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THE SEAWORTHY Offshore SailboatA GUIDE TO ESSENTIAL FEATURES, GEAR, AND HANDLING
By JOHN VIGOR
McGraw-HillCopyright © 2001 John Vigor
All right reserved.
Most mass-produced sailboats are based on the coastal cruiser philosophy. Their design and construction is governed by the theory that they will not stray far from a safe port and that their owners will seldom want to be at sea for more than a couple of nights. This philosophy calls for a light, fast, stiff, weatherly boat with spacious accommodations, plenty of auxiliary power, and sufficient crewmembers to handle her.
An ocean cruiser, on the other hand, must look after herself and her shorthanded crew in all types of weather for extended periods of time far from land. This calls for sturdier construction, stronger spars and rigging, more stowage, less need for weatherliness, and more need for seakindliness. In short, an oceangoing sailboat needs to be more seaworthy than a coastal cruiser.
That statement would border on the banal were it not for the fact that seaworthiness is poorly understood and difficult to define. If seaworthiness were merely the ability to stay afloat in the worst conditions of wave and weather, then a corked bottle, an empty eggshell, or a scrap piece of plastic foam all would qualify as supremely seaworthy.
Unfortunately, that's not a practical definition for our purposes, although it does illustrate that seaworthiness has more to do with design and construction than with size. It is a fact that a good big boat is more seaworthy than a good small boat, but size alone is not a reliable indication of seaworthiness. Many very small boats, including at least one less than 6 feet (1.83 m) long, have crossed oceans. But without going to that extreme, it is safe to say that boats of 20 feet (6 m) in overall length have proved themselves seaworthy enough to sail around the world.
There are two more important characteristics of a seaworthy boat: the ability, even in extremely heavy weather, to maneuver clear of dangers such as rocks and shorelines; and habitability, the ability to accommodate human beings. And there is a very desirable third characteristic: the ability of a sailboat to right herself quickly from the upside-down position and to continue her voyage.
There is no guarantee that even the largest yachts are immune from capsize, since (according to tests carried out at Southampton University, England) they can be turned turtle by a breaking wave with a height equal to 55 percent of their overall length. Thus, a 35-foot (10.7-m) boat would be capsized through 180 degrees by a 20-foot (6-m) wave, which could be generated by a 40-knot wind blowing for about 40 hours. Even a breaking wave with a height equalling only 35 percent of the boat's length (a 12-foot wave for a 35-footer or a 3.7-meter wave for a 10.7-meter boat) will roll her 130 degrees—from which position she may recover or turn turtle. And a 12-foot (3.7-meter) wave can be generated by a 24-knot wind blowing for 24 hours. Even large ocean liners and tankers have fallen victim to freak waves off the South African Wild Coast, between Durban and East London, where the swift-flowing Agulhas Current rears up in frenzy when confronted by southwesterly gales.
Development of a plunging breaker. Because of the rapid change in the steepness of the wave, a small boat caught between stages 2 and 7 would have little or no chance of avoiding capsize.
It is prudent, in any discussion of seaworthiness, to take it for granted that a boat may be rolled upside down at some stage. The chances of this happening depend on the size of the yacht, where she is sailed, the experience of her crew, and the time of the year. Along the well-used trade wind routes during the recommended times of passage, the likelihood of capsize is extremely small. Around Cape Horn in winter, it is infinitely greater.
A seaworthy boat, therefore, is one that is:
Able to recover from the inverted position without serious damage to her hull, deck, rig, rudder, or interior, and without shipping substantial amounts of water.
Strong enough to look after herself while hove-to or lying ahull.
Seakindly—that is, free of violent, extravagant, jerky rolling and pounding.
Well balanced, docile on the helm, and easily handled under sail at all times.
Agile downwind, to maneuver out of the way of plunging breakers.
Able to beat to windward, or at least hold her ground, in all but the heaviest conditions.
Habitable—able to carry ample crew with good headroom and comfort, plus water and supplies, for extended periods.
Capable of good average speeds on long passages.
No boat can fulfill all these requirements to perfection, since many are mutually exclusive. For instance, the long keel that makes a boat hold her course well also makes her less maneuverable. The widely spread-out sail plan that helps with helm balance also makes her less efficient to windward. Everything in boat design is a matter of trade-offs. One desirable feature must be sacrificed for another. But the most successful designs spring from a kind of mysterious resonance that occurs when sacrifices, judiciously made, add up to a net gain.
The requirement for a boat to be self-righting from a capsize would also disqualify most multihulls, because they are just as stable upside down as they are standing the right way up. Although some multihulls may be able to regain their feet by methods such as flooding one hull or inflating a masthead float, this is often more difficult than it sounds—especially under the conditions likely to cause a capsize. In their favor, it can be said that, without a heavy ballast keel, multihulls will not sink; but they will not be capable of going anywhere. Even if their crews are capable of finding shelter on board, they will be totally dependent on outside help for rescue.
