nonmagical engines are actually capped at 100 tons in WCC as i recall. there's a table that lists possible tonnage, and iirc it has a limit for nonmagical engines.
that said, one of the books has rules for towing objects too large to move with a single helm, but again, that's at tactical speed. it would take months to get to a place that SJ speeds could get you in under 1 round (about 69,500 miles per round at SJ speeds... equivalent to SR 244,640 by some quick math, so about 170 days travel at SR 1 equals 1 minute of travel at spelljamming speed).
anyways, the series helms in astromundi cluster i was referring to were... antillans i think they're called? anyways, the big sun empire around the core of the system has the ability to link major helms to make giant superships (those ships may need to be made out of crystals). i was not referring to the thoric tradesman at all.
of course, given the nature of the astromundi cluster, even if you can find one and have the resources to use it, getting it out of the sphere is a whole different story
