I did a bunch of calculations before I realized that the Core Rules should be indicating those ship movements as being in "yards per round" on page 95, rather than in "feet per round", or else they won't come out consistent with the results for "miles per day". I feel like I did the exact same thing a fewy years back!
Minor error, but if there's an errata list being prepared for the next revision, no sense in not fixing it so I don't do the same thing again a few years in the future.
Went back to look this up in older editions and apparently it's consistent in both B/X and BECMI. For whatever reason, all the tables across editions switch into feet/round instead of yards/round for ship movement.
So it's not a "typo" in ACKS, it's just the way things have always been done in Basic. Still confuses me though, since all other outdoor movement switches to yards.
I've also noticed that both B/X and BECMI have the "sailing" and "rowing" combat speeds reversed for galleys. ACKS seems correct here, since it wouldn't make sense for galleys to suddenly get slower when they start rowing.
OD&D handles this more precisely, by breaking down rowing speeds into slow/cruise/sprint and breaking down sailing speeds by point of sail. I really need to houserule this back in somehow.
OK, after doing a few more calculations, I still feel like there's something "off" about the ship movement table, and I'm wondering if there's some mixture of movement in feet and movement in yards listed here.
First, here's a rough conversion list based on distance and time conversions:
10.7 knots ~ 60 yards/rnd ~ 180 ft/rnd ~ 83 mi/day
8.9 knots ~ 50 yards/rnd ~ 150 ft/rnd ~ 69 mi/day
7.1 knots ~ 40 yards/rnd ~ 120 ft/rnd ~ 55 mi/day
5.3 knots ~ 30 yards/rnd ~ 90 ft/rnd ~ 42 mi/day
3.6 knots ~ 20 yards/rnd ~ 60 ft/rnd ~ 27.6 mi/day
1.8 knots ~ 10 yards/rnd ~ 30 ft/rnd ~ 13.8 mi/day
This is based on the conversion factor 1 knot ~ 1.15 mph ~ 13.8 mile/day, which assumes 12 hours of travel per day. I've also assumed that rounds are 10 seconds.
Based on those numbers, it looks like ships that travel around 72 miles/day are probably traveling at about 9 knots, a bit optimistic but not impossible for a quality sailing ship in good wind. That corresponds to a speed of 150 ft/round. If we look at the ACKS table for a large sailing ship, however, we see a combat speed of 45'. This looks much more like the correct speed in "yards/round", rather than ft/round. Incidentally, B/X and BECMI both list this as 150', so it was decreased by a factor of 3 for some reason in ACKS, not just copied from those earlier sources.
On the other hand, ACKS does use speeds of the 120' to 150' for galleys in combat, which feels just about right in ft/round. They should be able to sprint up into the 8-10 knot range for short times. As noted above, this looks like a correction in ACKS from the B/X and BECMI values, which seem obviously wrong. They have sailing speed for galleys being higher than rowing speed -- which might possibly be true outside combat, but certainly not during combat. I'm not sure about speeds in miles per day. ACKS optimistically lists galleys at 40-50 miles/day, but B/X thinks they can only make 12-18. I would have guessed somewhere in the middle, about 20-30 miles.
So at this point my assessment is that ACKS fixed galley speeds, but at the same time introduced an error in sailing speeds, possibly due to giving values in yards/round rather than ft/round. (It's possible this adjustment is related to some other correction factor, but I'm not sure what kind of correction would only reduce combat speeds but not daily movement...) I'm curious to see if there's anything I missed, but otherwise I think I'm going to house rule the sailing ship speeds back to B/X speeds. This gives them much better odds to escape a galley, unless caught upwind or without any wind at all.
If Alex is around, I'd be very interested in the rationale behind the changes from older editions.
My research into ancient naval warfare found that galleys were the ships-of-war and that sailing ships were largely useless in battle during that time period. They were outpaced and outmaneuvered by galleys. However, over long distances, the sailing ships were preferred for mercantile activity.
In the absence of a complex game mechanic to reflect wind gauge, sail hoisting, direction, facing, and so on, reducing the combat speed of sailing ships seemed like a good proxy for the low mobility of sailing ships. Hence, the speed of sailing ships in combat is reduced so that they are prey to galleys. Conversely, the speed of sailing ships out of combat is somewhat optimistic. A player is therefore encouraged to use sailing ships for his trade routes, and galleys for his combat needs. (The cargo capacity of sailing ships was also increased to a realistic level from the tiny cargo of BX / BECMI ships.)
If you are planning to create a detailed naval system, I would recommend working from primary sources to create outcomes that fit the time period you want to simulate. ACKS just tried to update existing D&D values a bit to simulate the ancient setting.