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WEIGHT FOR WEIGHT Hugo du Plessis (The following article first appeared in somewhat condensed form in the Royal Cruising Club's journal, Roving Commissions. However it seemed of such obvious relevance to OCC members that I asked Hugo for permission to repeat it in Flying Fish. As always, not every member will agree with Hugo's views, but they are certainly food for thought.) When preparing for a long cruise a boat becomes like those old fashioned coal holes in a London pavement. You keep pouring in car load after car load of stores, equipment and possessions until the designed waterline is just history. Except for racing freaks, painting a new waterline inches higher is a price owners are willing to pay for the joys of blue water cruising. With a sturdy old boat this does not matter, but boats are no longer built like that and overloading can be a serious and seldom considered danger. Extra weight has a greater effect on the performance of a modern lightweight cruiser/racer, but far more important is whether the boat is strong enough to carry the weight necessary to cross the ocean at all. The advice that one should not go to sea in anything but a strong, heavy boat purpose-built for ocean cruising - a Colin Archer, Saorse or Wanderer, has gone by the board. This is largely because it is difficult to find such a design among modern yachts and beyond most ocean dreamers' generally slender resources to have one specially built. Most who cross the ocean today do so in whatever boat they already possess, a boat often bought for a very different purpose, round-the-buoy racing, weekend sailing or short holiday cruises. That too is the market for which the popular fast, lightweight, fun-to-sail cruiser/racer is built. It does not mean the type is not seaworthy, but being technically seaworthy for the open ocean and being able to carry enough stores and water to cross without being dangerously overloaded are two very different aspects. Factors of safety Under the twin competitive pressures of greater speed at lower cost, yachts have become lighter and flimsier. Sophisticated construction backed by computerized design has narrowed the parameters and lowered factors of safety. Admittedly these were conservative, based on generations of experience by proper seafarers of the worst the sea could do, summer and winter. Now they are often ridiculed by summer sailors on the grounds that they make boats too heavy, too slow. It is convenient that decreasing margins of safety also reduces cost. Factors of safety mean just that. They are usually taken to be a margin on the right side to allow for errors in manufacture plus storm conditions or moderate damage. But with GRP there must also be allowance for the natural slow loss of strength due to water absorption, stress relief, ageing and other effects, let alone fatigue and overstress due to hard use. What is seldom mentioned (it would be lethal for business) is that on many fashionable cruiser/racers these factors of safety have already been pared away to such an extent that they can be exhausted after about ten years. Beyond that the boat is on borrowed time. Effect of overloading The merits and demerits of light and heavy displacement have been discussed endlessly. However there is one very important aspect which is seldom considered but which has for some years been worrying me - the danger of catastrophic structural failure due to overloading a light boat not designed for it. Certainly some good strong cruisers are still built, but it is not those boats I am talking about. I am worrying about the many lightly built cruiser/racers, race bred and more racer than cruiser, built down to a price and with `marketable performance'. Nothing steals speed like weight. In particular designers scream `Keep weight out of the ends', (not because the boat is not strong enough but to reduce pitching which slows the boat). Yet one of the hard facts about ocean cruising is where else do you put those tons of extra weight? You need the middle for living. Inevitably there will be a lot of weight in parts of the boat not designed to take it. To an ocean sailor, any empty space is somewhere crying out to be filled. Tons of extra weight Tons? Oh very much so. On an ocean crossing the boat has to be entirely self-contained for weeks, providing not only basic life support for the crew but comfort and pleasures as well, plus everything necessary to keep a fairly elaborate and complicated machine working smoothly and safely until it reaches its destination. One may hope for a twenty day tradewind Atlantic crossing, but only a fool or fanatical racing skipper prepared to accept the risks would not allow for a good deal longer. Damage, bad weather, head winds