Eric Flint: Grantville Gazette.Volume 22

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; cast 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)

With water ballasting, it is very important to keep the tanks full; any partially full tank of liquid is subject to the "free surface effect"; the liquid sloshes in the direction of the tilt and moves the center of gravity in the "wrong" direction. Likewise, solid ballast, whether sand or cargo, must be kept from shifting.

Fins

You can increase lateral resistance by placing a fin below the main body of the hull. The fin increases the lateral area and simultaneously lowers the center of lateral resistance. This improves resistance to leeway. Unfortunately, fins increase wetted area (thus, skin drag) and draft.

These disadvantages are somewhat muted by use of retractable fins. Seventeenth-century Dutch coasters were equipped with leeboards. These were fins which hung on either side of the boat and could be let down as needed. A centerboard is a retractable fin mounted in a (hopefully) watertight cabinet inside the hull. It could slide up and down (daggerboard, drop keel) or pivot on a bolt at one end (pivot keel). (Gougeon 33-6). The centerboard first appeared in 1774 (ChappelleHASS 166-8), and the pivoted type around 1809 (360), but wasn't really popular until the nineteenth century.

Chapelle says that the centerboard made it possible for a coaster to sail well when light. (ChapelleSSUS 279). He is also of the opinion that "extremely high speed-length ratios became possible only after the centerboard was introduced (412). Simpson's ironclads have two big centerboards (1634: TBW, Chap. 44).

Bilge keels are fins which are mounted at the boundary between the bottom and the sides. When the ship heels, the leeward bilge keel is submerged, and then resists further rolling. However, the submerged keel also increases resistance to forward motion.

Hydrodynamic Lift