46 C.F.R. § 169.631 – Separation of machinery and fuel tank spaces from accommodation spaces.

(a) Machinery and fuel tank spaces must be separated from accommodation spaces by watertight or vapor tight bulkheads of double diagonal wood, marine plywood, steel plate, or equivalent construction.
(b) On vessels less than 90 feet in length, segregation may be by means of a watertight or vapor tight engine box.