Our good friends at Elliott/Pattison Sailmakers put together the following primer on downwind sail trim:
Spinnaker trim really falls into two categories; sails flown on a pole and sails flown on a fixed sprit. There are also a very few boats that fly asymmetrics on an articulating sprit but in general those articulate through a small enough range that flying them is nearly the same as flying a sail on a fixed sprit. However what you are trying to accomplish is the same in all cases. For that reason this article will focus mainly on trimming spinnakers flown on a standard spinnaker pole with some ideas on how to incorporate the same ideas for sprit sails.
There are four main controls you use to obtain the shape you want; pole position vertically, pole position horizontally, sheet tension, and sheet lead position. Basic wisdom has always been to fly the spinnaker pole pulled aft so that it is perpendicular to the wind, fly it high enough so both clews are level (on symmetric sails), then ease the sheet out until the luff of the sail just starts to curl over. While this will get you close it will also leave you lagging behind the good trimmers that take full advantage of all the controls at their disposal.
For pole position fore and aft, once the apparent wind is aft of 90 degrees, the pole should be pulled back just far enough that when the sail is sheeted so that the luff is just starting to curl the sail maintains an even shape from the middle down to the foot. If the pole is too far aft the trimmer will have to sheet the sail in tighter to keep it from breaking and you will see the foot of the sail is stretched out straighter than the middle of the sail is. If the pole is too far forward when the sheet is eased until the luff just starts to curl you will see the foot of the sail is much deeper than the sail is in the middle. The other thing to look for is that the luff goes straight up nearly vertically from the end of the pole to the middle of the luff but this doesn’t work for asymmetric runners on a sprit. These sails are usually designed with a lot of luff round so that as the sheet is eased the sail will rotate well out to weather of the end of the sprit and let you sail deeper.
Setting the pole so that both clews on a symmetrical spinnaker are flying at the same height is a good starting point but the more important point is to get the sail to fly square so that it takes the shape it was designed with. I like to look at the horizontal seam where the head is sewn on and the vertical seam down the center of the sail. The pole height should be adjusted so that the head seam is level and the center seam goes straight up and down; they should be at 90 degrees to each other. This technique is very important when sailing on a boat that uses asymmetric spinnakers on a standard pole since the tack and clew will never be at the same height. It also works fine and well for a properly shaped spinnaker flying with the wind aft of 90 degrees, but that is not always what you have. By changing the pole height you can also change the location on the luff where the sail breaks first, you can pull the draft further forward which also opens the leech, and you can change the amount of horizontal camber. You want the sail to break first on the shoulder, just above the head seam where the luff profile has the most curve. If the pole is too high the upper leech will twist open and break well before the rest of the sail; if the pole is too low the upper luff will be stretched too tight and will break later than the lower luff.
What about on reaches you might ask. Well since the pole height determines how tight the luff is stretched it acts something like the halyard on your headsail. As your lower the pole and tighten the luff it pulls the draft forward in the sail and also opens the leech. This is an advantage if you are sailing on a leg that is a tight reach because the open leech lets the air exhaust without closing off the slot between the main and the spinnaker as much which will give you better speed. However if you are sailing on a reach that is very tight, where you can barely hold the spinnaker you need to remember you high school math and the Gauss-Bonnet Theorem. It says that the total amount of curvature in a surface remains constant. If you increase curvature of a surface (your spinnaker) in one direction curvature in other directions has to decrease. So sailing down a reaching leg you start out with pole slightly lower to open the leech and go faster but as you approach the mark you find the leg is getting tighter and tighter and you need to keep the spinnaker up because the next leg is also spinnaker leg. By raising the pole up in this condition you increase the vertical camber in the sail which decreases the horizontal camber which will let you sheet the sail tighter and point slightly higher. You will also go a little slower so this technique should only be used for short distances. The rest of the time it is better to sail slightly lower and keep the spinnaker working at maximum efficiency for a little longer then the switch to you headsail and come back up for the remainder of the leg.
In positioning the spinnaker sheet lead block there are two things to consider. First, the further aft you have the lead the wider the sheeting angle becomes, which helps keep the spinnaker spread as wide as possible when running; and second, the correct lead angle for reaching. It is best to sail the boat on a tight reach with the apparent wind just forward of the beam and check to see how the lead is. In this condition you adjust the lead much the same as you would for a genoa. When the sail is sheeted in tight to the point where the foot is just pulled out tight the leech curve should follow the twist in the mainsail. If the lead is too far forward by the time the foot is in tight the sheet will be pulling down too much on the leech which closes off the slot between the main and spinnaker. If the lead is too far aft the leech will be twisted too open and you lose power from the top of the sail. If this position ends up being near the transom then you can get by with just the single lead; however in most cases you will find that you want to sheet the spinnaker quite a bit further forward for tight reaching. The simplest way to accomplish this is by using tweakers, basically a block that the spinnaker sheet runs through before it gets to the sheet lead block. The tweaker block is attached to a line that runs down through another block fixed to the rail about half way between the transom and the shrouds, and then to a cleat. By pulling the tweaker block down you effectively move the lead position forward. Tweakers also give you the ability to move the lead aft while reaching if you are getting overpowered (just like moving your jib lead aft to depower) and when running in heavy air conditions you can move the lead well forward which chokes the spinnaker down and helps prevent oscillations.