I wanted to follow up with my fix on the broken nut embedded in the hull in case someone else has this issue.
I decided that rather than try to secure a new nut in the hull, it would make more sense to bring a bolt up from the bottom. My pre-2016 Tandem Island has a 1/4" bolt, while newer models have upgraded to 5/16". I decided to go with the 5/16" bolt size. When I removed the mast base, I found the nut was broken with the base remaining.
Procedure:
1. Remove v-brace.
2. Remove cross bar and mast base.
3. Enlarge hole by drilling 5/16" hole at location of original nut/bolt. I worked my way up through a few sizes.
(Base of original nut remains embedded in hull, which should provide extra resistance to head being pulled through hull.)
4. Enlarge holes through mast base and v-brace to 5/16".
5. Insert bolt through bottom with stainless/rubber washer + silicone sealant.
6. Set hull on something so that significant force is applied to bolt.
7. Install mast base, v-brace, and nylon lock nut with Loctite.
(Use hex key on bottom to tighten lock nut once nut is threaded)
8. Reinstall crossbar and complete v-brace install.
I'm currently pending step 5. Bolt below is on order. I tested fit with carriage bolt from hardware store. I'd appreciate any advice on waterproofing the hull penetration. Is there a better option than silicone? I do have some left over ZIP system Liquid Flash that I may try.
5/16"-24 x 2" ASTM F879 Hex Drive 18-8 Stainless Steel Button Socket Cap Screw
https://www.fastenal.com/products/details/73798