
ID	0396
Class	KNOWN-LIMITATION
Severity	2
Status	blocked

Models: RbCable/Cable, TrawlCable

Found: 2026-10-16, during planning of E3 (plan E3, Q12 and Q17)

Decision needed: whether a rigid element should get the drag moment of a distributed cylinder. This is a physics choice for the owner. It is not fixed in E3.

Evidence

RbCable/Cable, TrawlCable, MooringCable, RundDorg, TrawlBeam, Cable6Dof and Ring take the water drag at the element centre velocity only. At HEAD (64b0117):

src/RMCable/CableRMWaterLoad.h:79 reads the water velocity at the element centre P, and :107 calls cylinder_hydro::DragForce with the relative centre velocity v = V - current. The only moment added is the skin-friction moment on the spin about the axis (TorsionDragMoment, MARE-0376).
src/trawl_mooring_interaction/CableSegmentWaterForce.h:107 reads the water velocity at P, and :119 calls DragForce with V_relative. Again the only moment is the axial TorsionDragMoment.

Effect

An element that rotates about an axis normal to it, in still water, gets no drag moment. A distributed cylinder would get -∫ k |ω s| ω s · s ds over its length. So rotation of a rigid element about a normal axis is undamped by the water. This is an old simplification. It is separate from the CableElBasics per-node fix in plan E3 (Q12, Q17).

Possible fix

Integrate the cross-flow drag along the element (or use a closed form), giving a moment from the angular velocity. This changes baselines of every model above.

Test that would prove it

An element spinning about a normal axis in still water: the moment must oppose the rotation and match the integral.

Risk

Moves the baselines of all cable and rig models. A physics choice, so the owner decides.