If those webs were indeed only 5 mm and there were only three of them . . . and add in the difficulty of doing a proper tabbing job in that deep narrow keel stub . . . it was a time bomb.
Every time the boat heeled, the keel stub would try to bend -- putting those webs under compression -- and that's just the static load. When the boat's moving thru waves, you have additional dynamic loads. Sail across the Atlantic a couple of times and how many cycles is that?