Code zeros for boats with non-overlapping headsails

The Code Zero is a close-reaching asymmetric sail designed to fill in the gap between the headsail and the asymmetric kite. The problem with genoas and especially jibs is that they don't have enough area to power the boat up in light to medium air.

This void is filled by the code zero which measures as a spinnaker under IRC/IMS, but really works like a headsail, tacked on the bow of the boat, hoisted on the spinnaker halyard and sheeted in with the spinnaker sheet, using the tweaker for control.

The design of the code zero has changed a lot since it was first used on EF Language in the Whitbread.
Gone are the days of masts breaking under halyard tension. Now designs have moved on to make an easier-handling sail to cover a wider wind range, from 40 to 120 degrees apparent. One major change in design is the amount of depth in the sail, which is critical to its flying shape, without this depth the leech will flap.

Most design programs will work the camber out to be around 21.6-25.2 per cent of the overall cord length which keeps the sail stable and holds the leech roach up, stopping it from flapping. The draft position is normally around 38-41 per cent.

When a designer works from these parameters his most important objective is the amount of leech roach he has to work with, under IRC it is 75 per cent of the foot length, under ORC (IMS) it is 65 per cent of the foot length. That extra roach is a hard area to keep stable; you need to push depth into the back of the sail to keep the leech from flapping. With a bad design it flaps all the time. Good design will keep this area stable.

Less compression
Halyard loads are critical when flying a code zero; in times gone by they were mast-breaking; now we have moved on from that to lower loads. This helps the sail set better, with more leech load to keep it flying better and getting the luff to project forward, flattening the entry to windward.

Having the draft forward and keeping the knuckle in the sail stops the sail collapsing so easily. This knuckle is critical to the design giving power and ease of handling. The picture illustrates the code zero in reaching mode, showing the twist in the sail. Normal twist is anywhere around 10 degrees from the clew, depending on the boat upwind, on a reach it's about 30 degrees.

The illustration shows the sail shape and design, including the projected luff on the bottom left hand corner - which is critical to the overall flying shape. Without the looser luff, the leech load will not hold up the leech roach.

The set up

When setting up the sail on your boat, consider that it works just like an A-sail; tacked at the bow, halyard at the head, and spinnaker sheet in the clew with the tweaker.

When hoisting the sail I recommend double the amount of rubber bands or string to secure the sail. Some mast builders in days gone by suggested a 2:1 halyard. On maxis this may still be the case, but for boats under 60 feet 1:1 is fine.

Adjusting the halyard to get the luff to project forward is the key to setting it, but using the tack line is also useful for this. The problem is the tack tends to drift to leeward of the boat losing control of the sail and pointing ability. Easing the halyard is a lot more effective for getting the luff to project forward. The luff length is normally the same as the forestay length, so easing it up on the tack line or down on the halyard by one foot, will provide instant luff round.

The sheet just needs to be worked as a normal spinnaker sheet but when trimming, less movement is better than more. This keeps the sail stable and develops more power.

When in full upwind mode, the halyard needs to be taken back on, until it's tight. Normally setting the tweaker or the guy at half height to the clew of the Code Zero, about a 45 degree sheet angle is needed for full upwind mode. This angle helps pass the sheet load up the leech of the sail, keeping it more stable.


Most code zeros will start to curl in at this point, the trick being to ease the tweaker up until you have the least amount of return in the sail.

Having the knuckle in the front of the sail makes it easier for the helm to sail the apparent wind angle without crashing out in light air. This apparent wind angle is great for building speed and power, but it also decreases pointing ability.
To increase pointing ability you need to burn off this apparent wind angle by luffing up more or slowing the boat down a little. Normally with the knuckle in the luff of the sail this is done automatically for you, as the knuckle will roll back and then flip out again. As the sail is trimmed in all the way, the helm can follow this easily, with the trimmer doing nothing.

Following the telltales and burning off the apparent will give you boat speed approx 1 knot above the wind speed for most boats.

Tricky tacking

Given that the sail is only set on one side it impossible to tack without losing everything. The only way to achieve this is to set the sail inside the forestay and use two spinnaker halyards one for each side.

Break off as much apparent wind as possible before tacking when trying this, as you have to reach off on the new tack before heading upwind again. Then build boat speed to the max before going into pointing mode.

Sailmaker Barry Hayes runs the Hong Kong loft of UK-Halsey

Optimum settings

Settings for upwind: 40-45 degrees, tack line on, spinnaker sheet angle to the clew 45 degrees, when the sail is set ease the halyard down until the luff load eases and starts to project, or ease the cunningham on the tack, until the luff starts to get wrinkles.

Setting for reaching: 50-80 degrees, halyard up, tack line eased, sheet angle about 30 degrees, cunningham on, luff projecting forward, draft forward, some twist in the leech.

Blast reaching: 90-110 degrees, tack eased, halyard tight, sheet eased, no tweaker, cunningham on.

NOTE: With a furling code zero remember to have a very tight luff when furling as the unit will not work properly without a lot of load on the bearings.