Yuck! Old mast tangs

[woodster]

i am not an engineer nor do i have any formal education in such matters ....but i can assure any and all that over kill or what ever you want to call it is by far better than being non attentive or saying it has always worked before when the mast is in the water and some one may have died in the event...we just had a thread a little while back about a man and his daughter that were taking there dream cruse south that got caught up in a very bad storm....and it was a very sad day when the storm finally ended.......haveing been in the structual steel erection/fabrication business for over 40 years and seen what weak or insuffent rigging can do when it doesnt stand up to the work at hand .....i think its great that we are lucky to have a navy design man on board ...i for one learn a lot from his posts....right now i am facing a mast tang problem on my refit of my s2.....and this will give me lots of food for thought .....having said all of this ....i just want to say thank you for shareing your knowledge with us....

this is not a statement pointed at any one person just my way of saying thank you

regards

woody

 

[tommays]

What do you thing Rodge does for a living and consider that a lot of this is being done for its learning value

 

[Roger Long]

Why don't you run this by a structural engineer ... Hopefully he will give you some calculations to go with the warm fuzzy feeling.

I can do that myself (See post 45. The direct Permalink isn't working for some reason.)

http://forums.sbo.sailboatowners.com/showthread.php?t=125126&#post773106

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

This is a follow on to a couple of earlier threads. I'd already posted my concept sketch. I hadn't drawn the sketch just above when I started this thread. I was laying out my existing rigging for reference and startled to discover how bad the alignment was. Later, I'd finished the final drawing to deliver to my tang fabricator and included it in a response.

Curiosity satisfied?

Please satisfy my curiosity, why the smarmy and unfair crack about my not deeming anyone here worthy? Even though I design boats professionally, I learn stuff about sailboats here and there are things on my boat that are different because of things I have learned from forum members. If I didn't think there were people worth listening to here, I wouldn't be here.

 

[catsailor]

I can do that myself (See post 45. The direct Permalink isn't working for some reason.)

http://forums.sbo.sailboatowners.com/showthread.php?t=125126&#post773106

This is a follow on to a couple of earlier threads. I'd already posted my concept sketch. I hadn't drawn the sketch just above when I started this thread. I was laying out my existing rigging for reference and startled to discover how bad the alignment was. Later, I'd finished the final drawing to deliver to my tang fabricator and included it in a response.

Curiosity satisfied?

Please satisfy my curiosity, why the smarmy and unfair crack about my not deeming anyone here worthy? Even though I design boats professionally, I learn stuff about sailboats here and there are things on my boat that are different because of things I have learned from forum members. If I didn't think there were people worth listening to here, I wouldn't be here.

In part, I apologize, since I have a terrible cold and said too much. You are right. I guess I was not in the mood to see a simple matter flogged again. My head hurts.

Yes, I had guessed you did structural work, simply from your writing.

You've savaged the original design, I think, unfairly. No, it is not pretty or efficient, but it is on a generally a well designed boat and the odd design. I hate being too hard on other people's designs. I was fine with your first post, on the old thread and felt no need to comment on a simple solution. Simple. I too am an engineer and I spend a lot of time comparing theory with results. Fatigue is not a simple thing to model. I spend a lot of time with fatigue related corrosion and I understand that it is as much statistical as calculated. The original design is common and not very prone to failure. Might it age the upper end of the wire? Yes, I suppose so. I have seen more failures at the bottom, where the fitting holds water.

Really, without full engineering analysis and a design target, this is simply water cooler conversation. Is it over built already? I can't tell but I would think class history would tell.

 

[Bill Roosa]

Fatigue faliure

Given that there are no dimensions on the drawing I'm guessing here but it does not seem to be that big a flex. If it was flexing outside the elastic limit for SS then if would surly have failed by now. Which would indicate that flexing of the tangs is within the elastic limit for SS. As some one pointed out the off center load on the shroud end is the problem as the crimp connection there is already stressed beyond the elastic limit to get it to hold the shroud wire.
As a general rule just flexing stuff elastically (like an engine valve spring for example) does not cause a fatigue failure, ever! Ask yourself how many times you would have to bend a valve spring to get it to fail. At 60 MPH and 2000 RPM that is 1000 cycles/min for each spring. Drive 100,000 miles @ 60 MPH means 100,000 minutes of driving or 100,000,000 cycles on that spring. Now you have literally millions of cars ...... a whole lot of flexing and NO fatigue failure. It is truly a rare event to see a broken valve spring. A similar argument can be made for suspension springs and a whole host of other things.
Moral of the story: Keep your flexing within the elastic limit for the material and you will have NO problem. Even if you score the surface with a "bend to" line!!!!!!!!

or try this http://en.wikipedia.org/wiki/Yield_(engineering)

 

[Don12364]

Roger,
The original design at least has the advantage of redundancy. i.e. one tang can let go through fatigue and leave the other still holding the mast up. Of course you would need to get the load off it pretty quickly. Not so with the new proposed brick outhouse solution.