or calms can dangerously extend the most careful schedule. Not long ago thirty days was usual - nowadays anything more than twenty seems to be regarded with contempt. Even so, the minimum calculation for an Atlantic crossing should still be thirty days on full rations plus a generous reserve. Moreover, on the return journey eastwards the legs are longer, slower and much less predictable. The recommended allowance of water is one gallon per person per day, but that may include showers which, on a small boat, are luxuries at sea. By careful conservation, plus extensive use of seawater, many crews manage on half a gallon, but that is about the minimum. In either case there needs to be a substantial reserve. Too many production yachts around the 30ft mark are built with only a 20, or perhaps 30, gallon tank. With insurance companies insisting on a minimum crew of three, and some on four, where does an extra 50 or 100 gallons go? The problem becomes even more acute with a 25ft boat, and it is with these smaller sizes that the dangers of overloading are most serious. Many experienced cruising people claim a yacht under 30ft cannot safely carry enough water and stores for a crew of more than two. A gallon of water weights 10 lbs, so that means 300 lbs per person. Food and drink (only really ruthless skippers include beer in the water ration) will weigh about the same. Like water, there must be a reserve. If water runs short there is the possibility of catching rain, but if you run out of food there are no supermarkets at sea. Iron rations may sound the epitome of adventure, but most crews would prefer to fight catastrophe on a full stomach. On a typical Atlantic circuit the crew will be away for about a year, more if intending to go further. That means far more personal possessions than for weekend sailing. The dream may be to need no clothes, but you have to get there and back through waters even colder and wetter than at home, perhaps leaving late in the season. Also you still have to look respectable at times even in a warmer climate. Entertainment, from a stack of paperbacks for mid-ocean watchkeeping to television and video, plus innumerable other possessions and a mounting weight of souvenirs, add yet more weight. A lot of extra equipment will be needed - sails, awnings, electronics, radio, perhaps a refrigerator - and more and heavier anchors strong enough to ride out a storm or even hurricane, an important addition if normally a marina mouse. As many books and charts are needed to navigate a small yacht across the ocean as a supertanker. Finally the boat must carry spares and tools to be virtually self-contained for a year or more of continual hard use in a potentially destructive climate. Apart from the obvious fact that there are no chandleries or boatyards in mid-ocean, there are precious few either when you get the other side. The cost of sending spares is high, the hassle with customs formidable and the delays long and critical. In practical terms this means an absolute minimum of a ton of extra weight has to be stuffed down that coal hole and one 1 1/2 or 2 tons is a more realistic minimum. Table I gives suggested figures. They will vary greatly from boat to boat and even more from owner to owner, so a margin of +/- 25 % is shown also. They may well be an understatement, especially the + side. You may not agree, even think they are absurd. All the same, before dismissing them as unrealistic work them out for yourself - and do not overlook all the extras and toys that have been stuffed into the toy chest already. Table I Weight for 30 days at sea Assumed size of boat 25ft 30ft 35ft Persons aboard 2 3 4 ------------------------------------------------------------------ Water (1 gallon/day each) 600 lbs 900 lbs 1200 lbs Food and drink (10 lbs/day each) 600 lbs 900 lbs 1200 lbs 25 % reserve of food and water 300 lbs 450 lbs 600 lbs Spares and tools 200 lbs 400 lbs 500 lbs Extra equipment/sails/anchors 200 lbs 400 lbs 600 lbs Dinghy/outboard 100 lbs 200 lbs 200 lbs Personal possessions (50 lbs each) 100 lbs 150 lbs 200 lbs Fuel 100 lbs 200 lbs 500 lbs Miscellaneous 200 lbs 500 lbs 1000 lbs ------------------------------------------------------------------ Total 2400 lbs 4050 lbs 6000 lbs ------------------------------------------------------------------ - 25 % 1800 lbs 3000 lbs 4500 lbs + 25 % 3000 lbs 5000 lbs 7500 lbs Minimum displacement of yacht for load not to exceed 25 % 9600 lbs 12000 lbs 18000 lbs (4.3 tons) (5.4 tons) (8 tons) Modern lightweight cruiser/racers are just not designed to carry weights like that. The lighter and more race-bred the design the less it is safe to load it down and the greater the effect of the extra weight. Not only may this make the boat structurally dangerous, but unseaworthy as