by J E Gordon
Trusses in shipbuilding
There is a land of sailing ships,
a land beyond the rivers of Cush
which sends its envoys by the Nile,
journeying on the waters in vessels of reed.
Isaiah 18.1-2 (c. 740 B.C.; New English Bible)
As a matter of fact, trusses of various kinds were used and understood by shipwrights for many centuries before builders and shore-going architects got round to the idea. Most histories of shipbuilding begin with the boats which the Ancient Egyptians made for use on the Nile. As the prophet Isaiah seems to have been well aware, these boats were constructed by tying together several parallel bundles of reeds. Actually, these reed boats, which developed from rafts, date back to long before the time of Isaiah, probably to somewhere between 4,000 and 3,000 B.C. Similar boats are in use today on the White Nile and also on Lake Titicaca, in South America. Since the bundles of reeds naturally tapered towards the ends, a roughly boat-shaped form was achieved more or less automatically. Often the long, wispy, ends of the reed bundles were tied in such a way that they turned upwards so as to provide a vertical decoration at the bow and stern. This feature survives today, sometimes not very much changed in shape, in the high stemposts of Mediterranean rowing boats -especially in the Venetian gondola and the Maltese dghaisa.
Although most of the buoyancy of a ship is provided by the middle part of the hull and comparatively little by the tapering ends, nothing will ever prevent people from putting heavy weights into the ends of a ship. One result of this is that many vessels tend to ‘hog’ (the two ends tend to droop and the middle of the hull tends to rise). This state of affairs is the opposite of that which exists in roofs and bridges, where the middle of the truss is usually trying to sag below the level of the end-supports. This condition is called ‘sagging’ by engineers. Although in hogging and sagging the forces and deflections are acting in opposite directions, it is clear that in both cases the beam or truss is being bent and that precisely analogous principles and arguments apply.
Structurally speaking, a ship’s hull is a sort of beam, and the effect of the hogging forces on the flexible reed hulls of the Egyptian boats must have been very obvious. A hogged ship is a depressing thing to look at, and this state of affairs needs to be prevented for all sorts of other excellent reasons, so that it was necessary to do something about the situation even in 3,000 B.C. In fact the Egyptians tackled the problem extremely sensibly. They provided their ships with what is now called a ‘hogging-truss’. This consisted of a stout rope which was passed over the tops of a series of vertical struts, its two ends being looped under and round the ends of the ship, so as to prevent them from drooping (Figure 10) This rope could be tightened by some form of ‘Spanish windlass’. The latter device is a skein of cords which can be twisted – and so shortened – by means of a long stick or lever thrust through its middle. Thus the big reed hull could be strained to any degree of straightness or vertical curvature which the skipper happened to fancy. As the art of shipbuilding progressed, the Egyptians came to construct their hulls from timber, rather than from bundles of reeds. But, since most of the planks were very short and nearly all of the fastenings might be described as wobbly, the need for the hogging-truss remained.
Figure 10. Egyptian sea-going vessel, c. 2,500 B.C. This one is built of wood but retains the vertical ornaments at stem and stern characteristic of reed-built boats. The wooden planks are very short and badly fastened, hence this ship also retains the traditional Egyptian hogging-truss. Note the A-shaped mast.
Greek shipwrights were more advanced than the Egyptian ones and they built the splendid triremes or fighting galleys upon which the sea-power of Athens depended. However, these ships were also built from short lengths of timber, and their light hulls were very flexible and much inclined to leak. For these reasons the Greeks retained the hogging-truss in the sophisticated form which was called the hupozoma. This was a substantial rope which ran right round outside the hull, high up and just beneath the gunwale. Again the hupozoma was set up by means of a Spanish windlass which could be adjusted as needed by the helmsman. Since Greek warships fought mainly by ramming each other, they had to be able to withstand a great deal of structural abuse. The hupozoma was therefore an essential part of the hulls of these ships; they were unable to fight, or even to go to sea at all, without it. Just as it used to be the practice to disarm modern warships by removing the breech-blocks from the guns, so, in classical times, disarmament commissioners used to disarm triremes by removing the hupozomata.
