by Eric Flint
An advantage of iron plating, over wood planking and decking, was that the iron plates could be bent easily. But of course the iron added more weight to the hull.
Mild steel was 25-30% stronger than iron, allowing a saving of 20% in weight of scantling and 13-15% overall, but in 1880 it was 50% more expensive (White 429ff).
Hull Form
As the ship moves, water is parted by the bow, and passes around and under the hull, rejoining at the stern. If there is separation of the flow from the hull, eddies will form where the water returns. The energy to form these eddies comes from the ship's propulsion, so these flow disturbances are felt as "form resistance". Separation tends to occur where there is an abrupt change in underwater hull form. (ChapelleSSUS 49).
The choice of hull shape isn't easy. For example, the 3D shape which would yield the minimum wetted surface for a given volume, and thus the least skin drag for its displacement, would be a hemisphere. However, the fluid flow at the fore and aft "ends" would be kinked, creating significant form drag. (Gougeon 32).
To reduce form drag, water must be moved out of the way and back again more gently, i.e., the ship needs a streamlined shape. Fish bodies have offered inspiration to ship designers for centuries (and there are old plans which actually depict a fish body beneath the hull diagram).
Tapered shapes reduce form drag and frictional resistance, but also reduce both capacity and the ratio of capacity to resistance.
Midship Section Position. Imagine that the ship is sliced into vertical sections, like a loaf of bread. The section with the largest beam is called, somewhat misleadingly, the "midship section."
A good ship, old salts said, should have a "cod's head and a mackerel's tail" (a teardrop shape, with the midship section forward). In keeping with the adage, the seventeenth-century midship section was actually located about one-third keel length from the fore end of the keel. That yielded a short full (broad) bow and a long fine (narrow) stern. (BakerCV 20-21). A Dutch merchantman shown in Furttenbach's Architectura Navalis (1629) exemplifies this shape. I would estimate that quarter-length from the bow, it is a third broader than a quarter-length from the stern. (Landstrom 146). On the Mayflower replica, based on Matthew Baker's manuscript, the midship section was placed 21 feet from the forward end of the 58 foot keel. (BakerNM 80).
In later centuries, the midship section was moved aft, to true midships, or even somewhat aft (the "wedge" shape), the latter being touted by EB11 "Yachting." For our purposes, the key point is that the position of the midship section is something that the designers are going to argue about.
Midship Section Shape. On a ship plan, the sectional view of the ship is the view from the front. We need to consider both the underwater portion (the bottom) and the abovewater lines (the sides).
Bottom. A semicircular underwater section requires the least "wetted area" (which determines frictional drag) for a given capacity, and this was recognized by Georges Fournier (1595-1652) in his treatise Hydrographie (1643)(Laing 162). Unfortunately, it provides no stability, and hence is practical only on a multihull or if there is substantial ballast. Rounding makes the hull "tender"; a small degree of tilt produces only a small righting tendency so the hull heels easily and recovers slowly. However, if deep-ballasted (see below), then its resistance to heeling will increase as the angle of heel becomes larger.
The bottom may instead have one or more chines (angles). A simple-V bottom has one chine, a square bottom two, and a shallow-V has three. A square (flat) bottom maximizes capacity and also makes it easier to shelve the ship on a beach, if need be. Additionally, if the bottom is flat (or a shallow-V), the hull is "stiff"; the center of buoyancy moves sharply in the direction of a small tilt and thus creates a strong righting moment. However, if the tilt continues to increase, the righting moment will decrease once more. (Gougeon 39-42). Generally speaking, flat bottomed hulls experience more leeway than V-shaped ones.
In the early seventeenth century, most "blue water" ships had a short flat portion at the bottom (the floor), then a nearly circular underbody. (BakerCV 20; BakerNM 31). The Dutch merchantman shown in Furttenbach's Architectura Navalis (1629) had a more pronounced flat bottom, almost as wide as the maximum beam (Langstrom 146). Square bottoms were found on colonial workboats since they made the boats easier for neophytes to build.
However, there were ships, such as the Dufyken (1595), whose bottoms were partially V-shaped, not flat. The slope and rise of the "V" is called deadrise, and in later years there was much disagreement as to how much was desirable. (Duhamel 7, ChapelleHASN 406). Deadrise reduced resistance, but at the cost of stability.
