Eric Flint: Grantville Gazette.Volume 22

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).

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 constructed 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).

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.