I had a similar issue with my Hartley Queenslander, of damage in the anchor
well area.. I repaired the area by removing the ferro
back to nice white/grey coloured strong material, and cutting back all the rusty armature till it was at a position of no damage, and cleaned it to a bright finish.
I then placed 1 layer of 4mm galv square section (about 30mm x 70mm holes) wire tied to the original armature and stem, and another layer inside of that (off-setting the holes) and overlapping the hull by 100mm. I then plastered with Ramsett Epxoy Grout, following their instructions to the letter and like another guy has written, it sticks REALLY well, it is designed for concrete and doesn't mind minor moisture content. After plastering I painted the inside with the black bitumin type paint
that I purchased from Supercheap and is painted on the under body of cars, (fibre reinforced to stop stone impact damage), then lined it with 15mm thick holed rubber door mats, cut & cabled tied to form the inside shape.
This is stronger than the original now, very happy... But if I did it again I would not use the GROUT, but PASTE, the grout is designed to flow into every nook and cranny and not hang up..and I had to use formwork. Several Companies in Australia
have the good quality Epoxy
Paste designed for concrete repair, Ramsett, Parchem, Sika to name a few.
Another thing I would do next time is to put a couple af layers of half inch galv bird netting on the outside it will hold the plaster in better, gently push one in from the outside first and fair, then after the first layer is cured, push from the inside untill all covered. Unlike cement you don't need to do all the epoxy
it one hit, as it sticks very well to itself, just need to carefully work the second layer in, to eliminate voids
A lot of ferro boats go here and I think it is the chain impacting on the #1 frame in the well, chipping away and eventually exposing the armature (I had evidence of this), so when you have finished I suggest rubber mats and other protection as required.