Ancient ships: less than 1,000 km per month, potentially, assuming good weather, shoreline hugging, known waters, and a pressing need to do so.
More likely is 250 km per week, especially assuming some serious R&R at the end of that week, to include time for planning and time to let weather blow over, etc.
Here are some notes I made some years ago when trying to figure out ancient ship speeds. The ships I was interested in were all pretty much galleys -- ships that were typically rowed, but had a primitive square sail that could be used when the wind was going your way.
The other major factor was that a galley was designed more or less for day-travel only: it typically hugged the shoreline, and carried a day's worth of water, thus you had maybe 6 to 8 hours of travel time before having to knock off for the day and sleep on the shore.
With all those caveats, I drew numbers from a book on ancient ships that for the life of me I cannot remember the name of. It seems that longships, triakonters, liburna, biremes and triremes had roughly similar speeds. The pentekonter may have been a bit faster. To wit:
7kph rowing average, and perhaps 8 to 9 kph for pentekonters, maybe.
Double that if the wind was favorable, and for very short rowing bursts.
Double THAT if the wind was ideal, which I assume is rare to nonexistent.
Assume, then, that that's 50 km per day.
In a week, a galley could therefore cover as much as 350 km. Although I bet actual weekly distances would be a standard deviation shorter (250 km).
Therefore, in a month, I'd say no more than 1,000 km -- and probably less, since I doubt the ships and rowers were built to operate 30 days straight with no downtime. But your ship, if keeping to shorelines with known characteristics, could cruise around for several hundred km in a month if pressed to it.
EVEN MORE NOTES:
For a rule of thumb, realistic speed is roughly the same as marching speed. 2-5 knots. Occasionally faster under full steam with the wind at your back, even up to 15 knots, but not consistently.
Ship ratio appears to be a good metric for predicting speed as well as carrying capacity, by the way: Larger ratio = faster ship, Smaller ratio = more payload. Ship size also apparently affects speed. And of course, skill also affects speed.
Galley Era: 3000 BC to AD 1500, approximate
- Galleys: "Vessels larger than boats, propelled primarily by oar power and only occasionally by sail."
- Coast huggers.
- Light. Operates in shallow waters, rivers, small bays.
- Can be dragged on short overland routes.
- Navigation by the sun, prevailing wind, and stars.
- Stored in "ship sheds" on the shore.
- Ships are transports. Wooden platforms for melee combat.
- One (or more) rowers per oar.
- Length implies => number of oars per side (1 per 1.5m?)
- Up to 3 levels of oars per side; more levels = more upkeep, lower endurance (3 levels = typically just a day's worth of fresh water)
- Can have multiple rowers per hull
- Catamaran hull doubles the hull size, increases carrying capacity more.
- Maximum rowing speed is 2x average rowing speed?
- Speed under sail is 4x average rowing speed?
- Some ships have a ram...
Light, fast, Scout/Raiding/Pirate vessels:
Longships in general
- 6 to 34 benches (on a side?)
Longship & Triakonter
- 24m x 5m
- 16 oars per side, 32 rowers total.
- Crew of 40, maximum capacity 70.
- 28 kph under sail max
- 14 kph rowing max
- 38m x 4.5m
- 25 oars per side, 50 rowers total.
- ?? kph under sail max
- 18 kph rowing max?
Bireme or Liburna
- 33m (or more) x 5m
- 18 oars (or more) per side, 2 levels of rowers, 1 rower per oar.
- 28 kph under sail max
- 14 kph rowing max.
- 7 kph rowing avg.
- 3 levels of oars, 85 oars per side, 1 man per oar.
- 14 kph rowing max. 7 kph avg.
- Good offensive strength. Ram.
- Only carries one day's worth of supplies (i.e. fresh water).
- Lots of upkeep.
- Tactically inferior to Liburna.
- 24m to 37m or more. 3m to 5m wide.