by Eric Flint
Increasing breadth and depth increases the weight, and therefore the tendency to bend, but also the ability of the hull to resist the bending forces.
The usual antidote to bending was reinforcement. The thickness of the main deck and the keel could be increased (McCutchan 37). The French frigate L'Oiseau (1772) had diagonal planking (ChapelleSSUS 207), and the USS Constitution (1797) had diagonal risers (Otton), both to inhibit hogging. This became common in early nineteenth century. (ChapelleHASN 365).
Since hogging was feared more than sagging, from time to time, builders experimented with laying the keel with a slight sag in the middle. This expedient was recommended by Griffith in the 1850s. (ChappelleSSUS 366).
Handiness
The longer the ship, the slower it turns (Laing 32) and the larger its turning radius. A ship half the length is probably about four times as maneuverable. It also helps to have fine ends, and weight concentrated amidships. (Atwood).
Weatherliness
When the force of the wind upon the sail is not parallel to the keel, the ship will be pushed, not just in the direction its bow is pointing, but also laterally. This undesirable lateral motion is called leeway and a ship with minimal leeway when traveling upwind is said to be weatherly. The resistance to leeway increases with the draft and underwater length of the hull.
Windage (the force of the wind other than on the sails) is also important, and it is strongest when the wind is on the beam. In general, the ship with the higher freeboard or greater superstructure is going to suffer more leeway. If a single and a double decker were both making five knots close-hauled, in an hour the former might be pushed two miles to leeward, and the latter three. (Laing 75).
Weatherliness is especially important for fighting ships because it determines who obtains the weather gage. (ChapelleHASS 47).
Hull Dimensions.
The basic dimensions of the hull are its length, breadth (beam), and depth, all of which vary as a result of the curves of the hull.
Length. Seventeenth-century sources generally quote the keel length. The length of the gun deck limits the number of guns which can be carried upon it. (MurrayS 6). A late seventeenth-century naval regulation required a minimum spacing of 6.5 feet between gunports to accommodate the gun crew. (Grieco 110). Based on data for nineteenth-century British warships (Creuze 53), the length on the gun deck is perhaps 20% greater than the keel length. For the waterline length, which affects wavemaking resistance and pitch stability, I would split the difference.
Raleigh advised against building ships much longer than 100 feet (Creuze 17). For large warships launched between 1600 and 1640, a typical keel length would be 100-130 feet (Temmu). A first rate in Nelson's navy might have a 175 foot keel. (Longridge 7).
Breadth. For large warships launched between 1600 and 1640, a typical maximum beam (B) would be 35-45 feet, usually closer to 35.
Length/Breadth Ratio. This ratio determines the overall shape. Chapelle says that a ship with too wide a hull was slow, and one too narrow was an unsteady gun platform and couldn't carry sail well, presumably because of lack of stability. (ChapelleHASS 46). Vasa had a ratio of about 4:1. (Franzen 74ff).
The Portuguese nao had a length: beam ratio of 3:1 (Brigadier 11; Konstam 7). William Burroughs, the controller of the British Royal Navy, around 1596 suggested relative proportions for three "orders" of ships: (1) pure merchant ship, keel length twice breadth amidships, depth in hold half breadth; (2) all-purpose, length 2-2.25 times breadth, depth 11/24ths breadth; and (3) warship, length three times breadth, depth 0.4 times breadth. Looking at seven merchantmen (1582-1627) of 130-200 tons "burden" (see Capacity, below), these had length/breadth ratios of 2.14-2.92, and depth/breadth of 0.42-0.5. (BakerNM 8-9; Myers 106). The anonymous Treatise on Shipbuilding (c1620) called for length to be 2-3 times breadth, and depth 0.33-0.5 times breadth, and said the ideal warship was KL/B 2.78, D/B 0.43 (107).
"By 1634 it was very difficult to find a [British war]ship with a keel/beam ratio of less than 2.90, and there were several higher than 3.00." (Myers) In 1841, the typical length/breadth ratio for an English warship was 3.15, whereas in America, even merchant ships averaged 4.6. The extreme clippers which developed later in the nineteenth century reached a ratio of 5.7. (Laing 54).
