"Aft: in the direction of the back (stern) of the ship"
Yes, but you should also define "After" which is a location as in, perhaps, "the after swimming pool." You walk aft to get to the after pool.
"Keel: a heavy steel shaft along the bottom of the ship that keeps it upright"
No. On a SAILBOAT the keel -- a heavy, fin-like hunk of lead -- balances the force of the wind on the sails, and keeps the boat from tipping over. On a SHIP, the keel is the heavy steel beam along the bottom to which everything else is attached. The design, and loading of the ship keeps it from tipping over.
"COURSE;direction in wich the ship headed,usually expressed in compass
Not quite. The COURSE is the path over the ocean you would LIKE the ship to follow. The HEADING is the compass direction you point the ship in order to make good your desired COURSE. The tides, the current, and the wind push the ship where they want, and you have to compensate with your HEADING in order to end up travelling along the COURSE you'd like to follow.
"FANTAIL;the rear or aft overhang of the ship."
Should be: "FANTAIL; the rear or AFTER overhang of the ship." Remember, aft is a direction, After is a place.