well. Effect on performance Table II gives the quoted displacement of a number of popular designs, followed by 25 % of that figure - the realistic safe load. This shows that in the critical smaller sizes a well-built older boat like a Vertue, SCOD or Twister can carry a heavy load safely, as well as being more strongly built in the first place, whereas a lightly built, high performance cruiser/racer will be heavily overloaded, even at the minimum weight for two people. The load could be greater than the yacht's own displacement. The same applies to larger yachts where there will be less limitation on stowage and less incentive to be ruthless about weight. Also, bigger crew may well be aboard. The 35ft Oceanis can carry a smaller safe load than the Nicholson 31, and little more than the 25ft Vertue. Table II Change in performance with an extra 3000 lbs displacement Safe % performance change Class LWL Displacement load SA/disp disp/LWL --------------------------------------------------------------------- First 235 21ft 8in 2800 lbs 700 lbs 35 % 51 % Spring 24 22ft 9in 4500 lbs 1125 lbs 26 % 40 % Vertue 21ft 6in 10080 lbs 2500 lbs 16 % 23 % Nicholson 31 24ft 2in 14785lbs 3700 lbs 11 % 20 % Etap 35 28ft 6in 11460 lbs 2860 lbs 14 % 20 % Rival 36 27ft 2in 14250 lbs 3560 lbs 11 % 17 % Bavaria 32 32ft 3in 8158lbs 2040 lbs 15 % 26 % Moody 376 31ft 3in 16250 lbs 4060 lbs 10 % 15 % Oceanis 370 34ft 9in 11220 lbs 2800 lbs 15 % 21 % SA/disp (sail area/displacement) is the `power' factor Disp/LWL (displacement/LWL) the `performance' factor Note the effect on performance of extra weight, based on the sail area/displacement and displacement/LWL formulae used in Practical Boat Owner boat reviews. It is assumed in each case that the boats are carrying the same extra load of 3000 lbs, though admittedly this is unrealistic as small boats would inevitably be carrying less and larger ones more. Nevertheless, it does show how weight kills performance on a light cruiser/racer compared with a heavy displacement cruiser. It will also affect seaworthiness. There have been many claims for the dynamic safety of modern lightweight boats - how they can use their speed to sail out of trouble. However this only applies when sailing at their designed displacement, not when overloaded. So despite the insurance companies' concern, it may be safer to sail a small boat singlehanded, or with a couple, than heavily laden with all the essentials plus adequate reserve necessary for the larger crew often required by the insurers `in the interests of safety'. It is instructive to calculate the reduction in power and performance figures given in the PBO new boat reviews when the displacement has been increased by an extra ton or two, representing the ocean going condition. With the aid of a calculator it is quite easy to work it out for other designs too. Remember also that quoted figures for displacement are largely academic. They are the designer's figure before a single boat was built, but once into the brochure they stick. Few builders ever check the displacement of a completed boat, or alter the brochure if they do. Manufacturing tolerances alone can be +/- 10 %, and most designs get modified during production. Neither is there any standard basis for the quoted displacement, and what it includes. Normal seagoing equipment will increase displacement. So will added gadgets. The lighter the boat the greater the relative increase. In years of surveying I have seldom found a yacht floating above her waterline, but any number floating below, sometimes the entire class. Yet as these figures show, although size is the usual criterion in choice of yacht, the important factor is displacement. When buying a boat with ocean cruising in mind, consider carefully what weight you will want to carry. Then double it to get closer to what will actually go on board. If it is more than 25 % of the designed displacement you may want to think again about that particular boat. Effectively this puts the minimum displacement for ocean cruising with a crew of two at around 4 to 5 tons. Note that this is Displacement Tonnage, not Registered or Thames, which are based on volume. If weight carrying is important, such as for liveaboards who want comfort and habitability, the choice may be between the limited speed and sailing performance of a heavy displacement small cruiser or the expense, both capital and running, of a much larger yacht of similar weight. Would you do better with a larger secondhand boat than a smaller new one? For the same money you can buy a lot bigger boat. Or to put it more bluntly a lot more suitable boat to go dicing with the ocean. Also the more weight the boat can carry the greater the comfort and habitability, not only at sea but in harbour which is where all