By Tom Neale
In August of 1979, a fleet of 306 boats was hit by a massive, low-pressure system during the Fastnet Race along the south coast of Ireland; 136 people were rescued from 24 boats, 15 people died, and five ballasted sailboats were lost. The 60-plus-knot winds created giant waves that capsized dozens of boats, calling into question the safety of yacht design at the time and launching a massive study including extensive tank testing, the results of which changed the way we look at boat design today. This study emphasized monohull sailboats. Here are some facts learned from the Fastnet tragedy:
- Breaking waves matter. While any large waves relative to the size of the boat can be problematic, breaking waves are always dangerous. Waves break when they become too steep — think surfing waves rolling onto a beach (see Figure 1). At that point, the wave collapses down its front, crashing the entire weight of all that water and the force of its motion onto a boat caught in the wrong place. Breaking waves often occur where large waves enter shoaling water or the wind blows against a strong current, but they can also occur in other situations as when the wave grows too high to support itself, thus toppling over. This can occur even in very deep water. Breaking waves should be avoided if at all possible.
- Size matters. Larger boats can better handle larger waves.
- Direction matters. Boats are very vulnerable to being rolled or capsized when taking waves on the beam. When struck beam on by a breaking wave of a height equal to or greater than 35 percent of the boat's length overall (LOA), every model tested was rolled to 130 degrees or more — well past the horizontal. But if those same waves were taken over the stern or over the bow, most hull forms remained upright.
Learning To Work With Waves
The wave height danger zone would begin at a smaller relative size for an unballasted powerboat. A 24-foot powerboat hit on the beam by an eight-foot breaking wave will almost certainly capsize. In fact, a much smaller wave could put such a boat in danger.
With any boat — power or sail — the first and best tactic is to stay out of large waves, with "large" being relative to the boat's size, shape, power, ballast, and structure. Tactics to avoid large waves include staying in the lee of a windward shore for as long as possible, traveling with wind and current running together, timing the entrance and exit to inlets and rivers so that the current is running with the wind and waves, waiting untill slack tide before navigating strong inlets or rivers, or simply staying in port until conditions improve.
Second, don't take waves on the beam. If possible take them on the bow, or it may sometimes be better to take them directly astern or at an angle to the stern rather than the beam. Usually, when heading into waves, it's better to meet them at an angle off the bow to minimize pounding, hobby horsing, and burying the bow. If taking waves astern, it's extremely important to avoid losing directional control as the wave overtakes you. This may require a high level of seamanship skills. If you must change course, watch the waves carefully; time the move when you see a group of smaller waves or a long trough that you can turn in before the next wave comes.
Third, don't get caught in breaking waves. Breaking waves can occur when the wind is opposing a strong current, when waves are passing over a shallow bar, when they are ricocheting off a shore or rocks, when they reach a height too tall to sustain themselves and when they are leaving deep water and meeting shallow ground. Do everything you can to avoid areas where breaking waves might form.
Beyond these general guidelines, there's no single set of tactics applicable to all situations. Here are some tactics that you might want to experiment with, when it's safe to do so, depending on your boat and situation.
Operating an open boat in waves presents significant flooding issues. A decked boat can often plow through waves, taking green water over the decking and cabin without serious consequences, but that water may quickly swamp an open boat. Even in a well-balanced boat with positive buoyancy, a heavy load of water changes the center of gravity, and the sloshing of the water in rough seas can cause a sudden loss of stability and, potentially, a capsize. Particular care must be taken in an open boat to avoid boarding waves and to quickly bail if it happens.
If you're heading into the seas, one option may be to run at a speed that will elevate the bow just enough so that it goes over each wave. This requires careful attention to the throttle and, in many cases, constant throttle adjustment. Too much throttle can put the boat on plane, which may cause loss of stability. Approaching waves with the bow too high could result in its falling backward, to the side, or being blown out of control. Approaching with the bow too low could result in boarding seas.
The type of bow influences tactics. For example, a flat pram bow (as in a jon boat) will be more likely to pound. But if run into the waves, with the bow elevated at an ideal angle for the sea, it may give you a safer ride under the circumstances than a boat with a sharper bow, which will have a tendency to bury as it meets the wave.
The sharper bow may do better if you meet the sea just off the bow. Instead of the bow parting each wave and possibly plowing under, the waves will meet the boat in a broader area on the boat's forward section, which should allow the bow to rise better. The angle at which you take the waves will depend upon the wave shape and period, as well as what's necessary to maintain a safe heading. You want the bow to be able to rise easily to each wave while keeping the waves well forward of the beam. Careful attention to each approaching wave and each gust of wind is critical. An unexpected gust may catch the bow, particularly if it presents a lot of windage, causing loss of control at the top of a wave.
Running with the sea astern is generally more dangerous than heading into the seas in an open boat (as well as larger-decked boats). If running with the seas, too much or too little throttle at the wrong time could result in plowing under a wave you're overtaking. Inadequate control in any angle of approach could result in the bow sliding or being pushed off the wave, with the possibility of flooding or capsizing. The entire boat could slide down a large enough wave, out of control. Taking a large wave over the transom could flood the boat, but riding a sea that lifts the transom could, in combination with your engine, push the bow under the back of the wave ahead, causing the boat to spin sideways (broach) or even flip stern over bow (pitchpole). Your wake or a following sea could flood the boat if you slow or stop suddenly.
