Mast tang problem animation

RichH · Feb 12, 2011

Calculate the ELASTICITY / deflection of the rig / wire and mast top (& lowers) between 'normal' and max. (including SF) loads. Then, establish a 'free body' diagram for each individual 'section' of the attachment components, back-calculating the deflection values INTO the component ........... and perhaps youll begin to 'see' my position of BALANCED and non-flexing or minimal-flexing 'tangs'. etc. As this is the way just about all 'tang systems' are engineered on cranes, massive cable stays, gantries, and other potential 'lethal' applications etc. etc. etc.

PURE tension/compression without ANY flexure (minimizing by 'geometry' of the component) is always the 'best' method --- structurally and with respect of fatigue included ... pure tension/compression is 'fundamentally' THREE times STRONGER than 'beam flex' (four times for a 'cantilever')

When I was engineering 'mega-lift' and 'overhead' stuff I would not ever accept any design from my staff that included 'undue' flex at any such connection .... it prematurely 'breaks' and such without verification via 'physical destructive testing' is wholly unpredictable especially when fatigue is involved from cyclically applied loads.

I would suggest you go back and reconsider the 'evolved' Seldén tang system, etc. - easy to duplicate by 'fabrication' - simple, elegantly 'pure'.

;-)

Bill Roosa · Feb 12, 2011

Undue flexing

One mans undue is another man's design.
I'd bring up engine valve springs which flex quite a bit and I've never seen on fail under normal operation.
It is all about keeping the flexture below the elastic limit. You can flex it all you want and NEVER get a fatigue failure.
Tension flexture is not the same as bending flexture. In tension the loads try to straighten out the member and so you can absolutely determine the max flexture. In bending or compression fexture you can't and more engineering would be required to assure a fail safe member.

RichH · Feb 12, 2011

Sorry, ... go to the back of the class.
1. Fatigue can occur WELL BELOW the FATIGUE ENDURANCE LIMIT, starting with a few % of UTS or UYS. ... its the 'nature' of the grain structure of most metals. Fatigue fracture is a Statistical Additive Function (hysterisis - or 'rate dependent elastic memory') and can begin on a microscopic level WELL below the endurance limit.
2. Elasticty is Elasticity and no matter the geometry, its the stress risers and geometric forms that multiply the stress is what leads to the failure in 'brittle failure' ... Young's Modulus remains constant in an isotropic solid (less hysterisis), until the stress bearing properties of the material FAIL either by ductile or brittle mode, and usually in a combo of 'both' .

alexco38 · Feb 12, 2011

If you are able to put a 90 degree shackle type of thing between the two you won't have this issue. You'll have to design it in such a way that they can twist without deforming. My boat has a 1/2 moon mushroom thing that it basically rotates to fit the angle.

See RigRite site.

Ed Schenck · Feb 12, 2011

Not sure what you guys are debating but I think it makes me appreciate my keel-stepped mast even more.

But a question related to the forces at work on my tangs. When I replace the standing rigging this spring are the tangs automatically candidates after thirty-one years? Growing up I thought Tang was what the astronauts drank. So I did.

Shell · Feb 12, 2011

As a child I learned the best way to break a piece of metal was to keep flexing it.
Some metals will flex more than others with out breaking but I've never seen a tang made out of spring metal. I found 2 cracked tangs on the inner shrouds of my 1986 Catalina. Fortunately, the shrouds are attached by separate double tangs so the rig didn't come down. Need less to say, all tangs were replaced.

jimmyb · Feb 12, 2011

I think it was chevy chase that said, I cant argue with that......

.....cause I dont know what the hell your talkin about!

joking aside I love to follow these discussions even with my incredibly limited grasp of serious engineering I can always pull out a crumb of useful information, and the way you pro's talk the talk that is satisfying for me. Looking forward to more...

jimmyb

catsailor · Feb 12, 2011

The only fittings I have needed to replace on standing rigging due to failure--twice on my last boat, about every 8 years--were the style toggles shown above (unless I am miss-reading the drawing). Fortunately I caught the cracks during mid-season inspections each time. There seemed to be considerable residual stress and perhaps the beginnings of fractures in the bent areas. They predictably began cracking in the bent area.

