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Old 10-13-2003
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Fine-Tuning the Autopilot, Part Two

This is the second part of a series on sailing with an autopilot. To review Part One, click here.


Getting your sails trimmed so that the helm is balanced is the first step to getting the most out of your autopilot.
It hardly needs to be pointed out, but the goal for using an autopilot in all conditions is to sail the shortest course to your destination, and to get some rest along the way. The shortest course means that the boat is moving in a reasonably straight line. To make the boat go straight, the sail plan has to be in balance. If the mainsail has more pressure on it than the headsail, the boat will turn into the wind until the pilot senses the problem and makes a correction. If the mainsail is too eased and the jib overtrimmed, the boat will keep trying to bear off. The same basic issues of sail plan balance are true when you have a human steering the boat, but the human has a more complex response system and a tendency to compensate for bad sail trim with bad helming. When this happens, the boat will go straight despite the fact that the rudder is dragging through the water and only half of the sail plan is functioning.

Sails work to propel the boat, but they also work to turn the boat. The lift on the foils and the heeled shape of the hull work to steer the boat as well. If you take away the sails and push the boat with the propeller alone, the auto pilot equation becomes much simpler. This is why you'll find that in the trouble-shooting section of autopilot manuals the authors often suggest that you reduce sail area. In a cruising context, that is often the right advice. But in the interest of going as fast as possible, we use the following general guidelines for sail combinations while underway with the autopilot engage:

Upwind (light to moderate air)    Choose the Apparent Wind Angle setting (sometimes called "Vane Mode") for steering input. This is the condition where the pilot can do a better job than most humans, and you can hang out below deck listening to the water rush along the hull. The key to trimming the sails in light upwind conditions is to keep the boat at a constant angle of heel, with some load on the rudder. The mainsail should be trimmed very close to centerline and the headsail trimmed tightly enough that there is a minor amount of backwind showing on the mainsail. This is pretty standard sail trim advice. But the subtle difference with the autopilot steering is in the amount of leech twist. Both sails should be set up with the top third of the leech twisted open. With a good human helmsman driving in the same condition, the sails can be trimmed a little more tightly allowing the helmsman to sail closer to the wind when the boat is going fast (burning speed) and lower when the boat is slow (building speed). However, the autopilot wants to be locked into a lower "footing" angle all the time.

"By using the halyards instead of the sheets to adjust the leech tension, the sheeting angles remain constant and the autopilot has less work to do."
Once the boat is set up this way, any changes in wind strength between six and 12 knots can be compensated for by simply moving the halyards. In the puffs, both halyards (main and headsail) are tightened and in the lulls, both halyards are slightly eased. By moving the halyards at the same time, the pressure on the leeches of both sails stays balanced. And by using the halyards instead of the sheets to adjust the leech tension, the sheeting angles remain constant. Also, the halyards have the effect of moving the draft location in the top of the sails at the same time that they adjust the leech twist. (Tighter halyards pull the draft forward as they firm up the leech, looser halyards allow the draft to move aft and allow the leech to move to leeward).

Upwind (moderate to breezy conditions)    In this situation, you should continue to use the Vane Mode steering input. "Breezy" is a purposely vague term. The perceived wind strength is largely a function of the boat you are sailing on. What is a pleasantly "breezy" day for a Little Harbor 54 might seem like a real battle on a 30-foot race boat with a short-handed crew. Whatever your boat, when it comes time to depower the sail plan it is still important to keep things in balance fore and aft so that the boat wants to go straight. In the light conditions one of the sail trim goals is to set the boat up with some load on the rudder. In breeze, the rudder will have too much sideways load so we work to reduce it.

On a boat with an adjustable rig, your first depowering move will be to pump up the backstay until the mainsail draft is minimized. The tight backstay will have the effect of flattening the mainsail by bending the mast while simultaneously flattening the headsail by straightening the headstay. Don't be shy; pump it up until the sails get flat. Move No. 2 is the same for boats with and without adjustable rigs; tighten the halyards. The tighter your halyards, the flatter the sails will be. In this condition the goal is to reduce the load on the rudder (and hence the autopilot) and to reduce the amount of heel. Flatter sails, and particularly flatter leech sections, will go a long way toward reducing the helm load.


