Farr X2

And given the cassette design, the lateral loads are taken by the cassette, the bolts just carry the vertical load - the weight of the keel and bulb. As the boat heels, these loads decrease too.
This is true in the ideal world. In the real one, given the gaps and deformations, the bolts may be subject to not negligible parasite bending/shear stresses, the more if they are loose.
 
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SCANAS

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And given the cassette design, the lateral loads are taken by the cassette, the bolts just carry the vertical load - the weight of the keel and bulb. As the boat heels, these loads decrease too.
Yeah I was thinking the 16 ton was static not slamming over waves & tip outs.
 

LeoV

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Oh, damn, I did calculations for that 20 years ago. I think the safety rule was 2.5 times to static load to compensate for g forces and groundings.

Cassette keel, mount strips of Norlon or something like that, to take slop totally out. For offshore work, shape it to the foil form and install near bottom of cassette.
 

The Dark Knight

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By way of comparison, here is the Inglis solution to fixing a "cassette" keel.

1657193741714.png


Same design but different boat.

1657193765503.png


1657193815469.png


1657193838420.png




from this thread.



A big horizontal pin looks and feels a lot stronger than just two vertical bolts if that's all that's holding the X2 keel... Better to be way over engineered in a critical area like keeping the keel on.
 

DickDastardly

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This is true in the ideal world. In the real one, given the gaps and deformations, the bolts may be subject to not negligible parasite bending/shear stresses, the more if they are loose.
No doubt you’re right but if one bolt can carry 8 tonnes and the keel and bulb weigh, say 1.5 then that’s still a large safety margin.
 

Parma

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Yeah, looks like 2 12mm SS bolts. that's about 16 ton capacity for a 900kg keel, should be sufficient.
Okay, I'm no expert but I use 2x8mm SS for each one of my clutches so each bank of 6 cabin top clutches uses 12x8mm, which makes 2x12mm for a keel sound low, even given the differences in the directions of the loads & underlying structures.

I wonder if the width of the female receiver inside the top "tab" reduced/weakened that part to the point where failure was unavoidable?
 

221J

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It is more probable that whatever the bolt threaded into failed than the actual bolt. The strength of a Grade 8 M12 bolt is quite a bit higher than most laminates especially when it come to bearing strength. There may have been inserts involved too, one of the many unanswered questions. I am curious if there is evidence that the bolts were torqued to a value and what thread locking material/feature was used. That design looks dependent on the bolts being snug and staying that way. Sometimes failures occur because of construction errors and execution and not design flaws.

If the cassette insert snapped, the likely location is flush with the hull. The two (fortunate) sailors may have seen the stub of the cassette either in place but broken or missing. Also it is possible that the hulk can reveal what the status of the cassette is.
 
Just as a comparison, although for a totally different type of joint (flanged) and fin section (much larger) : my J/92S has 4xM27 SS bolts for an 1100 Kg keel. J/Boats recommends 350 Nm torque. By comparison with Farr280's recommended value at 99 Nm, and assuming same bolt material, Farr280's bolts should be around M18 size. I would be surprised if FarrX2 had just 2xM12. Too small in my view. Unless they use a much stronger material.
 

Snowden

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Just as a comparison, although for a totally different type of joint (flanged) and fin section (much larger) : my J/92S has 4xM27 SS bolts for an 1100 Kg keel. J/Boats recommends 350 Nm torque. By comparison with Farr280's recommended value at 99 Nm, and assuming same bolt material, Farr280's bolts should be around M18 size. I would be surprised if FarrX2 had just 2xM12. Too small in my view. Unless they use a much stronger material.


4? Deathtrap! My very similar boat and keel has 9:

IMG_0204.jpeg


It's a totally irrelevant comparison, of course, given how suboptimal the bolt loading is on this design when heeled compared to a cassette-type construction.
 

The Dark Knight

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Zonker

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Garden variety AISI 316, tensile strength is about 500 MPa. Yield is about 250 MPa

A4-70 are 700 MPa UTS and are pretty common in Australia/Asia.

M12 - tensile stress area (the threaded part) = 84.3 mm2.

So guess A40-70. Yield strength = 350 MPa. Yield load 84.3 mm2 x 350 MPa = 29505 N = 3007 kg x 2 bolts = 6014 kg

Pretty good for ~900 kg of ballast. That is NOT a big bulb and fin. (Really need a person standing beside one for scale)


I don't know much about thread strength of cast iron but it's going to be a fair bit less than A4-70. Yeah beginning to believe a failure of the threads of the keel MIGHT be possible. Especially with less than great drilling and tapping and thus not nicely formed threads.
 

Rawhide

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No doubt you’re right but if one bolt can carry 8 tonnes and the keel and bulb weigh, say 1.5 then that’s still a large safety margin.
Yep, you have to assume that one bolt carries all the load. so still less than 20% of the bolt proof strength including dynamic loads. Well within the fatigue limit for SS. All seems to point to some type of manufacturing f up, stripped, overcut thread?
 

Frogman56

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Guys,
Normally reliable sources involved in the retrieval process suggest that there was no structural failure of the hull parts and that alignment, movement and metal fatigue just in the pins the likely culprits.

I guess that the convenience of 'easy keel removal' might need to be reduced a bit; more or less to interference fit in the cassette and additional mechanical locking for the reduction of fatigue and potential stress concentration?

IIRC, the ISO fatigue standard was the equivalent of sailing upwind on one tack for 365 days x 24 hours...

Frog
 

duncan (the other one)

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you've got to wonder if bolts being removed and inserted from a tapped, cast keel strut is a wise choice for something that is designed to be repeatedly removed and reinstalled by various yards of variable skills.

A more robust, idiot proof, and inspectable solution would be studs semi-permanently left in the keel head with nuts for installation/removal.
 

Zonker

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If the fit in the cassette isn't really tight and the keel is twisting around the 2 bolts then yeah, that's not the right way to do it. If there is any slop in the fitup and they are relying on a layer of Sika or silicone to fill the gap between cast iron there will always be some movement.

Properly tensioning bolts does reduce fatigue - but it really needs a solid thing you are clamping to. It reduces the alternating stresses in the bolt. Solid e-glass might not be rigid enough (the stuff that makes up the top of the socket)
 

Kududine

New member
Still a lot of design/manufacturing information unclear. Sales web site talks about a "milled" steel keel. That, to me, means a keel machined from a solid block of steel. Others talk about cast iron. Was it manufactured to the drawings and specs? Then, there is the photo which seems to indicate maybe a steel core with an outer FRP shroud. The photo in the slings also seems to indicate a small "tab" inserted into a relatively small cassette, and held in place with vertical hex head screws. The recovery photo indicates that the cassette itself was intact, and the screw holes undamaged.

So my reading is that the the screws either vibrated loose or the thread connection failed.
As others have said, you would get a lot of noisy warning if the screws came loose, and there was movement occurring.
Conclusion then is thread failure. Surely not if steel to steel. Maybe that "tab" was cast iron?
Sherlock, where are you?

Ah, but then, access to the bolts does not look easy. Would the crew have a suitable spanner handy to tighten? The engineering knowledge to work out what was happening? The plot thickens...
 




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