Perhaps they don't flex in service, but they sure can fail. Either over size them chose a toggle style that does not require bending stainless. It could work, but I would feel no better about it than the original design, based upon my field experience with this style.

jibes138 · Feb 13, 2011

Defects and Stress Loading

Reread Rich's post. He makes some excellent comments. There and macro level defects you can see such as surface finish, discontinuities such as holes, radii, and such. Then there are defects at the grain level where the grains form creating grain boundaries and such along which slippage can occur under loading. In the engine industry they invented directional solidifcation and then single crystal casting to solve these problems. But more important is what causes embrittlement in ductile materials and this is typically at the atomic level. In any crystal it is loaded with atomic defects, also known as vacancies ( missing atoms) impurities (contaminating atoms) alloy segragation, and dislocations (missing rows of atoms). Dislocation entanglement is involved in the work hardeneing of a piece of sheet metal as you repeatedly bend it back and forth until it fractures and breaks.
In Rogers design I'd be concerned with the loading running into the formed radius of the tang that will put a bending moment on the tang driving it into the sharp corner of the bolt holding it to the mast. This will be a concentrated point loading that could likely lead to a fracture. Use a large washer and preferably a spherical washer under the bolt head this will act as a sacrificial part that if it cracks from the sharp corner of the bolt it likely won't propagate into the tang material. Why not eliminate the tang all together and replace the bolt with a lifting eye and shackle directly to that. These are use to lift massive loads like machines and containers off ships and such.

catsailor · Feb 13, 2011

Roger Long said:
Thanks. That's good information. It's also the first I've heard of a problem. I was going to use forged bronze toggles but Rigging Only assured me that they have not had any issues with these. I also have this style toggle on the lower ends of my current rigging and could find no visible problems after 30 years of service.

Was your previous boat a cat? They have higher rigging loads.

In any event, I plan to replace standing rigging at about 5 year intervals from here on out but I don't want to do the tangs and chainplates again in this lifetime. I'm also using the largest pin size toggles available for the wire size, 1/2" vs. 3/8" in the case of the uppers.

Specifically, the boat was a Stiletto 27, 1200 pounds, with a 5/16-inch forestay.

The fitting that cracked twice was very similar to this (http://www.jamestowndistributors.com/userportal/show_product.do?pid=52123&SHOPPING), but I'm not certain of the brand, since I sold the boat a few years ago. I never saw a crack anywhere else in the rigging (owned the 30-year-old boat for 17 years), though I replaced the standing twice (once soon after purchase, once just be fore I sold her, as a courtesy).

Yes, the boat had sharp snap roll, but the rigging was heavy and I didn't sail the miles a cruiser does.

Bill Roosa · Feb 13, 2011

OK guys, I'll admit that all that stuff (mostly) happens. However, I think you need to understand that engineering design ALWAYS incorporated a design life. A good engineer will look at what is being built and design it so that it does not fail during its lifetime. NASA rocket engines are designed for a lifetime of 110 seconds and the TVA dams where designed to last 100 years. Without that crucial piece of information you really can't make comments like "it will fail by xxx eventually" and have any real credibility. Sure if you leave a structural member there long enough it will fail or you can design it so that the fiberglass will be destroyed by solar radiation (1000 years) and the tang will still be working fine. Neither one of these extremes is good engineering practice
So Roger, what is your design life?
Also, I see that you still have that asymmetrical loading of the longer tang.

Bill Roosa · Feb 13, 2011

Stress risers and engineering design

Rich, see my post above on design life. And I agree all that stuff happens but is irrelevant in proper design which keeps those issues in check during the life of the member.

Roger: You can't just isolate one structural member like you did. Sure the shrouds stretch but so does the mast and the deck flexes under loads like that. I'd like to see a FBD before I went and made statements on what the loads on member x are. Perhaps you have done this and I missed it. What about mast crushing under the spreader during a knockdown? Just the compression of the mast section under the spreader could cause it to fail at a much lower shroud load than you have calculated. I'd note that if the mast proper buckles due to loss of cross section moment of inertia (I) any shroud design is pretty much moot. "I" is a 4th power type factor and even small changes to the cross sectional shape will result in large changes in its ability to resist bending.

There are a whole host of other failure modes that have not even been talked about yet. The engineering of "make it strong" is great but you have to define "it" as the whole rig not just parts of it. I'd, again, bring up my example of a massively strong rig attached to a weak deck. When de-masted the mast is carried away and takes a chunk of deck with it. This leaves you with a big hole in the boat at exactly the time you really don't want one. A fail safe design would have a chain plate that is designed to fail before the deck to cause a rig failure to not result in a deck failure also.

Bill Roosa · Feb 13, 2011

FWIW, wouldn't it be easier to just put a solid wedge under the attachment point of the upper tang to align the tang and shroud loads. You could then deal with the load being applied to the mast out of plane with a shear connection and avoid the whole issue of bending failure.