More rigging horror.

[Roger Long]

I decided that I'd better take a look at my lower tangs as well since I'm going to all this trouble and expense to start out next summer with a zero time rig that I won't need to worry about. So, this morning I went down and took off the lower tangs:

They have the same ugly score lines as the upper tangs I'm replacing so, in for a penny, in for a pound, this is the time to have new ones made, going to single thicknesses and using toggle forks.

Preparatory to laying out my new tangs, I drew up the old ones to see just how bad the lead angles were. Wow!

Think of the bending cycles these have been going through since 1980. I doubt that the pre-tensioning of the rigging is sufficient to pull them fully straight but sailing hard probably does. Then, when the rigging goes slack on the other tack, they spring back in. All this work was being done by the wire nipping at the end of the swage eye sleeves. Despite all this abuse, I can't see any sign of cracking or other problems either in the tangs or at the swage ends. It's been disturbing to find out how common this kind of rigging detailing is and it's a testament to the materials that you don't hear more calls to SeaTow to assist dismasted boats.

 

[Mike B1026]

They seemed to have held up fine, especially after 30 years. Are you sure you want to try to "improve" upon that?

 

[Roger Long]

They seemed to have held up fine, especially after 30 years. Are you sure you want to try to "improve" upon that?

I'm sure they will continue to hold up just fine until a millisecond before they break.

Most of this boat's sailing miles are still ahead of it. Yes, as a boat designer, I do want to improve on it and be able to look up at my rig and know that it is not an engineering horror show.

 

[dscribner]

Roger,

That "score line" is just where the male die struck it to form it. It's real common with thicker material especially when the brake press operator refuses to change the die out for the correct radius. I bet your paycheck that the score has a 1/32" radius. The operator used a "knife die" to form it. I'm a designer, not an engineer, but I'm screaming down on 50 and I've been swimming in the P.E. pool for a few decades. I wouldn't worry about the indent.

 

[RichH]

Same arguments apply as with the mast top tangs

Those OEM tangs were most probably appropriately annealed and then stress relieved.
Either replace with a design without the unnecessary stress risers or send out the direct copied replacements to have them properly annealed, etc. ... or both.

 

[Roger Long]

I wouldn't worry about the indent.

Given the stress levels at these components generally experience, I wouldn't either if, big IF, they had been bent to an appropriate geometry. If new tangs were available that aligned with the wire so the two halves shared the load properly and weren't bending every time the load changed, I'd probably just start with a new set. Since I've got to have tangs made, I may as well have them made right.

The most significant hazard with these, same as discussed with the uppers, isn't likely to be the tangs breaking but premature weakening of the wire. The tangs hold the terminals at an angle so the wire turns sharply where it enters the swage. Every time the load changes, it bends.

The reason these horrible designs work is that the wire seldom gets up to the 30,000 psi where fatigue becomes an issue. Before it does, the tanks and wire are pretty much pulled straight. Boats aren't heeled beyond deckedge very often so the critical fatigue cycles mostly occur at lower overall stress levels although there can always be local stress concentrations. The fact that most boats don't get sailed much also helps. I hope to sail long and hard.

I don't see a significant risk of this rig coming down after a simple wire replacement but I do see a virtual certainty of it nagging at the back of my mind in the otherwise stressful situations I expect (and hope) to encounter. I wouldn't let these details be put on a boat I designed for someone so I'm not going replicate them at this point.

 

[RichH]

........ gets up to the 30,000 psi where fatigue becomes an issue. .....

Thats only normal a 'go-by' as fatigue 'can' start to readily propagate at stress levels at much lower than the endurance fatigue limit (~30ksi) and especially if the material has been cold-worked and not annealed / stress relieved.

 

[jibes138]

Roger,
Mechanical design is an interesting field. Sometimes you design things so that at rest they look distorted but under load they take the shape you want them to have. An example of this is gear teeth. For highly loaded gears is is often the practice to modify the involute shape so that under load the bending of the gear results in a true involute shape. Perhaps the designers of your rig purposely modified the angles so that under load they are exactly the way you want them. Run some FEA analysis and see what you get. Bending from tacking would be very low cycle fatigue, fatigue is usually looked at in millions of cycles not in one cycle evey many minutes or hours.
To Rich's point cold working puts the atoms in the material at a high energy state, then you apply a load so they are even more energized. Annealing or stress relieving puts them back into a low energy state. Cold working can be beneficial for fatigue in some cases if you apply a residual compressive stress to a apart that is subsequently loaded in tension or if subject to high contact stress.

 

[Roger Long]

Perhaps the designers of your rig purposely modified the angles so that under load they are exactly the way you want them.

You are either kidding or you know a hell of a lot more about mechanical engineering than the state of the art of low end production boat building. Believe me, there was NO engineering involved in the design and building of this boat.

It was a very interesting suggestion above to have these tangs destruction tested. I wish I knew someone who would do that without much expense. It would be very instructive to the boating field in general. It wouldn't surprise me to find them either at nearly full strength or greatly degraded. There are a lot of imponderables in both engineering and individual boat history.

I'll be the first to admit that a lot of this project is about peace of mind and aesthetics. A lot of boatowners go crazy if they look forward and see a run in their varnish. As someone who has designed rigs, I couldn't look up from my cockpit and know that there was such ugly engineering holding up something my life could depend on, even if the actual risk were low.

