The way I see it, the designs provided with the base game are just reasonably typical examples, but the game comes with a starship design system for a reason.
The standard designs are, excepting the Type M, fairly serviceable. The A & R both turn a reasonable profit on full load, and are J1, so the per_parsec/per_jump issue isn't. The A2 is within a couple KCr of profit on full load; if it tramps, rather than routes, it can and does turn a hefty profit long term.
The M in 77 only makes any economic sense at all by charging more per jump.
The changes in '81 are just enough to put it into workable... if you allow multi-ticket postings on route so you can fill, and keep it between high pop worlds.