Quick reference
The two tables sailors ask for most: what the air is doing around your sail, and how a monohull and a catamaran behave differently.
Reading the flow
The four ways air can behave around a sail, side by side. Angles are for the simulator's default sail shape (10% camber, deepest point at 40%, aspect ratio 3.5); a different sail shape moves them a little.
| Luffing | Attached flow | Stall onset | Stalled | |
|---|---|---|---|---|
| Angle of attack | Below about 2° | About 2° to 17° | About 17° to 20° | Above about 20° |
| Air on the back of the sail | Hits the front edge from the wrong side, sail flaps | Follows the curve smoothly to the back edge | Starts breaking away near the back edge | Breaks away early and swirls in eddies |
| Suction | Almost none | Strong, peaks near the mast | Highest it gets | Collapses to a flat, weak plateau |
| Drag | Low, plus flapping losses | Low | Rising fast | Very high |
| Telltales | Windward one lifts and flutters | Both stream straight back | Leeward one starts to lift | Leeward one spins; back-edge one hides behind the sail |
| What to do | Sheet in, or bear away | Hold it | Ease a touch | Ease the sheet, or head up |
Monohull vs catamaran
How the simulator's two boats differ, and why it matters for sailing them.
| Monohull 40 ft | Catamaran 44 ft | |
|---|---|---|
| Based on | Beneteau Oceanis 40.1 class cruiser | Lagoon 440 class cruising catamaran |
| How it resists heeling | Leans over until its keel weight and hull shape balance the sail (righting moment) | Stays almost flat; the wide stance resists until the windward hull lifts |
| Near hull speed | Hits a "wall": its own bow wave traps it | Slim hulls make smaller waves, so it can push past more easily |
| Pointing (closest angle to the wind) | Better: less windage, deep keel | Worse: tall cabin and flybridge add air drag |
| Keel | One deep fin keel: resists sideways push with little drag | Two shallow keels: more drag and more sideways slip for the same push |
| Wake | One foam trail behind the stern | Two foam trails, one behind each hull |
| Warning sign when overpowered | Heel past 25°; deck edge in the water from about 30°; round-up past about 35° | No heel warning; watch the hull-lift percentage instead |
For the reasons behind these tables, read How a sail works, Reading telltales and When to reef.