diff --git a/src/pathfinder/yapf/yapf_ship.cpp b/src/pathfinder/yapf/yapf_ship.cpp
index 335a589ef6..fcb2e3759c 100644
--- a/src/pathfinder/yapf/yapf_ship.cpp
+++ b/src/pathfinder/yapf/yapf_ship.cpp
@@ -388,6 +388,13 @@ public:
 		uint8_t speed_frac = (GetEffectiveWaterClass(n.GetTile()) == WATER_CLASS_SEA) ? svi->ocean_speed_frac : svi->canal_speed_frac;
 		if (speed_frac > 0) c += YAPF_TILE_LENGTH * (1 + tf->tiles_skipped) * speed_frac / (256 - speed_frac);
 
+		/* Lock penalty. */
+		if (IsTileType(n.GetTile(), MP_WATER) && IsLock(n.GetTile()) && GetLockPart(n.GetTile()) == LOCK_PART_MIDDLE) {
+			const uint canal_speed = svi->ApplyWaterClassSpeedFrac(svi->max_speed, false);
+			/* Cost is proportional to the vehicle's speed as the vehicle stops in the lock. */
+			c += (TILE_HEIGHT * YAPF_TILE_LENGTH * canal_speed) / 128;
+		}
+
 		/* Apply it. */
 		n.cost = n.parent->cost + c;
 		return true;