The sides of the ship may be vertical (wall-sided), or, as they near the top, they may creep in (tumblehome) or out (flare). Tumblehome reduces topside weight, and also might make the ship more difficult to board (Millar 20). If taxes are based on breadth on deck, then tumblehome gives you a bit of "free" cargo capacity. It lowers the center of gravity (Millar 20), but it reduces the reserve of buoyancy and the "righting arm" at large angles of heel (Walton 144ff). The curved section also increases strength in compression, which may be helpful in supporting heavy guns. (Harland 44). Flare has the reverse effects. Both tumblehome and flare increase the cost of the hull.
In the early seventeenth century, the ships usually had, starting above the level of maximum beam, itself a little above the load waterline, a straight tumblehome at about twenty degrees from the vertical. (Baker).
Eighteenth-century British warships also had substantial tumblehome. After the Napoleonic wars, the British found themselves short of compass timber suitable for curved futtocks (or at least of the funds for buying such timber), and switched to "wall-sided" ships. (Kirby 98). Modern ships, while made of metal, also tend to have vertical walls. However, icebreakers curve inward both above and below the waterline, to protect them from ice pressure. (Rogers 28).
Full and Sharp Ends. To reduce form resistance, the hull tapers as you move from the midship section to the ends. The seventeenth-century shipbuilder determined what taper to use where by a combination of geometrical rising and narrowing algorithms, and judgment by eye (the latter assisted, in so-called "whole moulding," by the use of flexible wooden ribbands to ensure that the curves were smooth and plankable).
The combination of the bow (and stern) shape (horizontal) and profile (vertical) determines whether the ends are "full" (boxy) or "sharp" ("fine", tapered). The sharpness reduces form drag but also reduces local buoyancy; the ends will sag if local buoyancy is inadequate to support their weight. (ChapelleHASS 44; Darcangelo 1-3; Villiers 105). As to stability, a wall-sided ship with a diamond shape would have half the buoyancy, but only one quarter the metacentric height (and thus, roughly, initial stability) of a ship of rectangular shape. (Simpson 36ff). Sharpness of profile likewise reduces stability.
The sharpness of the design may be summarized in terms of the midship section coefficient (ratio of actual underwater area of midship section to that of the corresponding rectangle), prismatic coefficient (ratio of underwater volume to that of a prism with the same midship section and length, but without any taper), and block coefficient (ratio of actual underwater volume to that of the corresponding block).
We know that the wargamers in Grantville have several of Chappelle's books. They aren't identified, but given the nature of their hobby, they almost certainly own The History of the American Sailing Navy (ChapelleHASN) and The Search for Speed Under Sail (ChapelleSSUS). These books are valuable in that they detail the "lines" of numerous successful sailing ships, both warships and merchantmen.
While the designers of the 1632verse will no doubt be fascinated by the data in this book, it is hard to extract from them any overarching principles. A fast ship can have a full midsection (high midship coefficient), and relatively fine ends (low prismatic coefficient), or the reverse.
Looking at the fast ships of ChapelleSSUS chapters 6-8, which include clippers and their predecessors, the block coefficients are. 30-. 7
6, the midship section coefficients. 53-. 93, and the prismatic coefficients. 56-. 82 (ChapelleSSUS 406ff). The Surprise (1850), despite a. 82 prismatic, was considered a "fast sailor." The Eckford Webb (1855), with a. 72 prismatic, is known to have made 16 knots. (385), for an SLR of 1.43! (411), although it might have been blessed with a light load at the time. The most that can be said is that the short-term speed champs Sovereign of the Seas, Lighting and Sweepstakes had lower prismatics:. 62,. 61 and. 64, respectively. (409).
ChapelleSSUS teaches that for fast sailing downwind, a low prismatic coefficient (implying a ship with relatively fine ends) is desirable, but on other points of sail, a higher prismatic is better. (45ff). And the effective hull length of the ship, for computing hull speed, is the product of the actual hull length and the prismatic coefficient, implying that with enough wind power, ships with full ends will go faster.