It is quite clear that the Athenian shipbuilders, down in the Piraeus, were familiar with the principles of trussing, and one might well ask why the Athenian architects, such as Mnesicles and Ictinus, did not latch on to the idea for the roofs of their temples. Perhaps the analogy between hogging and sagging never struck them, or perhaps they just never hobnobbed with shipwrights. After all, how many house architects today ever talk to a naval architect?
When the fragile oared fighting galley went out of use, hogging-trusses disappeared. However, the American river steamboats of the nineteenth century were every bit as flexible as the Greek trireme or the Egyptian vessels on the Nile. Their shallow wooden hulls presented exactly the same problems, and the Americans solved these problems in precisely the same way as the ancient Egyptians did. All the American river steamers were provided with hogging-trusses of the Egyptian pattern. The only difference was that the tension members were made from iron rods, rather than papyrus rope, and they were tightened by means of metal screws instead of a Spanish windlass. Racing skippers claimed to be able to squeeze an extra half knot out of their steamboats by adjusting the shape of the hull by screwing or unscrewing the hogging-truss. The fact that the hulls of these steamers leaked, in consequence, even worse than the hulls of the triremes did not matter very much because they were provided with steam bilgepumps.
Trusses also occur, of course, in many different forms in connection with the rigs of almost every kind of sailing ship. Very probably, the sail is another Egyptian invention, for on the Nile the wind blows upstream for most of the year, so that cargo vessels can sail up the river with a fair wind and drift back downstream with the current – as they still do today.
The first problem in constructing a sailing ship is to erect some kind of mast upon which sail can be hoisted. The second, and much more difficult, problem is to keep that mast in place. Broadly speaking, the masts of conventional sailing ships are, structurally, simple poles or struts which are supported from a sufficient number of directions by the system of fixed ropes which seamen call ‘standing rigging’: that is to say, by ‘shrouds’ and ‘stays’. If one has a hull which is rigid enough to withstand the pull of the shrouds and stays, this is nearly always the best arrangement, and (as we shall see in Chapter 14) it can be shown mathematically to minimize the weight and cost. However, the Egyptians had not done this sort of mathematics, and, furthermore, they had no preconceived ideas about the subject. All they knew was that they were rather tired of rowing and they wanted to find some way of supporting a new-fangled thing called a sail above a hull which was made from reeds.
Having spent a good deal of time in developing sailing rigs for the pneumatic rescue dinghies which were carried by bomber aircraft,* I can sympathize with the ancient Egyptians about this business of masts. The blown-up hulls of the pneumatic dinghies were probably just about as flexible as the Egyptian reed boats. One cannot really expect to be able to attach highly-loaded ropes to a thing like a soggy balloon or to a floppy bundle of reeds, and in these circumstances the whole idea of ‘standing rigging’ becomes rather laughable. Very sensibly, therefore, the Egyptians merely planted a sort of tripod, or sometimes an ‘A ‘-shaped truss, on top of the rather squidgy hull (Figure 10). This affair worked perfectly well on the Nile; I used to envy the ancient Egyptians their solution to the problem, which, unfortunately, was never practicable with the rescue dinghies. The Egyptians did not have to arrange for the whole of their sa
iling rig to be folded up and packed inside a small bag, which, in turn, had to be stowed in a crowded aircraft.
The hulls of Greek and Roman merchant ships were generally sufficiently strong and stiff” to resist the loads imposed on them by conventional standing rigging, and so these vessels had their masts stepped in the middle of the ship and supported by shrouds and stays in the usual way. For some reason, however, even large Roman ships seldom got much beyond the stage of a single mast, carrying one large square sail, set from one long yard. It was not until the great expansion of sea voyaging at the time of the Renaissance that the rig of large sailing ships was elaborated by multiplying the number of masts and sails. About this time the single mast was replaced by three, called the fore, main and mizzen masts. Eventually, each of these masts was extended upwards so as to be able to carry, above the lower square sails or ‘courses’, first, square topsails, then topgallants, and finally royals. (The even loftier skysails and moonsails came much later, an affectation of the clipper era.)
