Without learning actual nautical stuff, simple rules for sea travel are below. Rules are arbitrary, abstract, simplified, and based on assumptions.
1. Top Speed. We assume in optimal conditions (meaning ideal wind and ideal water), a vessel can travel its top speed. We assume the top speed of an average vessel is 24 miles per day (or 1 hex) and that it sails for a 12 hour day rather than an 8 hour day. A very fast vessel sails at 32 miles for day (or 1 and 1/2 hexes).
2. Other Speeds: In sub-optimal conditions (meaning poor wind and/or rough water), a vessel travels at either three-quarters, half, or quarter speed, which ever narratively seems more reasonable. If there is no wind or if the wind is blowing in the entirely wrong direction, the ship can only travel its intended course if it has ores.
3. Storms: Sailing in a bad storm can put wear and tear on a ship, causing damage to the ship. In a bad storm, a ship may sail at quarter speed and the crew risks exhaustion. An exhausted crew cannot row and must rest, and are more likely to lose morale.
4. Crew and Passengers: A ship requires a skilled crew for maintenance and operation. Each crew member earns 1 gp per day. Officers and navigators earn 10 gp per day or more. The size of the crew depends on the size of the ship. This crew size may be completely arbitrary. A small ship may be crewed by 10. A Medium ship may be crewed by 30. A Large ship may be crewed by 50 to 100.
5. Supplies: The ship must have supplies to feed its crew and passengers. This costs 1 gp per day per crew member and passenger. Supplies means food, medicine, tools, rope, gunpowder and shots for cannons (called guns) if applicable, and spare parts and materials for maintenance.
6. Cargo Space: The size of a vessel's cargo space is limited by the size of the vessel. Cargo space limits how long a ship can sail before needing to resupply. We assume a ship has enough cargo space for supplies for a 30 day voyage. The amount of supplies in your cargo space is therefore represented as X / Y days of supplies, or just X days of supplies.
7. Damage and Repair: Ships have HP and a Damage Threshold which are usually dependent on the size of the vessel. Damage Threshold means a ship does not take any damage unless it takes a certain minimum amount of damage. A ship may be mended at sea if you have the supplies, up to 1/2 it's max HP. You must return to port to get your ship completely repaired. Assume a ship requires one week for full repairs and the cost of repairs is 1 gp per missing HP. Ships may be damaged by ship combat, running aground, hitting a reef, or sailing in a bad storm. A ship at half HP or less may not sail at its top speed. A ship at 0 HP begins sinking.
8. Navigation. Make one navigation check per day or night. If you fail, you identify that you're off course after 1d6 hours (and losing that much distance in your travel day) and can re-roll your navigation check.
Navigation may take a penalty when land is not within sight or when the sky is cloudy, and a big penalty when there is a storm or fog. Ships ordinarily travel within sight of land to judge their speed, to drop anchor for the night or when needed. Traveling across a sea or ocean has certain risks of getting lost and running out of supplies, and benefits from having a crew with a night shift, and a sea chart.
9. Purchasing of a Ship: In a period setting, shipbuilding was controlled and expensive. A rowboat may cost 50 gp, but a small sailboat may cost 2,000 gp. A small vessel may cost anywhere from 3,500 to 5,000 gp. Medium vessel costs 10,000 to 15,000 gp. A Large vessel costs 20,000 to 30,000 gp.
10. Combat and Chases. In most cases, out-maneuvering another ship relies on having a faster ship. The referee may make any assumptions or decisions about the speeds of any ships at the scale the chases and combats take place. A faster ship automatically intercepts a slower ship in time, and a faster ship automatically escapes a slower ship in time. If you're engaged in a chase with a ship of a comparable speed, the referee may run a chase scene.
Chases: The referee determines the starting ranges. There are three ranges: out of range, firing range, and boarding range (alternatively, you may use four ranges that include long firing range and short firing range). Ranges are all arbitrary and abstract. Each round of the chase, the ships' helmsman (basically a boat pilot) makes a vehicle check. The ship that wins the check, changes the ranges in their favor by 1 range. The pursuer may elect to automatically stop their quarry if they succeed at the closest range. The quarry may elect to automatically escape if they succeed at the furthest range.
Combat: When a ship is in any firing range, the referee decides if the ships are in position for the guns. Ships at firing range may spend a round getting into firing position. Most guns on a ship are in fixed positions on the gun deck (below the main deck) and are pointed either to the left or right (starboard or port) sides of the ship. Guns on the main deck, if any, may be pointed in any direction. If the ship is not in position for the guns, then no guns on the gun deck can hit.
Each round, gunners fire one gun each. A gunner may make one d20 attack roll + their intelligence against the opposing ship's AC for one gun. Once all the hits are tallied, roll one d10 damage roll for each gun that hits, total and apply the damage. The number of guns is restricted by the size of the gun deck, and the size of the gun deck is restricted by the size of the ship.
