(a)to a person who is in possession of a second-class master's certificate granted under section 21 or section 22, and has, by virtue of such certificate, acted as master of an inland [mechanically propelled vessel] [Substituted by Act 35 of 1977, Section 5, for " steam-vessel" (w.e.f. 1.5.1978).] having engines of forty or more nominal horse-power for a period of not less than five years, or