Angle of attack
The mainsheet is the only sail control for bringing the mainsail to the proper angle of attack with the apparent wind. This is the familiar easing out or bringing in of the boom to attach airflow to the sail’s surface. With time in the boat you will develop a feel for when the boat is performing well, however, you will need telltales to actually see the wind on the sail.

Telltales should be mounted on both sides of the main well back from the mast as well as on the leech. Usually four sets of telltales are sufficient; two above the throat and two below the throat, while only two are needed on the leech. Be sure to offset the starboard and port telltales from each other so you can tell them apart when the sun is behind the sail. Color-coding is also helpful.

The gaff-rigged catboat sail is very forgiving and generally has a wide groove upwind and down, making it very easy to sail. However, to get the best performance close-hauled, you need to sail on the edge of the groove closest to the wind. This is a very narrow part of the groove where the windward telltales should be lifting 50 to 80 percent of the time. It takes a lot of concentration and practice to stay in this narrow slot of performance through the constant changes in wind direction and strength.

Draft
Draft is the curvature of the sail, described as both the position of maximal depth along the length of the sail and how deep the curve is relative to the length of the sail.

The depth of the draft on a gaff-rigged catboat is only controlled by the outhauls, and there are two of them. Since the gaff is in a remote location while sailing, set your gaff outhaul tension before you raise the sail to match the expected winds. If needed between races, drop your sail and re-adjust the gaff outhaul. The boom outhaul adjustment is very effective to change the depth of draft in the lower half of the sail while sailing. In light air upwind, the boom outhaul should be tight enough to just remove the deep wrinkles coming up from the boom. As the wind increases and the boat starts to heel, tighten the foot with the outhaul proportionately. With 12 knots of wind the outhaul should be as tight as you can get it.

The position of the draft is easy to control in a four-sided sail. The twin halyards of the gaff-rig allow you to easily tweak the draft position and entry shape. This is the major unique tool of the gaff-rig; there is nothing like it on a three-sided sail. Watch your sail as you adjust the peak halyard and observe the resulting change of draft position. The four photos of a sail viewed from the cockpit demonstrate this progression of draft position as the peak halyard is raised. With increased peak halyard tension the draft will move forward, affecting the entry shape that we call “deep throat.” This amazing control of the draft and entry shape by the peak halyard is a major key to maximizing your sail’s performance.

In practice, to measure the position of the draft, use your reef points to estimate the length of the sail in percentage points. For example, if your sail has five reef points, they divide the sail into roughly six equal distances of about 17 percent each.

Twist
Twist is the tendency of the sail to rotate away from the perpendicular of the boom as it rises from the deck. This is a more complicated problem to control in a four-sided sail since the gaff is free-floating around the mast at its saddle, and there is no vang. Twist is necessary for proper sail shape because the apparent wind direction actually changes at different levels of the sail. The difference in the direction of apparent wind from the deck to the peak is greatest in lighter winds, and is minimal in strong winds. Therefore, twist should be greater in light air and less in a breeze.

In a marconi (three-sided) sail with a vang, the amount of twist can be confidently matched to the wind above the deck. In a gaff-rig catboat (four-sided) sail the only controls effecting twist are the mainsheet and the peak halyard. When sailing close hauled or on a close reach, the mainsheet actually pulls down on the boom more than it pulls the boom toward the center of the boat. This is the only time that you have good control of the twist by mainsheet tension. The peak halyard also tightens the leech, and as a result controls twist. However, the proper tension on the peak halyard to position the draft in the sail is so important to performance that its function to control twist is usually ignored.

Trimming to conditions
Depending on the strength of the wind, you’ll want to adjust the sail shape to most efficiently take advantage of the conditions. In light air, first raise the throat halyard to tension the luff loosely, so that the sailcloth seems to sag slightly between the mast hoops. Then raise the peak halyard to remove the long wrinkle from the throat down to the clew. The body of the sail is now smooth and the draft should be around 35 to 40 percent aft of the luff. This is referred to as “flat throat.” The wind does not have to bend very much to stay next to the sail. The mainsheet is eased, keeping the boom well off the transom corner, allowing a smooth exit of air off the trailing edge of the leech. This is your drifting, light air shape, sometimes requiring a slight leeward heel to let gravity assist to shape the sail.

In moderate breeze a deep throat can be very fast. First, tighten your throat halyard to create a smooth entry in the luff. Then tighten the peak halyard to move the draft into deep throat. You will find that there is extra sailcloth forward along the luff that allows the luff to slide to leeward of the mast creating a smooth flow of air along the backside of the sail as it passes the mast. The sheet does not have to be eased as much to keep the leech open in moderate breeze. The best indicator of the match of twist with the apparent wind is your leech telltales. Upwind the high leech telltale should stream aft more than half of the time, and the lower leech tell tale will only appear 20 percent of the time.

