Fiberglass construction

Interesting. Missed the issue in your item 1 - what mins/secs in the video is it? Thick glue seams are not good in general terms. Imho should have used all epoxy resins and fillers etc not mixed with polyester based products. Your item 3 they are putting cloth each side of the top deck see the initial part of that near the end of the clip. Would be concerned at the relative lack of structural continuity of the whole lot onto the original transom - ok there is glass under the hull to the sail drive and up the topsides, and the top deck overlaps the original deck, but 3 decent longitudinal stringers through the original transom each side above the hull skin providing good structural continuity to the hull would be more than useful. But with the inspection ports fitted from the inside their plan might be to have the whole lot shear off if they get a bad load case from a seaway or an impact from a wharf or rock etc. Albeit inspection ports are not designed to be loaded in that direction - only the thread will take the load not the whole lip onto the body fastened to the panel. Such a feature inserted into the transom must be the same load rating as the transom panel construction.

There are lots of good composite articles on line eg;

boatbuildercentral.com/support-tutorials/Tutorials/foam-sandwich-how-to.pdf

Diab publish a sandwich handbook (heavy going) and guideline to core and sandwich.

This book is excellent but also heavy going - see Ch 5, 8, 9, 10 for practical aspects.

www.diva-portal.org/smash/get/diva2:1366187/FULLTEXT01.pdf

That second pdf is pretty comprehensive r13, much of it way over my head, but interesting, thanks. Those pass-through stringers you're suggesting, would have to be pre-constructed one-piece foam sandwich members, wouldn't they? Otherwise, they wouldn't have the required structural strength.

Gday

I have built four composite multihulls and have learnt a few things long the way. There are some good bit that the builders do but I would do things differently from them in a few regards.

The mould. They did this reasonably well. I would have liked them to pull the stern in in the waterlines so that the transom is less beamy. The flat on the bottom will likely be a real pain in a chop. Granger likes flat bums on his cats and he has had owners put little pods on them to stop slapping. He just ensures the bums are in the water, which is a bit slow in light airs.
Nidacore - this is not a great core. It is nowhere near as solid as foam or cedar. I have built a few things from it and you can tell, they just flex more than foam structures do with same laminate. It aint Nomex. in fact Nida core os a bit of a pain in many ways. The sandwich made is more flexy than foam and the resin sort of falls into the voids between the hexagons. You worry about getting a good bond with the core and glass - not recommended by me. There were a couple of cats built with this stuff and one or two were quite flexy. Use it for a dinghy (I did) but not for anything you can't throw overboard.
I liked the way they used the mould and laid up the outer skin. I would have liked to see the nidacore drilled with holes and then see the guys get an orbital sander and go over the core when they stuck it on the filler. This agitates the core and gets a better bond to the filler/laminate. Just pushing is not great. It is pretty easy to make a vacuum bag which is the bees knees but at least some plastic bags full of sand after agitating with the sander which would help get a good bond.
I feel they miss a really important stage next. They then put the little longitudinal bulkheads in which is pretty silly. IMHO they should have run a laminate across the inside of the core. This would make the core a continuous structure sideways. What they did was then install the bulkheads and then laminate them and the inner laminate. This means they have no continuous inner laminate. But they have lots of bulkheads so this should help but it would have been better with an inside skin all the way across.
Gluing/glassing the bulkheads on is fine. There is no need to get into the old transom and continue an existing structure. Glass is incredibly strong if you do it correctly. Tying on the structure to the hull top and bottom will be more than enough in tension and the bulkheads will stop oilcanning of the extension. Compression is no worries.
What really worries me is the mixing of poly and epoxy. That really is a super no no. It will be fine to get the epoxy laminate to stick to the poly one but then you have to keep using epoxy because poly won't like sticking to epoxy. So unless they keep on using epoxy for all glues and layups there will be issues.
Also you shouldn't just smush glue on the ends of the bulkheads and expect it to work. The bloke puts glue on the core and then the deck will be mushed onto these pieces. This will do almost nothing in tension or shear, but will be okay in compression. The core cannot take much load and so the glue will crack and pull off the core at the first flex. This is not good practice at all. Good practice is to make up some top flanges, maybe out of glass or at least cedar and get a pathway for the loads for the deck piece to get into the bulkhead laminate.

So there are some good bits to this video. With the amount of glass it will probably be fine but there are some issues with resin use and some understanding of how cores work. I would not be surprised if the deck lifted off the bulkheads when his heavy mates walk on the deck so it could get springy.

Thanks guys, I've learnt a lot just from these few replies.