
ID	0082
Class	BUG
Severity	2
Status	blocked

Models: Trawl/TrawlDoorBase

Found: Review 2026-09-28: FISH-R-1, I-8. 2026-09-28 review of feature/net-hydrodynamics at 73aa0c7 (worktrees/nethydro/review/fhsim_fishery.md and interactions.md); rechecked at 1e6770c; filed by phase K WP K4 (owner ruling R42 item 39)

Decision needed: Owner (Karl-Johan): TrawlDoorBase buoyancy from Rho (a physics change for inputs with Rho ≠ 1025; the review asks to wait for the owner)?

Update 2026-10-04: The CenterWeight part is resolved in cbdf038 (FISH-0102, feature fluid-body-interaction): a Rho parameter (default 1025) sets its drag, buoyancy and wake, following the set-wide ruling of BASE-0054 (resolved 2026-09-30, a Rho parameter default 1025). Only the trawl door remains; lifting bodies are out of that feature's scope.

Review 2026-09-28: FISH-R-1 (severity 2, tracker scale) and the fishery hard-codes named in interactions I-8 (CenterWeight). Confirmed by code trace. The cross-layer density question of I-8 is fhsim_base BASE-0054.

Evidence

src/trawl/TrawlDoorBase.cpp:88: creator->GetDoubleParam("Rho", &m_rhoWater, 1025);, used for the hydrodynamic forces at :324, :591 and :600.
:449-454 (AddInternalForces): rhoAmbient = 1025 or 1025 * (submergence + m_height / 2) / m_height; :458 builds buoyancy from rhoAmbient. With Rho="998" the hydro loads are at 998 and buoyancy at 1025. The option text (src/trawl/TrawlDoorBase.h:79) says "Density of sea water" without qualification.
(Resolved, FISH-0102) at 1e6770c src/trawl/CenterWeight.cpp:38, :61 and :150 hard-coded 1025 for the weight's buoyancy, drag and drag Jacobian; it now reads Rho.

Effect

A door in fresh or tank water (Rho ≠ 1025) mixes two densities.

Possible fix

Use m_rhoWater at TrawlDoorBase.cpp:450 and :454 (identical results at the default), as BASE-0054's set-wide ruling and CenterWeight (FISH-0102) do.

Test that would prove it

After the decision: a test of the chosen behaviour.

Risk

Result changes only in the cases named above; check the regression baselines.