Short answer
Source quality is claim-specific. A shipyard plan may be strongest for construction dimensions, an inquiry for disaster testimony, and a passenger list for who actually sailed. No source type wins every question.
A useful source hierarchy is conditional
Primary technical recordsShipyard plans, registers, specifications, logbooks, manifests, and official records are often strongest for the facts they were created to document.
Institutional archives and museumsStrong custodianship and provenance make these especially useful, though catalog descriptions can still simplify complex histories.
Scholarly secondary worksMost valuable when they identify sources, explain conflicts, and distinguish inference from documentation.
Company publicityExcellent evidence for how a line presented a ship, but not automatically neutral evidence for performance, luxury, or competitive claims.
Contemporary newspapersEssential for chronology and public reaction, but vulnerable to early errors, sensationalism, and provisional disaster figures.
Collector and enthusiast sourcesCan preserve valuable specialist knowledge, but claims should be traceable before they become evidence.
Ask what the source was trying to do
A source can be authentic and still be poor evidence for a particular claim. Advertising material is excellent evidence of marketing language; it may be weak evidence for an objectively measured speed claim. An inquiry transcript is excellent for testimony; a witness may still be mistaken.
Repetition is not independence
Ten websites repeating the same figure may all descend from one uncited book. Agreement only becomes powerful when independent source chains converge.
Curator standard: prefer traceable evidence over repeated certainty. If the evidence chain cannot be reconstructed, lower the confidence of the claim.