The displacement volume can be estimated if the length ("between perpendiculars"), beam and draft are known; multiply these by an assumed block coefficient (. 6-. 7 for a merchant,. 5-. 6 for a battleship,. 4-. 6 for a cruiser; White 4; Ridler 62).
Bow Shape. A curious practice, lasting the entire seventeenth and eighteenth centuries, was a cutoff upper bow. The lower bow was round, but above the beakhead, the upper deck had a square Which should have carried a sign which read "Shoot Me!" since it was a weak point, vulnerable to raking. Nelson's flagship, the Victory, when brought in for repairs, was much more heavily damaged in its upper bow than its lower bow (Fincham 203). The English shipwrights smacked their collective foreheads, and English ships constructed after 1811 featured a complete round bow. (Anderson 181).
Stern Shape. In medieval times, big ships had rounded sterns. Beginning around 1500, such ships were given a flat, square, "transom" stern, which the Dutch called spiegel (looking-glass), perhaps because of the reflection from the windows set in the stern cabin.
Hydrodynamically speaking, the transom stern was inferior to its predecessor, the round stern. The water pushed out of the way by the bow reunited abruptly behind the stern, creating a turbulent wake. This increased drag, reducing speed. Moreover, part of the rudder was in the area of turbulence, and hence steering was impaired. (As a palliative, at some point the aft edge of the rudder was thickened.)(BakerCV 19).
In the seventeenth century, the English created a hybrid stern, in which the lower part was rounded and the upper part square. This can be seen in Sovereign of the Seas (1637), whose stern flattened out about ten feet above the waterline, but it wasn't common on English ships until the 1650s (Anderson 144; Langstrom 152; Millar 17). The hybrid stern solved the hydrodynamic problem, but bear in mind that the transom upper stern was probably a weak point in combat. The English made the final step to a round stern in the 1820s. (Millar 20), and the Americans followed suit. Besides being stronger, the round stern offered the prospect of placing gunports in the rear quarters, eliminating that blind spot.
Unfortunately, the change also eliminated the traditional quarter galleries for the ship's officers, and this wasn't borne (especially by senior officers) in silence. The plans for the early round stern warships were revised so as to mask the new sterns with quarter galleries. (ChapelleHASN 365).
Bow Profile. Usually, the bow and stern overhang, that is, they rake outward as you go upward. Compared to a ship of the same deck length but without an overhang, the raked ship will have less frictional drag because the water flows under it more readily, but also less resistance to leeway, reserve buoyancy, and cargo capacity. A ship with an excessively raked stern or bow might not be able to carry guns at those ends (ChappelleBC 43).
Bow profiles may be plumb (vertical), "V" (straight; slanting outward); clipper ("concave"; "hollow"; starts vertical and arcs outward); spoon ("convex"; starts horizontal and arcs upward) or tumblehome (spoon bow which ultimately curves inward).
John Smith wrote in 1627, "fore Rake is that which gives the ship good way, and makes her keep a good wind, but if she have not a full Bow, it will make her pitch her head much into the Sea; if but a small Rake forward, the sea will meet her so fast upon the Bowes, she will make small way…" (226).
The seventeenth-century bow was typically a "V" bow terminating in a beakhead, a structure similar in appearance to the ram of the ancient galley. It wasn't used for ramming, of course, but rather to provide a platform for sailors working on the bowsprit. It also "shattered the seas at each plunge and kept them from sweeping fore and aft." (Masefield; AndersonRS xvi, xvii) You could also find spoon bows (xv).
As ships got longer, a pronounced rake became problematic from a strength standpoint. Rake went out of fashion in the 1820s and 1830s, and packet ships sometimes had almost vertical stem and sternpost. (ChapelleHASN 423).
The clipper bow was popularized by John Griffiths, who argued that such bows would have minimal head-on resistance, and would resisting burying when the ship plunged in a heavy sea. He tested his ideas in tank experiments, but the proof of the pudding was his Rainbow (1845), which made the round trip from New York to China in 7 months, 17 days. (Laing 170-9).
A twentieth-century variation is the bulbous bow. Above the water line, you have a clipper bow. Below it, there is a protruding bulb which creates its own bow wave. If properly designed, it reduces the normal bow wave at speeds near the hull speed (GlobalSecurity).