But sometimes putting the seas on your stern is the only way to get home. If you have to do it, constantly glance astern to see what's coming and plan a response to each wave. Tacking by putting the waves on one stern quarter for some distance and then switching to the other stern quarter may be a good approach. This tactic can make it easier to watch the oncoming waves, give you more maneuvering options, and present the transom corner to the oncoming sea, which provides a higher gunwale and sharper wave entry than the flat transom with the low cutout for the outboard. This may also diminish the wave's impact on the flat stern, which can tend to push the bow under and impair control. Do so with care, as running cocked to a stern sea may increase the likelihood of broaching or pitchpoling. Each encounter with waves may require a little experimenting to see where you get the best control and best ride.
Tacking and changing course obviously involve turning. This can be dangerous because it presents the beam or other vulnerable area to the sea, if only for a brief time. Waves travel in sets, with smaller waves coming in for brief periods of time. Study the passing waves to pick the least dangerous time. However, if you're heading into a lee shore or shallow water, start looking for the right wave pattern early so that you'll have plenty of time to safely execute your turn.
Much of what we've said about smaller boats applies to larger boats, whether sail or power, operating in large waves. Larger boats should have sufficient decking to keep boarding seas out, but a boarding sea can still wash down the deck, damaging equipment, washing people overboard, and even crashing through windows. Avoid boarding seas using tactics such as tacking to present the side of the bow or running with bow slightly up.
If handled well, the nimble and more powerful sportfish may do better in breaking seas, as in a bad storm or confused inlet, because the available power/speed bursts can be carefully and skillfully used to keep the boat optimally positioned relative to each wave.
A decked powerboat can make headway in large seas that might swamp an open boat.
But care must be taken to avoid heavy boarding seas, which can wash away equipment or break windows. (Photo: David J. Shuler)
Your engine can help you power through seas that otherwise would knock you back and greatly reduce your overall speed. Throttling down to meet an oncoming sea and then slightly up to power through can maximize efficiency upwind in large seas. The engine can also be used to position the boat to take each wave just off the bow. If running with waves, maintaining a temporary position on the back of a wave or a position well ahead of a break can keep you out of more dangerous parts of the wave. Many times we've surfed our motor sailer into an inlet on the back of a wave, not because we wanted to but because we had no choice. This sort of maneuvering requires great skill, understanding of the waves, familiarity with the boat, close attention, and some luck. Many larger powerboats have broad and tall flat transoms. Given all that surface area, waves astern can slap the stern around or force the bow under the next wave. Boats with rounded sterns may handle following seas better, although they may be more likely to be pooped.
Use Of Other Equipment
It's seldom, if ever, a good idea to use the autopilot when steering in large waves. Although a good unit can, within limits, sense the effect of waves on the boat, it isn't keeping all-around visual watch, doesn't have the sensitivity of a good helmsperson, and can't make the informed decisions and instant power and rudder changes that may be necessary. Using stabilizers on a trawler or sails on a sailboat when traveling distances in open water can greatly increase comfort in large swells, but again, nothing takes the place of vigilance by a person in charge, anticipating the waves.
Whether driven by an outboard or an inboard, a propeller can cavitate or ventilate if it leaves the water or gets into the churnedup surface of the wave, causing excessive and sometimes damaging vibration and sudden loss of power and thus control. Full down tilt of an outboard or sterndrive reduces the risks by keeping the props as far below the water surface as possible. That option is not available with an inboard, and when it cavitates, the engine races wildly and the prop shaft vibrates, perhaps even whipping within its strut and gland. It's usually more likely to occur in following seas; a good way to prevent it is to keep an eye astern for each wave. One that's going to pass under the boat lifting the stern as it does so is likely to cause this. But a boat with powerful engines can often maintain a position in advance of such a sea until it levels out or breaks behind the boat.
Whatever boat you have, give it optimum trim by arranging weight and lowering the center of gravity as much as practical. Use of trim tabs may be helpful but the orientation of the boat changes so frequently that a helpful trim tab setting one moment may be dangerous the next. The extent to which you rely on trim tabs will depend on your ability to quickly adjust them. For example, it may be helpful to use trim tabs (or outboard trim) to keep the bow low when heading into waves if the conditions are likely to cause the bow to go airborne as you pass over a wave. You would want to avoid trimming this way if there's a likelihood of plowing under the seas.
Many of the things we've discussed regarding different hulls and tactics for different conditions apply to sailboats, which have the stabilizing benefit of ballast and a long keel. Sails stabilize the boat's motion, dampen the roll, and maintain forward progress into large seas. Going upwind in large seas, sail just far enough off the wind to keep the sail powered up, If you sheet in too tightly a wave hitting your bow may make you come about unexpectedly causing other problems. Running dead downwind in a large following sea is not necessarily good because of dangers of broaching or pitchpoling, jibing, and twisting of foresails; tacking off the wind may be much safer. Going downwind, sails sheeted in too tightly will tend to turn the boat up into the wind, increasing the likelihood of serious problems. Too much sail sheeted out too far can also render the boat out of control because of combined influence of wave and wind. If you must run downwind in large seas, consider having the motor on, ready to help with thrust and control. Reef early; having too much sail up not only increases the chance of capsize but also the likelihood of dismasting as the boat rises from a deep trough to the top of a large sea and the resulting sudden wind rise snaps the sail. Entering an inlet with an incoming large sea under sail alone is a bad idea; always have the motor on and use it to keep yourself well-positioned in the waves.
No Substitute For Experience
Having a grasp of the tactics that should keep you and your boat out of harm's way is a good start. Now it's time to practice and master your skills, when the weather is breezy and the waves are present but not threatening. Confident, conservative boat handling is part of the challenge and thrill of being on
Tom Neale, a technical and lifestyle writer and liveaboard cruiser, leads our distinguished "Ask The Experts" team.
— Published: June/July 2014
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