I came up with the concept for just such a ship once for an RPG. It did not require any "antigravity" to make it float either.
The basic premise that allowed the ships to fly and navigate was that there was a magical "keel" material that would easily move back and forth along one axis, but was very much harder to move in any other direction, though somewhat less difficult to turn. Thus, by holding the keel material in a particular attitude and pushing, an entire assembly could be made to gain in altitude and manoeuvre under wind power. The performance parameters of the airship would depend on the amount of sideslip that the keel material allowed amongst other things.
The ship itself, as I envisaged it, would be a hull shaped like a boat-tail rifle bullet within which cargo and the keel mechanism would be housed, with masts that hold the sails. It made sense that there would be masts on all sides of the ship, from two extending laterally. to three in an Y (upright or inverted), or four in an X or a +. Masts may also be duplicated moving back along the hull, thus 4 or 6 or 8 with two sets of masts, or 6 or 9 or 12 with three sets. The masts would carry spars, sails and rigging that would catch the wind.
A Lateral-only mast plan would have the advantage that the masts would not interfere with landing greatly, while two sets of X- or inverted-Y- masts could act as landing gear if they were properly rigged and reinforced.
The ship would be controlled by using two or more independently controllable keels, mounted in gimbals fore and aft at a minimum, though the more keels there are, the more stable the ship. Since the keels move easily in only one direction, and resist movement (relative to their environment) in other directions, steering can be accomplished by properly angling multiple keels. With both keels pointing slightly upwards, the thrust provided by the wind will cause the ship to sail along in level flight. Angle them further upwards, and the ship will climb, and angling them down will cause the ship to descend. Angling the fore and aft keels up and down independently can cause the ship to pitch, and angling the keels sideways can cause the ship to sideslip or turn. With more keels, say 4, the ship can also be made to roll.
There would be issues of stability that would require constant trimming of attitude, though this need not be accomplished by the crew; the gimbals could be equipped with damped pendulums that would hang downwards in the local gravity, and could be used to automatically keep the ship in the correct attitude, and might even provide automatic banking so that centripetal forces would not lead to objects sliding across the decks, but would automatically bank the ship.
Like any sail-and-wind-powered ship, there would be issues of wind speed and direction, necessitating tacking or wearing in order to sail upwind, and affecting military tactics. In such ships, altitude would be as important a factor as being to the windward of an enemy, as it is a factor common to both sail and our self-powered flight that altitude may be traded for speed. By reducing sail in order to reduce drag, and descending, a ship's speed will increase significantly, and speed can also be traded for altitude. Navigators will also have to consider local wind conditions, including updrafts and thermals, though they would typically have lesser tactical implications than altitude and wind direction.
Landing would be a matter of descending, shedding speed by furling sails and correctly angling the keels (extra keels beyond 4 would help here) to create additional drag, and allowing the keels' inherent side-slip to gently lower the ship to the ground. Take-off would be a little more complicated - there would need to be landing gear that would be retracted to allow the ship to float freely long enough to gain sufficient headway for the keels to provide enough lift to climb.
Considering the weapons that these ships might carry, Age of Sail had as its best weapon system black-powder smoothbore muzzle-loading artillery. The shot used in this period was initially stone or iron round shot, and progressed to explosive-shell round shot with time-delay or impact fusing. It would also be reasonable to suppose that we could have black-powder gravity bombs with time similar fusing. Rocketry existed in the AoS, but was lightweight and inaccurate, and used primarily for night-time signalling.
Given the complexities of air-to-air combat with smoothbore open-sighted artillery, we could expect that most combat would occur either at roughly equal altitudes with artillery broadsides fired from long cannon or short carronades, or at high/low altitude, with unguided bombs and downwards-firing guns against high-angle guns.
It would be possible to mount a smoothbore artillery piece so as to be able to fire steeply downwards, but this would require ramming the bore with a tightly-fitting wad over the shot to prevent the shot from simply rolling out. In addition, the gun would have to be loaded horizontally, then tilted downwards and primed in an rear-facing flash-pan or flintlock before being run out. A downwards firing gun would require a rather different gun carriage, which would allow the gun to be run in and out vertically yet still remain secured (We don't want a loose cannon) and able to be tilted for loading. The breeching would involve counterweights so that running the guns in and out by hand would be possible. Downward-firing guns would be slower-firing and heavier than broadside-firing guns or upwards-firing guns.
