0393 — Net/Disk soil friction multiplies the disk's own velocity by its speed relative to the water

ID	0393
Class	BUG
Severity	2
Status	ready

Models: Net/Disk

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

Decision needed: Owner ruling Q5f (2026-10-15): Disk follows the cable element law with its blend near the bed; that also fixes this. Fix with the ME switches of D1 phase 2.

Evidence

• src/net/Disk.cpp:107 relSpeed is the speed of the disk relative to the water, |v - v_water|.
• :133-136: below the seabed the friction is tau D T times X[m_VelIdx + i] (the disk's own velocity) times relSpeed: F_i = -tau D T v_i |v_rel|.
• A disk at rest on the seabed in a current feels no friction (v = 0) although the water moves past it, and the force is quadratic in a mixed quantity (own velocity times the relative speed) that has no physical reading.

Effect

The friction of a disk in the soil depends on the current speed even when the disk sits still in the soil, and has the wrong dimension for a force law (tau D T has Pa m2 = N, times m2/s2).

Possible fix

Q5f option 2: use the cable element law (L6 blend, drag in the soil). Minimal alternative: use |v| (own speed) for both factors.

Test that would prove it

Test first: a disk below the seabed in a 1 m/s current at rest has no friction today; the pin SeabedContactPin.L5_DiskFrictionIsTauDTTimesRelativeSpeedTimesOwnVelocity in tests/SeabedContact_Pin_Test.cpp (friction = tau D T |v_rel| v) moves with the fix.

Risk

Moves Disk baselines on the seabed.