Depth/breadth ratio. The depth determines the number of decks (Glete 52), the height of the gun deck, and the amount of freeboard. Duhamel (1764) says that the depth of a warship is usually 0.5B; but in the list of warships he provides, it is usually a bit less than that (4-6). In nineteenth-century British ships, depth was 0.55B for sloops and smacks, 0.58-0.75B for schooners and brigs, and 0.66-0.75B for large schooners and ships. This evolved under tonnage rules which penalized breadth and ignored depth. (Creuze 36).
Body
The body of the ship consists of the hull, deck, and any superstructures other than rigging. The hull is the backbone of the ship. It's partly out of sight, beneath the waves, but it should never be out of mind. Without a hull, you're swimming!
Frame vs. Monocoque Construction
In the classic truss frame construction, an internal skeleton carries the load, and the skin of the structure just keeps out wind and water. In a monocoque ("single shell") construction, it is the skin that bears all or most of the load.
Generally speaking, the truss frame is superior (on weight and cost basis) for resisting compression and bending, and the monocoque shell for resisting shear and torsion. In the Thirties, airplanes got large enough and fast enough for the monocoque strategy to prevail. (Gordon 311-3).
While classical ships were monocoque, their construction was labor- and timber-intensive, and hence this construction strategy was gradually abandoned. By the seventeenth century, the transition to frame construction was almost complete. The principal exception was that the Dutch used a hybrid process in building flutes; a few bottom strakes were attached to the keel before doing any framing.
Chinese junks have been characterized as monocoques because they lack the keel, stem and sternpost, but their strength is not attributable just to their skin; they are reinforced by transverse bulkheads. (Thomas). Modern European monocoques are mostly open boats made of plywood or fiberglass, but there are also some monocoque minesweepers.
The backbone of the framed ship was formed by the keel, stem and sternpost. The length of these pieces determined the length and depth of the ship. Beginning in the early eighteenth century, there was a false keel under the true one. The idea was that if the ship ran aground, the false keel would absorb the impact, like the bumper on an automobile. (Mondfeld 74).
If a ship has a strongly tapered stern profile, it may have "deadwood," a vertical extension of the keel, to connect the aft end of the keel to the buttock of the ship. (After 1860, the bow was tapered enough so deadwood was needed in front, too.) The sternpost is connected to the aft end of the deadwood and the rudder mechanism is attached to the sternpost. Both the stem and sternpost are likely to be made of a single log of first class oak. (Longridge 11). Deadwood reduces leeway but increases frictional resistance. (Winters).
Transverse vs. Longitudinal Framing
There are two basic framing methods. In transverse framing, curved ribs run up from the keel, forming the load-bearing elements of the sides of the ship, and then deck beams bridge the tops of the ribs. It is called transverse because the ribs are (viewed from above) perpendicular to the keel.
The alternative is longitudinal framing, in which the sides of the ship are established by longitudinal (parallel to the keel) stiffeners. This was introduced in the nineteenth century (Young).
Bear in mind that a ship would not be purely transversely or purely longitudinally framed. Even if a ship has transverse ribs, they are attached to a longitudinal keel. And if a ship has longitudinal girders, then these must be linked by transverse "webs."
In wooden sailing ships, and early steel ships, transverse framing predominated. However, that meant that most of the structure did not offer any resistan
ce to longitudinal bending. (NavArchWeb). That's fine for an accordion, but not good for a ship.
Bulkheads
These divide the ship into watertight compartments, and also increase its hull strength and its stability after being damaged. However, they also make it more time-consuming to load and unload cargo. Both transverse and longitudinal bulkheads were used regularly in Chinese junks since at least the second century (Temple 190), but prior to the nineteenth century, their use in European ships appears to have been spottier. Bulkheads were initially made of wood, but iron ones were introduced in the 1830s. (Young; Gould 79).
Planking
The framing must be covered with wooden planks or iron plates. There are two basic construction methods. In both, the planks run fore-and-aft. Carvel construction, invented in the ancient Mediterranean, fitted the planks or plates so they met edge to edge, forming a smooth surface. In clinker (lapstrake) building, used in northern Europe and in China, the planks or plates overlap their edges. (Svensson, 8). Clinker is not as streamlined as carvel, but it is stronger, and hence the planks can be made thinner.