Traditionally each sail – course, topsail, topgallant and royal -is set from its own separate section of mast. That is to say, each lower mast is surmounted by a topmast, each topmast in turn by a topgallant mast and so on. Each of these upper masts constitutes a separate piece of timber, and each is supported in its proper position by means of elaborate and sophisticated sliding fittings. These were arranged so that all the upper masts and yards could, on occasion, be lowered and sent down on deck. Since the larger spars each weighed several tons, it needed both skill and nerve to raise and lower such unwieldy objects in a rolling ship. However, a big warship would have a crew of 800 men, most of whom could have put both steeplejacks and trained athletes to shame. The sail-drill in the Mediterranean fleet in the 1840s has become legendary. It is alleged that, when the admiral had finished his breakfast, he was apt to signal ‘All ships will strike topmasts. Report time taken and number of casualties’. However this may be, it is certain that crack battleships like H.M.S. Marlborough could be stripped to their lower masts by their own crews in a matter of minutes and re-rigged as quickly. These competitive exercises were by no means a waste of effort. Ships carried ample supplies of spare spars, and the safety of a ship in an emergency, or the outcome of an action in time of war, had repeatedly depended upon how quickly crippled masts could be replaced. A limited number of casualties during peace-time drills had to be accepted, as we accept accidents in riding or rock-climbing.
The structural technology behind all this was superb of its kind, and it is worthy of the attention of modern engineers, who are apt to be rather snooty about it. The complexity of the rigging which was needed to support all the tophamper in the later sailing ships can best be appreciated by going to look at the Victory (Plate 14) or the Cutty Sark. The total height of Victory’s mainmast, for instance, is about 223 feet (67 metres). The length of her main-yard is 102 feet (30 metres), but this can be extended at will to a total width of 197 feet (59 metres) by means of sliding stunsail booms. All this immense mechanism worked, and worked reliably, for years on end and in spite of the most appalling conditions of wind and sea, being much more reliable than most modern machinery.
The masts of big sailing ships represent perhaps the most elaborate and certainly one of the most beautiful systems of trussing which has ever been developed. At the cost of considerable complexity, the total weight of structure up aloft was kept down to a safe figure. However, when big guns, mounted in revolving turrets, had to be introduced into sailing battleships around 1870, the network of shrouds and other ropes was found to restrict unduly the arcs of fire of the guns. For this reason certain ironclads, notably H.M.S. Captain, were fitted with tripod masts which could be arranged so as to permit a better field of fire. This was a reversion to the Egyptian method of masting, if you like. However, the extra top-weight (if these tripod structures had a bad effect upon the already precarious stability of these ships. This top-weight undoubtedly contributed to the capsizing of the Captain, under sail, one dirty night in the Bay of Biscay. Nearly five hundred men were drowned.
Cantilevers and ‘simply supported’ beams
It is evident that, functionally, it does not make much difference whether a ‘ beam’ is in the form of a long continuous piece of material – a solid tree-trunk or a steel rod or tube or joist – or whether it takes the shape of some kind of open-work truss. This latter might be a wooden roof-truss, a sea-going arrangement of ropes and spars, or some modern Meccano-like lattice, such as a bridge or an electricity pylon. As we shall see, there are plenty of both kinds of beams in animals as well. The fact that bridges and roof-trusses and horses’ backs and dachshunds are usually more or less horizontal, while ships’ masts and telegraph poles and pylons and ostriches’ necks are quite often vertical, does not make much difference. The essential purpose of all these structures is the same: that is to say, a load which acts at right angles to the length of the beam is supported without putting any longitudinal force upon whatever is supporting the beam. This is essentially what all beams are for.
It might be thought that a thing like a ship’s mast was an exception to this, because a mast thrusts downwards, forcibly, upon the hull of a ship. But then the shrouds and stays pull upwards on the hull just as much, and so there is no net vertical force upon the hull, which does not rise or sink in the water in consequence. Similar arguments apply with many animal structures. A horse’s neck, for instance, is very much like a mast. The vertebrae, like the mast, are in compression and push backwards on the horse’s body, but they are stayed, like the mast, by the neck tendons, which pull forwards on the body with an equal and opposite force.
