How big can a yacht be (max length) and why (CFR, etc..)?

California does license mechanical engineers, and interprets some engineering work done on boats and ships as mechanical engineering. This is straight from the California Board of Professional Engineers:

"Stability and structural calculations for a ship or boat would be mechanical engineering since the vessel is not a fixed work (which would require a civil license)."

All employees, contractors, and consultants of manufacturing corporations are exempt from licensure in California. So a license is not required to perform engineering services for a boat builder. A license is required to perform engineering services for a private individual. Naval architecture tasks such as stability calculations and structural design are considered to be mechanical engineering in California.

U.S. flagged recreational vessels over 300 gross tons are inspected under 46 CFR Subchapter I. See 46 CFR 90.05-1(a). A PE license is not required to submit drawings and calculations to the Coast Guard for inspected vessels. However, the Coast Guard may accept, as the basis for plan approval, the certification of a registered PE or a duly authorized ABS employee that vessel plans, designs, calculations, etc. meet applicable Coast Guard requirements. See NVIC 10-92.