Gould contends that the adoption of gunports favored the adoption of carvel planking (Gould 215). In the seventeenth century, the English used carvel planking everywhere, but the Dutch used clinker for their upper works. (Anderson 153).
John Smith's Sea Grammar (1627) says that a ship of 400 tuns requires four inch planking; one of 300, three inch; and smaller ships two inch.
Waterproofing
Wooden ships are made watertight by caulking them. Traditionally European ships were caulked by filling the seams with oakum (fibers from old ropes) soaked in pitch. The pitch can be distilled from pine resin, or from asphalt. Lime was sometimes used in place or in addition to pitch. The Chinese instead used tung oil or fish oil. Modern sealants include silicone and polyurethane.
Deck
In profile, the deck of a ship may be flush (horizontal), or it may have a sheer (upward curve) toward either or both ends. Judging from contemporary illustrations, seventeenth-century vessels will most likely have a very pronounced stern sheer. The sheer was gradually flattened out over the course of the eighteenth century.
Viewed from the front, the deck will usually be either flat or slightly cambered (convex upward). Camber slightly increases the structural strength, and reduces the recoil distance of the cannon (Dodds 89). Sheer and camber both permit water to drain away.
Hull Material
Wood has the advantage of being naturally buoyant; wrought iron and steel, that of considerably greater tensile strength and stiffness, both absolutely and in proportion to weight. Wood is vulnerable to biological attack; metal, to corrosion.
Wood. The deck could be made of any of variety of woods, such as pine (BakerCV 95). The materials requirements for the hull were more stringent. The official march of the eighteenth century British Navy proclaimed, "Heart of oak are our ships…" Oak was the principal hull wood for European navies in the seventeenth century, too. Pine was used, especially in the cost-conscious Dutch flutes, when oak was unavailable or deemed too pricey. (Unger), but it was definitely inferior.
However, beginning in the sixteenth century, Portuguese ships built in Goa shipyards used teak. The teak hulls lasted a decade longer than those made of oak or pine, perhaps because of its resistance to teredo worms. (Brigadier). Moreover, oak contains tannic acid, which corrodes iron, and teak doesn't (Jordan). Teak requires less seasoning than oak, it doesn't expand significantly when heated, and it is extremely durable. (Bowen 143). The British displaced the Portuguese, but it wasn't until the early nineteenth century that the British permitted warship construction in India. Soon thereafter, the British use of teak surpassed that of oak. (Schlich, 578).
Mahogany was used by the Spanish in the New World, it being readily available in Cuba and the Honduras. It is more buoyant than oak, easy to bend and carve, and resistant to dry rot. It also doesn't burn or splinter as easily as oak. (Glete 31; Fine Woodworking 25).
As commemorated by a Bermuda postage stamp, native Red cedar ( Juniperus bermudiana) was used to build the Deliverance and Patience , which went to the rescue of the Jamestown colony in 1610. Many sloops for the West Indian and African trade were constructed from this wood. (ChapelleSSUS 65).
Long, straight timbers are used for the keel, and are also sawn to make planking. For large ships, several pieces had to be "scarfed" together; HMS Thunderer needed seven baulks, each 26' (Dodds 58).
There is also a demand for "compass" (curved) timber for use in framing. Foresters would survey forests and mark the trees which had branches of a particular desired curve. In like manner, they looked for "knee" (angled) timber, taking it from the junction of branch and trunk. The knee timbers secured the deck beams to the frame (BakerCV 95). Warships had a particular need for crooked timber for reinforcement (Glete 52).
Wooden planking can be curved, but the curves must be gentle. If the curve is too sharp, the wood will break rather than bend. (Henderson 118). In the seventeenth century, "green planks were often scorched or heated in wet sand to render them pliable enough to be fitted around the customary bluff bows…" (BakerCV 31).
The natural supply of compass and knee timber was gradually depleted, and hence means were sought to produce it artificially. Unfortunately, the heavier the timber, the harder it is to bend. According to Baker, "the steam bending of frames was unknown" in the seventeenth century, and Griffiths (16) says, "from time immemorial shipbuilders have bent their planks by a due application of heat and moisture but it is not… until the present [nineteenth] century, that any of them discovered how to bend frame timbers and knees."
