Handling Storms at Sea

The 5 Secrets of Heavy Weather SailingBy Hal Roth

The McGraw-Hill Companies, Inc.

Copyright © 2009 Hal Roth
All right reserved.
ISBN: 978-0-07-149648-3

Contents

Chapter One

An Overview

Before we begin, let’s put this storm business in perspective. I know my own experiences best, so let me talk about them. During the past 40 years I’ve sailed some 200,000 miles on the world’s oceans either alone or with my wife. These voyages include eleven trips across the Atlantic, five voyages across the Pacific, and three trips around the world—including two via the Southern Ocean when I sailed to 58° south. I’ve gone around Cape Horn three times, anchored in the outer Aleutian Islands, circled the big island of Newfoundland twice, and had a hard look at the long coast of Labrador.

Impressive, huh? A wonderful or a foolish way to spend one’s life? Yet in all those seagoing passages—some up to 52 days in length—I’ve never seen prolonged winds of hurricane strength and only one violent storm of Force 11.

If I close my eyes and think hard (and refer to my old logbooks), I remember a strong gale (Force 9) in the Gulf of Alaska during the summer of 1968. There was a Force 10 problem in 1970 off the Oregon coast west of the mouth of the Columbia River. In August 1974, while passing through the Strait of Le Maire between Tierra del Fuego and Isla de los Estados near Cape Horn, Margaret and I ran off under bare pole before a severe Force 10 storm from the southwest. On that same run north to Mar del Plata, Argentina, we sailed mostly in gale conditions (Force 8) and flew a storm trysail for seven out of eleven days.

The strongest storm I’ve been in was a sustained Force 10 to 11 wind of 55 knots and extraordinary seas about 1,200 miles south-southwest of Perth, Australia, in the Southern Ocean in January 1991, during one of my solo trips. A month later I ran off to the north in front of a turbulent Force 9 strong gale a little northeast of the southern tip of New Zealand’s South Island.

In March 1992, a strong gale (46 knots from the northwest) stopped me while sailing northward between the Argentine mainland and West Falkland Island. My choice was the scary lee shore of Jason West Cay to the east or being pushed off to the southwest (see Appendix 2 for the log of this account).

More recently, Margaret and I sailed from the east coast of the U.S. to Turkey in the eastern Mediterranean and back. Except for short-lived blasts from local winds at the entrance to the Strait of Gibraltar and in one place in the Aegean for a few hours, we had no winds over 35 knots during the 18,132 miles of the voyage.

My point in this wind recital (six strong storms in 40 years) is that violent weather is infrequent, and that with care in planning, bad days can be avoided or certainly minimized. When a storm does appear, I’ve worked out a system of five steps to follow. We do one thing, and if that doesn’t handle the situation we do the next.

I suggest the following:

Step 1. Deep reefs in the mainsail; a smaller headsail.

Step 2. Heave-to.

Step 3. Lie a-hull.

Step 4. Run off.

Step 5. Employ a parachute sea anchor from the bow or a drogue or drag device from the stern.

The first four actions are what I propose to call onboard control methods, steps that you can take on the boat to keep the sailing under control. Number 5 involves off-boat control methods, which require special equipment and techniques. In the chapters that follow I will do my best to detail these steps, sort the good from the bad, and explain my thinking and reasoning.

Let me speak plainly. Over the years I’ve come to know that many small-boat sailors grossly exaggerate wind speeds. They throw around words like “gales” and “hurricanes” and “50-knot winds” when what they really mean are strong breezes, near gales, passing squalls, and 30-knot winds. The emphasis is always on peak gusts, not the minimums when the wind drops.

Some readers may accuse me of being a mean old man, but I’m tired of reading articles in sailing magazines by writers who endlessly circulate hokum about winds and storms and who seem to be more conversant with a thesaurus than a reefing handle. Some of these writers actually talk of hurricane winds and having sails up in the same paragraph.

I’ve learned that true, sustained wind speed is usually far less than what I imagine, especially if there’s spray flying around, the wind is cold, it’s blowing in my face, and I’m tired. Consider the Weather Channel on television and how often the traveling announcers go to a hurricane site along a beach somewhere and talk about a huge storm when behind them you see an almost flat sea. (“We’re early,” or “The storm’s already gone,” they say.)

In magazine stories (“It was really blowing …”) you can generally halve the reported strength of winds and be closer to the truth. I own a carefully calibrated Swedish wind-measuring device and when I use it, I’m always crestfallen about the true strength of my deck-level readings, even when I correct them for the standard height of 33 feet, or 10 meters, above the water. I’ve learned to judge the speed of the wind by carefully evaluating whitecaps, sea conditions, the heel of the yacht, and how much sail the yacht can stand.

