I was once teaching a crew of sailors advanced racing techniques. It was Friday and we had spent all week learning how to start, turn a mark, and slam-dunk another boat. The only thing we hadn't been able to practice was broaching. This particular Friday the northerly was fluctuating all over the shop, first gusting and coming ahead, then dying and moving aft. A typical northerly pattern. By now the crew had become confident in their own ability and had, as many sailors do, started to sit on the rail and talk among themselves without heed to the trim of the boat.

I was standing near the backstay watching the helmsman as he attempted to control the boat, while a single crewman stood at the shrouds playing the spinnaker. I watched a puff of air stir up little whitecaps as it moved toward us and called out "Puff coming. Standby on the main."

The crewmen were too busy telling jokes and ignored my call. I tried again to no avail. The helmsman heard the call and didn't bother to relay it. Complacency had set in.

The puff hit. It was a lot harder than the crew expected, and I hung onto the backstay. As the boat heeled it quickly became apparent that nobody was tending the mainsail and the helmsman had the helm hard over. We broached while I watched the crew tumble from the rail. The next time I called out. "Puff coming," three crewmen leapt for the mainsheet.

Once we were back under control, we discussed what had happened and why. We looked at what to do to reduce the potential to broach and how the crew could maintain control even in conditions that might lead to broaching. These points are the subject of this story.

The causes of a broach

When a boat is sailing along in a straight line, be it upwind or downwind, the forces on the hull and sailplan are in equilibrium. As they move slightly out of equilibrium, the helmsman uses the rudder to move them back in to a steady state. If the rudder is too small or if the sail forces become very large, they overcome the ability of the helmsman, and ultimately the rudder, to balance them. This is when the boat broaches.

A broach can happen when the boat is sailing in any direction: to windward, reaching, or when sailing downwind.

A windward broach

When a boat is sailing to windward, the wind is coming over the bow at an angle of about 30 to 45 degrees. The wind speed is exerting a force on the sails. For the sake of simplicity, this force can be resolved into two major components, driving force and heeling force, as shown in Figure 1.

The driving force from the sails is moving the hull through the water at a slight angle known as the leeway angle. The action of moving the hull forward at a few degrees of leeway creates a force on the hull, keel, and rudder, known as sideforce. It also creates drag, as shown in figure 2. (Again, for the sake of simplicity, we resolve these forces into two components: sideforce and resistance or drag.)

When these forces are in perfect opposition, the boat sails at its maximum speed, as shown in Figure 3. Unfortunately, because of wind fluctuations, wave action, and other external factors, these forces rarely stay perfectly aligned and the helmsman must use the rudder to bring them back into line. If the sail force moves forward for any reason — for example, a gust of wind or an increase in wind strength — it tends to heel the boat and drive the bow of the boat into the wind. The heeling moment tends to move the sail's center of effort (CE) aft, and the center of lateral resistance (CLR) of the hull forward. If the movement is small, a quick correction of the rudder angle by the helmsman brings the bow back on track.

But if the force is much larger than the force the rudder can exert, the boat continues to seek equilibrium. This is when the boat broaches. The exact sequence of events goes something like this.

A boat is sailing along, as in Figure 3, with all forces balanced. A gust hits and the boat heels more. As it heels the hull's center of lateral resistance moves forward. At the same time, the heeling motion causes the center of effort of the sails to move slightly aft. The helmsman turns the rudder to correct, but the sail force is too high, so the helmsman turns the rudder even further. The sharp rudder angle causes the rudder to stall, and with no helm control the bow swings up into the wind. Figure 4 shows how the forces are aligned at this stage.

To reduce the CE's tendency to move aft, a quick mainsheet trimmer can ease out the sheet, which moves the CE forward again. Figure 5 shows how the CE moves forward once again and the boat comes under control again. Quite often this movement is enough to enable the helmsman to regain control. If the trend is toward consistently stronger puffs, then a mainsail reef should be taken in. Not only will this reduce the tendency toward broaching, but it will also help the boat stand more upright and sail faster.

A downwind broach

The forces acting on a boat sailing downwind are slightly different to those acting on a boat sailing upwind. Figure 6 shows how downwind forces are aligned. When a boat broaches downwind the broach is often preceded by a rolling motion. The roll moves the sail's CE away from the hull's CLR. The helmsman attempts to correct this movement, but quite often lags behind the roll period. The slight lag tends to emphasize the rolling moment until a couple is formed. Depending upon the direction of roll, this couple will force the boat into a turn. While the couple is mild, the helmsman can control the boat by use of the rudder. But as soon as the forces have become high enough to require additional use of the rudder, control is lost because the rudder stalls. At that point everyone hangs on as the roll turns into a broach.

How to avoid broaching

1. Carry enough sail, but not too much.

2. Make sure the mainsail is reefed in heavier winds to reduce its effect when sailing upwind.

3. Have somebody ready on the mainsheet when sailing in gusty conditions.

4. Make sure the sheets get thrown off or eased out quickly as soon as the helmsman has more than one turn of wheel on.

5. When sailing downwind, reduce sail in a prudent manner.

6. Set your flattest spinnaker in heavier winds and sheet it well forward. Aft sheeting will help pull the stern around.

7. Put your best helmsmen on the wheel for short periods of about half an hour each.

8. Keep all the crew weight aft when going downwind to keep the rudder and stern immersed.

9. Don't cleat the sheets and guys.

10. Ease the spinnaker sheet out as a gust hits and the boat heels. As soon as the gust has passed, trim the sheet in again to keep power on.

If your boat broaches, here's what you can do to reduce or eliminate the tendency.

1. Fit a larger rudder.

2. Cut down the length of the main boom.

3. If the boat has a bustle, you might want to have the boatyard fill in the bustle to provide smooth flow over the rudder blade.

4. Move the mast forward slightly to reduce the tendency toward weather helm.

Next month we will look at the causes and cures of broaching in powerboats.