Another issue was how to attach the planking to the frame, and indeed the various frame elements to each other. Baker says that in the early seventeenth century, there was extensive use of "treenails" (wooden pegs), with iron nails being used mainly in fastening down the planking of the superstructure. (BakerCV 33). However, the Portuguese apparently just used iron nails in the hull, and the Spanish San Martin (sunk 1618) used both kinds on the same planking (Crisman). Treenails were cheaper than iron ones even in the mid-eighteenth century (Dodds 24).
The great enemy of the wooden ship was dry rot. (Dodds 13). Three expedients were used to minimize it. First, shipbuilders selected resistant woods. Teak and greenheart are good for perhaps twelve years but weren't used by Europeans (except as noted) in the early seventeenth century. Oak is durable if seasoned (English practice was three years), lasting perhaps a decade, but unseasoned oak can be destroyed in a few months. (Which didn't stop the American colonists from using unseasoned wood for smugglers and privateers, ChapelleSSUS 13.) The heartwood was preferred even though that meant that the tree had to be allowed to grow longer to get enough of it. Elm doesn't rot if it's constantly immersed in water and hence it was used for the keel and the lowest planks. (Dodds 18; Murray 72). The Sparrowhawk (wrecked 1626) had an elm keel (Riess 71).
Secondly, at least by the nineteenth century, they experimented with various preservatives. Metallic salts didn't work well, but cresoted timber was resistant to both rot and marine worms. (161).
Finally, they grudgingly recognized that they needed to hold down the moisture level in the wood by forced ventilation. (162). In the nineteenth century, steam fans were available (Lewis 112).
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Iron. Iron use in seventeenth-century ships was mostly in cannon, bolts, hinges, chainplates, hooks and the like. (ChapelleSSUS 14). Iron knees were used in the first rate Royal James in the 1670s, but weren't routinely used in England until after 1800. At first the knees were a hybrid of iron and wood. The complete iron knee appeared in the Unicorn (1824). (Goodwin) By the end of the eighteenth century, iron had also been used in the cross-bracings of warships. (Dodds 7).
An iron-hulled pleasure boat was built as early as 1777, but little is known about it. Wilkinson's Trial (1787) weighed eight tons yet drew only eight inches empty. Unfortunately, it and the three additional barges he con
structed in 1788 cost at least three times as much as its wooden counterparts. (Barker)
In 1810, Sir Samuel Bentham unsuccessfully urged the Admiralty to switch to iron-hulled warships, in view of the shortage of timber. (It took 2-3 loads, each fifty cubic feet or one "standard" oak tree, per ton of ship, to build a warship, and 1-1.5 for a merchant ship, and the cost even in the 1750s was almost 10?/load. Dodds 13)..
Nonetheless, iron hulls started popping up a few decades later. The Aaron Manby, a wrought iron steamship, was built in 1820, and the 218 ton bark Ironsides in 1838 (McCutchan 111; Young). Ma Roberts (1858) was the first steel paddle steamer, and the 1271 ton Formby (1863) the first steel square-rigger. It cost twenty pounds to the ton. (McCutchan 36). The first iron warship was HMS Warrior (1860).
Iron had advantages other than availability, of course. It was stronger than wood, and hence could be used to build longer (hence faster) ships. While iron was more costly per unit volume, its strength meant that less was needed, so 19c iron ships were 10-25% cheaper. Iron hulls also lasted two or three times longer than wood ones (White 412).
Iron ships usually came in two basic flavors, the all-iron ship, and the composite, which had an iron frame and wooden planking and decking. (Svensson 62). Composites were favored for tropical waters, where copper sheathing was necessary to protect against marine borers; iron set against copper would experience bimetallic corrosion (Lewis 117). Another variation was seen in the Great Republic (1853), 335' long; it was mainly wood, but its hull was reinforced with diagonal strips of iron.
It is important to recognize that there wasn't a rapid transition from wooden to iron ships; the two types co-existed for decades. Iron ships were not only more expensive to construct than unsheathed wooden ships, they had nasty effects on ship's compasses. The bottom of the all-iron ship was a haven for barnacles and seaweed, increasing skin drag if not cleaned frequently. So their maintenance cost was higher than for sheathed wooden ships. And iron corroded three times as fast as copper. (McCutchan 110, Atwood 299, White 415).
