Spelljammer has been around for a while, and it's amassed a bunch of Olias-of-Sunhillow style ships over the years. The 5e edition of Spelljammer adapted or converted 15 or so, but that leaves over 30 ships lost in the shuffle. Luckily, a resourceful fan called ffwydriadd has converted a bunch of ships and posted a free PDF online.
One ship that I thought hadn't been converted was the man-o-war, an elven ship from the 2e Lorebook of the Void source book. As it turns out, it actually was included in Spelljammer 5e, but under a different name, so I didn't discover it until ffwydriadd pointed it out to me. It's interesting to ponder how the official conversion was done, though, and what's involved in doing a conversion, so I'm posting my process nevertheless.
A 2e man-o-war is a big ship, and there's no reason to change that for 5e, so the keel and beam stay the same.
It's got 2 medium ballistae requiring 2 crew members, a catapult requiring 3, and a jettison requiring 3. That's a lot of weaponry, but then again it's called a man-o-war. The problem is, 5e's weapon stats differ from 2e weapon stats. Either the crew required for each battle station is going to be different, or else there will be fewer battle stations.
I think keeping the battle stations the same is best. After all, it's a man-o-war and should be well suited to attack from any position.
A ballista is a ballista, so no change is required there except that 5e ballistae tend to require 3 crew members while the 2e medium ballistae required 2.
A mangonel is a catapult, so that translates easily, aside from the increase in crew requirements (from 3 to 5.)
Finally, a jettison is a ship-to-ship Spelljammer technique of basically dumping rubbish onto an enemy deck in hopes of injuring crew members. There's no such weapon in 5e, but in my mind this is potentially more a trap than a weapon. I can imagine imposing a DEX saving throw for players to avoid the rubbish thrown at your vessel, rather than making a targeted attack roll. I'd scale damage depending on level. For tier 1 and 2 players, the enemy just threw some rocks. For tier 3 and up, the ammunition is caltrops, scrap metal, glass, and so on.
The weapons:
The 2e man-o-war has an armor rating of 7, which translates roughly to a 14 or 15 in 5e.
This may be a little off, due to materials. The man-o-war has a ceramic hull, while other ships have wood or metal or crystal hulls. I'm not sure what effect, if any, that has on AC. I'm working under the assumption that the material has no effect on hardness and, if anything, affects resistance and immunity (a metal hull is likely immune to fire damage, while a wood hull is not.)
Hull points in 2e indicated how much damage a ship could take before its hull was breached. In 2e, numbers range from, say, 25 to 60. There doesn't seem to be a direct correlation between the 5e HP number and 2e hull points. For instance, the squid-ship in 5e has 300 HP, while in 2e it has 45 hull points.
Monster manual stats by AC, as provided in the DMG on page 274, renders very low AC values compared to ships converted by the Wizards of the Coast authors. For instance, a hammerhead ship in 5e has an AC 15 (wood) and HP 400, but a monster with AC 15 according to the DMG is expected to have 175 HP at the most.
In my experience, ship combat usually goes on too long, so the fewer HP you have to punch through to end the encounter the better. For that reason, I think lower HP is preferable, and I wish the authors had just followed the monster stat estimates.
Then again, a ship is basically a moving building, and buildings tend to be pretty hard to hammer through, so I understand why the HP numbers should arguably be bigger. The ballistae of 5e definitely do more damage than they did in 2e, so that helps.
In an attempt to provide myself with some guidance on what number to start with, I tried adding 2e hull points to the ship's beam measurement, and then multiplying the result by 5. It's not far off, in the few cases of direct (or near direct) comparisons between 2e and 5e ships.
The "hammerhead" ship in 5e is the "hammership" in 2e:
The "flying fish ship" in 5e is essentially the "tradesman" in 2e, although the 5e version is physically larger.
The nautiloid in each version:
The wasp in each version:
It's obviously not a perfect formula, but it does seem to get similar numbers to what's been demonstrated by the authors. And that number is closer to the target than anything I tried to make the DMG monster table work. (In short, it's pretty obvious that a ship's AC has nothing to do with its HP.)
