porpoising

Porpoising

/ˈpɔːrpəsɪŋ/ · noun, verb

A steady rise and fall that feeds itself: lifted, let go, caught again.

What porpoising is

Porpoising is a repeating up-and-down bounce of something moving through water or air. A force that holds it up, or presses it down, depends on its own height or angle. So a small disturbance gets amplified into a cycle of rising, losing that force, falling, and regaining it.

It is named after the porpoise, which rises and dips at the surface as it swims. The loop is the same wherever it appears: a bump, a late or over-eager response, an overshoot, and a swing a little bigger than the last. It stops growing only when something gives, such as airflow stalling, a hull leaving the water, or a floor meeting the ground.

Where it happens

Boats

A planing hull rides up on a small patch of water. If its centre of lift drifts behind the centre of gravity, the bow pitches down, wetted area and drag jump, lift drops, and the bow rises again. Boat designers still argue about the exact causes and cures.

Formula 1

The 2022 rules brought back ground effect, where tunnels under the car pull it toward the track. Too close, and the airflow underneath can stall, so downforce vanishes and the car springs up. Then the flow returns and it is pulled down again. For 2023 the FIA raised the minimum floor-edge height by 15 mm.

Aircraft

Pilots also call it a phugoid oscillation, usually slow, around 20 seconds. A 2007 study of a small gyroplane found a faster cycle of roughly 6 to 8 seconds, which grew with a small tail and settled with a larger one.

Porpoises and dolphins

The original. They leave the water in low arcs as they swim fast, breathe, and slip back in. Here the bounce is not a fault. It is the plan.

Sources

Summarised and paraphrased from the work below.