
ID	0080
Class	BUG
Severity	2
Status	ready

Models: FlukeAnchor, TorpedoAnchor

Found: 2026-10-16, feature seabed-pins (plan D1 phase 0, step 4, defect candidate D-c)

Decision needed: Ruling 5 (2026-10-16): fix in D1 phase 2 (not only filed). The owner decides the rule of the cap (physics).

Evidence

src/rigidbody/PolyplateEnvironmentForces.cpp:244 (plus side, normal pointing up): soilWeight = rhoPlus * Aplus * 9.81 * h * n(2) - 2 * sqrt(pi * Aplus) * depth * h where depth is the sea depth (GetSeaDepth).
:253 (minus side): soilWeight = rhoMinus * Aminus * 9.81 * h * n(2) + 2 * sqrt(pi * Aminus) * tauMinus * h with the shear strength tau.
The two sides are not mirror images: the plus side uses the sea depth in metres where the minus side uses a soil strength in Pa. The cap on the plus side grows with the depth of the sea.
Pinned in tests/src/SeabedContact_Pin_Test.cpp (L11, the q2 cases).

Effect

A fluke pulled up through the soil is held by a force that depends on the depth of the water.

Possible fix

Replace depth with the soil term the minus side uses (tau), as a mirror of the minus side. The rule is the owner's (physics).

Test that would prove it

Test first: a fluke with its normal up and the same soil at 10 m and at 1000 m sea depth: the capped force must be equal. Today it differs. The q2 pins move.

Risk

Moves anchor baselines that reach the seabed (none today: start at z = -500, seabed at 10000 m).