I have seen cases where rigging fittings failed whilst the boat was on its mooring.
Sorry to criticise.
Don.

 

[Roger Long]

The original design at least has the advantage of redundancy. i.e. one tang can let go through fatigue and leave the other still holding the mast up.

Good lord. I wouldn't put two cents on the chances of the rig staying up if one of the dual tangs let go. The stress that would break even a fatigued tang would instantly be transferred to the other as a shock load together with rotating it around so the eye could slide off the pin. If it was the cotter pin side, the eye would suck that pin right out of there.

Don't worry, I don't consider that criticism.

 

[Ross]

Why not use a fork cable terminal and a one piece tang?

 

[Don12364]

IMHO its not simple direct stress that breaks them but fatigue or if they develop cracks due to crevice corrosion. In which case they seem to fall apart on their own.

 

[Rick486]

Given that there are no dimensions on the drawing I'm guessing here but it does not seem to be that big a flex. If it was flexing outside the elastic limit for SS then if would surly have failed by now. Which would indicate that flexing of the tangs is within the elastic limit for SS. As some one pointed out the off center load on the shroud end is the problem as the crimp connection there is already stressed beyond the elastic limit to get it to hold the shroud wire.
As a general rule just flexing stuff elastically (like an engine valve spring for example) does not cause a fatigue failure, ever! Ask yourself how many times you would have to bend a valve spring to get it to fail. At 60 MPH and 2000 RPM that is 1000 cycles/min for each spring. Drive 100,000 miles @ 60 MPH means 100,000 minutes of driving or 100,000,000 cycles on that spring. Now you have literally millions of cars ...... a whole lot of flexing and NO fatigue failure. It is truly a rare event to see a broken valve spring. A similar argument can be made for suspension springs and a whole host of other things.
Moral of the story: Keep your flexing within the elastic limit for the material and you will have NO problem. Even if you score the surface with a "bend to" line!!!!!!!!

or try this http://en.wikipedia.org/wiki/Yield_(engineering)

Actually, the material property of interest here is called the "Endurance Limit", or "Fatigue Strength", which for steels is related to the ultimate strength, not the yield. Steels typically exhibit the type of behavior you describe when loads are kept below the endurance limit. However, care must be taken to avoid stress concentrations, which increase the cyclical load above the endurance limit.

 

[Roger Long]

IMHO its not simple direct stress that breaks them but fatigue or if they develop cracks due to crevice corrosion. In which case they seem to fall apart on their own.

Exactly. That's why good design minimizes movement in situations where stress is applied cyclically. A sailboat rig in which nearly all stress is removed on the lee side is a perfect example. Even if overall stress is less than the fatigue limit, local stresses in corners like the brake line grooves visible on many tangs can be higher in small regions. Get some salt in the groove and you are headed for trouble.

Engineering is more mysterious than people think. The world is full of example like this where all the rules are broken and failures are still rare. There are also massive structures which fail due to overlooking a small detail.

I think the service record of production boat rigs like this is due to the fact that most boats are not sailed much or hard and the stability makes such an excellent shock absorber and load limiter. You can also probably get away with things far up in the rig where they get more rain than salt spray than down on deck.

If I could buy a brand new set of matching tangs, I probably would get 2-3 sets and just replace them frequently. However, I know someone who can make up the simple tangs I designed for less than that would probably cost and I can design them so they should be about the last thing on the boat to ever fail.

