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Your TruDesign Specialists Say That Gaining Control Is Not As Difficult As You Think

Raritan Engineering Company your TruDesign analysts would like to share with you these topics we thought would be of interest to you this month regarding the best ways to trim your symmetric spinnakers.

Your TruDesign professionals know that in run mode, wind flows vertically in the spinnaker, entering near the head and exiting from the foot.

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. 

Your TruDesign professionals understand that 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). 

Reach to VMG Downwind

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. 

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. 

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. 

Heel angle: Keep the boat flat, or heel it a little to leeward. Your TruDesign experts feel that 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. 

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.

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. 

Your TruDesign Specialists Help You to Learn How to Pass Boats In No Time!

You can find more information as well as get assistance on seacocks and on the best ways to trim your symmetric spinnakers at Raritan Engineering.

Your seacock experts know that kite control is the determining factors in the success of your downwind legs. Understand the flow of the spinnaker better, and you’ll be passing boats in no time.

Symmetric Spinnaker Run Mode

Pole angle: Experiment with moving the pole aft of the apparent-wind being perpendicular to the pole. The idea is to get it rotated to windward and away from the main’s wind shadow.

Pole height: Set the topping lift so the luff clews are parallel to the water. In this mode, the boat will be heeled to weather, so relative to the boat, this will mean the luff clew is higher than the leech clew, yet relative to the water, the clews are the same height off the water. 

Sheet trim: Trim a little bit tighter than curl. The trimmer can ease to luff curl occasionally to make sure it’s in the right range, but ultimately the spinnaker should look symmetric. The challenge is to balance trimming hard enough to get the spinnaker clews spread to get projected area while still ensuring the foot is loose enough for air to escape. 

Switching Modes

Because the two modes have different setups and trims, recognizing when to be in true run mode versus when to use reaching mode to VMG down a run is not easy. The decision is made be by feel. On our boat, if we are in reach mode and the wind increases enough that I think we can transition to run mode, I bear off and ease the main. 

Another excellent clue is the shape of the spinnaker. When fully supported with pressure, the spinnaker flies high, which is a sure sign to be in run mode. When sagging, it is time to go high onto VMG reach mode. If we can’t decide which mode, we default to reach mode because high and fast is not a disaster.


Here’s what we see in the wind tunnel when the 3D sail is trimmed on a reach, with the apparent wind at approximately 90 degrees (viewed from the bottom of the sail plan).

1. Smoke source: A horizontal line of smoke flows across the main and spinnaker, about halfway up the mast.

2. Support to hold the 3D printed mainsail and spinnaker in place during tests.

3. Wind attaches to the spinnaker and splits, flowing along both sides of the spinnaker (green) and mainsail (blue).

4. Backside spinnaker flow: The wind attaches to the back of the spinnaker for about half the distance to the leech before it detaches.

5. Movement in the slot: On a reach, the wind flows across the face of the spinnaker, keeping flow attached all the way along the back of the main. Wind flows across the spinnaker, not downward from head to foot, as it does on a run.

6. Stagnation is the area where separation happens behind the spinnaker.

7. Note how the flow is turbulent for quite some distance to leeward and behind.


Run Mode Steering Technique

Your TruDesign analysts understand that the driver is constantly experimenting with sailing as close to dead down wind as possible without loosing speed. If it’s marginally windy enough to be in run mode, slightly hotter angles need to be sailed, or the boat stalls. 

I look at my masthead fly to see if it the main is blocking the spin or not. If it is, I head up until the wind has a clear shot. My trimmer is usually barking, “Head up 5 degrees” anyway because what I see with my fly he feels as no pull on the sheet.

So don’t forget these helpful tips on how to trim your symmetric spinnakers. 1) Set the pole perpendicular to the apparent wind, and then a fraction aft of that;  2) keep the boat flat, or heel it a little to leeward;  and 3) trim a little bit tighter than curl.

Raritan Engineering always has more information on TruDesign, seacocks, and the best ways to trim your symmetric spinnakers.

via Symmetric Spinnaker Flow Control