Mast tang problem animation

[Bill Roosa]

OK Roger

A man with an experience is never at the mercy of a man with a theory. Me being the latter. I would note however that it is common engineering practice to design load attachment tangs as "dog bones" and to separate loads on different axises into separate members I believe your factory set up had just such a design.

I'm sure you get the concept Roger. As an engineer you are doing lots of things in your design that are not being documented here. All fine and good but the rank armature that re-engineers his rig is not gong to be doing any of that out of ignorance. I'd hate to suggest to such persons that you can just "make it over strong" and they are going to be OK. My example is on one of many fail safe considerations. I use it because it is dramatic and illustrates the potential magnitude of problems that over strong engineering can cause. History is replete with this mistake.

My only point in all this (except for the lateral torsional buckling of your tang design) has been that over strong design is not necessicarily better design and can lead to some pretty bad unforseen consequences. I'd advise anybody out there considering rig re-engineering that if you don't KNOW what your doing seek professional help. You are dealing with life safety issues.

Or more philosophically, A great way to contemplate infinity is to consider the magnitude of human stupidity.

 

[kloudie1]

Low normal loads except for the occasional AAAHHHHHHHEEEEEE !!! and low cycles.. enough square inches of stainless, and ya have long life.. The times when this stuff is even close to max are seldom.. Roger, you have a good reason to be concerned and to be changing them... but the new design looks lits better .. the old one has not failed yet.. so my conclusion is that you've got much more than original life... alhhh ..unless the new metal has some internal linear dislocation or inclusion flaws .. !! Ohhhh I didn't mean to say that out loud.. Inspect the new ones very carefully (which has been discussed before) and they should be fine..

 

[Roger Long]

common engineering practice to design load attachment tangs as "dog bones" and to separate loads on different axises into separate members I believe your factory set up had just such a design.

It's pretty clear from looking at the parts and knowing the industry, however, that this design is driven by being able to have the parts inexpensively stamped out of thin plate and bent to an approximate angle. Their flexibility lets them accommodate to different rigs without custom bending but at the expense of longevity and reliability. Get the boat into the hands of the buyer as inexpensively as possible. If the rig goes over the side ten years later, it will be because the owner failed to inspect it carefully. Right.

I'm enjoying these discussions and the opportunity to shed some light on the issues involved but, damn, I'm beginning to wish I'd just stuck new rigging on the old tangs. Realistically, it probably never would have failed.

The bent tang on a bolt(s) is such a ubiquitous marine fitting with good enough service history that it's a surprise to find out how much stress can develop around the bolt hole and bend. Trying to do a retrofit with in the constraints of existing fittings is always a challenge and usually involves some compromises. I'm also working with a pretty limited budget here.

The precision required to insure that dual layer tangs are actually sharing the load would be extraordinary. That's one of the reasons I'm avoiding the design. In probably 99.9% of these installations, one tang is taking most of the load with the other just keeping the pin aligned. I can see this in the wear patterns of my parts. However, they are sized as if both parts were fully effective. If one side goes, the clevis pin is not going to keep the stay connected for long. Most rigs with this design have tangs half the strength the engineering numbers would indicate.

If you go through the same offset analysis I did above (BTW folks, this isn't real engineering, just a quick way to get a handle on the probable magnitude of the numbers) on the thin dual tangs, if proves that they all bent, i.e. "failed". In practice, they don't bend as much as I show in my animation because the slop in the clevis pins makes them sort of a toggle. I can see this in the worn hole edges and knife groove marks in my removed clevis pins. None of that is good either.

Good point about not trying any of this at home kids.