 

[jibes138]

Wow, a few thoughts on fatigue and design limits. First of all I had a car once that had a valve spring fracture from fatigue. So either I hit the lottery or it is more common than you might think. The elastic limit is never used to design parts in fatigue. The elastic limit defines the stress limit beyond which you start to get plastic deformation. The fatigue limit as noted in a previous post is much lower than this, on the order of 50% of the elastic limit. Fatigue is also a function of the surface conditions in terms of surface finish or material defects. Materials can not be assumed to be perfectly homogeneous. They often have inclusions, impurities, alloy segragation and all kinds of problems not visible to the naked eye. The state of surface stress has a lot to do with fatigue resistance. For instance shot peening or cold rolling of some metals improves the fatigue resistance by imparting a residual compressive stress in the metal. This can be thought of as starting the stress strain curve below the zero stress point so it takes a certain amount of tensile stress to put the metal in a zero stress state. It will not necessarily efeect the "yield point" but will do wonders for fatigue.
Roger you may want to shot peen the new tangs, even glass bead peening or sand blasting will give a little improvement.
Here's another thought for you. One of the great inventions of man is carbon fiber. Why not make a little mold, get some carbon fiber and make your new tangs such that the carbon fiber will be tensile loaded. It will be corrosion resistant, way stronger than stainless steel in tension, it will have wonderful fatigue resistance. Check out the Island Packet website and look at how they bond the chainplates to the hull. I can picture an elliptical shaped mast tang with a couple bushings in it that will reduce weight aloft and give you a much stronger design than the original.

 

[jibes138]

Roger,
If you stick with the stainless tang you designed consider using a cogsdill roller burnishing tool to cold work the holes. It will provide a wonderful surface finish on the order of a 2 to 4 RMS microfinish and will impart cold working to the holes to improve fatigue resistance. This is a simple process on a drill press, you set the tool to a small interference fit and the rollers provide high contact stress above the elastic limit of the material. The resulting residual compressive stress is marvelous for improvement in fatigue. You can practice on a plate as too much tool interference will cause spalling of the metal, an undesirable condition. Stainless is highly ductile so you can usually put quite a bit of tool interference, a few thousandths anyway. Hole will increase in finished size a small amount. Good luck with the new design.

 

[Snotter]

Roger, thanks for giving us a glimpse into what a naval architect have to go through during the design process. It is great that you share this stuff with the rest of us. This thread and forum, and the others like it, are the only places I am exposted to this material.

 

[Roger Long]

If you stick with the stainless tang you designed consider using a cogsdill roller burnishing tool to cold work the holes.

Another excellent idea I'll file away for when I'm doing something very weight critical and close to the bone. Some of the other responders above will have a hard time believing I'm saying this but there is a point when enough is enough. The service record of the really gross production boat designs out there indicates that you don't have to go to extreme lengths with these fittings as their design is fairly conservative.

I'm using a tang 1/16" thicker than the best practice standard from custom yacht designing in the 60's and then drilling out some lightening holes to compensate. I am adding the extra weight to carry the tank up to a second bolt, one which holds the mast head fitting in. This is back up in case the main tang attachment should lose it's nut or otherwise be compromised. The upper bolt should hold the mast up until I notice that the rigging is slightly slack because of the tang bending out from the mast and see the gap. (Bill Rosa, are you listening? )

 

[RichH]

Roger -
Starting with a clean sheet of paper .......
Seldén probably makes the best tang system in the industry. I have an older design SeldTang system on my boat .... Ty37 Bob Perry design. In most of the rigging for this boat I keep back calculating a Safety factor approaching 4 (probably because the rig size calculated is just under 3 and he used the 'next larger' available wire, etc.) It is apparent to me that Perry (and probably most others, including yourself) determine rig strength by pulling a theoretical force parallel to the horizontal plane applied to the mast head and calculate the moment arm about the 'full load' metacenter with the boat held at the ~45 degree angle. I did actual heeling/force testing (not to 45 degrees over) .


I have the 'old' style of SeldTangs and never doubt them as most boats that have them are multi-circumnavigators, so that should give you some confidence in sailing in the high latitudes. Although these are usually quite expensive, it might be a good solution .... buy & install, including the 'lowers' connections as I deem this to be equally important strength-wise.

We've discussed in your ‘chainplate thread’ the need to minimize all stress risers, bends, etc. and make all the critical parts 'straight' or inline tension, without flexure ... all to lessen fatigue endurance failure. We also discussed the importance of surface finish, etc. to lessen fatigue propagation.


