Arcus M overran runway after self-launch was not aborted at briefed point

Two pilots self-launched an Arcus M from Bellechasse RWY 26 (520 m grass), near the 800 kg max mass, in 25 °C heat (~ISA+13), on slightly damp 20 cm grass with 80 cm corn beyond. The engine ran normally but acceleration was sluggish; at the briefed abort point the required speed had not been reached, yet the takeoff continued. The glider overran, briefly flew over a terrain depression, brushed the corn, and ground-looped ~100 m into the field. Both pilots were experienced but with limited time on type; the cumulative penalty was underestimated and the briefed abort point was not honored. Minor winglet damage; no injuries.

1. Self-launch takeoff roll: Self-launch takeoff run on Bellechasse RWY 26 (520 m grass) with two pilots near 800 kg max takeoff mass and ~10 L of fin ballast.
2. Adverse runway conditions: The grass along the runway was about 20 cm tall and still slightly damp from recent rain, with possibly softer ground than normal, raising rolling resistance and takeoff distance.
3. High weight and density altitude: Mass ~785 kg vs 800 kg max; 25 °C at 1421 ft AMSL = ISA+13 °C. Flight-manual takeoff distance over a 15 m obstacle at these conditions, with the +20% grass increment, is ~660 m — already beyond the 520 m runway. With ~80 cm corn beyond and no wind, no margin remained.
4. Limited time on type: Both pilots were otherwise very experienced (~4850 h and ~7000 h total) but had limited time on the Arcus M (~50 h and ~200 h), the likely explanation for underestimating the cumulative performance penalty.
5. Briefed abort point not honored: The pilots had briefed an abort speed and the point at which the takeoff would be aborted at the latest. At that point the required speed had not been reached and the engine was producing normal power but no further acceleration was felt; the pilots elected to continue the takeoff rather than abort.
6. Runway overrun and low flight: The glider rolled past the end of RWY 26, lifted briefly aided by a terrain depression, intermittently brushed the ~80 cm corn beyond, and remained partly airborne / partly rolling for about 100 m.
7. Outlanding - damage: Settled into the cornfield ~100 m beyond the runway end with a ~180° ground-loop. Minor winglet damage; both pilots uninjured.