SailNet Community banner

1 - 20 of 66 Posts

·
Registered
Joined
·
905 Posts
Discussion Starter #1 (Edited)
The keel bolts held. The structure failed.

Just saw this today on Facebook, posted by a salvage operation. This Beneteau apparently hit a reef and sunk in 5 minutes when the keel snapped off. What suprised me in the photos is that it doesn't look like keel bolts failed - instead it looks like the bottom of the boat ripped away with the keel. I would have thought the bolts would pull before the structure itself would fail.
 

Attachments

·
Registered
Joined
·
845 Posts
The safety factor for keel bolts is 7 or more. That means they can carry 7 times the load they are currently carrying. It is generally the body that is broken. You can see many images of keel bolts nearly half gone due to corrosion but still doing their work.
 

·
Registered
Joined
·
905 Posts
Discussion Starter #3
Yes, but the bolts held in this case. What failed is the structure they were bolted to.

I probably should have used a different title to the thread.
 

·
Registered
Joined
·
4,647 Posts
I agree that it is good engineering to have a large factor of safety in the keel bolts to account for possible corrosion over the years. What I don't think is good engineering is these deep small cross sectional area blade keels with a bulb on the bottom. The forces applied in a grounding are huge and the small hull area that must carry that load is just not enough
 

·
Registered
Joined
·
629 Posts
I'm not a GRP engineer by any stretch but I am surprised to see how cleanly the keel stub snapped off. You would think that (when properly engineered) the FG would have torn away up the hull a bit as it's supposed to be laid up in a way that disperses the load to a large area of the hull. I would picture it snapping off more like peeling a large sticker in any number of directions along the FG layers rather than snipping off almost cleanly like a matchstick.
 

·
One of None
Joined
·
8,045 Posts
Pics shows very small keel stub opening which makes me think it was a bulb keel and it was snapped off laterally. Guessing if a keel were wedge between two rocks, a few rocking actions port and starboard would probably snap it off in short order
 

·
Administrator
Joined
·
8,999 Posts
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.gov.uk/media/55408664e5274a157200005b/MAIBInvReport_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.gov.uk/media/554086e1e5274a157500003d/AnnexesToMAIBInvReport08-2015_CheekiRafiki.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
Undefined 40
Welded fin failures 11
Grounding or collision 8
Hull/internal structure 8
Keel bolts 3
Keel cant system 2
Total 72

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.

Jeff
 

·
Administrator
Joined
·
8,999 Posts
The safety factor for keel bolts is 7 or more. That means they can carry 7 times the load they are currently carrying.
I have to ask, how did you come up with this statement?

After all, when you say that keel bolts are designed to safety factor of 7 do you mean that in all loadings (static, steady-state and impact- whether vertical, lateral and from fore and aft) the keel bolts are designed to take 7 times the expected load?

Or are you saying that keel bolts are simply designed to carry 7 times the weight of the keel, regardless of the induced forces due to heeling and dynamic loads?

Are you saying as a fact that the keel bolts on this boat were designed to be 7 times stronger than the any of these loads?

Are you saying that ABS (which this boat was designed to) required a safety factor of 7 for the keel bolts (ABS didn't) or that the ISO standards require a safety factor of 7 for keel bolts (they don't)?

So what is it that you are trying to say?

Jeff
 

·
Registered
Joined
·
905 Posts
Discussion Starter #11
Jeff -

Looking at the photos it seems that the antifoul around the keel area flaked off.

Possible sign of flexure near the keel before it broke off?
 

·
Administrator
Joined
·
8,999 Posts
Jeff -

Looking at the photos it seems that the antifoul around the keel area flaked off.

Possible sign of flexure near the keel before it broke off?
Good point, I had seen the bottom paint flaked off around the keel. You may be right that there was flexure near the keel before the failure which led to the paint failure.

I actually has blown up the close up photo trying to get a better sense of what could have caused this. When I did there were a few more oddities visible. For example if the failure resulted from a lot of flexing of the hull, you would expect to see a lot of spider cracks in the gelcoat and barrier coat, I don't see those.

When you blow the picture up, it looks like you can see the second and third from the stern keel bolt washers in place and intact. That would suggest that this was in part a keel bolt failure, but if this was a keel bolt failure, then what would have caused the hull to tear away.

The shoal draft version of this boat had an iron keel with iron keel bolts, and perhaps they rusted and dropped the keel, but I still have to wonder what then would have caused the comparatively clean cut that is visible in the picture.

It will be interesting to see what is figured out. Anyone know where this occurred and what happened to cause the keel to separate?

Jeff
 

·
Registered
Joined
·
905 Posts
Discussion Starter #13
Found out a little more information. The boat is a 2006 Beneteau First 36.7 per the salvager who pulled her up.

Freshwater boat.
 

·
Doesn't sail enough
Joined
·
623 Posts
Those French guys are dang good engineers. Lots of math, finite element analysis, safety factors.

But that's not how you attach a high-aspect, bulbed keel. This is how you attach a keel.

(the keel bolts to the bottom. the sides bolt to the longitudinal stringers. posted earlier today, from Francis Lee)
 

Attachments

·
██▓▓▒▒░&
Joined
·
13,645 Posts
"I would have thought the bolts would pull before the structure itself would fail."
And in fact, just aft of center, it appears that one of the keel "stud bolts" that B inserts into the top of their cast iron keels, is still attached to the hull on the starboard (down) side of the keel mounting area.
Is the saildrive normal on that model? Or does it reflect a boat in the EU market?
 

·
Registered
Joined
·
276 Posts
It just seems strange to see a sunken boat pulled up with the main still covered and the jib furled up like that, almost looks like it sank at a dock. The tear does look a little more jagged on the forward side. Can’t wait to hear more details on the circumstances leading up to that.


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
1,188 Posts
It just seems strange to see a sunken boat pulled up with the main still covered and the jib furled up like that, almost looks like it sank at a dock.... Can’t wait to hear more details on the circumstances leading up to that.


Sent from my iPhone using Tapatalk
It could have happened while motoring and they ran it into something.
 
1 - 20 of 66 Posts
Top