In order for Ishmailites to decide for themselves from reliable sources about the speed of whaling ships the following links are offered. The internet now offers a number of original logs of whaling vessels contemporary with Melville's voyages as well as before and after.
To keep ocean passage speeds in perspective please consider that the fastest ocean passage ever recorded for a 19th century vessel was the clipper ship Thermopylae from Britain to Australia. She averaged 9 knots (9 nautical miles per hour). The Thermopylae was a tea clipper with hollow entry forward and fine form to allow her to approach the theoretical speed limitations of a displacement hull (19 knots in her case). She was the race car of her day when compared to the plodding floating factories called whaling ships. Yet the best average day's run over a long ocean passage was 219 miles per day. Although a vessel in ideal conditions is able to approach her maximum hull speed, her average for any passage is usually half of her maximum speed. The whaler Charles W Morgan at Mystic had a theoretical max of 9+ knots.
For the purpose of calculating average speed and nautical miles run per day I have chosen the log of whaling ship Susan V of Nantucket outward bound in 1841 returning 1846. Choosing the fastest passages of the voyage for example from Dec 25,1841 to Jan 1, 1842 the log gives first fix as
Saturday Dec 25 ; 34° 31"N, 44° 35"W
a seven day run to Sat Jan 1; 31° 10"N, 31° 42"W
Calculating the great circle route at the navigation site
gives 1259 km which converts to 680 Nautical Miles (nautical mile = 6080 ft)
680 divided by 7 days gives 97 miles per day at an average speed of 4 Knots
One can convert km to nautical miles at
Homeward bound The Susan V running well loaded in ideal conditions made the following 4 day run
Feb 18 43° 35"S, 83° 24"W to Feb 22 48°12"S, 80°50"W
giving 550km converts to 297 nautical miles for 4 days run or 74 miles per day for an average speed of 3 knots
You can calculate for yourself from a number of other logs using the sites mentioned above.
The logs are well worth reading for the art and perspective of life aboard.