diff --git a/docs/source/api/python/tanks.rst b/docs/source/api/python/tanks.rst
index 05a4398..b8bb7c8 100644
--- a/docs/source/api/python/tanks.rst
+++ b/docs/source/api/python/tanks.rst
@@ -99,6 +99,21 @@ Tank
 
       Longitudinal free surface moment (m⁴).
 
+   **FSM Configuration**
+
+   .. py:attribute:: fsm_mode
+      :type: str
+
+      Current FSM calculation mode ('actual', 'maximum', 'fixed').
+
+   .. py:method:: set_fsm_mode(mode, t=None, l=None)
+
+      Sets the free surface moment calculation mode.
+
+      :param mode: Mode string ('actual', 'maximum', 'fixed')
+      :param t: Transverse FSM (m^4), required if mode is 'fixed'
+      :param l: Longitudinal FSM (m^4), required if mode is 'fixed'
+
    **Geometry**
 
    .. py:method:: get_vertices()
diff --git a/python/navaltoolbox/navaltoolbox.pyi b/python/navaltoolbox/navaltoolbox.pyi
index ade5811..ffa271e 100644
--- a/python/navaltoolbox/navaltoolbox.pyi
+++ b/python/navaltoolbox/navaltoolbox.pyi
@@ -1391,6 +1391,21 @@ class Tank:
     def free_surface_moment_l(self) -> float:
         """Returns the longitudinal free surface moment in m⁴."""
         ...
+
+    @property
+    def fsm_mode(self) -> str:
+        """Returns the current FSM calculation mode ('actual', 'maximum', 'fixed')."""
+        ...
+    
+    def set_fsm_mode(self, mode: str, t: float | None = None, l: float | None = None) -> None:
+        """Set the FSM calculation mode.
+        
+        Args:
+            mode: Mode string ('actual', 'maximum', 'fixed').
+            t: Transverse FSM (m^4) (required for 'fixed' mode).
+            l: Longitudinal FSM (m^4) (required for 'fixed' mode).
+        """
+        ...
     
     def get_vertices(self) -> List[Tuple[float, float, float]]:
         """Returns tank container vertices [(x,y,z)]."""