Read an Excerpt
ADLARD COLES' HEAVY WEATHER SAILING
By PETER BRUCE
The McGraw-Hill Companies, Inc.Copyright © 1991K. Adlard Coles
All rights reserved.
Yacht design and construction for heavy weather
HEAVY WEATHER HAS TAKEN ITS TOLL among vessels of all the shapes and sizes that one can imagine, and the survivors have been just as varied. Is it worthwhile, then, to consider the design characteristics of yachts that should best survive the worst that weather can offer; or is handling the only factor? It seems clear that the boat counts too, and could be decisive, although once at sea, the action of the crew is what counts.
To declare the obvious, to survive means to stay afloat, to keep water out of the hull; and further to remain in, or at worst return to, an upright condition. Strength and range of positive stability are first requirements. In the course of this study we shall try to determine how these essentials can best be refined and combined with other characteristics to provide for the safety and comfort of the offshore crew.
A lifetime around the water has shown me the many types of yacht that have come through the extremes of weather on long cruises. I think it must have been in 1926 when my brother Rod and I spotted Harry Pigeon's Islander lying in New Rochelle harbour, not far from our home. We were quick to greet him from a borrowed dinghy and to take him for a tour of the nearby countryside after inspecting the home-built 10.4m (34ft) yawl that he had sailed singlehanded around the world. Neither Islander's light displacement nor simple vee bottom form made for survival difficulties. One was most impressed by the simplicity of the construction and equipment: no engine or electrics of any kind, no speedometer, or even a patent log. We admired the man who made it all seem so easy. Soon we heard that Alain Gerbault and his Firebird were at City Island, so we went there. The contrast was in every way disappointing, but the older, heavier boat had made it through some very bad weather.
I had, and retain, a great deal of respect for the work of Dr Claud Worth, the owner, during the 1920s, of several yachts called Tern. He must have been a thorough and meticulous student, as well as a practitioner, of offshore sailing. He advocated moderate beam, plenty of displacement and a long keel. I read and re-read his books: Yacht Cruising and Yacht Navigation and Voyaging.
This background, reinforced over the years, had led me to believe that size and shape can vary widely though I like to avoid extremes. If structure and handling are sound, then the larger the better but the bigger vessel demands more of the builder and crew as the loads increase geometrically with size. Big sails supported by great stability require strength and skill to control; small sails can be manhandled. Similar observations apply to hull, spars and rigging.
Analytical studies, such as those carried out in the course of the joint United States Yacht Racing Union (USYRU) and the Society of Naval Architects and Marine Engineers (SNAME) study on Safety from Capsizing, and by the Wolfson Unit of the University of Southampton, have noted two characteristic conditions of capsize that have a bearing on design, ie those that occur due to the force of the wind on the rig, and those resulting from the jet-like force of a breaking sea. In the first condition the light structure of a small boat may not be overloaded, but in the second the hull or deck may be smashed, destroying its ability to float like a bottle.
As the terms are often used, size and displacement mean about the same thing, although the terms 'light' or 'heavy' displacement usually refer to the displacement/length ratio, expressed as tons of displacement divided by the cube of 1 per cent of waterline length in feet. Though extreme, one can accept a range of 500 to 50 in that ratio over a range of 6–24m (20-80ft) in waterline length. In geometrically similar hulls the righting moments increase in proportion to the fourth power of length, while the heeling moment grows only as length cubed. Because of this, small boat designs need more inherent power, ie beam and displacement, while big yachts with similar proportions need very large rigs. Thus smaller boats should avoid the bottom of this range and the larger boats should avoid the top. Figs 1.1 and 1.2 show a possibly over-liberal suggested range. I say 'possibly over-liberal' with the personal feeling that the area just below the middle of the suggested range, say 125 to 250 in the same length range, is best of all.
Displacement is determined primarily by the requirements of strength and stability and, further, for comfort in the sense of motion and of roominess. The yacht's total weight must provide for an adequate structure together with the weight of crew, stores and equipment, and for sufficient ballast to ensure stability for sail carrying power and a good range of positive stability. Truly efficient use of the best materials can give a light hull and rig and, with enough ballast, appropriate design can provide stability, adding up to a lower limit on safe displacement. Though structural materials are not the subjects of this study it should be said that, in the hands of a competent builder, sound light hulls can be built of many different materials including wood, GRP and aluminium alloy. The high-strength, high-modulus materials often used as composites, such as carbon fibre, offer, when used with care and experience, strength and light weight. Steel and concrete are inherently heavier, especially the latter. Boats with light displacement must have light hulls so as to carry a reasonable amount of ballast for the sake of stability. In heavier boats material selection is less critical.
The vertical centre of gravity and hull geometry combine to establish the range of positive stability. A good range, for illustration's sake, over 120° at least, will virtually assure that a capsized boat will right herself in conditions that have caused a capsize. Much lower values may ensure the reverse. The determination of range depends on a calculation that may be more or less complete in its application to deck structures, cockpits and allowance for some flooding. It also depends on the existing centre of gravity as affected by sails such as those that are roller furled. This suggests the need for some allowance over a stated minimum. It seems unfortunate that racing influences, earlier on the IOR, and still, although to a lesser degree, the IMS, have led to wide beam, and a shoal body: the conditions for a poor range. At least the IMS rule favours hulls with a low centre of gravity, a better situation than the IOR which encouraged an unwholesomely high centre of gravity. Ballasted narrow, deep hulls of the older International Rule type like a decked over 12 metre could go to 180°, representing the full 360° rollover. In beam and hull depth moderation is the best course. Beam offers initial stability and roominess, but too much of it reduces the range of positive stability and results in quick motion. Depth provides easier motion, headroom, structural continuity and space for some bilge water; all desirable, but they are less conducive to high speed. Heavy ballast contributes to range of stability, but also gives quick motion.
A moderate ratio of beam to hull depth seems ideal; say, a beam of not more than three to four times the hull body depth, with the centre of gravity low enough to give a positive stability range of at least 130°. Here one may note that Adlard Coles' three Cohoes fall within the range of proportions that I have recommended. Also, that the damage to Vertue XXXV, described in earlier versions of Heavy Weather Sailing, was caused by a breaking sea that threw her over and down, smashing the lee side of her cabin tru
Excerpted from ADLARD COLES' HEAVY WEATHER SAILING by PETER BRUCE. Copyright © 1991 by K. Adlard Coles. 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.