A properly designed and built monohull yacht, however, is capable of righting herself and continuing toward land under her own power, even if that power is a jury rig. Many have done it, some more than once. Again, the likelihood of a multihull's capsizing is small if she is in the right places at the right times: There are many well-documented accounts of catamarans and trimarans weathering prolonged storms without damage.
In the end, it boils down to making choices. Does the lack of heeling and the inherent positive flotation of a multihull compensate for the risk of remaining upside down after a capsize? Many people think so, and who can say they are wrong?
How Design Affects Seaworthiness
The yacht designer's vocabulary is full of moments, righting arms, lines of buoyancy, centers of gravity, and so forth. But simply put, two things counterbalance the overturning force of the sails: beam and keel weight.
Wide beam gives a boat initial stability—it's hard to get her started heeling.
Keel weight gives a boat ultimate stability, the ability to right herself from a 180-degree capsize. Keel weight starts to work only after the boat has begun to heel, and its maximum efficiency occurs when the keel is sticking straight out sideways. The deeper the keel and the farther it sticks out, the more effective it is.
Incidentally, the pressure of the wind in the sails, even the sudden blast of an unexpected squall, is unlikely to cause a 180-degree capsize. The sails spill wind as they become more horizontal, and the rig puts up great resistance to further heeling as it hits the water surface.
Cruising yachts should be designed for minimum stability upside down, in the opinion of British expert John Lacey, former honorary naval architect of the Royal Naval Sailing Association (RNSA). Lacey, a member of the Royal Corps of Naval Constructors, said in the fall 1982 issue of the RNSA Journal that until the disaster of the 1979 Fastnet Race, few people had explored the stability characteristics of yachts sailing on coastal waters beyond a 90-degree knockdown.
But on the night of August 13, 1979, that complacency changed. Sixty-three yachts each experienced at least one knockdown that went substantially farther than 90 degrees. Many did not right themselves quickly and remained upside down for significant periods.
Lacey said the influence of the International Offshore Rule (I0R) had radically changed the shape of yacht hulls by greatly increasing the proportion of beam to length.
"Increase of beam gives great sail carrying power without additional ballast," he pointed out. "It also provides the benefit of greatly increased accommodation in a given length."
But the shape of such a hull made it very stable when inverted: To bring the boat upright again would require about half of the energy needed to capsize the yacht in the first place.
"Since the initial capsize may have been caused by a once-in-a-lifetime freak wave, one could be waiting a long time for a wave big enough to overcome this inverted stability."
By way of contrast, Lacey calculated that a narrower cruising hull with a lower center of gravity, like a Nicholson 32, would require only one-tenth of the capsize energy to recover from a 180-degree capsize.
Beamy, shallow-bodied boats, he said, "may increase the size of the wave needed to initiate capsize, but in the end the sea will still win if the wave is awkward enough. It therefore seems in my opinion that we should tackle the problem from the other end, and design yachts for minimum stability when upside down."
To talk about narrow beam and excess beam is meaningless unless we know what normal or reasonable beam is. For purposes of comparison, it is usually related to length on the waterline (LWL). In ancient times, ships were generally about three times as long as they were broad.
The longer a boat's waterline length, the less beam she needs, proportionately, because a hull's stability, its resistance to being overturned, varies as the fourth power of the waterline length.
In other words, all else being equal, if you double (× 2) the waterline length, the stability increases 16 times (2 × 2 × 2 × 2). At the same time, the heeling moment of wind pressure on the sails varies only as the cube of the waterline length. For example, if you double (× 2) the waterline length, the heeling forces increase only 8 times (2 × 2 × 2) for the same amount of breeze. This is why a boat only a few feet longer than yours is able to carry much more sail with proportionately less beam.
Small sailboats need a waterline-length-to-beam ratio of less than 3 to 1 to stay on their feet in moderate winds. The smaller the boat, the greater the beam, until you end up with a circular coracle. In addition, there is a modern tendency toward beamier boats that is driven by a demand for interior space rather than seaworthiness.
The waterline length-to-beam ratios of three well-known "traditional" deep-sea designs indicate what has proven to be acceptable in practice:
Pearson Triton 28 (designed by Carl Alberg): LWL 20 feet, 6 inches (6.25 m); beam 8 feet, 3 inches (2.51 m) (ratio 2.48 to 1)
Southern Cross 31 (Tom Gillmer): LWL 25 feet (7.62 m); beam 9 feet, 6 inches (2.89 m) (2.63 to 1)
Valiant 40 (Bob Perry): LWL 34 feet (10.36 m); beam 12 feet, 4 inches (3.76 m) (2.76 to 1)
While the amount of beam is an important element of a deep-sea design, it is not the be-all and end-all of seaworthiness. Its role is modified by other design elements so that a range of ratios becomes acceptable for any one waterline length. That is why beamier coastal cruisers—such as Frank Butler's Catalina 27, which has a ratio of 2.46 to 1 on a waterline length of 21 feet, 9 inches (6.63 m)—are still capable of sailing around the world after suitable modification.
