Floating island - Trimaran or Catamaran?

If this is a cost conscious build, you might consider the ancient and venerable practice of strapping metal drums to the bottom of a triple strength joisted structure, filling them with styrofoam., or some other inexpensive material that would assist in flotation. Then, building a concrete dock somewhere that you could berth her in storm season. Thats silly, of course, but its an idea to think about.

In my case, I might build the platform in 10x10 sections, and then bolt or pin them together, as well as mount full length stiffeners ( like, I-beams or trusses )across all sections, in both directions for use. Erect the demountable bar structure ( i would do it out of tubular steel ) on top each season, tear it down each season. That way I could haul the whole thing out in pieces and store it on shore, safely away from hurricanes. Have a nice tow boat available to move it around easily and to get supplies, as well as to do setup and tear down. Flotation is super cheap, so supporting more weight shouldn't be an issue.

The nice thing there? Its not a vessel. plus, you can easily get the sections out in the water or stored with minimal equipment. You just have to work out flotation needed for max expected weight. The down side is she might have more than 1/3 of her weight aloft ( with people on board the second story) , so you'd have to seriously moor her down once you found a spot.

FYI, though it wasn't on water, I have done bigger demountable structures than this at Burning Man many times, and only the largest ones took more than a day to put up. We tested a few ideas on water like this, and they were "hella fer stout" once we got done.

Good luck, and make SURE you, or a competent NA, does the math!!

 

Wrong. It is a commercial vessel. It needs to comply with whatever local regulations and insurance requirements are.

Just having enough buoyancy for the expected load evenly spaced is not enough. The calculations need to consider the worst case condition of all the passengers and crew being on one side, plus the windage force. Further, a mooring does not have any effect of stability.

 

.... and many other things. In addition, it is necessary to find out what the "worst condition" is for each study, since the worst condition for the study of people on one side does not have to be the same as when studying the action of the wind. On the other hand, the study of wind and waves is more restrictive than that of wind alone.
Not only the stability must be studied, which requires checking several load conditions, but also the freeboard and buoyancy.
It is easy to forget, or not know, that the minimum freeboard or buoyancy must be checked despite being conditions that are sometimes difficult to meet.