2 min read · Updated 19 May 2026
The A4 is a heavy-air running asymmetric spinnaker — smaller, flatter in the shoulders and more strongly built than the lighter running sails, designed to keep a yacht fast and stable running deep when the breeze is up. It is the strong-wind end of the running range, the heavy-air counterpart to the all-purpose A2, and the sail that lets a crew keep pushing downwind in conditions that would overwhelm a lighter kite. It completes the asymmetric range in the sail wardrobe.
What the A4 does
Running deep in strong winds is fast but unforgiving. A big, full running spinnaker becomes hard to control as the breeze builds — it rolls, oscillates and can trigger a broach. The A4 is built to tame that. It carries less area, has flatter shoulders and is made from heavier cloth, so it stays steady and predictable while still driving the boat hard downwind.
On a planing boat the payoff is real: with the A4 up in a breeze, the boat stays on a fast, controllable plane down the run rather than wrestling an overpowered sail, which is both quicker and far safer.
Where the A4 sits
Following the convention that even numbers run and higher numbers suit more wind, the A4 is the heavy-air running sail, sitting beyond the all-purpose A2 and alongside the heavy-air reaching A3. A crew sets it when the leg is a run and the breeze is genuinely up.
It is handled with the same techniques as any asymmetric — see spinnaker hoists and drops — and in a strong breeze a clean set and a controlled string drop matter more than ever. Whether a boat carries a dedicated A4 depends on its rules; a strict one-design runs a limited inventory, while an offshore yacht may carry the full range. For more, see the sails pillar and the boat page.
Frequently asked questions
- What is an A4 spinnaker?
- The A4 is a heavy-air running asymmetric spinnaker. In the standard naming system even numbers are running sails and higher numbers are for stronger winds, so the A4 is the heavy-air running sail — built smaller, flatter in the shoulders and from heavier cloth so it stays stable and controllable running deep in a breeze.
- When do you use an A4?
- An A4 is set for downwind running in strong winds, when the lighter A2 would be overpowered and unstable. It keeps the boat driving deep and fast while remaining manageable, which matters most offshore and in fresh inshore conditions where a blown-out or collapsing kite can be dangerous as well as slow.
- Why are heavy-air spinnakers cut flatter?
- A flatter cut and tighter shoulders make a spinnaker more stable in strong winds. A very full sail becomes hard to control as the breeze builds, oscillating and collapsing, which slows the boat and risks a broach. Flattening the sail and building it from heavier cloth keeps it steady and lets the crew carry it safely in more wind.