Eric Flint: Grantville Gazette.Volume 22

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-. 76, 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).