In the sense which we have just been discussing, all beams, living or dead, do the same job; yet beams as a whole tend to fall into two main categories: ‘cantilevers’ and ‘simply supported* beams. There are in fact further variants and sub-divisions, which are frequently useful for examination and other purposes, but we shall ignore them for the moment.
Figure 11. A cantilever beam with distributed load.
A ‘cantilever’ is a beam one end of which can be considered as being’ built in ‘to some rigid support, such as a wall or the ground. This end-condition is called by engineers encastre – which is merely French for ‘built in’. The other end of the cantilever, of course, sticks out and supports the load. Electricity pylons, telegraph poles, ships’ masts, turbine blades, horns, teeth, animals’ necks and trees and cornstalks and dandelions are cantilevers, and so are the wings of birds and aeroplanes and butterflies and also the tails of mice and peacocks.
Figure 12. Simply supported beam.
A simply supported beam (Figure 12) is one which rests freely on supports at both ends.
Structurally, the two cases are closely connected. From Figure 13 we can see that a simply supported beam is simply equivalent to two cantilevers, back to back and turned upside down.
Figure 13. A simply supported beam may be considered as two cantilevers back to back and upside down.
Bridge trusses
The road is carried across valleys hundreds of feet in depth on rude trestle bridges, which creak and groan beneath the weight of the train. Anything apparently more insecure than these structures can hardly be found else-where, and I always drew a long breath of relief as I found my self safely on the other side. It is a fearful thing to look out of the carriage windows into the dizzy depth below, and feel that if the frail fabric were to collapse, as it seemed on the point of doing, we should all be dashed to pieces with no possibility of escape. Even in the Eastern States many of these primitive bridges yet remain, and it is said that few accidents have happened from their use. They are, however, very liable to destruction from fire, caused by burning coals falling from the engine.
Rev Samuel Manning, LL.D., American Pictures (1875)
The English railways were built straight and level across the rolling English landscape by the lavish use of cuttings and embankments and splendid viaducts of masonry and i
ronwork. All this engineering luxury depended upon supplies of capital and labour, both of which were plentiful in Victorian England. Conditions in America were totally different.* The distances were enormous; capital was scarce; the wages, even of unskilled men, were high. In the Land of the Free, where every man was an amateur, skilled craftsmen of the European type scarcely existed. Iron was expensive, but there was unlimited cheap timber. Above all, the American railroad engineers, like their steamboat colleagues, were prepared to take risks with other people’s lives and property which made the hair of British engineers rise up under their stove-pipe hats. Yet these British engineers were certainly not unduly cautious men; nowadays we should consider them rash. Nineteenth-century Americans, of course, were in the habit of living dangerously – but this was more on account of their engineers than of the Red Indians or the bandits.
The railroads were pushed westwards as fast as they could be built and with a minimum of expensive cuttings and embankments. When conditions were suitable, the valleys were bridged by means of those enormous timber trestle viaducts which alarmed the Rev. Dr Manning. They will always be associated, in tradition, with the American railways; a fair number of them survive today (Plate 15). Once they had been constructed, the American railways were vastly profitable – the Central Pacific Railroad is said to have paid dividends of 60 per cent – and so they were soon able to convert many of their precarious trestle bridges to solid earth embankments by tipping soil from the top from specially constructed trains until the whole wooden structure was encased in earth and could be left to rot away.
Wide and rolling rivers could not be crossed by the trestle viaducts and so there was a need for large, long-span bridges. Permanent bridges of the European type were often impracticable for lack of money and skilled labour, and so there was a very active requirement for long – and cheap -.wooden trusses, which could be made by ordinary joiners. Since the construction of these trusses was potentially profitable and since the Americans are an incurably inventive people, a very considerable number of nineteenth-century Americans seem to have spent their time in inventing trusses. There are therefore to be found in the textbooks a very considerable number of designs for bridge trusses, each slightly different, and each called after the name of its inventor. We need not go through them all in detail, for they all work upon somewhat similar principles, but two or three types are worth mentioning.