Trimming Your Asymmetrical Spinnaker
Okay, your aspin is rigged. Let’s get out sailing in a light to medium breeze.
Let's assume that you led your tackline around the port side of the forestay, and you'll hoist the sail while on starboard jibe. Once clear, aim downwind with the breeze over your right shoulder (starboard jibe). With the main eased out appropriately for a broad reach, let someone else take the helm (or let Mr. Autopilot take over). Before you hoist the aspin, locate the loop of braided line, hanging out of the mouth of the snuffer. This is the "snuffer line". It's a continuous loop, so pulling on one end raises the snuffer, releasing the sail, while pulling on the other end pulls the snuffer back down, collapsing and containing the sail. Make sure you have chosen somewhere where you can tie this line off once the sail is hoisted, so it's always close at hand (on the mast, for example).
Now raise the sail on its halyard. It will now be hanging straight down like a long limp sausage. Check that the leeward sheet runs free and clear of everything, back to its winch. The tack line should have several feet of slack, and be cleated. Now pull on the end of the snuffer line that hoists the snuffer. Raise the snuffer mouth as high as it can go, releasing the sail completely. But look aloft and make sure that the mouth itself is not jammed between the mast and the forestay. It's okay if you have to lower it a few inches, to avoid this.
The sail will now billow out freely, luffing until you trim the leeward sheet in. Just trim until the sail stops luffing—just like a headsail. Back on the helm, you'll notice that the sail is pulling the boat along convincingly, without much heel. Wow, nice! What about that tackline? Well, the luff of the sail has been cut short enough that a) the sail shouldn't reach the water, even when hanging straight down (in no wind!), and b) you will be able to pull the tack down closer to the bow when reaching, tightening the luff, and pulling the sail into a shape more similar to a headsail than a spinnaker.
So how tight or loose? Bear away to a run, with the wind just off the quarter a bit, so you don’t jibe unintentionally. Ease the sheet appropriately (out to a luff, then back in to eliminate it). Now cleat the sheet off and ease the tackline slowly. Notice whether the tack of the sail moves straight up, or up and to weather, or up and to leeward. If the sail moves straight up, it will pay to ease the tackline and let the sail fly higher, which will in turn allow it to spread its girth and catch more wind. If it moves up and to weather, that means you are very close to dead downwind, and again it will pay to ease the tackline to allow the sail to rotate even more around to weather, exposing more area. If the sail moves up and to leeward, you are still a bit high of a "run", and you should probably leave the tack flying from 3- to 5-feet above the bow, depending on the size of your boat. If you’re reaching, pull the tack line so that the tack of the sail is about 2- or 3-feet above the bow. So, how much to ease or tighten? There is no exact formula for that. Just use tack height to adjust the sail shape. The lower the tack, the straighter the luff, and the more like a headsail your sail shape will be (good for reaching). The higher the tack, the wider your sail will fly (good for running). Too much of either won’t get you into any trouble, but the boat's speed will suffer a bit. It will take a bit of experimenting until you find the optimum tack height for each wind angle. Another option, for those who like to keep their sailing simple, is to set the tack height at some average height and then just leave it there. You’ll find the sail works quite well (but not optimally) without the extra fiddling.
Now for that first jibe! You can jibe with the sail flying, just as you would with a symmetrical spinnaker (or your genoa)—simply easing one sheet and trimming the other. Or you can snuff the sail, then jibe it (pull it around in front of the forestay to the other side), and then un-snuff it again. Your choice will depend on whether or not you have crew to help you. The “other” choice was whether you led that "other" sheet, over or under the tackline, before hoisting the sail.
If you led the “other sheet” over the tack line when you rigged the sail, you are set up for an “outside” jibe. This is where the lazy sheet is outside the entire flying sail. For this type of jibe, bear away slowly from a reach to a run. Ease the active sheet out completely, so the sail’s clew flies straight downwind in front of the boat like a flag. See why those sheets had to be that long? Now, as you slowly complete your turn onto the new course, pull in quickly on the new sheet until the sail fills and is no longer luffing. Pretty simple eh? (I’m Canadian). But you might have noticed that you could end up sailing over the lazy sheet, if you’re not careful to keep it out of the water. That’s about the only drawback with rigging and jibing in this manner.
The other type of jibe ('inside') is where the lazy sheet was led under the tackline, inside (aft of ) the flying sail, but outside the forestay. To jibe, bear away slowly from a reach to a run, easing the active sheet as you go, but keeping the sail full. At a certain point the sail will start to collapse, being blanketed by the main. At that point, and while completing your turn, ease the active sheet completely, and pull the new sheet in as quickly as you can. You’ll see that you’re having to pull the sail through the gap between its own luff, and your forestay. The difference is that here there is no risk of sailing over the lazy sheet, but getting the sail through that gap and filled and trimmed again is a question of timing and practice.
You might find it helpful to add telltales at regular intervals, up the luff of the sail, just like on a headsail. These will tell you whether the sheet can eased (telltales stalled), or whether it needs to be trimmed, long before the sail actually luffs.
You’ll also find that the shape of the flying sail will respond to changes in sheet lead, just like a headsail. Normally, aspins are designed with a clew at about boom height, in order to have the sail appropriately (but not excessively) twisted when flying. The sheets are normally led aft somewhere near the stern (like the sheets for a symmetrical spinnaker). But under certain reaching conditions (and with certain sail shapes), you might find that the sail’s leech is twisting off to leeward excessively, spilling power. This will depend on the cut of the sail. In this case, some people like to rig “tweakers” or “Barber haulers” on their sheets. These are extra, light trim lines attached to a small block that rides on each sheet. Each trim line is then led to a point along the toe rail, about halfway from the aft sheet block to the chain plates, and then turned through another block there and led to a cleat in the cockpit.
By pulling in on the tweaker attached to your active leeward sheet, you can effectively change the sheet lead angle, as though you had moved the sheet block forward. By trimming the tweaker, you add tension to the leech, while reducing tension on the foot. Experiment. If this gives you more drive and speed, without excessive heeling or “backwind” in the main, it can be an advantage in some conditions.
Okay. Now go out and play with that aspin. The key points with trim are again:
1) Ease the sheet until the sail luffs, and then trim it in just enough that the luffing stops.
2) Adjust tack height so that the sail is flattened for close reaching (tack down, tighter luff), or deepened for running (tack up, luff loose).
You will discover that unless you have a specific point you HAVE to steer to, you can simply trim the sail to your boat’s optimal point of sail, and then cleat the sheet and steer by your luff telltales, just like a headsail.