To be honest, I'm not confident that there is a formula. Considering that the 5e Spelljammer books take a very casual approach to ship rules (and by casual, I mean it foregoes it almost entirely), I suspect that the hit points were alotted by feel rather than any kind of mathematical conversion. And that's fine. These are ships, and there's a lot of variety in the multiverse. I can believe the HP values as easily as I can believe that ships can fly in outer space.
Almost all ships in Spelljammer 5e have a damage threshold of 15. The exception is the Bombard, which has 20.
The DMG describes damage threshold on page 246. Essentially, big objects (like a castle) ignore damage unless that damage exceeds its damage threshold. Because the DMG isn't organised in any way, it lists damage thresholds for ships back on page 119 (yes, over one-hundred pages before the concept is introduced.)
For a warship, the damage threshold is listed as 20, which corresponds to the Bombard. The man-o-war is indeed a warship, so its damage threshold is 20.
On page 119 of the DMG, the cost of a warship is listed as 25,000 gp. There's no reason to expect a man-o-war to cost more than that, unless you factor in the price of the Spelljamming helm. Unfortunately, the Spelljammer ships don't seem to always obey the table on page 119. For instance, the Bombard is 50,000 gp but there's no ship on the table over 30,000 (the galley.) The Bombard has a crew 12 people, while a galley has 80 and the warship has 60. Both the galley and warship have 500 HP, but no ship in Spelljammer 5e has an AC over 400.
The authors of Spelljammer have obviously decided not to follow the rules described in the DMG. If I follow the DMG, I break the economy of Spelljammer (why buy a Bombard for 50,000 when you can get a Man-o-war for 25,000?) For that reason, I set the price of a man-o-war at 50,000.
Again, the DMG lists speeds for ships but none of them align with Spelljammer. Spelljammer tends to set speeds at around 35 ft. per turn, or 4 miles an hour on average. There are some ships that move twice that, others that move a little faster. But it feels like a general-purpose Spelljammer moves at 4 mph.
I see no reason to deviate from that, and a few reasons to follow suit. First, I see a lot of value in standards. Ground combat in D&D is expressed in as around 30 ft. or 6 squares on a battle map. The more ship combat mimics ground combat, the less a player has to learn anew. 30 to 35 ft. movement seems like a great way to remove any confusion about how far ships move: they move the same as your character does. Secondly, deviating from estabished values in official Spelljammer would break the speed and chase economy.
For those reasons, I set the speed of the man-o-war at 35 ft.
Armor Class: 15 (ceramic)
Hit Points: 400
Damage Threshold: 20
Speed: fly 35 ft. (4 mph)
Cargo: 30 tons
Crew: 20
Keel/Beam: 200'/20'
Cost: 50,000 gp
Armor Class: 15
Hit Points: 50
Cost: 50 gp (ballista), 5 gp (bolt)
It takes 1 action to load a ballista, 1 action to aim it, 1 action to fire it.
Bolt: Ranged Weapon Attack: +6 to hit, range 120/480 ft., one target. Hit: 16 (3d10) piercing damage.
Armor Class: 15
Hit Points: 100
Cost: 100 gp (mangonel), — (stone)
It takes 2 actions to load a mangonel, 2 actions to aim it, and 1 action to fire it.
Mangonel Stone: Ranged Weapon Attack: +5 to hit, range 200/800 ft. (can't hit targets within 60 feet of it), one target. Hit: 27 (5d10) bludgeoning damage.
Armor Class: 14
Hit Points: 80
Cost: 100 gp (jettison), — (scrap metal)
It takes 1 action to load a jettison, 1 action to fire it.
Jettison Rubbish: All creatures not below deck of target ship must make a Dexterity saving throw against DC 15. On a failed save, take 7 (2d6) piercing or bludgeoning damage (DM's choice).
My conversion is pretty close to the official one, as it turns out. Here's how mine compares:
That's pretty close, actually. As I've said, the rules for Spelljamming ships in 5e are minimal, so conversion really isn't that hard. Don't over-think it, and you'll come up with something pretty reasonable.