In my fantasy world I have a humanoid species that can work pretty much non-stop. Assuming he has enough energy, can one of these creatures do most of the necessary things on a 25 meter long cog to sail it successfully? There would usually be about 4 other people on board who are competent at sailing, so they could help out when extra hands are needed, but could the tireless creature and maybe one or two others do the bulk of the work, or are there too many stations to be manned? I haven't done much research about ships yet, so I don't know what all the different tasks on board would be, but the purpose of the ship would be either for traveling or transporting cargo.
From the real world, a sloop the size of your "cog" can be singlehanded for long voyages, even circumnavigations.
The reason I specify "sloop" is because the rig is designed to be easy to sail; the number of controls is minimized with only mainsail and jib (you wouldn't bother with a spinnaker in this situation, and they're generally only used for racing anyway). The tiller or wheel can be "lashed" when the captain/pilot has to leave the cockpit to change the set of sail, and for long ocean legs one will often hold the same tack for hours at a time anyway (you get the same effect if you sail fifty kilometers and then tack forty as if you sailed ten five kilometer legs and ten four kilometers ones the same headings).
By the late 19th century, there were even "autosteer" rigs using a wind vane to manage the tiller and keep the wind on the same point, allowing the singlehander to get a short nap without going adrift.
Speaking as a sailor, if we're talking a 75-footer on medieval technology, the answer is "not really." Even with the aforementioned four unskilled hands to pull line, there are tasks requiring skilled hands both managing the sails and the tiller. And in a storm, all bets are off: you need a skilled steersman on the tiller at all times, and people who know what they're doing around to deal with other tasks. In shallow waters, you need a leadsman to boot.
In fair weather, light winds, calm seas and deeper waters with light traffic, sure, you probably could manage. Anything else, that's a recipe for disaster.