Of course, the number of guns carried on a deck of particular length depended on the spacing between the gunports, and how close they came to the bow or stern. On the Dauphin Royal (1735), 74 guns, there were 13 ports to a side, the foremost about 18 feet from the stem and the aftmost about 10 feet from the stern. The port width was 2'10" and the distance between ports (edge-edge) was 7'7". (du Monceau 4).
Gunport spacing was limited by the area and crew needed to work the guns; the more powerful the gun, the greater these were. Gunport breadth, for example, was 3 feet for a 48-pdr and 1.5 for at 4-pdr (Id). The spacing was also affected by the framing; you didn't want to cut through a frame and weaken the hull. A mid-seventeenth-century Dutch admiralty had these rules of thumb: gunport spacing (center-center) 20 shot diameters; height, six diameters; width, five (Hoving 104). A mid-eighteenth-century rule allows for 25 shot diameter spacing and 6.5 diameter width, with the sill 3.5 diameters above the deck (Davis1984,110).
In the British 1745 establishment, no warship had more than 28 guns on a single full deck. However, there were post-establishment British warships, such as the First Rate Victory (1765) and the "Large" class 74s, with 30 guns on the lowest gun deck (Lavery 121ff). And in 1764, du Monceau, said that a 112-gun French warship had 32 guns (24-pdr) on its second deck.
If you wanted more guns than a single full deck could accommodate, you put them on the quarterdeck or forecastle, or, if that still wasn't enough, you added a second (or if need be a third) full deck.
The mid-nineteenth-century introduction of iron and steel construction allowed warships to be lengthened, and thus history has some examples of some long "broadside ironclads." The longest of these was the HMS Minotaur (1863), 407 feet long, a sail/steam hybrid. There were also two-decker broadside ironclads, such as the French Magenta (1862), 282 feet long. (Neilson).
Broadside guns had a limited firing arc. In Napoleonic warships, the range of traversal was 40–45 degrees before or abaft the beam; this was apparently an improvement on earlier warships (O'Neill 71). However, in steam ironclads like HMS Warrior, the gunports were narrowed, thereby reducing the arc of fire (Lambert 46). The theory was that with steam propulsion, they weren't subject to the vagaries of the wind, and therefore could maneuver as needed to bring the guns to bear. Moreover, firing at extreme angles reduced the rate of fire. I also figure that the narrower gunports meant that the guns were less vulnerable to counterfire.
I believe the first turret ship was the wooden Royal Sovereign (1857); Eriksson's ironclad USS Monitor (1862) was the first to engage in battle. The advantage of the turret was that by rotation it could bring its gun(s) to bear in any direction, save for those obstructed by the ship's superstructure (including funnels, masts, and other turrets). Because of the size and expense of the turret, the tendency was for turreted warships to be fitted with a small number of very powerful guns. For this discussion "turreted" may mean a true turret (armor rotates) or a barbette (armor stationary).
Sails, masts, spars and stays would of course restrict the firing arcs; nonetheless, many early turreted warships were hybrids (sail/steam) because of doubts as to the reliability of the engine.
This led to a variety of curious expedients. On the ill-fated HMS Captain (1869), the two turrets were on the lower (main) deck, and the masts were stayed to the upper (hurricane) deck. I would imagine that this arrangement would limit how high the guns could be elevated. The hurricane deck was dispensed with on the double-turreted HMS Wyvern and HMS Scorpion (1863), on which the turrets flanked the main mast. Captain Coles proposed use of iron shrouds and stays. (Breyer 34).
HMS Devastation (1871) was the first turreted warship without rigging (it had a central mast for signaling and observation). HMS Inflexible (1876) had two screws driven by compound steam engines, and two masts that could carry 18,500 sf sail (Wikipedia). The latter was removed in 1885. (Breyer). While I am not aware of later turreted warships with sails, the broadside-armed HMS Calypso (1883) was ship-rigged (Ireland1997, 36) and the Russian cruiser Rurik (1892), barque-rigged.
Early turreted warships included those with one (USS Monitor), two (USS Onondaga) and even three (USS Roanoke) turrets. With multiple turret designs, one has the concern of where to place the turrets. The most obvious arrangement was to place them single file on the centerline. The obvious problem was that a bow turret couldn't fire directly astern, and a central turret (as on the HMS Monarch (1868)) couldn't safely shoot fore or aft.
One alternative was to mount the extra turrets on the side (wings). This increased the frontal fire at the expense of broadside fire. In theory, wing turrets could be staggered, and fire if need be across the deck. but that tended not to be too good for the deck. And the centerline design was structurally sounder.
Another option was to stack the turrets, like the tiers on a wedding cake. On USS Kearsarge (1898), the double-decker turrets turned as a unit. It's reported that the vibration of the 13" guns below interfered with the firing of the 8" guns above. (cityofart.net)
The 1870s Italian navy experimented with a "diagonal reduit", in which two turrets were mounted near the center of the ship but diagonally offset from it. (Breyer 33).
A single turret could carry one, two or even three guns, but if it attempted to fire multiple guns simultaneously, "invariably, one of the guns was thrown off target by the firing of the first weapon." (Kaufmann 5).
With muzzle loaders, the turrets had to be of large diameter, but the guns short-barreled, so they could be run back and reloaded inside. (Ireland1997, 38).
The mechanisms of turret gun laying and loading are discussed in part 2, and the armoring of turrets in part 5.
Vertical Arrangement of Guns
Positioning the gun on a higher deck has the advantage that the gunports are less likely to be forced to close as a result of rough sea conditions (Laing 76). Raleigh urged that the ship be designed and laden so that the lowest tier of ordnance was four feet above the water (Creuze 17). An upper deck gun will also have increased range (as predicted by Torricelli) and can take advantage of plunging fire (shooting at the flimsy enemy deck, not the relatively stout side). However, if the enemy is close at hand, the gun might not be able to depress enough to fire upon it, and the higher the guns are, the higher the ship's center of gravity must be, reducing its initial lateral stability (but the higher freeboard does provide some compensation by increasing the angle of vanishing stability).
In the fifteenth century, ships had guns mounted high up, in the aptly named forecastles and sterncastles. The size and number of these guns was limited by their effect on stability. In the early-sixteenth century, gundecks and gunports were introduced. Since the armament was lower, it could be made heavier. (Svensson 16). As broadsides became more effective, the superstructures became less useful and were reduced in size. The early-seventeenth century was a transitional stage in which the capital ships mounted heavy broadside armaments, but still had significant superstructures.
The depth (and draft!) of the ship limits the number of gun decks. Over the course of the sixteenth century, a second and then a third gundeck (~1591) was introduced. (Creuze 15). The Dutch didn't use three-deckers, but the English and French did. (Anderson 158). British designers of the late-eighteenth century found that three-deck 80-gun ships were top heavy; two-deck 80s were too long for their height, and hogged (drooped amidships); the two-deck 74s were ideal and, even though they were considered to be of the "third rate," became the most common "battleships" in "foreign service." (Millar 9).