Movement attrition is a form of attrition caused by movement. As with all forms of attrition, it is applied only at the end of the month; a unit may be moving for most of the month but if it stops on the 30th, it will suffer no movement attrition. See the article on monthly movement for discussion.
Movement attrition is 1%, and it applies to all military units with these exceptions:
- Units which are not moving at the start of the month.
- Naval units (and their passengers) in a fleet led by an explorer.
- Land units in an army led by a conquistador.
- Naval units (and/or passengers) moving in national waters. Passengers can move without the fleet moving, when they move ashore.
- Naval units (and passengers) moving out of a port (i.e. in the port province, not in any sea zone).
Note that fractional attrition is tracked for both land and naval forces. Fractional attrition applies to any unit which is too small to take losses from a small amount of attrition. Movement attrition still applies to them, but it may require many months of movement to add up to anything. For example, a fleet of 10 ships can move for 9 months straight without losing any to movement attrition. The total of 9% attrition is tracked, however. If the fleet moves again on a 10th month, the total attrition would be 10%, which is large enough to kill one ship. (The fractional attrition counter is reset to 0%). Fractional attrition is reset to zero whenever a fleet enters a port, and also when you recombine fleets. So it's pretty easy to avoid.
Because artillery are small in numbers like ships, artillery can also avoid movement attrition. Five units consisting of only 10 cannon can take 10 months of 1% movement attrition at which time five cannons are lost, but a unit of 50 cannon can only take two months of 1% attrition at which time a single cannon is lost. But the cannons can be repaired by merging, splitting, or reorganizing. So keeping your cannons in units of size 10 can be useful in preventing loss of artillery from whatever small amounts of attrition including movement attrition. Splitting also repairs artillery in a siege province but you need some cover force when you split otherwise one of the split forces retreats as does the other if it is below the required cover size.