A guide to naval warfare

I write this guide with two objectives in mind. Firstly to broach a subject not many are really familiar with, educatory in a way; secondly to clarify the uses of naval warfare, to say something about why one would invest in naval warfare in the first place.

Why would you embark?

That second objective is not something to be overlooked, for naval warfare is not something one has to invest in, it is not a necessity. People live on land. If you want to defend yours or to attack others directly you need to do that on land, for that's where you'll find them. Land warfare is also 'easy' in a sense: you only need troops. They can use their own legs to move and, if necessary, their own hands or tools to fight. Naval warfare is another matter. You need shipyards, vessels and you won't find people out on sea. In other words, you need to make investments to go somewhere where you won't find your target. So while land warfare comes natural to us land creatures, we need a reason to go out on the waves.

For most generals the primary reason would be crossing large bodies of water, so they can continue their land warfare on the other side. The sea can be an obstacle, to cross which you need a fleet. Why else would you use a fleet? A reason closely related to this first one is transportation along the coast. This can be highly profitable, for movement over water is much more efficient than over land. Ships are much faster and can transport more mass (troops and supplies) with less effort.
Another reason that may come to mind would be to build a fleet to defend your coast against enemy fleets. With some exceptions that is a fruitless idea. Your fleet can only be in one place at the same time and probably your coastline is long enough that your enemies could land anywhere along that coast. It'd probably only work if the port where your fleet is stationed is attacked directly from sea, if your coastline is very, very short, or if there is some other bottleneck through which the enemy has to pass in order to reach your coast.
More than that though, building a fleet specifically to fight enemy fleets smells of waste. If both you and your enemy would do that and you would fight, you would both invest a lot to contest a terrain that probably is no direct use and might contribute nothing to victory: a total waste. Still, it might just be of some use. Perhaps you want to use your fleet preventively to locate and destroy enemy fleets before they can be used to transport an army to attack you. Perhaps you want prey on mercantile shipping to harm an enemy who relies on it, or to counter such piracy. Or perhaps you want to enforce a naval blockade on an enemy to whom shipping is important.
Still, in most cases any such extra reasons will be secondary to the primary reason to bringing troops across the water.

The ships

Where on land the land is your world, a world with aspects that affect warfare, a world in which you move around, the ships and the fleets are your world on sea. Unless you're confident about swimming in sea the limits of the ships are the practical limits of your world. You also don't move yourself, but your ship moves for you. In short, the ship and its features are important.
In Arl the variety of ships is kept limited so as not to further complicate naval warfare, for everyone's benefit. This might change later, but is the situation now. There are various kinds of vessels.

Note that embarking horses on roundships can only be done in a harbour and that disembarking horses anywhere but in a harbour is a difficult and slow task (involving makeshift cranes and boats to ferry the animals to the shore). Horse transports are a specialist variation on the roundship design (less round for one) built for horses, but the practical troubles with horses remain. They aren't as high as other roundships and lack castles.

Small craft include all sailing craft of various (perhaps local) designs smaller than the roundships. Normally used for fishing and river traffic, certainly not built for war, they can find use in war fleets for reconnaissance and river duties. They're manoeuvrable and cheap compared to larger craft. Because small craft are not built for the open sea their action radius is limited to two days' sailing.

Naval battle

While ships are important and your ships' features heavily influence all aspects of naval warfare, your ships don't fight themselves. Regardless of the importance of ships, once you find yourself in a fight, it's always your men who need to carry the day. When one thinks about naval battles, one is quick to think about broadsides of guns blazing away or galleys ramming each other, but not so in Arl. While you can put a few war engines on your deck there are no such broadsides in Arl, nor are there ramming galleys. When there's a naval battle your ships aren't really weapons, but more platforms on which your troops do the fighting.
The first thing this means is your primary target isn't the enemy ship, but the enemy troops and sailors. Spears and arrows can't sink a ship, but they can kill the people operating it. A benefit of that is that accidents resulting in sinking ships aside, any ships you defeat are yours to use (of course you can sink them afterward if you don't want them). When you do target a ship it's usually not to sink it but to incapacitate it, like cutting stays, ruining sails or demolishing the rudder.

A good way to picture naval fights is to regard each roundship as a floating castle. You can imagine the higher ship has a distinct advantage. You can rain down arrows on lower foes; are harder to target by them; and you can jump down on their decks while they have to climb your hull akin to scaling a castle's walls. Thus a large roundship is a match for several small ones; and longships are worse off.

The first sort of weapons used in naval battles is missile weapons. Just like on land you don't need to meet an enemy in close combat if you can take him down from afar. Even if you do intend to close in eventually, the opening stages of a battle often see exchanges of missiles to weaken the enemy. If you're facing higher vessels bows have an advantage over crossbows in being able to make arcing shots over the higher hull.

Then there's boarding. A reasonably simple practice, unless you need to scale a higher ship of course. Fore and aft castles matter here too. You can fire as many missiles at an enemy ship as you want, but if you want to actually take an enemy ship you'll need to board it sooner or later. One important thing to keep in mind is that the space on deck is limited. A ship might be capable of transporting many hundreds of troops, but no more than some one hundred fifty men will actually fit on deck to fight.
Close combat weaponry isn't dissimilar from their uses on land either. Just keep in mind the restricted space and lack of large formations aside when you're defending the board. Weapons that need room are useful when trying to prevent an enemy from boarding, but once they've gained a foothold and the close quarters combat really start their use deteriorates rapidly; small weapons are at a premium then.
Body armour is a risk. If you fall in the water you will drown faster, but in the close quarters combat good protection will help keep you alive.