diff --git a/APMrover2/Log.cpp b/APMrover2/Log.cpp
index 4a6ecb4a20..4fffc249d6 100644
--- a/APMrover2/Log.cpp
+++ b/APMrover2/Log.cpp
@@ -40,6 +40,28 @@ void Rover::Log_Write_Attitude()
     DataFlash.Log_Write_PID(LOG_PIDA_MSG, g2.attitude_control.get_throttle_speed_pid().get_pid_info());
 }
 
+// Write a range finder depth message
+void Rover::Log_Write_Depth()
+{
+    // only log depth on boats with working downward facing range finders
+    if (!rover.is_boat() || !rangefinder.has_data_orient(ROTATION_PITCH_270)) {
+        return;
+    }
+
+    // get position
+    Location loc;
+    if (!rover.ahrs.get_position(loc)) {
+        return;
+    }
+
+    DataFlash.Log_Write("DPTH", "TimeUS,Lat,Lng,Depth",
+                        "sDUm", "FGG0", "QLLf",
+                        AP_HAL::micros64(),
+                        loc.lat,
+                        loc.lng,
+                        (double)(rangefinder.distance_cm_orient(ROTATION_PITCH_270) * 0.01f));
+}
+
 struct PACKED log_Error {
   LOG_PACKET_HEADER;
   uint64_t time_us;
diff --git a/APMrover2/Rover.h b/APMrover2/Rover.h
index 310d04ca1c..3910be1fca 100644
--- a/APMrover2/Rover.h
+++ b/APMrover2/Rover.h
@@ -478,6 +478,7 @@ private:
     // Log.cpp
     void Log_Write_Arm_Disarm();
     void Log_Write_Attitude();
+    void Log_Write_Depth();
     void Log_Write_Error(uint8_t sub_system, uint8_t error_code);
     void Log_Write_GuidedTarget(uint8_t target_type, const Vector3f& pos_target, const Vector3f& vel_target);
     void Log_Write_Nav_Tuning();
diff --git a/APMrover2/sensors.cpp b/APMrover2/sensors.cpp
index b5e7f6327c..c37cd7f3f0 100644
--- a/APMrover2/sensors.cpp
+++ b/APMrover2/sensors.cpp
@@ -226,6 +226,7 @@ void Rover::read_rangefinders(void)
     }
 
     Log_Write_Rangefinder();
+    Log_Write_Depth();
 
     // no object detected - reset after the turn time
     if (obstacle.detected_count >= g.rangefinder_debounce &&