We were at the end of a long week of teaching cruising sailors how to race, sailing upwind in a gusty northerly with a very relaxed crew sitting on the rail, legs out, telling jokes. They thought they had learned all they needed to know, but there was one more important lesson coming their way. The helmsman was the only person in the cockpit. I was making like a cool instructor, standing behind the helm holding the backstay.

"Puff coming," I called, expecting a crewman to uncleat the mainsheet ready to ease it when the puff hit. Nobody moved. "Puff coming," I shouted, much louder this time. Again, nobody moved. Do I uncleat the main myself? Nah. Let them learn.

The puff hit. The helmsman yelled, "I can't hold it!" The crew suddenly realized what was happening, but by then it was too late. The boat spun into a broach as the wind overpowered the boat's stability. All I saw of the crew was their feet pointing skywards. I leaned forward and popped the mainsheet, and the boat righted itself. The next time I called "puff coming" five guys leapt into action.

RALPH DEEDSAttack Mars broaches during an unexpected squall that hit boats during the first race of the 1997 Express 27 Nations on San Francisco Bay in July 2007.After we docked we had a session on why broaches occur. It's really very easy to understand. A sailboat is acted upon by the wind. To counter the forces generated by the wind, the hull, keel and rudder, generate lift and drag. If the wind force suddenly increases, as in a puff, the hull forces must be brought back into balance by turning the rudder. If the rudder force isn't big enough, the boat abruptly spins around, sometime facing the way it came.

But one thing many people don't realize is that broaches can occur not only on a sailboat, either upwind or downwind; they can also occur on a powerboat. That can be even scarier when you realize that you have no control and are looking at green water through the cabin's windows.

BROACHING A SAILBOAT

You've never really broached until you've experience it in a yacht 80 feet in length or larger. Only then do you realize the stupendous forces that can push a powerful boat on its side. And it will scare the bejesus out of you.

There are two situations that are likely to cause a sailboat to broach. One is when the boat is sailing upwind and the sail forces overpower the ability of the rudder to control the boat's direction. The other is when the boat is sailing downwind under spinnaker and the sail starts to oscillate, again causing the sail forces to overpower the rudder's control.

When a boat is sailing to windward, aerodynamic and hydrodynamic forces are created on the sails and the hull. As long as these forces are in equilibrium, the boat sails along nicely. But suppose there's an increase in wind speed. The aerodynamic forces increase, but there is no automatic change in the hydrodynamic forces to counter this increase. In order to rebalance the forces, the helmsman must turn the rudder until everything comes back into equilibrium.

RALPH DEEDSTaz!! is taken down by winds on the San Francisco Bay.Now what if you get hit by a big gust of wind. The increase in wind speed heels the boat far over, requiring a larger rudder angle. But the rudder is already hard over, so the only thing to do is to decrease the aerodynamic forces by easing the sails, usually the mainsail. If there is no one to do this, the boat will spin out of control. Literally spin, I've seen boats turn 180 degrees. I've also seen boats tip on their ear, and on one boat where I was holding the stanchion, I was basically airborne.

How do you overcome this type of broach? It's simple: have one crewmember hold the mainsheet and ease the traveler first. By easing the traveler, the mainsail will slide to leeward with its shape staying the same. As soon as you reset the mainsail after the gust, it will be set up properly. If easing the traveler isn't enough, ease the mainsheet too. This will change the mainsail's shape, requiring that you retrim it in order to get the horsepower back.

Downwind broaching generally happens when the boat is flying a spinnaker and the sail starts to oscillate. The oscillation of the spinnaker causes the boat to roll from side to side. Often, as the boat rolls, the spinnaker pole goes in the water on one side and the boom plows a watery furrow on the other side. As the boat rolls, the sail forces move away from acting directly above the hull forces. This creates what's known as a "couple," two forces spaced well apart from each other acting in opposite directions, which causes the boat to turn. If the turning moment becomes large enough to overcome the rudder forces, the boat will broach.

