Radar Arch Fail - Cautionary Tale

[Boatguy30]

Originally Posted by UNCIVILIZED

Posted by Boatguy30
"care to share your engineering data? The sheer strength if 1/4" machine screws is more than adequate. Most winches up to around 40:1 use 1/4". 2000 pound of Genoa sheet load is far more than an arch leg SHOULD take."

There aren't a lot of 40:1 winches out there with 2,000lb leads on them, especially not from genoas. When you get into sheet loads like that, you want winches with close to double that power ratio. Especially as with the shock loads which sheet winches see, they'd rip winches that size out of the deck. Unless, that is, you have some monster over-sized backing plates, & a Herculean deck laminate.

So I'm curious as to where you're getting your numbers. Because, generally, to get those kind of sheet loads, you need a racing boat in the mid 40'ish size range, carrying it's #2 Genoa (say 750sqft) hard to weather, in winds north of 25kts. Which is NOT a chore that a 40:1 winch can even begin to touch.

The sheet load formula is Wind Speed (in knots), squared x Sail Area (in square feet) x 0.00432 = Sheet Load (in pounds). Harken
750sqft x 25kts (squared) x .00432 = 2,025lbs




On this, from how I was schooled, the math is something like this:
(0.5 x diameter) squared x 3.14 x 0.85 x metal tensile strength = Breaking load.

The 0.85 is a constant used for bolts, due to their being notched, & thus not as strong as if they were straight cylinders, or smooth pieces of base metal.

The rule of thumb, strength wise, is that you want a safety factor of 4-5:1

And I'd bet a few thousand $, that those bolts were at best 304/306 Stainless. Which has a ballpark tinsle strength of 85k psi, depending on the grade, how it was machined, etc. As can be seen here
MatWeb - The Online Materials Information Resource

Using semi-exotic alloys for these kinds of apps is fairly uncommon I would think, as it's cheaper for the manufacturer to simply go with a bigger bolt, than it is to use those alloys.
And if such is in doubt, go & look up the price difference between 2 shackles, the same size, made out of the differing alloys of stainless in question.


So, back to the math at hand, AKA figuring out the approximate strength of the bolts.
If we skip the safety factors for a moment, & use TacomaSailor's assumed 1/2" of bearing surface, then.
(0.25" x 0.5") squared x 3.14 x 0.85 x 0.5" x 85,000psi = 1,772lbs/bolt

Where TacomaSailor's 1/2" diameter figure for the bolts in these calc's is coming from, I don't know.


If folks want to use stays in order to reduce some of the cantilvered hanging loading on the arch, then...
- Put a gusset/chainplate on the upper, forward section of the arch frame.
- Put a chainplate or reinforced (backed) padeye on the hull or deck, forward of the foot of the arch, by somewhere in the neighborhood of it's height+/-
- Run a stay, using standard rigging practices between said points, & tension appropriately

***NOTE*** - If you do choose to go this route, the whole load bearing dynamics of each leg of the arch will change. Most notably, that the stay will induce a good bit of compression loading in the forward most leg of the arch & it's attaching hardware.

Running back stays on any IMS style sloop with fraction rig. Used to grind with a double handled winch basically till the inch wouldn't turn anymore generations around 6,000 pounds of heads tension with a 2:1 cascade then 40:1 winch. 5 1/4" bolts.

[Boatguy30]

Those things do flex a ton. I was aboard 2 mono innthe Bahamas last winter with them. "wow I'm really glad I didn't buy one of these" was my rum soaked comment. One boat had even bought the "diagonal bracing" kit they don't really seem to advertise since they're so awesome you wouldn't need one.


My one piece are is extremely solid and 11' wide. Weighs less than 100 pounds and instead of using those cheap feet that come with all production arches. I had 2 plates welded to the front tubes that bolt to the hull to deck joint. I used the stock mounts aft but had the socket welded into the tube. I also had an additional brace bent and welded to match the standard inner reinforcement bar. Also had him space the feet 30" instead of 24"


It basically looks like part of the boat not a tacky ad on. Total price with shipping and welding mods, $2000

[hellosailor]

Polyana says to look on the bright side: Those mounts didn't tear a batch of hull out with them.