I remember my friend, the veteran sailor Peter Tangvald, laughing about storm stories. He mentioned one account in which the cook passed up a bowl of soup to the helmsman while he was sitting in the cockpit steering in a Force 10 storm.

“In the first place,” said Peter, “with all the rolling, the soup would probably have spilled. Secondly, if the man had managed to hang on to the bowl, the wind would have sucked the soup right out of the bowl. Finally, I think with all the water flying around, the helmsman’s appetite would have been pretty slim. People who write such stuff have never been out in real storms.”

Chapter Two

What Is an Ocean Wave?

It sounds almost biblical to say that in the beginning the sea is calm until the wind arrives. First there are a few ripples and trifling wavelets. Once these have formed, the wind has something to blow against. The tiny waves grow in size, and energy is transferred from the moving air to the water. Then with more wind, the waves grow longer and steeper until the crests commence to break and we watch a choppy sea unfold in front of us.

These surface waves have been formed by mechanical means. If the wind dies, the wave energy will spread out and become longer and smaller in amplitude, all due to dispersion and in part to gravity. Ultimately the waves die as a result of friction in the water and other dissipative effects.

If the wind grows stronger, most of its force is transferred to longer, higher waves, which can absorb more energy, and we see that larger waves are constantly replacing smaller ones. Depending on the wind strength and the span of water over which the wind can blow, this process goes on and on; the small waves disappear into larger ones until the size of the waves suits the force of the wind and we have a condition called a fully developed sea.

This often means that in front of us is an ocean with orderly whitecaps and an endless procession of waves rumbling in lockstep from the horizon. But what is the nature of waves? Are they a real menace for small boats? Or is all this talk overblown and exaggerated? Let’s see what the scientists can tell us.

In 1802, a pioneering investigator in fluid mechanics named Franz Gerstner worked out the first wave theory and found that water particles in a wave move in circles. Although much of Gerstner’s early work has been superseded by modern hydrodynamics, this pioneering researcher described how the particles at the crest of a wave travel with the wave, while the particles in the trough move in the opposite direction.

From his studies, Gerstner learned that although the appearance of waves sweeping ahead on the ocean might suggest that the wind is pushing lots of water, in truth the forward movement of the water is trifling.

If you watch a resting bird or a stick of wood on the surface in a 20-knot wind, for example, you will see that the bird or piece of wood going with the drift hardly moves forward at all. “Waves are only moving forms,” writes the authority Willard Bascom in his book Waves and Beaches.

In deep water, most waves run in a smooth, regular fashion according to laws and formulas that scientists have learned from their studies. These mathematical connections help us understand that waves are packets of energy moving through the water—energy that’s being transferred from one water particle to another.

In later years scientists improved on Gerstner’s early work and built long boxes with glass sides that served as flow channels. The investigators generated waves with an adjustable-speed paddle at one end that at first was operated by hand and later with a variable-speed electric motor. As the researchers examined the relationships between wave period, height, length, and velocity, they learned that all these things were tied together by physical laws that could be expressed by mathematical formulas. Wavelength, for example, varies directly with wave period—the time elapsed between the passage of successive crests past a given point—whereas wave height does not.

Wave velocity varies with wavelength according to the formula:

V = √gL/2π

where V is velocity in feet per second (fps) in deep water,

g is the acceleration due to gravity, 32.2 ft/sec/sec (often written as ft/sec²),

L is the wavelength in feet, and

π is the constant 3.1416.

A formula for wavelength is:

L = 5.12T²

where T is the wave period in seconds.

Turned around, the formula reads:

T = √L/5.12

Thus out in the ocean, a wave with a length of 300 feet between crests will have a velocity of 39 fps (23.1 knots or 26.6 mph) and a period between wave crests of 7.7 seconds. You can easily examine these relationships between the speed, length, and period of waves in deep water in the accompanying graph.

Wave velocity (V) is therefore directly related to wave period (T). Since we know that L = 5.12T² and that V = √gL/2π, by substitution we can work through to V = √164.8 x T²/6.28 or V = 5.12T.

If you’re in the cockpit during a storm and wondering how fast the waves are moving, multiply the period between crests—say, 10 seconds—which you can get by counting or using a stopwatch, by 5.12 (5 is close enough). The answer for a 10-second period is about 50 fps or 34 mph or 291/2 knots. A close-enough approximation is that the wave velocity in knots is about three times its period in seconds. (Full disclosure: There should be a small subtraction from the period for the speed of the yacht; see page 16.)

Looking at the relationships still another way, the wavelength in deep water is roughly equal to the square of the period in seconds multiplied by 5. For example, a period of 10 seconds squared is 100. Five times 100 gives 500 feet, a good approximation of the wavelength.

