Draft, beam, bridge clearance, and interior heights (tradeoffs between bridge deck clearance and headroom).
Weight budget and weight distribution (especially with large battery banks and compute equipment).
Bridge deck clearance (slamming avoidance)
Use loaded displacement (full fuel/water/provisions). Marketing specs are often light-ship.
Common rule-of-thumb: 5-6% of LOA as a starting point for offshore comfort; many offshore discussions cite ~30 in (76 cm) as a minimum reference point, but it depends on beam/tunnel shape.
Red flags: nacelles/steps/protrusions in the tunnel, flat/vertical panels for waves to hit, or a long low bridgedeck carried aft.
Single-hull width: hull slenderness (LWL/BWL)
There is no single standard hull width for blue-water cats; designers commonly reason about the slenderness of each hull using LWL/BWL, where BWL is the waterline beam of one hull.
Rule-of-thumb ranges (per Richard Woods): ~10:1 for a good cruising boat; ~16:1 for a racing boat; and ~11:1 to 13:1 as an often-optimal range for normal cruising catamarans in average conditions. Source: Woods Designs - Hull Resistance and Hull Shape Comparisons.
For this project's baseline LWL 38-48 ft, that implies typical single-hull waterline widths of roughly:
At 10:1: ~3.8-4.8 ft (1.16-1.46 m)
At 11-13:1: ~2.9-4.4 ft (0.89-1.33 m)
Note: spec sheets often publish max hull beam (topsides) rather than BWL. Flared topsides can make the max hull beam look "wide" even if the waterline beam is relatively slim.
Build vs refit vs buy notes
Build: you can design for access and future service; but mistakes are expensive.
Refit: hidden core rot/water intrusion can dominate schedule and budget.
Buy: survey focus is structural moisture, bulkhead tabbing, and evidence of overload.
To be added later
Material options and cost ranges; known failure modes; inspection checklist.