Interesting (I think) side issue
....
You May remember the Super yacht called DRUM. ..... It's Keel fell off it seems because when the keel was bolted on, washers and FULL nuts were screwed on on the inside of the hull and then Half nuts were screwed on as locking nuts. In putting the nuts on in that order, when the locking action was performed with two large spanners, the half nuts ended up taking all the load and the full nuts were left doing nothing.
There were not enough threads in the half nuts for the job and in heavy weather, a sudden thread failure resulted. This sudden failure then put an IMPACT load on the full nuts which also failed because of that impact, so the keel fell off and the yacht turned over, as it would!!
Hmm.... I followed this failure with some interest at the time, and I have a somewhat different explanation on file.
The cross section attached below is taken from the official report, and one reason I was interested (apart from the marine engineering angle which was of professional interest) was that I had sailed on the Southern Ocean leg of the delivery to the UK of Drum's sistership (Lion NZ) which had, at least by design, the identical keel attachment detail.
We did have occasion to heave to a couple of times under stormsails, (just a trysail, on at least one occasion) about 2500 miles from anywhere, whereas Drum was close inshore when her keel fell off, so I was interested to know whether our keel had been compromised.
Even more interested was Peter Blake, skipper of Lion.
His understanding was that <<Drum's keel had snapped off at the hull when welds in the upper keel frame structure failed
(see sketch below)..... most maxi keels are fabricated in aluminium. The required amount of lead ballast is securely moulded into the lower part of the aluminium boxwork. The finished product is welded to an aluminium gridwork built into the floor of the yacht and through-bolted into the internal framing..... With Drum, however, it seemed that the work had not been done to specifications. The keel had flexed from side to side and finally snapped off,
leaving the keel bolts still in the hull. (My emphasis, seems to be confirmed by second photo)
The Woolfson Unit at Southampton University, in an independent investigation into the mishap, found that the all-important welds were porous and had insufficient penetration, or none at all.>>
Lion's keel was removed in any case in the UK prior to the race, as the configuration had been redesigned and a new keel built, lighter and further aft (but not the attachment detail, which was considered satisfactory)
I'd be interested to know who supplied the explanation you heard, if you can recall? I guess it's conceivable both failure modes were present, but in that case I'd be surprised to see the 'aluminium gridwork' external member sitting flat against the hull all the way along, as it seems to be in the second photo. I'd have thought it would have started to peel of at one end, perhaps triggering the weld failure referred to above.
Coincidentally, I was looking at Lion NZ just earlier today, and went on board her two days ago. She's just back from commemorating her Sydney Hobart win 25 years ago, her maiden voyage (in the course of which over 100 boats retired or broke, including her main competition).
This time, forty boats preceded her across the line at Hobart - partly a comment on the weather, which was too light for her liking, but also a telling indication of what has happened to the speeds achievable.
Boats are routinely losing keels today, but that's another story (canting keels, using engineering which -inexplicacably to me - completely fails to make use of one of the unique capabilities of hydraulic cylinders and their control circuits to limit the bending moments on the keel, and the loads on the ram and attachments)
