Part II - Crew Control and Boat Handling
Build 27 · 20 September 2026 · not yet reviewed
7Downwind and Gennaker Modes
Sections in this chapter
The 49er sails its downwind legs on apparent wind. With the gennaker set, the boat's own speed rewrites the wind it feels, and the quantities that govern the leg are the apparent-wind angle, the boat speed and the velocity made good toward the leeward gate, not the true-wind angle alone. The run is therefore a speed problem first and a routing problem second: the route available depends on the speed the boat can hold, and the speed depends on the pressure the route finds. Section 7.1 sets out the VMG mechanism, the high-road and low-road choice in conditions, and the tactical reasons for a deliberate low mode. Section 7.3 turns to the gennaker itself: the sheet as the throttle, the luff curl as the trim reference, the windward heel and chine the class manual specifies, and the oversheeted survival mode. Both sections describe one control loop, in which the helm's course and the crew's sheet are adjusted together to hold the apparent wind where the sail wants it.
7.1VMG, pressure, and tactical low modes
Apparent wind is the of the true wind and the boat's course S19, and as boat speed rises "the apparent wind speed increases and the apparent wind angle decreases, moving further forward" S26. The crew is "looking to really over the gennaker from quite early on in the wind range" S29, and across the range "the apparent-wind angle basically remains at 90 degrees" S27. Lift acts perpendicular to the apparent wind and S19, so with the apparent wind near the beam lift becomes drive and drag becomes side force for the daggerboard to react. Helm and crew therefore steer and trim to the apparent wind, and the true-wind angle behind VMG follows from the resulting speed.
Higher than the optimum angle the boat gains speed and loses depth, lower it gains depth and loses speed, and either departure lowers VMG S27. Ingham's rule, "Speed before depth is essential because once the boat is going well, the apparent wind goes forward and the boat can go lower" S27, sets the sequence: a boat that soaks before it is up to speed sees its apparent wind fall aft, loses drive and needs a large luff to recover; the low side of the is the steeper side. No published 49er polar exists in the fact base; the crew's logged targets per wind band, crew value, are the reference.
In the band neither mode is obviously faster. Bithell calls it "the high road or the low road" S29 and warns that the high mode can mean "sailing extra distance for not much extra speed gain" S29. Underneath is the hull's change of regime, buoyancy below a of about 0.4 and hydrodynamic lift above 1.0 to 1.2 S24: the high road bids for the lifted regime, where resistance falls and the apparent wind builds on itself; the low road accepts displacement speed and cashes it as depth. The high road pays only when the extra speed changes regime and holds it; otherwise the extra distance is pure loss. The equalising wind speed, crew value, comes from logged VMG on each road in the same pressure.
Pressure comes first: the biggest gains and losses on a run usually come from differences in wind speed across the course S51, and sailing in pressure is the key priority in light and medium air S63. 30 to 40 percent extra speed S50, so the crew scans S50 and routes through them. The second priority is phase: the boat gybes on lifts to stay on the headed gybe S51. A tactical low mode is a deliberate departure below the VMG angle with a named purpose: reaching pressure the VMG angle would miss, extending on the headed gybe when gybing would surrender the lane, or holding clear air. Every low mode is called, timed and ended on a trigger and costs less VMG than it buys; an unbounded soak is simply a slow mode.
7.3Gennaker trim, projection, and luff stability
The gennaker is the largest sail, 38 m² S11 against a mainsail of 16.1 m² and a jib of 5.1 m² S10, and its sheet is the throttle: it sets the angle of attack to the apparent wind and so the drive force. The trim reference is the luff curl. Eased until the leading edge just folds inward, the sail sits at the smallest angle of attack at which flow stays attached at the leading edge, which is . Sheeted harder, the leech hooks, leeward , drag and heel climb and drive falls; eased further, the luff collapses. The crew therefore plays the sheet continuously around the curl, easing until it appears and trimming until it disappears.
Sheet and helm form one loop because both move the apparent wind. When pressure arrives the apparent wind moves aft and the curl disappears; the helm bears away and the crew eases, so the pressure becomes depth and the apparent wind stays near 90 degrees S27. When the boat accelerates the apparent wind moves forward S26, and the luff needs trim or the helm heads up. The class manual warns: "Do not under trim the spinnaker as this will allow the leech to rotate up" S12. The mechanism is twist: with too little sheet load the leech opens, the head rotates to leeward, and projected area is lost where the gradient gives "a stronger apparent wind and a " S21. The curl is a leading-edge reference, not a licence for a slack sheet.
The manual states: "The key is to sail the boat heeled to windward with the windward chine in the water" S12. Windward heel moves the rig's to windward of the centre of lateral resistance, cancelling the yaw that leeward heel produces, so the rudder runs straight. It lets gravity hold the gennaker to windward, projected outboard of the mainsail with the luff loaded in light air. And it puts the windward chine in as the running edge, the turning water down for dryness and lift S14 and giving a clean release line. The heel is set by crew position on the wing, not by luffing the sail; the target by wind band is crew value.
In survival conditions the throttle slows the boat: "there's a risk of pitchpoling if you plough into the back of a wave at full pelt", so the gennaker is oversheeted S29. Oversheeting raises drag, closes the leech and sheds drive without letting the luff collapse, since a collapse and refill when the bow is deepest. "considerably prior to bearing away" S12 to keep mainsail leech load off the rudder in the turn. An observer sees a small steady curl on the luff, a sheet hand that never stops, the windward chine in and the rudder straight, and in survival mode a hard luff, a closed leech and a bow steered around the back of each wave rather than into it.