
ID	0374
Class	KNOWN-LIMITATION
Severity	2
Status	blocked

Models: RbCable/6DofCable, RbCable/Ring

Found: 2026-10-13, feature mare-owner-rulings (resolving MARE-0314 for RbCable/Cable at a78638e; split from MARE-0314, whose ruling names RbCable/Cable only)

Decision needed: Owner: should these two models' rigid elements carry the normal added mass Ca rho A L f (Ca = 1, DNV-RP-C205 (2021) Table A-1) in their inertia and take (1 + Ca) rho f V a_n, as RbCable/Cable (MARE-0314), TrawlCable and fhsim_base Cable (R93) now do? For Ring: and should Cm_IsAddedMassHack then go?

Evidence

src/RMCable/ElementWaterLoad.h:94 (WaterLoadOnElement, used by Cable6Dof.cpp:304 and Ring): InertiaForceFactor(element.waterDensity, volume, f, 0.0), the Froude-Krylov force only; the header says "There is no added mass in the elements' inertia".
src/RMCable/Cable6Dof.h:26 and src/RMCable/Ring.h:24 document the same.
src/RMCable/Ring.cpp:60: m_mass = m_length * m_weight + m_AddMassHack, an isotropic stopgap (Cm_IsAddedMassHack, default 0, MARE-0017) that adds the mass along the axis as well and gives no matching excitation.

Effect

As MARE-0314 before its fix: a steel element has about 13 % too little inertia normal to itself, a rope near neutral buoyancy half the inertia it should have, in still water as in waves. The excitation is consistent with that inertia (an element of mass rho V follows the water normal to itself).

Possible fix

As RbCable/Cable (a78638e): a per-element translational inverse inertia N N^T / m + (I - N N^T) / (m + m_a) wherever the dynamics divide by the scalar mass, Ca in InertiaForceFactor, and the Coriolis force -Ȧ (v - u) of the turning added mass.

Test that would prove it

As SimObject.CableRM_AddedMassIsInTheInertiaNormalToTheElements: two otherwise equal objects of different dry mass pushed by a starting cross current; their initial velocity ratio is the inverse ratio of m + Ca rho A.

Risk

Changes every RbCable/6DofCable and RbCable/Ring result with element accelerations normal to the elements.