captain of a small boat; skipper; coxswain
Refers to the person in command of a small vessel such as a midget submarine, torpedo boat, or lifeboat. Not used for large ships (see 船長).
彼は救命艇の艇長を務めた。
He served as the coxswain of the lifeboat.
length of a boat
Rare technical term for the overall length of a small boat or watercraft. In everyday contexts, 船の長さ or 全長 is more common.
このボートの艇長は5メートルです。
The length of this boat is 5 meters.
特殊潜航艇の艇長は、狭い艇内で指揮を執る。
The captain of a midget submarine commands from within the cramped vessel.
Compound of 艇 (small boat) and 長 (leader, head). The second sense 'length of a boat' is a rare extension using 長 in its meaning of 'length'.