles. One barbarian galley is enough to block it. Normally I won't trade lux/res with distant civ because of the high possibility the route will be blocked. Don't trade until you control all the coastline yourself or discovers astronomy/navigation/magnetism