Symmetric Spinnaker Flow Control
In the previous issue we visited Cornell University’s wind tunnel to see how wind flows around an asymmetric spinnaker. We learned a lot, of course, especially the importance of being dynamic with our trim, so we went back to the tunnel to explore some key points of symmetric spinnaker flow and trim.
Before stepping into the tunnel, I had a naïve vision of attached flow on both sides of the spinnaker. What I quickly discovered, instead, was that the smoke showed large areas of stagnation and early flow separation. Thinking our 3D printed plastic test spinnaker was too rigid, or its shape flawed, we went out and placed telltales on my J/24 spinnaker. Our real-world tests confirmed our wind-tunnel findings: The flow is there, but it’s less than ideal. The difficulty in getting flow to go the way we want, and keeping it attached as long as possible, emphasizes how important and attentive trimming really is.
Flow goes in a remarkably different direction in reach mode than it does in run mode. In reach mode, the wind flows roughly horizontal, entering the luff (windward, pole side) and exiting the leech (behind the main). In run mode, wind flows vertically, entering near the head and exiting from the foot. This is why, in strong winds, we see wind ripples on the water in front of the bow. There’s often a misconception about these basic flow concepts, particularly with run-mode, top-down.
Reach to VMG Downwind
With predominantly windward-leeward courses, reach mode is used in lighter winds to sail angles downwind to VMG. The basic flow experienced is from luff to leech horizontally across the spin. Keeping in mind this cross flow, here’s some guidance to achieve a good downwind VMG reach-mode shape.
Pole angle: Set the pole perpendicular to the apparent wind, and then a fraction aft of that. Use the masthead fly as a reference. In a lull, the apparent wind moves forward because the boat carries momentum, and its component of the apparent wind becomes bigger. When this happens, the pole needs to be eased forward for a short time, until the boat slows, and then readjusted once the driver finds the new wind angle.
In a puff, the pole won’t need to be moved much. Instead, the driver will be able to sail deeper as the boat accelerates. Once into the new angle, the pole will be able to be moved aft as the spinnaker is shifted to windward for the lower, VMG course. With the sheet trimmed correctly, the foot should have shape to it. If the foot looks too round, the pole is likely too far forward. If the foot is flat, or stretched, it’s too far aft.
Pole height: Pole height sets the spinnaker’s luff tension. Set the topping lift so the “tack” is slightly lower than the clew. A low pole tightens the luff, making the wind’s entry angle more consistent while keeping the draft forward and the leech open. Lowering it creates a shape that’s a little more like an asymmetric spinnaker. Because the spinnaker hangs on its weight, the luff falls in lulls and rises in puffs, so the topping lift will have to be adjusted to keep the height low relative to the clew as conditions change. The luff should break evenly along the leech, from top to bottom. If the pole is too low, the luff will be very sensitive to trim. If it’s too high, the top of the leech will curl well before the rest of the sail.
Sail trim: Starting from the luff, encourage flow toward the leech on both sides. To optimize attachment, trim so the luff barely curls all the time, indicating the front edge of the luff is parallel to the wind to get flow started on both sides. Trimmers often take this too far by trimming too loose with a lot of curl. Wind tunnel-results show under-trimming inhibits flow over the windward side of the spinnaker. Over-trimming results in early separation along the leeward side.
Apparent-wind angles change in reach mode, even in steady winds, so the sheet needs to be trimmed constantly to test the edge of this curl. It’s better to anticipate and make consistent small changes to trim so the sail is always at the edge, rather than reacting late and having to make substantial changes. A quiet ratchet block, a sheet with lots of intermittent large trims, lots of curl or no curl at all are all signs of sheeting incorrectly. Just a hint of curl with constant yet small clicking from the sheet ratchet is perfect.
Heel angle: Keep the boat flat, or heel it a little to leeward. With a flat boat, gravity brings the spinnaker to windward, which is good. Some boats do need a little heel to reduce wetted surface area and to balance the helm. If it’s really light, the main and spinnaker tend to fall in on themselves if they’re too flat, so extra heel may be needed to use gravity to help keep shape in both main and spin. You can’t go wrong with a perfectly neutral helm, and it’s always fast to keep the heel such that the tiller is dead-centered in the boat.
The mainsail: In reach mode, the spinnaker is helping direct flow through the slot between itself and the main. The main is therefore integral to the working of the spin. The apparent wind is further aft at the top of the main because the true wind is stronger up there, thus the apparent-wind vector is moved aft. Set the vang so the top batten is angled just slightly outboard of parallel-to-the-boom. Don’t set it and leave it: Puffs will open the leech so more vang will be needed, and it will need to be eased for lulls. Also, trim the main so it’s not luffing. My default is to ease until I see luff or back wind, then trim in until any luff is solidly gone. If there’s a big change in the spinnaker sheet, then a big trim in mainsheet is probably due, too.
Steering: While sailing wide angles to optimize VMG downwind, the driver is always steering as low as possible without losing too much speed. To do so, the driver is steering a lot trying to keep the apparent-wind angle constant. Essentially, the boat is heading up in the lulls and down in the puffs. In puffs, the boat accelerates and the apparent wind goes forward, allowing deeper sailing angles while keeping the apparent-wind angle about the same. A lull slows the boat, bringing the apparent wind aft, requiring hotter angles to keep the apparent wind the same. Even a small difference in wind can make a big difference in driving angle, so the steering angle is perhaps even more important than trim in this light-air, downwind reach mode.