diff --git a/README.ko.md b/README.ko.md
index f1e6dee..27a4998 100644
--- a/README.ko.md
+++ b/README.ko.md
@@ -52,7 +52,8 @@ export GZ_SIM_SYSTEM_PLUGIN_PATH="$PWD/install/lib${GZ_SIM_SYSTEM_PLUGIN_PATH:+:
 - 추종 차량 속도 상한: 11.9 m/s
 - 추종 목표 거리: 38 m
 - 거친 파도 속도 상한: 7 m/s
-- 얕은 수역·운하 속도 상한: 4 m/s
+- 측정된 얕은 수역 속도 상한: 6 m/s
+- 운하·수심 미확인 구간 속도 상한: 4 m/s
 
 추종 차량은 선행 차량의 실측 위치를 따라갑니다. 시계·진행·기울기·파도 정책은 실제 관측을 사용합니다. 복구 함수는 지형을 검사한 이동 후보를 제안합니다. 호출 앱이 보조 복구를 승인하고 표시합니다.
 
diff --git a/README.md b/README.md
index afcf370..af77a29 100644
--- a/README.md
+++ b/README.md
@@ -52,7 +52,8 @@ Send `gz.msgs.Double_V` to `/wwos/promotional/path` as `[mode, speed_mps, count,
 - Follower speed cap: 11.9 m/s
 - Follower target separation: 38 m
 - Rough-water speed cap: 7 m/s
-- Shallow-water and canal cap: 4 m/s
+- Measured shallow-water cap: 6 m/s
+- Canal and unavailable-depth cap: 4 m/s
 
 The follower follows measured leader poses. Clock, progress, tilt and rough-water policies use actual observations. Recovery helpers propose terrain-checked motion; the caller authorizes and reports assisted recovery.
 
diff --git a/src/posim_control/speed.py b/src/posim_control/speed.py
index 88a3c2f..fcd9e06 100644
--- a/src/posim_control/speed.py
+++ b/src/posim_control/speed.py
@@ -7,7 +7,8 @@
 ROUGH_MPS=7.
 APPROACH_MPS=CRUISE_MPS
 FOLLOWER_MAX_MPS=11.9
-SHALLOW_MPS=4.
+SHALLOW_MPS=6.
+UNKNOWN_DEPTH_MPS=4.
 CANAL_MPS=4.
 
 def sensitive_water(near_port,canal,clearance,caution=False):
@@ -15,9 +16,9 @@ def sensitive_water(near_port,canal,clearance,caution=False):
 
 def target_speed(near_port,canal,clearance,pending=False,rough=False):
  # Depth constrains safe speed; a port label and background downloads do not.
- if clearance is None:return SHALLOW_MPS
- if clearance<2:return SHALLOW_MPS
  if canal:return CANAL_MPS
+ if clearance is None:return UNKNOWN_DEPTH_MPS
+ if clearance<2:return SHALLOW_MPS
  return ROUGH_MPS if rough else CRUISE_MPS
 
 
diff --git a/tests/test_promotional_rough_water.py b/tests/test_promotional_rough_water.py
index e408079..c7f5292 100644
--- a/tests/test_promotional_rough_water.py
+++ b/tests/test_promotional_rough_water.py
@@ -57,10 +57,15 @@ def test_moderate_heave_peak_restarts_calm_hold(self):
    self.assertTrue(watch.update('a',float(t),dict(lead=pose(z=(-1)**t*.3)))['rough'])
   self.assertFalse(watch.update('a',84.,dict(lead=pose(z=.3)))['rough'])
 
- def test_speed_changes_do_not_lift_canal_or_clearance_limits(self):
-  for clearance,canal in [(1.,False),(None,False),(30.,True)]:
-   self.assertEqual(target_speed(True,canal,clearance,pending=True,rough=True),4.)
-  self.assertEqual(target_speed(True,False,30,pending=True),10.5)
+ def test_shallow_speed_preserves_canal_unknown_depth_and_rough_limits(self):
+  for rough in (False,True):
+   for clearance in (1.,1.999):
+    self.assertEqual(target_speed(True,False,clearance,pending=True,rough=rough),6.)
+   for clearance in (None,1.,30.):
+    self.assertEqual(target_speed(True,True,clearance,pending=True,rough=rough),4.)
+   self.assertEqual(target_speed(False,False,None,rough=rough),4.)
+  self.assertEqual(target_speed(True,False,2.,pending=True),10.5)
+  self.assertEqual(target_speed(True,False,2.,rough=True),7.)
 
  def test_calm_hold_excludes_window_peak_and_duplicate_frames(self):
   watch=RoughWaterWatch();watch.update('a',0.,dict(lead=pose(tilt=31.)))