Yuck! Old mast tangs

[Roger Long]

In the "What you can get away with" department":

I measured up my mast carefully today so as to lay out the exact angle for the new mast tangs including detailed measurements of the old so I can get the pins in the right place.

Look at what has worked for the last 30 years

I've been looking around at other mast more critically since starting this project and I think this kind of engineering is more common than I would like to think.

 

[Witzend]

When under load wouldn't the tang tend to flex out and correct the exit angle error your experiencing?

 

[Roger Long]

When under load wouldn't the tang tend to flex out and correct the exit angle error your experiencing?

Of course, and that's the problem. See earlier chainplate and tang discussions. When the lee rigging goes slack, the tangs flexe back. All those cycles will eventually take their toll in fatigue. The visible grooves from the bending in a straight metal brake don't help. It's a wonder there are any masts out there still standing

 

[Tim R.]

I had the mast down on my new boat for the first time a few weeks ago. The uppers have a pin that sits in a "U" shaped channel built into the mast cap. The shroud then exits hte mast through a slot below the cap. This eliminates problem with shroud angles. The lowers use a ball type fitting in the spreader ends next to the mast. This also eliminates angle problems.

 

[Witzend]

I would not expect that this amount of flex with such an obtuse break angle would be much of a concern. Have there been reported failures from this type of design/fabrication misalignment?

 

[Roger Long]

Have there been reported failures from this type of design/fabrication misalignment?

I don't know of any but, in engineering terms, this whole set up is sort of like those cartoons you see with the caption, "How many mistakes can you find in this picture?". From what I've seen looking around the yard as boats come out and during my visit to the mast shed to take measurements today, these are very typical rigging details.

As we learned in the other two threads on rigging attachments, stresses in these fitting aren't as high as you might think. The stability of the hull is an excellent shock absorber and load limiter. Still, when the wind vane or a crew member are driving my boat to windward in 10 foot seas and 30 knot winds, I'll sleep a lot more soundly

 

[Witzend]

Boy, I wish had the time to make my world perfect so I could sleep at night as well. I understand your dilemma, I will be lucky if I can get my new dingy davits installed before the season. I probable worry too much as well, but I'm trying to put things more in perspective lately when I worry, about how our children are going to pay for all the dept our country has run up, about whether my nest egg (60% still invested in dividend paying equities) is going to last me through my retirement.
My apologies for straying from your thread.

 

[Roger Long]

BTW the real problem with this kind of arrangement isn't actually tang failure. It's the way the tang design holds the swage fitting at an angle. The wire will then take a bend and flex at the end, always the most vulnerable part of a rig. The wire will probably wear out and fracture long before the tangs give up. It may never be apparent to the owner and the others picking over the wreckage that the wire might have lasted much longer with proper attachment design.

 

[Old Skool Neil]

Seems like a little more bend at the top would resolve the alignment.

 

[RichH]

"It's the way the tang design holds the swage fitting at an angle."

Yup, asymmetric loading of the clevis ends... a little bet of flex, a little bit of strain relief in that 'double bent' tang and noone can ever predict the 'actual' loading %.
BAD design. :-o

 

[jibes138]

Change it

Roger,
Why not change it to a stemball fitting? This has a spherical end swaged on the wire that in turn fits in a spherical seat that is attached to the mast. My O'Day is rigged this way, it is an Isomat spar so that seems to be the source of the design, not O'Day. Misalignment is not an issue except at the deck end that is more of a concern. I think you could design a tang that would bolt on yet have a stemball seat in it. Might be a little tricky machining, you'd need to machine a spherical hole for the stemball seat. I had Hall Spars in RI make a new set of uppers for me once because the yard kinked one of them, They made them while I waited in the lobby. They could probably help with your problem, great company.

 

[Bill Roosa]

Pole vaulting over mouse feces

How many years has it lasted? 30 you say. Hummmm.
And it has not broken or borken something else? No?

This clearly falls into the "if it aint broke, don't fix it" catagory.
Inspect for cracks and move on.

 

[Roger Long]

This clearly falls into the "if it aint broke, don't fix it" catagory.

Not necessarily. I have no idea how the former owners sailed the boat. Few cruisers drive their boats as hard as I do.

Inspect for cracks and move on.

Even dye testing won't reveal fatigue. New tangs of the simple design are a small part of this whole project and will make me feel nice and warm and fuzzy all over.

 

[kenn]

My initial reaction is also a shrug; I suspect the tang plates themselves would long outlast the shrouds. Having two of them is in itself added safety. But it's got me thinking a bit more about this. What sort of fitting was on the shroud end that connects to the tang?

We do want to you to be as warm and fuzzy as possible.

 

[Roger Long]

What sort of fitting was on the shroud end that connects to the tang?

Just a standard swage eye. No sign of problems but the rigging is 30 years old (thanks to most of its life being in fresh water). I plan to sail this boat a long, long time and money will be tight towards the end so I want the new rigging to last as long as possible. As the tragic accident a few weeks ago shows, a dismasting is potentially one of the most serious things that can happen to a long distance cruiser. Since it is practical to roll the clock back to zero on this aspect of my boat, why not? This craft is basically the basket in which I'm going to put all my life's eggs so I'm taking the project a bit more seriously than might be justified for many of you.

 

[Mike B1026]

Too much thought about a non-existing problem.

 

[Ron20324]

I'm assuming you plan to put a fork on that tang? Seems to me that in the forks I've seen, half of a fork is weaker than an eye like I believe you had between the two tangs originally.

 

[Roger Long]

I'm assuming you plan to put a fork on that tang?

Stick a fork in it. It's done

What else would I do?

 

[Roger Long]

Too much thought about a non-existing problem.

A Mac 26 on a lake in Texas is a lot different than a 32 footer somewhere north of Newfoundland in the dark.

 

[catsailor]

Why don't you run this by a structural engineer with a background in fatigue failure? You have already decided that no person on this forum is worthy. Explain that you sail hard. Hopefully he will give you some calculations to go with the warm fuzzy feeling.

Just curious... Why didn't you post your solution at the start of the thread?