Hi james, and thanks for the photos! (I had asked about it in a PM as I wanted to see what the factory tabbing looked like.)
To answer your question about the corners, it would not be for expansion and contraction; there isn't really any to speak of with fiberglass, and in any case it's fiberglass to fiberglass.
But.... corners are very hard to get to lay down properly. And probably not necessary anyway. Anyplace there is an air pocket is not really doing anything, and if you can't get the cloth situated properly there will be air pockets.
On the hidden areas: I hear you! On boats that is usually a key to check on boat building quality: What did they do in the supposedly hidden areas? (which are also often the devil to work in)
The one place there was tabbing in my Boler
was attaching a "septum" in the rear overhead cabinet. Once I had that cabinet out I could peel that off by hand. No doubt poorly prepped and polyester resin, which doesn't have great secondary bonding strength. On the other hand, it was only a divider/strengthener, and had worked fine while it was in place.
Another note is that in "the old days" fillets were not used. Fiberglass cloth does not like sharp inside corners, and a fillet basically fills in the corner and makes it a sweeping transition. Older fiberglass boats (and some new ones) don't have filleted corners either, but they do help quite a bit.
Backing up a bit here: Since fiberglass can be tabbed, the rivets in my Boler
were bugging me. When it was time to renew the caulk in the center holes, well.... I just couldn't bring myself to do it! So, in a fit of "but this is *fiberglass*" I decided to remove all the rivets and tab the cabinetry in.
Of course it would have been a lot easier in the first place, because coming along after the fact I have to first peel back the Ensolite and then remove the adhesive, which is a bit of a pain. Then I can sand/prep and tab. So far I have the closet done, and the kitchen overhead and rear overhead about halfway done. That leaves the kitchen lower cabinet (the benches are screwed to tabbed in ledges from the factory).
I already have about 75% of the exterior holes filled and waiting for gelcoat.... ahhh, that feels better.
I'm looking forward to lying in bed at night, listening to rain pattering on the roof and not thinking about any rivets. As a side bonus, it makes things much stronger (not that that's really needed, and the base fiberglass is relatively weak anyway).
The main thing is.... I just felt like it
PS: The reason I took the "septum" out of the rear overhead cabinet is that it divided the cabinet into two compartments, but since the doors are towards the sides, the middle was pretty much useless and hard to reach. So I made a second septum, and then put them both in about 18" apart, leaving the center free of septumry. I'm going to cut a hole there for a middle compartment. I think I'll just leave it open but with a bit of a "ledge" at the bottom to hold things in. I'm also thinking I may remove the two (ridiculously heavy) doors and make canvas "doors" instead, but haven't decided for sure.