 

[Witzend]

Emotions should not play a part in decision making regarding reliable engineering for critical components, nor should they play a part when using the end product during life endangering endevors.

 

[Snotter]

Roger, if I ever need a boat designed or modified, you will be the first guy I contact. I like the way you think.

 

[jibes138]

Roger, you should add some load cells to your rig so you can real-time monitor the cyclic loading and have it print out on a strip recorder as well as capture the data in digital form. Then you will know exactly what is going on. You could put some strain gages on various components and take measurements under verious points of sail and then modify your designs based on actual real world feedback of stress loading. This is how most critical products with life safety are designed. FEA has advanced dramatically but products are still destructively tested to see if all the theory and design work resulted in what you thought it should be. Boeing bends wings to see where they will break for instance, same thing with fuselages. The least you can do is some tensile testing on your rig to see what it will take. Or you can just take some risk and not sleep very well at night worrying.

 

[Roger Long]

The least you can do is some tensile testing on your rig to see what it will take.

No, the least I can do is just have some simple single tangs made and slap them on with Sta-Lok toggle eye fittings. These aren't going to cost me much. (The tangs anyway.) I'm not going to have them heat treated as Rich suggests as the specialist stainless steel pressure vessel and piping builder who is going to do the passivation for me laughed at the idea when I described the amount of bend and nominal loading. If I was 20 years younger and planning a circumnavigation, I might though.

I'm not going to worry about this rig when it's done even though I'm doing something between nothing and the ultimate.

 

[Mac]

wow!

and I worry about spillin my beer when i heel!!

 

[jibes138]

why not make a mold of your current tangs out of a low temperature alloy like cerro-tru and then lay up new ones using carbon fiber? you will reduce weight aloft that will reduce your heeling moment, and reduce your pitching moment. a couple pounds at say a 30 foot moment arm is a lot of moment (60 foot pounds). My sailmaker used to always rant and rave at me about minimizing weight aloft. didn't want sunbrella suncover on the jib for instance. so think about that rather than just settle for a minor change in your design. New materials may be a better solution. what other weight can you remove? maybe the shrouds can be fiber as well instad of steel?

 

[Bad Obsession]

Roger....If you are replacing all of this stuff you should take the old stuff, once you are done with it, and failure test it to see what type of load it takes. I bet that you will be surprised how it holds up.

 

[Roger Long]

failure test it to see what type of load it takes. I bet that you will be surprised how it holds up.

I wouldn't be surprised in the least to the parts reach their rated strength and no more surprised than I would to see my rig over the side. Engineering is not as precise a business as people think when it comes to individual components. Remember that Airbus rudder that just fell off? That was designed by people just one rung below rocket scientists with a virtually unlimited design budget. In fact, the Challenger shuttle that blew up WAS designed by rocket scientists

 

[Roger Long]

lay up new ones using carbon fiber?

Good lawd. The last thing I want to be thinking about on a dark and stormy night is the behavior of a material like carbon fiber. This is no place to be doing R&D. Oh yeah, see my post above about the Airbus rudder. That was carbon fiber.

The weight distribution business is a bit overblown for cruising boats. I could detect no difference in my boat's performance after installing 25 pounds of radar halfway up the mast. I've watched with interest as three big teenagers went from the cockpit to sit on the foredeck while going to windward in waves and been surprised to see almost no change in pitching amplitude.

Racing is different. It's all about that .05% margin.

 

[Roger Long]

Here is the first layout idea for my new lower tang design.

I'm somewhat constrained by the existing spreader mount so the 1/4" plate main tang needs a 3/16" doubler plate to get sufficient metal around the mounting bolt. This will be welded on and will also add metal in the bend area.

This design avoids a problem I've noticed on many masts in which the two lower shrouds come down to a diamond plate to gain sufficient separation for the terminals. Fore and aft lower shroud tensions change a lot relatively depending on backstay tension, especially with an adjustable backstay as I have. This means a wide diamond plate is constantly being rocked back and forth so a change in tension on one shroud effects the other. My design has both strain lines running true to the mounting bolt.

There is a slight difference in thwartships angle between the shrouds. My lowers are usually mostly along for the ride when the backstay is tensioned so I'll have the main bend made for the forward set. Use of Sta-Lok terminal forks will mitigate this slight misalignment.

The availability of a plasma cutting table makes this complex design possible.

 

[Bill Roosa]

Jonny Rancloud here again

Roger, you do know that you have asymmetricly loaded the longer (right) tang, right? The load line does not go through the center of the material where it necks down. This is classical "oopsee" failure. The load will be concentrated on the "short" (right hand side of the load line in the picture) side of the tang. This causes the material to act as if it is in bending and it will suffer lateral-torsional-buckling.
So as you load the left side tang you will notice the side to the left of the load line is going to act as if it is in compression and flex out of plane.
This is the result of having two areas that are of different sizes on either side of the load line. As the material is all the same the load tries to spread itself out across the surface (tension resistance surface) and the smaller surface (right side of load line) is under more stress and elongates more than the left side which is under less stress.
Basically the load line wants to (and should ALWAYS BE DESIGNED TO) pass through the centroid of the material. An extreme example is trying to pull apart the ends of a horseshoe. It matters not how much you get the forces lined up where they are applied to the material (the ends in this case) the middle is in bending and tension (off axis or lateral tension) Hence lateral torsional buckling
BTW this is statics 101 stuff.
You said that there is not much engineering going on in the design of your boat. That is what good engineering is supposed to look like.