Fantasy Trimaran

Calculating displacement can be looked up on the web. Cross arm structures are built in many ways. Look at Multihull Structure Thoughts thread and read several pages in, you will start to get the idea of what is required to build and hold a cat or trimaran together. Final thought. The righting moment (displacement multiplied by the distance between the centreline of a main hull of a tri and the centre line of a float) of a 40 foot tri is about 120,000 ft lbs. An 80 foot tri of 40,000 displacement 20 meter wide is about 1,200,000 ft lbs or 10 times more than a 40 footer. The cross beams have to be 10 times stronger to have equivalent safety factors. 10 times stronger often mean 10 times of any given material. Is it becoming clearer why something bigger may take a lot longer to build.

 

For the legal stuff your go to place as a british subject is the MCA. Here some links to get you started:
Vessel classification and certification https://www.gov.uk/guidance/vessel-classification-and-certification
Pleasure Craft Regulations | Regulations | Knowledge & Advice | RYA - Royal Yachting Association https://www.rya.org.uk/knowledge-advice/regulations/pleasure-craft/Pages/hub.aspx
As far as I know (but I could be wrong) there is no law stating that a big boat must be constructed by trained profesionals in a commercial yard. Backyard amateur construction is possible as long as you comply to all relevant regulations (wich you must clear beforehand with the MCA) for construction and operation.
The other "legal" aspect is insurance, but that is private law and negociated between parties.

The design and building is something completley different. As far as I know there are no theoretical size limitations for laminated wood construction. That does not mean that it always makes sense or that practical limitations do not exist. Multihull beams and masts are a good example, while possible in theory in practice their size and weight will impose severe compromises on the rest of the boat to the point where one can not fulfill the original design goals (just to be clear I am referring here to big boats with big loads).
Constructing the hulls in ply epoxy is certainly feasable even for a big boat, but after a size one must ask if other materials do not make more financial sense. At 24m and over aluminium is probably cheaper and faster to build if we compare cost per finshed square meter of hull. It might even be lighter than ply epoxy but that probably depends on the design brief (meaning how you prioritize weight vs durability vs risk). Normally I would add that location also has an impact on price, but with the quantity of materials involved in a 24+m trimaran, transport and import duties are irrelevant, you won't be buying local anyway.
Sails can be lower spec but after a certain size you need to divide the rig into smaller panels to manage the forces involved. Even so you must realize that the rig is going to cost as much as the hulls, no way around it.
Building time is measured in man hours. More workers less actual time needed. 30 000 man hours means 300 hours for 100 workers or 7,5 weeks of regular worktime. That is trained workers with all pro equipement, no head scratching about "how is this done best" or "where do I buy that". Of course in reality you can not have 100 workers working at the same time because some operations need to be done in a certain order and some jobs only allow so much people working at once, but you get the idea. If you need to do every job by yourself and learn how to do it beforehand besides tackling all those jobs that are not directly building like logistics, systems design, etc. the figure doubles.
Unfortunately a boat is not just a funny shaped plywood cabinet. Building the shell is the easy and cheap part.

 

I have read the replies in this thread and I want to thank you all for your input so far, and I would like to reply properly in the future but for this moment I want to ask about something. It seems that it may be better to start small at this time, and I am aware of the smaller, commercially available trimarans, specifically the CORSAIR and DRAGONFLY (and I forgot the third company),with their range of small to smaller sizes and specifications such as "sport" and "cruise" options, but let me ask this - What would be the sort of cost and feasability of something along the lines of a 12metre "backyard building" version from plywood, epoxy resin and glass cloth, of one of these smaller commercially available trimarans in the +/- 12 metre range?

I am wondering how much their price tags are accounted for by the labour, high specifications and "added" value of the extra technology and furnishings, and therefore how much a lesser-sophisticated but similar-sized home-built version of the cruiser/live-aboard type might cost, roughly.

As I said I realize that the commercial ones are packed with technology and high-spec, so I am not counting that into this question, especially as some such technology could theoretically be added later, depending. I am asking about a trimaran that has a hull, rigging, maybe an outboard motor of suitable hp, and the other "basics" and "works".

 

Fantasy. I will try and open up the question with more reading. there were many 40 ply tris plans available. Some designers still sell them, but the best were the Searunner, Seaclipper or constant camber series sold by John Marples Searunner Multihulls - Searunner Trimarans https://www.searunner.com/index.php/designs/searunner-trimarans . Attached samples below. Norm Cross also sold many good tri plans that may be available. Cross had an excellent build time versus size chart that will still be on the web somewhere. Dont buy/build an Arthur Piver trimaran. A Very General statement. A well finished/equipped home built ply boat will cost will be similar to a production built same design. Reason. Most production builders get big discounts on equipment and materials which balances out the labour costs. There are other ply tri designers but download two free pdf's one is at here-tay-vaughan https://www.yumpu.com/en/document/view/30192813/here-tay-vaughan/98 which is the Searunner study plan and philosophy book and the Searunner Construction Manual (photo attached will need to search on think outrigger media site). The real issue with ply boats is not their construction costs but their lousy resale values in a world of fiberglass. The rough cost build breakdown is 33% on hull structures, 33% on internal fit out, 33% on rig and equipment. Don't go cheap on hull materials, limit internal fit out and get second hand rig/deck gear that can be replaced. You will spend about 5000 plus hours build time. Costs depend on where you are in this world.