luwang00 · Oct 5, 2026 · Oct 5, 2026 · Oct 5, 2026 · Oct 5, 2026 · Oct 6, 2026
diff --git a/docs/source/user/api_change.rst b/docs/source/user/api_change.rst
@@ -41,6 +41,38 @@ AeroDyn                                       \*                            (-)
 OLAF                                          26       RegFunctionPart            2  RegFunctionPart - Particle regularization function {0: None, 1: Exponential, 2: Compact, "default": 1} [only if VelocityMethod=2,3] (switch)
 ============================================= ======== ==================== ==========================================================================================================================================================================================================================================================================================================
 
+The meaning of the HydroDyn ``WaveDisp`` input has changed, although its valid values (0 and 1) are unchanged.
+``WaveDisp`` now controls both where the wave kinematics and dynamic pressure are evaluated and the orientation
+of the strip-theory (Morison) members and joints. With ``WaveDisp=0``, the strip-theory members are placed at a
+reference configuration consistent with the potential-flow model—the reference yaw (from ``PtfmYMod``/``PtfmRefY``)
+plus a horizontal x-y drift of the platform reference point governed by ``ExctnDisp`` and ``ExctnCutOff``—rather
+than being held at the undisplaced position. With ``WaveDisp=1``, the exact instantaneous displaced position and
+orientation are used. The previous ``WaveDisp=0`` behavior is recovered by also setting ``PtfmYMod=0`` and
+``ExctnDisp=0``.
+
+============================================= ======== ==================== ==========================================================================================================================================================================================================================================================================================================
+Modified in OpenFAST `5.1.0`                             
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+Module                                        Line     Flag Name            Example Value
+============================================= ======== ==================== ==========================================================================================================================================================================================================================================================================================================
+HydroDyn                                      \*       WaveDisp             0   WaveDisp       - Method of displacing strip-theory members and joints {0: use potential-flow consistent displacement, 1: use exact instantaneous displacement} (switch)
+============================================= ======== ==================== ==========================================================================================================================================================================================================================================================================================================
+
+In previous versions of OpenFAST, HydroDyn's input file included the ``AMMod`` option to control the method of computing
+distributed strip-theory added-mass force. Setting ``AMMod`` to 0 forces HydroDyn to always compute the added-mass
+force up to the still water level (SWL). This switch was added because of numerical stability issues encountered when
+evaluating the added-mass force up to the instantaneous free surface with some hydroelastic models. With the improved
+solver stability of OpenFAST `5.1.0`, the ``AMMod`` option was deemed no longer necessary based on user feedback and was
+removed.
+
+============================================= ======== ==================== ==========================================================================================================================================================================================================================================================================================================
+Removed in OpenFAST `5.1.0`                             
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+Module                                        Line     Flag Name            Example Value
+============================================= ======== ==================== ==========================================================================================================================================================================================================================================================================================================
+HydroDyn                                      \*       AMMod                0   AMMod       - Method of computing distributed strip-theory added-mass force {0: evaluate up to SWL, 1: evaluate up to instantaneous free surface if WaveStMod > 0} (switch)
+============================================= ======== ==================== ==========================================================================================================================================================================================================================================================================================================
+
 OpenFAST v4.2.x to OpenFAST  v5.0.0
 -----------------------------------
 

diff --git a/docs/source/user/hydrodyn/appendix.rst b/docs/source/user/hydrodyn/appendix.rst
@@ -67,8 +67,7 @@ structure::
                    0             0             0             0             0             0
                    0             0             0             0             0             0
       ---------------------- STRIP THEORY OPTIONS --------------------------------------
-                   0   WaveDisp       - Method of computing Wave Kinematics {0: use undisplaced position, 1: use displaced position} (switch)
-                   0   AMMod          - Method of computing distributed added-mass force. {0: Only and always on nodes below SWL at the undisplaced position. 1: Up to the instantaneous free surface} (switch) [overwrite to 0 when WaveStMod = 0 in SeaState]
+                   0   WaveDisp       - Method of displacing strip-theory members and joints {0: use potential-flow consistent displacement, 1: use exact instantaneous displacement} (switch)
                    0   HstMod         - Method of computing hydrostatic loads. {0: Up to the still water level. 1: Up to the instantaneous free surface} (switch) [overwrite to 0 when WaveStMod = 0 in SeaState]
       ---------------------- AXIAL COEFFICIENTS --------------------------------------
                    2   NAxCoef        - Number of axial coefficients (-)

