
Class	KNOWN-LIMITATION
Severity	1
Status	ready

Models: TrawlCable, MooringCable, RundDorg

Found: 2026-09-30 review of f42360e..a6492d6 (four parallel correctness reviews)

Decision needed: —

Evidence

AddedMassRigidElement (src/trawl_mooring_interaction/AddedMassRigidElement.h) puts the added mass A = Ca rho V f (I - N N^T) into the body inertia; StandardRigidElement::Setup (fhsim_coribo) forms the Coriolis term from the whole mass and the element velocity v. The added-mass momentum is A (v - u), u the water velocity, so its rate has Ȧ (v - u), not Ȧ v.

Effect

A rotating element in current or waves takes a spurious force of about Ȧ u ~ omega Ca rho V |u|, nonzero even when it drifts with the water. Second order for slender, slowly turning elements; larger for the MooringCable buoy element. The change of f in time is also left out (documented in the header).

Possible fix

Add the correction Ȧ u to the environment force (EnvironmentForces), or keep it and document it in the header.

Test that would prove it

A single element turning at constant rate in a uniform current with no drag and no gravity: the net force from the added mass must be zero.

Risk

Low.