As others have noted, this is a very unusual failure for a small keel root detachment failure. Normally, in these failures there is a large area of the laminate that is pulled loose, and often the hull is pulled free from the frame and pan. In this case the keel bolt washers (which are rectangular plates that span between each pair of bolts and are roughly 5/16" thick stainless steel which was compliant with the ABS standard at the time the boat was built but which should have been 7/16" under the current ISO 12215-9 standard) sheered through both the hull and pan, and sheered through the bottom of the transverse frame as well. I can't even visualize what directional force would cause that type of failure except fatigue at the edges of the plates taking a toll on the matrix and causing it to weaken to the point of failure.
The laminate in this area was all solid glass and was made up of uniaixial and biaxial fabric. The pan and frame structure is glued to the hull skin with an engineered adhesive. The tan color is probably that adhesive.
A hard grounding would have been expected to pull the forward bolts downward tearing them out of the hull and tearing the joint between the pan and the hull, and would have pushed aft edge of the keel up through the hull tearing the matrix back there. But in the photo there does not appear to be those kinds of compression marks aft the aft end of the keel. If the boat was run aground in a surf and was sideward to the waves, you would similarly expect one side to have sheered outward and torn the laminate outward and had a inward sheer failure on the opposite side of the keel. You would expect the transverse frame to be completely mushroomed on one side, but it does not appear to be the case. And if the boat was dropping vertically on hard bottom (similar to one of the contributing causes of the failure on the Cape Fear 38 some years ago) you would similarly expect to wide spread delamination and crushing of the frames.
To me the mystery here is the comparatively clean cut opening which almost suggests a failure mode where the boat was lifted vertically while its keel was held down some how. (I am not saying that actually happened just at first glance, that is how the damage looks).
But I am also interested in the fact that the boat in the picture looks like a 40.7. The construction and inspection during building of the keel attachment is outlined in the report on the sinking of the Cheeki Rafiki. https://assets.publishing.service.go...ort_8_2015.pdf
What was evident in the report was that the Beneteau 40.7's went through an extraordinary testing and inspection process compared to most production boats, with layup made up of precut laminate and the resin weighed. Hull thickness measured at each keel bolt hole on each boat, and each penetration on each boat with the coupons from through hulls tested for thickness and resin/reinforcing properties. And lastly the construction process is observed during construction by an independent testing company. That level of care just is not done on production boats.
And in the Annex C&D to the report, the Wolfson Unit, did a detailed analysis of the structure of these boats. https://assets.publishing.service.go...eekiRafiki.pdf
There is a chart with their results of the analysis. The table summarises the results of assessing the structure design, and concluding that depending on the element the design was between 1.05 to 7.96 in excess of the requirements of the standards when the Cheeki Rafiki was built, and that only the washer thickness would not comply with current standards. It also noted that Cheeki Rafiki was built differently than the original design, (and alegedly other 40.7's) and as built one of the designed keel bolts was missing. While that would have met the ABS standard of the day, it would only meet 95% of the current EU ISO 12215-9 standard.
And yet, with all of this care in design, construction, and with values exceeding the standards of today, this is the second of these boats to lose a keel.
One other interesting item in the Cheeki Rafiki report is a table looking at boats which have lost keels since 1984, which are summarized in Table 4 below.
Table 4: Summary of ISAF data on keel failures
Cause of failure Number
Welded fin failures 11
Grounding or collision 8
Hull/internal structure 8
Keel bolts 3
Keel cant system 2
Its interesting to me that over 20% of the failures are due to grounding, collision, or hull/internal structure failure, while 4% is was actually caused by keel bolt failure. That said, there were a number of keel bolt failures last year in older boats that would skew that number quite a bit.
Lastly, while these small root footprint keels are definitely more difficult engineering problem, over my lifetime, boats have been losing their keels no matter how long their keel root and how they were built. My family's CCA era Pearson Vanguard lost its encapsulated keel in 1970 due to a hard grounding. A number of IOR era boats lost their keels (Drum being the most famous) despite having a comparatively large keel root, and yes, newer designs are losing their keels as well, which is why World Sailing (AKA the former IYRU) is studying this issue as we speak, and hopefully will come up with better standards.