diff --git a/docs/source/user/hydrodyn/input_files.rst b/docs/source/user/hydrodyn/input_files.rst
@@ -244,6 +244,14 @@ motion to prevent double counting the contributions from first-order
 structural motion already included in the second-order potential-flow wave 
 excitation.
 
+Beyond the potential-flow wave excitation, **ExctnDisp** and **ExctnCutOff** also
+govern the horizontal x-y drift applied to the strip-theory (Morison) members
+when **WaveDisp** = 0, as described under Strip theory options below. These inputs
+are honored regardless of whether potential-flow bodies are present, so a
+strip-theory-only model can use the same reference-position treatment. When
+**ExctnDisp** = 2, **ExctnCutOff** must be greater than zero even without
+potential-flow bodies.
+
 HydroDyn now supports large but slow (well below wave frequencies) 
 transient platform yaw motion with both strip-theory only and hybrid 
 potential-flow models. To enable this capability, the inputs 
@@ -302,24 +310,24 @@ should be set appropriately to ensure adequate angular resolution in
 platform heading. However, a high **NExctnHdg** can increase memory use 
 by OpenFAST substantially.
 
-Additional constraints on HydroDyn inputs apply when **PtfmYMod** = 1. 
-The strip-theory hydrodynamic load must be evaluated using the wave 
-kinematics and dynamic pressure at the displaced structure position 
-by setting **WaveDisp** = 1. State-space wave excitation cannot be used. 
-**ExctnMod** must be either 0 (no wave excitation) or 1 (frequency-to-time 
-domain transform using inverse discrete Fourier transform). Lastly, 
-full difference- and sum-frequency QTFs are not supported, requiring 
-both **DiffQTF** and **SumQTF** to be set to 0. However, mean- or 
-slow-drift loads based on Newman's approximation can be included through 
-the **MnDrift** or **NewmanApp** inputs explained below.
-
-Note that the inputs **PtfmYMod** and **PtfmRefY** also affect the 
-strip-theory hydrodynamic load. This is because the orientation of 
-the strip-theory members is updated based on **PtfmRefY** instead 
-of the instantaneous platform yaw rotation. Behavior of previous 
-versions of HydroDyn can be approximately recovered by setting 
-**PtfmYMod** = 0 and **PtfmRefY** = 0 deg, in which case, the 
-inputs **PtfmYCutoff** and **NExctnHdg** are not used.
+Additional restrictions on HydroDyn inputs apply when **PtfmYMod** = 1.
+First, state-space wave excitation cannot be used. **ExctnMod** must be
+either 0 (no wave excitation) or 1 (frequency-to-time domain transform
+using inverse discrete Fourier transform). Secondly, full difference-
+and sum-frequency QTFs are not supported, requiring both **DiffQTF**
+and **SumQTF** to be set to 0. However, mean- or slow-drift loads based
+on Newman's approximation can be included through the **MnDrift** or
+**NewmanApp** inputs explained below.
+
+Note that **PtfmYMod** and **PtfmRefY** also affect the strip-theory 
+hydrodynamic load when **WaveDisp** = 0. In that mode, the displacement
+and rotation of the strip-theory members and joints are based on
+**PtfmRefY** (together with the x-y drift from **ExctnDisp**) rather than 
+the full 6DOF instantaneous structure motion, as described under Strip
+theory options below. When **WaveDisp** = 1, the exact instantaneous
+member and joint orientations are used instead, so **PtfmRefY** does not
+affect the strip-theory load. The inputs **PtfmYCutoff** and **NExctnHdg**
+are not used when **PtfmYMod** = 0.
 
