[2] She was 108 feet 6 inches (33.1 m) long and of wooden construction, and her main deck had a cabin, a laboratory, accommodations for her officers and embarked scientists, and a mast fitted for dredging.

[5] She then made two oceanographic cruises in the Gulf of Maine under the direction of Dr. Henry B. Bigelow, the first in December 1920–January 1921 and the second in March 1921, steaming a combined total of 1,286 nautical miles (2,382 km; 1,480 mi) between Cape Cod, Massachusetts, and Nova Scotia, Canada.

[5] With it completed, she made a 1,243-nautical-mile (2,302 km; 1,430 mi) cruise between Nantucket, Massachusetts, Browns Bank, and Yarmouth, Nova Scotia, Canada, to collect bottom samples in support of a hydrographic survey of the Gulf of Maine.

[7][note 6] During March and April 1922, Halcyon was at Newport, Rhode Island, spending little time at sea but conducting work in support of flatfish studies by the Woods Hole station.

[8] In February 1923, Halycon provided valuable service in keeping channels open through ice in the harbors of Gloucester, Manchester-by-the-Sea, and Salem, Massachusetts,[8] and from April through June 1923, she tagged codfish near Nantucket.

[10] Halcyon continued her fishery investigations in the Gulf of Maine during fiscal year 1926 (1 July 1925–30 June 1926), working in concert with the BOF research vessel USFS Gannet and steaming nearly 3,000 nautical miles (5,600 km; 3,500 mi).

[11] She interrupted this work in February 1926 to visit Portsmouth Navy Yard in Kittery, Maine, where her crew reconditioned the newly acquired BOF research vessel USFS Albatross II.