(1) A device to support the boom. Sometimes takes the form of a plank with a notch at the top like a Y, whence cometh the name which is the same as the crutch of a tree or the crotch of a person. Sometimes takes the form of a metal strut or crossed struts. But a goal-post type of support with two verticals and a cross-piece is known as a Gallows.
(2) A rowlock crutch is the purist’s name for what I call a Rowlock. A metal object of familiar form (like a Y but with a top that’s more of a U than a V) which supports an oar for rowing. It is called a crutch because it is like one of nature’s crutches, and the hole it drops into is a Rowlock. ‘strue. But I’ll still call crutches rowlocks.