0365 — Trawl doors and clump weights take full buoyancy and drag whatever their submergence (no wet fraction)

ID	0365
Class	KNOWN-LIMITATION
Severity	1
Status	ready

Models: TrawlCable, RundDorg

Found: 2026-10-12, feature wake-receiving P1 review of a237d92 (owner ruling A1 of 2026-10-12: file, do not fix)

Related: owner ruling R110 (MARE-0313): the wave zone reaches the door and the clump weight only where their documentation puts them near the surface; the TrawlCable and RundDorg pages put them at depth. MARE-0366 (wave inertia).

Evidence

• src/trawl_mooring_interaction/CableSegmentWaterForce.h:199 (DoorForce): Force(2) = 9.81 * (door.mass - door.displacementVolume * kRhoWater); and :201-202 the lift and drag on the full plate area: no wet fraction.
• src/trawl_mooring_interaction/CableSegmentWaterForce.h:229 (ClumpWeightForce): Force(2) = 9.81 * (clump.mass - clump.length * clump.radius * clump.radius * M_PI * kRhoWater); and :231 the drag: no wet fraction. The doc comments say so (:170, :211: "No wave inertia and no wet fraction").
• The cable elements do it instead (InWaterForce, :78-112): WetFraction (:51-56, body_submergence::CylinderWetFraction against the wave surface, owner rulings R66, R104) scales the buoyancy (:87), the drag and the damping moment (:96-97); above the surface an element keeps only its weight (R109).

Effect

A door or a clump weight at or above the surface (a pelagic rig hauled in, a shallow-water or surface-towed rig, a foil or clump weight on a RundDorg line near the surface) keeps its full buoyancy and full water drag and lift in air, while the cable elements around it lose theirs. For the bottom rigs the pages describe (R110) the body is always submerged and nothing changes.

Possible fix

A wet fraction per body, as for the elements: the clump weight as a cylinder of its radius and length (body_submergence::CylinderWetFraction); the door as a plate (a wet fraction of its area over its corners, or a box). Scale the buoyancy, the drag, the lift and the damping moment by it. Needs the environment in the pure laws (or f as an argument).

Test that would prove it

TrawlCableWater: a clump weight half above the surface takes half its buoyancy and half its drag; fully above, its weight only. The same for a door lying across the surface.

Risk

None for submerged bodies (f = 1, bit for bit if f = 1 is exact). Moves results for runs where a door or clump weight reaches the surface.