When sailing upwind in moderate conditions it's important to open up the sheeting angle so that there's not an overwhelming amount of pressure on the rudder.
Next you will need to open up the sheeting angle by slightly easing the sheets and dropping the mainsail traveler to leeward. A good rule of thumb for the headsail sheet tension is that the last two or three feet of the middle of the leech should be pointing aft, parallels with the boat's centerline. The aft section of the top third of the sail will be angled well to leeward of the boat's centerline, but not so much that the head of the sail is luffing. Then drop the mainsail traveler as far as you can without it luffing. Having a small backwind bubble in the luff of the sail is actually the desired effect.

Reducing Sail    Faced with strong winds, you'll want to switch your pilot settings to Compass Mode for steering input. If the mainsail is eased out too far, the boat will have a tendency to sail too low, resulting in more heel than you are looking for, and in the case of a light boat, more speed than is comfortable for the wave conditions. If the jib is eased too much relative to the mainsail, the boat will turn into the wind and slow dramatically. If you can't sail with both sails trimmed so that they are not luffing, it is time to reef. The same simple issues of balance apply to reefing as they do to trimming the sails: Keep the sail plan in balance to reduce the workload on the pilot.

If you sail a fractionally rigged boat, or a masthead rig with a big mainsail, the first move will be to reef the mainsail. With a reefed main and full jib, the mainsail will need to be sheeted a little harder and the traveler positioned closer to centerline to keep the boat going straight. If your boat has a relatively small mainsail and an overlapping headsail, the first area reduction will be to roll up part of the headsail. The limit on the headsail reduction is dictated by your ability to move the sheet lead forward. With the car positioned at the front of the track, an imaginary projection of the line of the jib sheet should intersect the luff of the headsail halfway between the rolled up head and tack. If the sail is rolled up farther than your track allows, the projection of the sheet will aim more towards the tack of the sail and the top of the sail will end up luffing. In this case there will be more pressure on the mainsail than the headsail and the boat will tend to turn into the wind, making it difficult for the autopilot to do its job. If this is the situation, you've got two choices, both of them compromises: You can unroll more of the jib until you can trim the entire length of the leech, or you can sheet the foot of the jib as tightly as possible, accept a lower sailing angle and then depower or reduce the mainsail size until it balances with the poorly trimmed headsail.


Solo sailors like near-legend Christophed Auguin (above) are well versed in the complex machinations involved in trimming sails effectively for autopilots.

Upwind (very heavy air)   
At some point around 30 knots, most cruising boats are more comfortable going to weather with a small mainsail and very small headsail. In this much breeze the area of the mainsail is enough to keep the boat going but a headsail is required to force the bow down and keep the boat from rounding into the wind. Usually, once you reach this point, all roller reefing headsails are well off the front of the jib tracks, meaning the sheeting angle will be too close to horizontal to give you any effective leech tension. So, whatever amount of headsail you use will be sheeted very flat. While the tiny amount of headsail might have some influence on deflecting the wind toward the leeward side of the mainsail, you will be using it mostly for a steering foil to help the autopilot. When a wind gust or wave turns the boat towards the wind, the airflow will hit the over-sheeted section of headsail fabric and blow the bow back to leeward. Think of it as your cannard rudder in the air.

A storm jib or a small staysail makes sense in heavy air because it brings the headsail area towards the center of the boat at the same time that the reefed mainsail area moves down and forward. However, on our lightweight boat we get chucked around in these conditions, so we've found that we can actually maintain better average speed and pointing ability by using a tiny bit of roller-reefed jib way forward on the headstay rather than using our inner staysail. The staysail looked better and was clearly the right sail on reaching angles, but for going to windward the autopilot was better off with the steering help of the roller reefing jib's clew patch hanging off of the headstay.

On a heavier displacement boat, with a larger displacement motor, the autopilot will be most happy with a deeply reefed mainsail and propeller spinning at 2200 rpm. The prop wash over the rudder blade does half the work for the autopilot and the added momentum and more upright heel angle will keep the boat going straight despite the wave action.

Beam Reaching    In situations where the breeze is slightly forward of the beam, choose compass mode for the autopilot steering input. With the breeze forward of the beam, the same sail plan balance issues apply for reaching as for sailing on the wind—the leeches of both sails have to be evenly tensioned. Controlling the leech tension on the mainsail is pretty straightforward. As long as the boom is within about 15 degrees of the end of the traveler track, the leech tension is a function of the mainsheet. When the boom is eased well beyond the traveler track, the loads on the mainsail leech are progressively lower and the twist can be better controlled with the boom vang.

"To set up correctly for several hours of autopilot steering on a close reach, the jibsheet should be led as far outboard as possible."