 HydroDyn has two methods for calculating the radiation memory effect.
 Set **RdtnMod** to 1 for the convolution method, 2 for the linear
@@ -707,29 +715,29 @@ apply constant hydrostatic/buoyancy loads to the generalized modes.
 
 Strip theory options
 --------------------
-**WaveDisp** can be set to 0 to compute the strip-theory loads using the 
-wave kinematics and dynamic pressure at the undisplaced position of the 
-structure. If set to 1, the loads will be computed using the wave kinematics 
-and dynamic pressure at the instantaneous displaced positions of the strip-theory 
-members. Note that when wave stretching is not used (\ **WaveStMod** = 0 in 
-SeaState), only the *X*- and *Y*-displacements of the strip-theory member 
-nodes are considered when **WaveDisp** = 1, while the vertical *Z*-displacement is 
-ignored. This is done to avoid discontinuous nodal loads that can result in 
-unphysical structural vibration with a SubDyn substructure model. When 
-**WaveStMod** > 0 and **WaveDisp** = 1, displacements of strip-theory members 
-in all three directions are considered when computing the wave kinematics. 
-A load smoothing procedure is performed to avoid discontinuous nodal loads 
-in this case.
-
-**AMMod** controls the computation of distributed strip-theory added-mass force. 
-If **AMMod** = 0, the strip-theory added-mass force is always evaluated up 
-to the SWL while neglecting the vertical displacement of the strip-theory member 
-nodes, even if wave stretching is enabled. With **AMMod** = 1, the strip-theory 
-added-mass force is evaluated up to the instantaneous free surface if 
-**WaveStMod** > 0. The vertical displacement of strip-theory members will also be 
-accounted for if **WaveDisp** = 1. **AMMod** should only be set to 0 if wave 
-stretching is causing numerical instabilities with flexible fixed-bottom support 
-structures modeled in SubDyn.
+**WaveDisp** controls both the position at which the wave kinematics and dynamic 
+pressure are evaluated and the orientation of the strip-theory members and joints 
+used when computing the hydrodynamic loads.
+
+Setting **WaveDisp** = 0 evaluates the loads at a reference configuration of the 
+strip-theory members that is consistent with the potential-flow model. Rather 
+than holding the members at their undisplaced position and orientation, HydroDyn 
+positions and orients them using the reference yaw (set through **PtfmYMod** and 
+**PtfmRefY**) together with a horizontal x-y drift of the HydroDyn origin (PRP) 
+governed by **ExctnDisp** and **ExctnCutOff** (see above). This results in 3DOF
+horizontal-plane displacements only (surge, sway, and yaw). Both the member and 
+joint orientations used to compute the hydrodynamic loads and the node positions 
+at which the wave kinematics and dynamic pressure are queried follow this 
+reference configuration, so a mixed potential-flow/strip-theory model remains 
+geometrically self-consistent. The behavior of previous versions of HydroDyn with
+**WaveDisp** = 0 can be recovered by setting **PtfmYMod** = 0 and **ExctnDisp** = 0.
+
+With **WaveDisp** = 1, the strip-theory loads are computed considering the full
+6DOF instantaneous motion of the strip-theory members and joints. The wave
+kinematics and dynamic pressure are evaluated at the exact instantaneous positions
+of the strip-theory member nodes and joints, and member and joint orientations
+consider full 3DOF rotation of the structure. A load smoothing procedure is
+performed to avoid discontinuous nodal loads as members cross the free surface.
 
 **HstMod** controls the computation of distributed hydrostatic loads on 
 strip-theory members. If **HstMod** = 0, the hydrostatic pressure is always