How do you stop this kind of broach? As a veteran offshore sailor once told me, "Keep the boat under the sail, man!" This is often more difficult than it seems. It requires foresight on the part of the helmsman to predict where the bow of the boat is likely to be before it actually gets there. In other words, the helmsmen must lead the boat, not simply react to its movements.

If you wait until the sail forces have acted, and then try to stop the broach, it is unlikely that the rudder can exert a large enough force. In fact, by acting too late you can increase the turning forces and end up contributing to the broach. To stop a broach, the helmsman must act early and decisively to increase the rudder force while the turning moment is still low.

RALPH DEEDSA crew was hoisted up the mast of New Wave to retrieve a halyard in 20-plus knot of winds when this broach occured.The crew can also do their part to stop a broach. They can sheet the spinnaker farther forward at the widest part of the boat and pull the pole and sheet farther aft to make the spinnaker flatter. This tends to reduce the oscillations. The crew can also be attentive to mainsail and spinnaker trim so that the sails can be eased even before a broach starts.

The onset of a broach can be reduced or eliminated if you are prepared for the conditions that contribute to broaching and stay attentive to your sails. Avoid becoming overcanvased, and always sail within your experience level. A broach can easily be stopped if you understand the forces involved in creating one. But to continue going fast when racing, you may have to sail on the edge of broaching. This takes nerve and skill to do.

BROACHING A POWERBOAT

Several years ago I was returning to Newport aboard a large, wooden powerboat. We were heading north with the typical Newport southerly blowing up our stern at about 20 knots. I was standing next to the helmsman. Suddenly the boat surged down the back of a wave, buried the bow, and then rolled to port. We heeled over so far that the lee rail buried. Two guests in the cabin slid to leeward. A third was left hanging onto a handrail. Drinks and plates of hors d'oeuvres crashed to the leeward side. We had broached in the classic sense of the word.

When we recovered and cleaned up the mess, I learned that the boat was prone, thanks to its shape, to broaching in heavy seas when going downwind. The vessel had a shallow forefoot, a narrow transom with a small skeg, very little lateral surface, small rudders, and a single prop.

So what exactly causes a powerboat to broach? In the fourth edition of High Speed Small Craft, author Peter Du Cane says that a boat travelling downwind in a strong sea is liable to broach if a series of conditions are met.

The boat must be travelling at, or slightly faster, than the speed of the wave train. As such, the boat will spend a relatively large amount of time on the wave crest and on the wave front. As it travels down the front of the wave, the bow goes down and speed increases due to gravity. By the time the boat hits the bottom of the wave trough, it buries the bow in the wave ahead, which slows the boat. The following wave slams into the boat's transom and causes it to turn, creating a "couple," or turning moment, that starts the broach.

RALPH DEEDSStrega succumbs to the wind at the 2008 Express 27 Nationals on the San Francisco Bay.

The boat must also lack lateral profile. In effect, it must have a small skeg, or no skeg at all, and a rounded hull form, such as a rounded chine. With no lateral profile a broach can develop.

In addition, the boat must have small or largely ineffective rudders. Small rudders limit the helm's amount of control.

A powerboat broach can be prevented by slowing down, first and foremost. Make sure the boat is traveling slower than the speed of the wave crests. It might take you longer to get home, but you won't have that knot in the pit of your stomach every time a wave comes up astern.

Also, look for a boat with a lot of bow flare to keep the bow from digging too far into a wave. At the same time, it is beneficial to have a chine hull, large rudders, and a transom that is not excessively wide or likely to be pushed around in a following sea. If you have to get a round-bilge hull, make sure there is a large, effective skeg. A skeg tends to give a boat longitudinal stability that helps prevent broaching.

No matter the boat, you can likely avoid a broach if you recognize what's happening – that means understanding exactly what causes a boat to broach in the first place.