Breaking Waves

When scientists increased the height of the waves in the flow channel by moving the paddle faster in their big glass tank, they saw that the waves became very steep, collapsed, and formed breaking waves. The researchers found that the waves would break when the wave height exceeded one-seventh of the wavelength. In these circumstances the unstable wave breaks at the top, and masses of broken water cascade down its face.

Deepwater sailors have long described the front of a breaking wave as a waterfall. At sea this process is augmented by strong wind pressure from behind—the force that caused the wave in the first place. A yacht caught in a large breaking wave is liable to be pitched forward, out of control. Then the boat is not only subject to tons of falling water from above, but gravity adds to the force acting on the hull as the boat is dropped or catapulted into the wave trough below. The total forces of such impacts can be enormous.

But I must be careful not to exaggerate. In truth this breaking wave business happens only rarely at sea, because in deep water the crest-to-crest wavelengths tend to be hundreds of feet long and roll along smoothly day after day. Though the waves may be high, their height seldom approaches one-seventh of their length.

Another way of saying this is that a wave becomes unstable when its crest angle begins to fall below 120 degrees. You can readily understand this, because any nonbreaking wave has a rather gentle crest angle and does not threaten to explode and break. If you study the aerial photograph of Bass Strait in truly horrifying wind and sea conditions on page 20, you will see that the first enormous breaking wave to the right of the yacht has a crest angle of perhaps 80 degrees. In other words, the boat is threatened by a waterfall.

In the Force 10 to 11 storm that I mentioned in Chapter 1, I remember counting the time of the passing wave crests over and over while I was steering. The apparent wave period was 10 to 11 seconds, which meant that the apparent wavelengths were on the order of 565 feet. Because of the relative motion between the wave period and the yacht, however, I needed to subtract the distance covered by the boat while I counted. My matchstick on the waves was traveling at about 7.5 knots or 12.7 fps. (To convert knots to feet per second, multiply by 1.689.)

In 10.5 seconds, therefore, the boat would have covered 133 feet. From 565, subtract 133, which gives 432 feet, the actual wavelength. The graph on page 13 tells us that a 432-foot wavelength has a period of 9.5 seconds. In Table 2 on page 15 we see that a Force 10 storm blowing long enough over a wide enough expanse of ocean to create a 9.5-second wave period will have produced a maximum wave height of about 38 feet, which—as the helmsman during that storm—I thought was about right. But that height would have been a little less than one-eleventh of the wavelength—not one-seventh—so the waves weren’t breaking.

My point is that if the wind is steady and is blowing across deep water, breaking waves do not appear to come from the one-seventh rule, but from other causes. To wit: troublesome cross-seas, leftover seas from old storms, an ocean current, a tidal stream, shoals, a salinity imbalance, a large passing ship—anything that upsets the roughly straight-line flow of the waves. Eventually my boat capsized violently because old waves from a different direction mixed with new waves. Finally one of these menacing combinations broke under the yacht and flipped the boat upside down.

If a crest breaks loose from a large wave, the separated water can move faster than the wave itself and in a slightly different direction. The two masses of water may then collide with disastrous results to a nearby vessel.

Three things govern the magnitude of waves caused by wind: the speed of the wind; the length of time that it blows; and the fetch, or distance, over which it blows. When the wind velocity increases, the waves become higher and the periods longer. The amount of energy in larger waves is much greater, because the energy is proportional to the square of the wave height.

Moderate-size waves that exhibit a white breaking crest are not breaking waves, and there’s some misuse of these terms. With wind of any significance, the tops of most regular waves show white breaking crests. We’re all familiar with the whitecaps (or white horses) of a Force 5 fresh breeze of 17 to 21 knots.

Depending on the fetch, the height of Force 5 waves runs from 3.5 to 8 feet (Table 1, page 14). We know that the tops of waves running before a steady 21- knot wind simply crest and tumble at the top. There’s no danger for a seagoing yacht in our size range.

As the wind increases, however, the waves begin to look a bit more alarming. The wavelength—the distance between two adjacent crests—increases as well, and the toppling crests grow in size. By the time we climb the scale to Force 8 (34 to 40 knots) and examine a fetch of, say, 100 miles, with the wind blowing for 13 hours, the significant wave height has grown to almost 21 feet. One of these bundles of energy whooshes past the boat every 6.9 seconds. The most frequent wave height, however, would be about half the significant height, or 10.5 feet; 10% of the waves would be about 25% higher than the significant wave height, or 26 feet.

(Continues…)

Excerpted from Handling Storms at Seaby Hal Roth Copyright © 2009 by Hal Roth. Excerpted by permission of The McGraw-Hill Companies, Inc.. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.