As the wind strength increases the rig becomes overpowered, and deep throat and the extra power it creates is no longer needed. Therefore, in heavy air depower by easing the peak halyard, which will move the draft aft to 35 to 40 percent. Keep the throat halyard tight enough to allow a smooth entry along the luff. Ease the mainsheet to increase twist, spilling power from the top of the sail to decrease heel and helm. Sailing deep and keeping the boat flat seems to be fastest upwind in heavier air, particularly since there usually is a chop to sail against in these conditions. If you are sailing light or solo in a breeze (12 or more knots) and do not wish to reef, then sailing with a bubble in the luff and pinching in the puffs is your only option.

Points of sail
Now let’s look at what we need to do to when sailing on different points. When reaching, ease the peak halyard and the throat halyard as you bear off to maintain the same relative position of the luff curve to leeward of the mast, and a powerful deep throat shape. On a close reach, deep throat draft forward is max power. It doesn’t get any better than this. Whoever is trimming your halyards needs to pay attention, since you usually have your hands full with the tiller and the sheet.

As the reach broadens, difficulties with twist control rapidly take over and the game changes. The twist in the sail almost never matches the apparent wind in the sail. The bottom third of the sail is constantly stalled, and the top of the sail is near luffing. The trick is to keep as much of the sail properly trimmed and powered up as possible. The high leech telltale should be streaming and easily visible most of the time. The low leech telltale will be constantly hidden behind the leech, stalled like the lowest luff telltale pair.

Running downwind, the large main is stalled and functions as a very flat and inefficient spinnaker. Deepen the draft by easing the boom outhaul and both halyards to improve potential circulation around the sail. Speed test with another boat of your class in various wind and wave conditions to see which is fastest: sailing by the lee, broad reaching or heading dead downwind. Move crew weight to windward and forward to balance the helm in light to moderate air. As the breeze freshens and the rig loads up, move the crew back into the cockpit and keep the boat flat.

The principles of sail trim for a four-sided gaff-rigged sail are no different than those for the more familiar three-sided marconi sail: angle of attack, draft and twist. The biggest difference is the amazing function of the peak halyard to position the draft far forward for power. The greatest drawbacks of the four-sided sail are the lack of an effective vang and the inability to de-power the rig as the wind gets above 15 knots, except by reefing. The four-sided sail is very sensitive to sail controls, so constant fine adjustments are required to get the most from your rig.

If your goal is maximum performance, then I recommend the following:

1. Use low stretch Dacron or Dacron/Spectra braid, in the appropriate size, for all
running rigging.
2. Make sure all of your blocks run smoothly and all controls function easily.
3. Increase the purchase on your halyards to improve fine control.
4. Use cam cleats on your halyards and outhaul adjustment.
5. Have telltales on your leech and near the luff, and know how to read them.
6. Add marks on your control lines and adjacent deck for reference to a standard trim condition.
7. Practice with your crew to improve sail trim in all conditions.
8. Speed test with another boat of your class.
9. Spend time in the boat.

In this article the Sandpiper (Marshall 15, www.marshallcat.com) catboat is used as an example. It is representative of small, gaff-rigged racing catboats. The trim techniques for four-sided sails shown here should translate to other similar classes. However, each class may have individual variations in trim techniques that may be more successful; consult your sailmaker and national champions in your class, then experiment.

A look at the parts of a four-sided gaff-rig sail reveal similarities to the more familiar three-sided sail, with a few additional points. The reef points divide the sail lengthwise, and can be used to measure the position of the draft.

In an under-peaked sail the peak halyard is too loose, creating a fold in the sail from the throat to the clew. This is an inefficient sail shape with the sail basically folded into two triangles.

With a flat throat the peak halyard has been tightened just enough to eliminate the under-peaked fold. The draft is aft in the sail at about 40 percent and the entry shape is shallow. This shape maximizes pointing but is not as powerful as the “deep throat” shape.

With a “deep throat” shape the peak halyard has been tightened further, moving the draft forward so that it is affecting the entry shape and creating maximum power for upwind performance.

When over-peaked the peak halyard will be too tight, creating a fold from the top of the gaff to the tack that will cause turbulence around the luff, affecting the efficiency of the whole sail. There is little difference in the peak halyard tension between deep throat and over-peaked sail shapes, so constant attention to sail shape and halyard tension is necessary.

George Francis and his crew aboard Black Pearl show incredible focus as they round the leeward mark in the Callahan Regatta on New Jersey’s Barnegat Bay. Catboats and catboat racing has become more popular, and a better understanding of sail shape and trim will make sailing these traditionally rigged boats more fun and competitive.