In the AoS, the primary armour material was wood, though some ships had double timber hulls with the space between filled with stone; these were less successful. AoS naval ships could have very thick wooden hulls, often several feet thick, yet this armour was also a liability, as shot that penetrated the hull would be accompanied by a spray of splinters which caused more crew injuries and fatalities than the shot itself. Short barrelled carronades capitalised on this, firing large, relatively slow (and short-ranged) shot that maximised splintering and crew casualties, as paradoxically faster, smaller, longer-ranged shot caused less damage to both ship and crew. Ironclad wooden ships occurred near the close of the AoS, and were nearly invulnerable to roundshot, driving the development of ogival shells. In the AoS, there were also varieties of shot designed to maximise damage to an enemy ships rigging, including bar shot and chain shot.
As much combat would occur on a level or high/low, warships may be designed with diamond hulls that would more readily deflect shot from the same level or from above/below, though to be most effective, the ship would have to be rolled to present the plane passing through the points of the diamond toward the projected trajectory of the incoming shot. This may be effective in 1 on 1 combat scenarios, but in a massed battle, the flat faces of the diamonds would be more of a vulnerability than a defence, and it could be expected that heavy combatants designed for massed combat would have round cross-sectioned hulls, those being most likely to deflect a shot coming from any trajectory
In the AoS naval battles, a common formation was the line of battle, with ships lining up so that if an enemy did manage to "Cross the T" (to pass in front - or less effectively astern - of an enemy ship with its broadside facing the smaller, less heavily armed bow or stern aspect), only the lead or trailing ship would be particularly vulnerable.
To extend the line of battle from 2 dimensions on the sea to 3 dimensions in the air, we would have a wall of battle, with lines of ships stacked so that the broad side of the formation would face the enemy, most commonly vertically or horizontally, but angled walls would have their place in some combat situations.
Neutralising Enemy Ships
In combat, an enemy ship can be neutralised in several ways:
- The hull can be broken to the point where the ship comes apart - this would be the most difficult outcome to achieve through gunfire alone.
- The crew could be injured or killed to the point where the ship is no longer an effective fighting unit.
- The ship could be set on fire, which would damage all the equipment and injure the crew, and in the worst case, could result in a magazine explosion that could destroy the ship.
- The sails, spars, masts and rigging can be attacked, reducing manoeuvrability and speed, and ultimately resulting in the ship being forced to land or crash.
- The keel gimbals could be damaged, resulting in a loss of manoeuvrability.
- The keels could be dismounted or destroyed, resulting in a loss of 'grip' that would cause the ship to crash.
Given that the keels would be the most important and vulnerable components of such a ship, it can be anticipated that in military vessels, the gimbals would be armoured in addition to any other armour the ship may carry.
In combat against ground installations, airships have a distinct advantage in that they can attack from high altitude, beyond the range of ground-based guns. However, accuracy at these altitudes would be low, and a great deal of ordnance could be expended for only a few good hits. There would be a balance between altitude and accuracy, and it is likely that larger, longer ground-based guns could make effective attacks at ranges not effective for ship-based upwards-facing guns which would be more limited in size and mass.
Ground-based guns would also have the advantage that they could fire red-hot iron shot that would be highly likely to cause a fire in their target, however while this was an effective tactic in naval combat where misses would go harmlessly out to sea, in aerial combat, misses are quite likely to come back to earth and cause fires there, so this is more likely in coastal fortifications than those inland.
Parachutes would be invented earlier; they are simple devices that could be invented in the Age of Sail with the correct impetus, namely airships. As an interesting note, in World War I, parachutes had been invented and were issued to personnel manning barrage balloons (and to German pilots), but were not worn by allied fighter pilots, as it was thought that they encouraged cowardice, and to encourage fighter pilots to fight to save their aircraft rather than bail out, they weren't issued. It may well be the case in some aerial warship forces that the crew are not issued parachutes so as to encourage them to fight rather than flee by the expedient of jumping overboard and trusting to their parachute. It may also be possible that the more highly trained officers and petty officers would be issued parachutes while common airmen would not be; it would all depend on the attitudes of their leaders.
We can also expect that there may well be a branch of military service for paratroopers; ground troops deployed from aircraft. These troops could be deployed far more rapidly than marines from an AoS naval ship, who would have to rely on rowed boats.
Conversely, we can expect that parachutes would be standard issue for merchant crews; a parachute and a delay for search and rescue could be cheaper than replacing a crewman in the event of a fall from the rigging.
We can expect that ram-air parafoil parachutes would be rare even after their invention. In a military situation, they provide too much manoeuvrability that would lead to the excessive dispersal of a mass-drop of paratroopers, and they are more complicated to use and train than round 'chutes. Their primary use might be in small special-forces drops.