However, on reaching angles, controlling the headsail leech is a little more difficult. In order to trim the headsail effectively on a close or beam reach, the jib lead has to be moved outboard while the sheet is being eased. If you simply ease the sheet while using the same lead position that you use to go upwind, the top half of the headsail will twist open and flap in the wind. To set the boat up correctly for several hours of autopilot steering on a close reach, you will need a changing jibsheet and a padeye on the rail as far outboard as possible. The padeye should be located a few inches forward of where the jibsheet car is for sailing upwind.

In more breeze, set both sails with tighter halyards and more eased leeches. The open leeches let the sails absorb gusts without causing the boat to heel and turn toward the wind. And having a staysail set on an inner stay can pay dividends for close reaching with an autopilot. Because the staysail is narrower than the primary headsail, and because the tack attachment is moved aft, increasing the sheet lead angle, the leech tension is much easier to control.

In lighter air, set both sails with tighter leeches (eased halyards, tighter vang). The slightly tighter leeches will load the rudder and allow the pilot to stay primarily in one position, holding the rudder against the slight sideways pressure. Leeches that are too open on a light-air reach will result in an overly balanced rudder and the pilot will steer more of an S-curved course.


Almost anytime you've got the autopilot engaged and a spinnaker flying, it pays to oversheet the sail.

Broad Reaching     On this point of sail, you should use compass mode for the autopilot steering input. On the single-mast, marconi-rigged sailboats that most of us spend time aboard, the headsail becomes progressively less effective as the breeze goes farther aft of the beam. With an aft breeze the rudder and keel are not consistently loaded and waves have more of a steering influence on the hull. The pilot typically spends more time "hunting" for the correct course as the boat's natural corkscrew motion in the following waves takes over. Because a large part of the headsail area is taken out of play due to the mainsail's shadow, the rig is out of balance in favor of the mainsail. Remember, the mainsail tends to turn the boat towards the wind so your only recourse is to overtrim the headsail. As soon as the boat starts to turn into the wind, the leech of the overtrimmed headsail will load up and the blow the bow back down, somewhat diminishing the tendency for the boat to travel in an S-curve fashion.

Running     While sailing below a broad reach, you should use the compass steering input, specifying high-steering response level and low rudder angle limit. It is safe to say that anytime you put up a spinnaker and leave the steering to the autopilot, you are risking the reality of a large repair bill from the sail loft. But that's not to say that it can't be done. The problems associated with autopilot steering downwind are that the boat has a natural tendency to wander off course in ocean waves and a free luffed spinnaker has a much less forgiving steering groove than an upwind headsail. On an upwind angle the sail plan helps correct off-course steering as the balance of pressure shifts from mainsail to headsail. With a spinnaker pulling the boat, the mainsail has very little steering effect, so the pilot is left to do all the work.

In light air, most boats achieve a better VMG sailing higher angles and jibing more often than sailing low. Sailing jibing angles is pretty safe while using an autopilot because both sails can be overtrimmed a little so that there is some sideways load on the rudder.


Downwind in a breeze is when it's most difficult to utilize an autopilot effectively.
Downwind in breeze, however, is when things start to happen too quickly and it gets exciting. The biggest danger with the autopilot is when the boat steers too low and the kite collapses behind the mainsail. There are two likely outcomes, one bad and the other worse. In the first case, after the kite collapses the boat will turn slightly towards the wind. The spinnaker will then either luff, or it will refill starting at the leech, and begin rounding the boat into the wind. This can be a very loud, nerve-wracking event, but typically there is no disastrous consequence. The other scenario is that the kite can wrap around the headstay after collapsing. If it does wrap, you will eventually have to get on the other jibe to reverse the airflow through the foretriangle, causing the kite to unwrap. Then you'll have to get the thing down.

As a general rule for trimming spinnakers while sailing with the autopilot engaged, keep the sail slightly overtrimmed, and the leech twisted. The slight overtrim should allow a wider steering groove, and minimize the chance of a wrap. The eased leech will reduce the tendency for the spinnaker to round the boat into the wind. If necessary, you can ease the leech by moving the spinnaker block as far aft as it can go on the rail, and releasing your twing line if you have one or easing the mainsail boom vang if the boom is deflecting the spinnaker sheet.

There is good reason why nearly all boat owners have cute, human nickname for their autopilot. The pilot becomes a trusted part of the team. We call ours "Arnie." We used to worry about Arnie, but now we take him for granted. He stands longer watches than any other crew member and never gets in a bad mood or uses too much water. In return, we do what we can to reduce his workload. You should too.

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