Code zeros and asymmetric spinnakers are two very different Sails. One specialized for reaching and higher angles, the other is specialized for reaching and deeper angles. One is specialized for getting upwind in light winds, the other is specialized for getting downwind in light air. Both of them will reach, if designed to do so.
Both are free flying, meaning you don’t attach them to a structural forestay.
First of all, a code zero: is designed to be flown is when the apparent wind is approximately at or above the beam. The area is roughly, very roughly, equivalent to 1.5 times as big as a 155% Genoa, and has a deeper draft for more power compared to a Genoa. It is a very powerful sail that can double your speed through the water in light winds, for example, in 3- 8 nets of true wind.
With a code zero, you can’t sail quite as close to the apparent wind as with a Genoa. you can sail as close as 35 or 40 degrees to the apparent wind, which corresponds to more than 50 degrees to the true wind.
Shapewise, the code zero has an almost straight luff with just a little bit of round, with a very low stretch, very strong flexible line seized in, around which it can be furled (usually). (Genoas have a hollow in the luff luff). The leech is shaped like a skinny spinnaker, with a bump at the shoulder. The draft, as mentioned above is deeper and more powerful than a Genoa. The shape is designed for use in light winds up to about 8-10 kts of true wind, which means it has to be strong enough for use in apparent wind speeds of 15 or higher.
Codes zeros made of cloths that don’t stretch can sail closer to the wind than stretchier ones. That’s why heavy weight spinnaker nylons 1.5 oz and 2.2 oz) are used in the less expensive ones, with 1.5 oz and heavier laminates used in the pricier ones.
Code zeros are almost always used on furlers because they are hoisted while sailing into the wind. (You can’t blanket them in the wind shadows of the mainsail during the hoist, as you can when sailing downwind.). The furler is usually a bottom up furler, with a flexible Anti-torque line between the top swivel and bottom swivel. The furling line is usually a continuous loop, not a single line, because you have need a lot of turns to deploy and furl it, more than a Genoa on a rigid furler.
One last thing about code zeros, if you want to sail really deep angles, you pole them out like a Genoa using a big whiskerpole, you can. But then you’re using the sail in parachute/push mode (drag) , and you’re not generating any lift. You can go faster than the wind by generating lift, but that doesn’t work in push mode.
in Contrast.... asymmetrical spinnakers are designed to be used when the apparent wind angle is on the beam or further aft. They can be used to sail at much deeper angles than a code zero. You can get versions of Asymms that are “all purpose”, or for either reaching or running, either in light air, or for racers usein heavy air. All purpose means it’s a compromise between either reaching high or running deep. An all purpose racing asymm is called an A2; an all purpose asymm for cruiser sees is often called a cruising Chute or gennaker.
The shape of an asymm varies depending on the angles that it’s designed to sail, but in general it look like lopsided spinnakers, with positive curves on both the luff and the leech. The luff has a moderately rounded shape, and the leech has a prominently wide shoulder, like a symmetric spinnaker. The leech is shorther than the luff, and the clew is fairly high, so that it sheets towards the back of the boat. The draft is comparatively very deep, and is closer to the luff than the leech. The total area of the asymm is much, much more than the code zero. It generates a lot of its power from lift rather than drag when it’s trimmed properly to go down wind.
As for what gear to use, remember that a furler for either a code zero or asymmetric has to clear the pulpit in a straight line from the tack at the bow to the top of the mast where the halyard goes. Most traditional pulpits get in the way unless you either a) modify the pulpit by removing the top rail or b) install a bow sprit kit.
So the cheapest free flying sail for a Hunter 420 (for example) with a traditional bow pulpit would be an asymmetric with a snuffer/ sock. You would need to put a block in front of the jib furler for an adjustable tack line, have a spinnaker halyard block on a short crane at the masthead, and install ratchet blocks near the stern for the sheets. You will need winches in front of the sheet blocks.
In summary, here are a couple of generalizations, but remember it depends on the individual hull shape, displacement and rig design of your boat.
- fractional rigs with small jibs and big mainsails really benefit from having a code zero to work upwind in very light winds.
- boats that can easily go faster than "hull speed" benefit from Code Zeros more than "lead mines" because the apparent wind shifts forward easily if the boat is easily driven and is "fast"
- Heavy displacement boats with big genoas typically benefit a lot from having an asymmetric spinnaker because they really are relatively slow at deep angles downwind.
I’m out of time for now. Feel free to ask more questions.
Below:
Here’s a short video of us reaching with an asymm . You’ll notice we have the luff pulled tight, with the tack down as far as it goes to get the luff straight.
Below:
Here’s a short video of us running deep wth the asymm in super light winds outside the Golden Gate Bridge. The tack is eased up t allow the luff the curve, and the sheets are eased to let the luff rotate across the centerline to,windward. That allows us to sail really deep angles.