but ruthless record breakers spend more than half their time. Catamarans have vast space for their size and make excellent mobile homes but are not good weight carriers. There is no ballast to offset extra weight, the shape does not have the strength of an ordinary boat and in many cases the design and construction are weak. Often there is emphasis on speed, but when overloaded they tend to wallow and may become unseaworthy. What effect is this going to have? I suppose most of us have fears as we lie awake on a dirty night while the boat crashes and thumps to windward. How can she stand such a hammering? How can it all hold together? How indeed, when every time the bow lifts it has to raise that heavy extra weight and when it crashes down there is that extra force behind it - forces for which the boat may not have been designed. In a race around the buoys a boat will be driven hard. The bow lifts and falls about every two seconds - say one thousand times. But on an ocean passage it may go on for days. The classic tradewind passage is a glorious run, but before getting down to the tropics there can be a thousand miles or more of rough, closehauled sailing. The number of times that bow lifts and crashes will get towards a million. As part of the preparation for ocean cruising in a light boat add extra stiffeners, especially in the forward sections. So as well as overstrain from the substantial extra weight, fatigue will become an important factor. The result of sailing closehauled for days on end when heavily overloaded would be progressive weakness, aggravated by the occasional overstress from extra heavy thumps. It is a feature of GRP that it fails progressively, starting at a comparatively low level and finally breaking at well below the theoretical strength. Eventual failure would be sudden and catastrophic. There should be warning in the form of movement and noise, probably building up over a long time, but this could be overlooked, or considered normal and discounted. I believe this may already have happened in some cases. Sudden failure, especially at night, could easily be put down to collision, perhaps with submerged debris. When a boat is sinking fast there is no time for a carefully considered analysis of what may have happened. What is the answer? Unless a racing fanatic prepared to accept the risks, do not go ocean cruising in the modern lightweight cruiser/racer, especially those sold with an emphasis on speed, unless she is large enough to have ample displacement for the loads likely to be carried. Remember also that there is a very important difference between a heavily built, seaworthy cruiser designed to sail comparatively slowly and a light race-bred boat slowed by being overloaded. In very general terms older boats were more strongly built. If she has not fallen to pieces in the first ten years she is not likely to in the next, and even if she is going to the signs ought to be fairly obvious. Fibreglass deteriorates more rapidly in tropical waters, and that a boat has managed to get there is no assurance that, after a few years in warm waters, a lightly built boat will be safe for the tougher sail back. Tropical waters also exploit deficiencies in design and manufacture, whether poor moulding or wrong choice of materials, which one might get away with for many years in home waters. When you leave the Canaries westward-bound and pass the point of no return it is too late to find out if the boat can carry the load safely. Leave Britain fully loaded. That allows you a thousand miles of probably hard sailing across Biscay and beyond to test the boat, and plenty of ports of refuge. Do not be tempted to race or adopt racing-style voyages in a heavily laden small boat. Sail at your own pace. More to the point, sail at the boat's pace. Someone to whom sailing has always meant racing may not be able to bear the thought of a slow boat. Yet a good cruising yacht need not be a sluggard. Many older ones were good club racers in their day, and some were designed as ocean racers. When there is nobody to compete against the marginal difference between fast and fastest, so critical when racing, becomes unimportant. The critical factor is that irreducible extra weight necessary for life support as a fraction of the boat's displacement. You can reduce the size of the boat and her weight, but humans are still the same size, with the same appetite, the same thirst and the same personal requirements. There is a minimum below which you cannot reduce the weight of stores, water and spares, and whether the go-fast designer likes it or no, that is what the boat has to carry, in all weathers and under all sea conditions, without being unseaworthy or breaking up. (3316 words)
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