
ID	0367
Class	BUG
Severity	1
Status	blocked

Models: TrawlCable, RundDorg

Found: 2026-10-12, feature wake-receiving P1 review of a237d92 (owner ruling A2 of 2026-10-12: file, do not fix)

Decision needed: the law of the rotational damping of a door and of a clump weight (dimensionally a moment, ½ rho C L⁵ |ω| ω or similar, and its coefficient), or keep the present term as a numerical damper and document it as such.

Evidence

src/trawl_mooring_interaction/CableSegmentWaterForce.h:205 (DoorForce): Force.segment<3>(3) += -0.5 * kRhoWater * Area * 0.7 * Area * W.norm() * (1.28 * Wxy + 0.1 * Wz); with Area = door.length * door.width (:200).
:234 (ClumpWeightForce): Force.segment<3>(3) += -0.125 * kRhoWater * Area * 0.7 * Area * W.norm() * (1.28 * Wxy + 0.1 * Wz); with the clump's drag area (:230).
Dimension: rho [kg/m³] · Area² [m⁴] · ω² [1/s²] = kg·m/s² = N, a force, not a moment [N·m]; a rotational drag of a plate or cylinder scales with rho · (length)⁵ · ω². Origin: the factors 0.7, 1.28 (the door's post-stall Cd, :148), 0.1 and the clump's 0.125 are carried over unchanged from the original TrawlCableConstraintSets.cpp (2011, JHJ); no source, no document, no test of their meaning (TrawlCableWater pins only the values).
The cable elements do it instead (InWaterForce, :97): -kRhoWater * radius² * length * 0.1 * |W| (W·N) N * f, damping only the spin about the axis, rho · r² · L · ω², which is also N (the same dimension defect at a much smaller scale; a cylinder's spin damping goes as rho r⁴ L ω²).

Effect

The damping moment does not scale correctly with the body size: a door twice as large in every length gets 16 times the damping, where a moment of the form rho L⁵ ω² would give 32 times. Its value in N·m changes with the unit of length. For the default door (TrawlDoorArea 4 m², Area² = 16 m⁴) the term is a numerical damper of unknown physical size; the door's pitch and roll response and its settling depend on it.

Possible fix

A dimensionally correct rotational drag: integrate the drag over the plate (½ rho Cd w |ω| ω ∫ r³ dr over the length, i.e. rho Cd w L⁴/64 |ω| ω for a plate rotating about its centre) and the analogue for the clump cylinder.
Keep the present term, documented as a numerical damper with its unit, and the coefficient named.

Test that would prove it

TrawlCableWater: the damping moment of a door scaled by a factor k in every length grows by k⁵ (option 1).

Risk

Option 1 moves every result with a door or clump weight (orientation and its transients). Option 2 changes nothing.