Starting with a blank sheet of paper plus your mast .... below is my interpretation of the ‘SeldTangs’ for local manufacture whose only 'tricks' would be the special eccentric spacers that are milled from bar or rodstock ... whose interfaces at ~10 deg from the axis of the bore. These spacers will eliminate virtually ALL indeterminate flexure in the tangs proper. The bolt length increase to accommodate the fixture probably would need an 'auxiliary tang' to help support vs. flexure in the bolt .... my sketch is bent but I dont deem this to be critical in this particular member as it only there to as a 'helper' to reduce the bolt stub-end from flexing as a 'cantilever' .... the helper tang removes the (weak) 'cantilever' action so the tang put you into supported beam formulas instead of the 'inherently weak' cantilever requiring 4X the strength of the bolt in flexure - changes the weak cantilever to an end supported beam (mechanical/stress advantage = 2+). Stub ends and cantilevers are too unpredictable and vulnerable for my 'conservative' design mind. Ill leave it to you to calculate the lateral flex and shear in the bolt stub (using standard beam flex calcs.) at full rig wire load (including FS) to see if indeed you actually need this aux. tang. Seldén in their ‘simple’ tang systems usually has this 'aux' tang to support the stub end flex. .... I alway did this when engineering gantries and overhead cranes, etc. (about 35 years ago - groan).

The 'spacers' would be milled from rod stock. Simple operation to mill @ 10 degrees (the probable mast intercept angle). Thickness of the spacers would be critical for clearance to mast, clearance to bolt head, and clearance for rigging eye in between the two tang plates. A bolt loaded at 10° off, isnt going to put that much of an axial thrust load against the bolthead ... but you can do the trig. to verify as Wire load X Sin10 = 17.4% of wire load. For the tangs proper, just for 'spits and giggles' Id increase the FS by ~17% because of the ~10° eccentric bore holes - just a SWAG.

Forgot/edited - You'd also need to consider to apply a small ferrule over the thread contact area of the bolting on the thread end of the SHOULDER bolt - just in case the torque/strain relieves the pressure on the friction joint.



Summary of my 'cobbled' design:
•No flex in tang plates – reduction/elimination of stress risers
•Needs milling of the 'spacer faces', spacial ‘stack up’ of combined length of spacers is important
•Torque to 80% creates a 'friction connection' against the compression ferrule
•Compression ferrule creates large cross section to prevent ‘mast bore compression failure@ projected load bearing surface’
•Aux. Tang is ‘helper’ to lessen flex in bolt stub ends – can be bent.
• As in all fatigue service applications, surface finish will be important - mirror polish.

 

[Roger Long]

Starting with a clean sheet of paper .......

Very nice design. I'll probably stick with what I have though because it is straightforward and easy to get fabricated.

Although I hope to get up into some high latitudes, it will be long distance coastal cruising, anchored most nights with infrequent passages. I won't be doing the non-stop days and weeks of a circumnavigation. Looking at how the atrocious designs that are so common hold up, I think my simple tangs will be fine. If I were headed around the world, I would do something like this.

My upper bolt is just redundant back up BTW. I'm actually drilling the hole slightly oversize since the spacing on each side is slightly different. It will add some friction between the tang and mast but is primarily there in case something happens to the primary bolt. In that case, the tang will probably flex enough away from the mast to slacken the rig a bit but it should hold long enough for me to unload the rig. It will also keep the tang from swiveling back and forth and breaking the TeffGel or other bedding material that keeps water out of the interface between the stainless and aluminum.

It is apparent to me that Perry (and probably most others, including yourself) determine rig strength by pulling a theoretical force parallel to the horizontal plane applied to the mast head and calculate the moment arm about the 'full load' metacenter with the boat held at the ~45 degree angle.

Actually, I (we,maybe) distribute the load over the luff, pretty much ignoring the stiffness of the mast as far as shroud loads are concerned. It's been a long time since I did it so I've forgotten what typical load sharing is between uppers and lowers but the 65% in the lowers that is listed on the design data sheet I recently ran across from my days at Rhodes sounds right.

I'm sizing my new tang tos take the full load as if it were a dinghy mast without lowers to it's pretty conservative.

The lowers are pretty important as you point out, 65% of the load. However, there are two of them so there is a higher level of redundancy. I may have new tangs made for them as well but I'll have to wait until the mast comes out of the shed in the spring to cross that bridge. I can reach the masthead but the lowers are sandwiched between two other masts. I'm sure the lower tangs are good for a few more years so I might do them in a year or two. The one set of uppers I took off for patterns look remarkably good on close inspection despite the design and brake score marks.

 

[RichH]

Suggest then you at least send the tangs out for heat treating to stress relieve/anneal so to return the crystals in the area of a the bends back into alignment and to relieve the cold-worked zones.
Small parts such as these should be inexpensive to do, especially if the heat treater is doing a lot of stainless heat treating .... but could take some time as the parts may have to wait until a larger lot is being treated.