Unfortunately, it's more useful on powered ships, since they can consistently maintain the right speed. A sailing ship is at the mercy of the winds, and at speeds much different from the "design speed," the bulb increases drag. For a Napoleonic era warship (say, 170'LWL, hull speed 13 knots, 0.60 block coefficient), you would need to reach a speed of 10 knots to see a 10% reduction in resistance.
With short ships, there will also be increased pitching at high speeds, and the bulb does no good when it's out of the water and actually creates drag on each reentry.
Finally, at all speeds, the bulb increases turning resistance. (Killing 37-9), and frictional resistance, so light wind performance will be poorer.
Stern Profile. In the seventeenth century, the most common stern profiles were rounded or slightly vee'd sterns (AndersonRS xv, xvii), and one which angled or curved outward and then turned vertical to create a transom (xvi).
Subsequently, shipbuilders constructed ships with "finer" sterns, the theory being that this reduced the turbulence behind the ship and thus reduced form drag. The trend was reversed by Griffiths, who favored matching his sharp bow with a blunt stern (leading detractors to suggest that his ship would sail better backward).(Laing 176).
Girdling. As early as 1622, the English added an extra layer of planking (girdling, furring) at the waterline to increase stability. (BakerNM 6; OED). For example, the 76-gun Royal Katherine (1664) had a normal beam of 39'8" and a girdled beam of 41' (Temmu). Lewis (200) says that this substantially increases stability while adding "very little" to the draft.
Ballast
The height of the center of gravity is determined not by how much weight is being carried, but where it is located. In general, the lower the center of gravity, the greater the stability of the ship. Merchant vessels have the luxury of stowing heavy cargo deep in the hold. Warships have the problem that guns are heavy, and needed to be high enough on the hull to be above the wave action. They therefore need to carry ballast to compensate; ballast and water was typically 12-14% displacement (White 84).
Ballasting lowers the center of gravity, and thus increases stability, but at the cost of increased mass and thus reduced speed. Also, too much ballast will make the ship too stiff. (Walton 168ff). Ballast is most effective when deep in the hull, and so as more ballast is added, the return in stability diminishes.
The most commonly used ballasts were gravel, coarse shingle, sand and rock. (ChapelleHASS 247). They had the advantage that they could be laid wherever desired.
Iron and lead were only rarely used as ballast in the seventeenth century, most likely on account of their cost (Lavery 186). In the late eighteenth century; ca
st iron ballast cost?27-5s/ton (Dodds 23). The most efficient ballast would be lead, because of its high density, but on account of its cost it was then limited to royal yachts.
In 1796, Samuel Bentham had iron ballast bolted outside the hull, beside the protruding keel. That moved the CoG more than the same weight of internal ballast would have. This deep ballasting eventually evolved into the uncapsizable hull. (This has a deep fin with ballast attached at the end; a completely watertight hull is uncapsizable if the external ballast moves the CoG below the center of buoyancy.)(Gougeon 39, 51; ChapelleHASN 236).
The crew's water and victuals may also be placed low in the hull, to augment the ballast, but of course they diminish over the course of the cruise. HMS Endymion (1797) carried 120 tons iron, 26 shingle, and 124 water. (Gardiner 145).
Cargo can also serve as ballast, if dense enough, and has the advantage of earning revenue. The Portuguese found Chinese porcelain to be a useful ballast for their East Indiamen. (Brigadier 54). Madeira is a fortified wine, and in the eighteenth century it was recognized that unlike other wines, its taste is improved if it spends, say, three months "cooking" as ballast for a ship traversing the tropics. (NewScientist 114). The improved Madeira was called vinho da roda ("wine of the round trip"). The nineteenth century frozen water trade from New England to the American South, the Caribbean, and even India, was profitable because the ice served in place of stone ballast (which had to be paid for).
1911EB "Ballast" notes that "in modern vessels the place of ballast is taken by water-tanks which are filled more or less as required to trim the ship." For example, a tank in the bottom of the screw-propelled icebreaker Ermack (1898) held 800 tons of water. Pumps could be used to shift this water to tanks fore, aft, port or starboard. (Rogers 29-30). Simpson's ironclads appear to have a similar feature. (1633, Chap. 4; 1634: TBW, Chaps. 31, 44, 48, 60,61)