The usual cause of a 180-degree capsize is wave action. If the boat continues to turn upside down, the ballast keel's efficiency (what the designers call its righting moment) tapers off until it is sticking straight up in the air, at which point it has no righting effect whatsoever.
If, however, your boat's beam is reasonably narrow and your keel is quite deep, any oncoming wave at this stage will tend to tilt the boat. The keel will then gain some righting moment: It will want to fall into the water and lever the boat upright again.
But if your boat has a very wide beam, oncoming waves will have to tilt the inverted hull much further before the keel exerts enough righting moment.
You may have seen some news footage of singlehanded Vendée Globe 60-footers drifting upside down in the stormy Southern Ocean with their keels sticking up in the air—and seeming to stay that way permanently. Such boats have an excess of beam that gives them the ability to plane at high speeds; it also provides enormous resistance to a capsize from wind forces alone. But this property makes them inherently unseaworthy, if you count in the possibility of capsize by wave action. Extreme designs like these sacrifice ultimate stability to speed, and they rely on the skill of their crews to keep them upright. But singlehanded crews have to sleep now and then, no matter how skilled they are, and they can't watch out for every rogue wave.
There are, naturally, other factors besides beam and keel weight that affect seaworthiness. Before we assess your boat's seaworthiness and determine what she might need for an ocean crossing, we must look at these other factors briefly. Here, expressed in nontechnical terms, are definitions of some of the most important elements of yacht design and how they affect performance and seaworthiness.
Gives greater initial stability.
Gives more interior room.
Gives more deck space.
Contributes to a jerky, tiresome movement at sea.
Enables a boat to carry more sail with a shallower draft.
Gives a wider, safer base for mast-support shrouds.
If carried down low into the water, slows the boat down by making bigger waves and offering more resistance to the water.
Excess beam makes it more difficult to recover from a 180-degree capsize before the boat fills with water.
Gives less initial stability.
Slips through the water more easily.
Contributes to a slower, easier motion at sea, but a greater range of roll downwind.
Usually means a deeper hull is required for the same volume of accommodations. Often means more heeling for the same windspeed.
Cramps decks and accommodations below.
Provides a narrower, less efficient base for shrouds.
Results in quicker, more positive recovery from a full capsize.
A cabin trunk, or coach roof, provides light, ventilation, and standing headroom in the cabin.
A wide cabin trunk restricts the width of side decks and makes access to the foredeck more difficult.
A high cabin trunk creates wind resistance and detracts from windward performance.
A cockpit provides shelter and seating for the deck crew. While it is pleasant to be able to stretch out full-length on a cockpit seat on a hot tropical night, a cockpit of that length can be dangerous. The less water the cockpit contains after a wave sweeps over the stern, the safer it will be.
It must be self-draining through at least two drains that are each a minimum of ½ inches (38 mm) in diameter, and all hatches must be dogged and made leakproof. How fast should the cockpit drain? As fast as possible—preferably before the next wave sweeps over the lowered stern.
The larger the volume of the cockpit in relation to the displacement weight of the boat, the faster it needs to drain. That's why some true cruising boats have very small cockpits—often no more than footwells. The problem then is that they provide little shelter for the crew from wind and waves.
A strong bridge deck at the height of the cockpit seats should separate the cockpit from the accommodations below, otherwise disastrous amounts of water will surge into the cabin after a pooping.
High freeboard provides a greater range of stability.
Too much freeboard adversely affects sailing ability, particularly to windward.
High freeboard means a lower cabin trunk for the same headroom.
Flare, the outward projection of the topsides from the waterline to the deck, promotes drier decks and provides increasing buoyancy as the boat heels.
Tumblehome, the inward inclination of the upper topsides, gives the hull great longitudinal stiffness.
Hatch sizes should be kept to a minimum. The absolute minimum size for a person to get through is 22 × 22 inches (560 × 560 mm).
The normal size for an access hatch is 24 × 24 inches (610 × 610 mm).
Large hatch openings weaken the deck or cabintop structure, unless properly braced, and invite disaster if the cover is broken or lost in a storm.
The washboards, or dropboards, that seal off the companionway beneath the main sliding hatch must have some arrangement to lock them in place at sea. Otherwise, they can be lost in a capsize.
Hard bilges (sharply rounded edges where the keel joins the hull) contribute to initial stability, or form stability.
Slacker (or softer) bilges have less wetted area to cause friction and resistance, make for less jerky motion at sea, and provide more headroom and more stowage space for the same beam.
Excerpted from THE SEAWORTHY Offshore Sailboat by JOHN VIGOR Copyright © 2001 by John Vigor. Excerpted by permission of McGraw-Hill. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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