Something to consider before reinstalling, is where do you want the weakest link in the chain to be? The mount, or the hull?


Among the other technical issues about the bolts (grade, metal type, etc.) no one has mentioned that threads can be cut into bolts, or rolled into them, and iirc that also makes a difference in ultimate strength.


Then too, even the airlines have lost aircraft because counterfeit bolts are so hard to spot. Without an analysis, no one even knows if those bolts are what the arch maker actually specified should be used. Even if they were, casting errors happen. Contaminants happen. So we don't know why they really failed, absent some lab time.


Making the mounts stronger is simple. Have someone weld up larger mounting plates, use eight bolts instead of four. Use larger bolts. Use 5200 under the plates, to mechanically bond them to the hull, actually making the stainless bolts redundant and superfluous once the 5200 sets, and spreading the "point contact" into a larger even load. Add backing plates the same way, sure.


But you'd still need to find out just why these bolts failed, and whether you want to shift the failure point to the hull (instead of the bolts) and then, just how strong do you want to make it. Strong enough to stay attached during a roll over? Or a swamping wave? Or, should it shear off cleanly at some point before then?


Truly elegant engineering anticipates all of that--but it certainly will impact the budget, either in time or money.


Sealing off tubing sections, or adding buoyancy and a leash, might also be considerations. Just in case there is a next time.

[TacomaSailor]

"Three corroded cheap (probably Chinese) crap stainless bolts of insufficient size. And one sheared bolt. Poor monkey bar design. Would you even allow small children to play on it? Let alone load it up with all that weight, dinghy, solar, radar etc. I'm sure the OP meant well and probably relied on the manufacturers recommendations and trusted the contraption would work for its intended use. Sad out come.... Terrible design."

I suspect you have never touched or even seen an Atlantic Towers Arch in person? If you had, you would not make such irrational comments.

My Atlantic Tower is IDENTICAL to the one that failed. It has done 7,000 miles in the Pacific Ocean and another 3,000 miles in Puget Sound during the last 16-years. I installed the tower in summer 1999 and I know that every part of the tower is sound and well engineered. But, ignoring that - let us look at my experience.

I have removed all the mounting bolts on two occasions. Those 16-year old "cheap ... crap stainless bolts" look brand new with not a bit of corrosion or wear.

I weight 230 pounds and climb up the side of my tower every week to clean the solar panels. I have hung from the aft portion of tower during installation just to check the integrity of the tower.

My tower has supported four 125-watt panels for the entire 16-years on the boat. The tower supported those panels in 60+ knots of wind while our boat boat was lying on the beach heeled at 30 degrees with 10' breaking seas striking the tower every 8-seconds.

During that time a 60-pound outboard was hanging from the tower and swinging wildly side to side in the wind and seas. Those breaking seas were coming over the high side of the boat and breaking into the cockpit. I sat in the cockpit and on shore for hours watching the waves and wind batter the tower. It never flexed or moved at bit!

Six-months later I removed the tower and inspected every component and the mounting pads and holes. There was not a sign of any excess wear or movement. Everything looked like new. 12-years has now past and the tower, bolts, and mounting pads still look like new.

We've been at sea in 12' breaking seas with wind gusts over 50-knots while the boat rolled 20-degrees side to side. The arch never moved an millimeter. It is solid, a permanent part of the boat and probably stronger than the boat itself.

We've set in many anchorages with wind gusts over 45-knots for 36-hours straight. The panels and tower never flex, wiggle, or move.

Your comment is ridiculous and uninformed. I can say with 16-years of absolute certainty that a properly installed Atlantic Tower solar panel/radar arch is bullet proof and will not fail under almost any conditions.

FACTS - not opinions!

Sorry for the vehemence expressed in this response - but you are impugning the integrity of a product that I have trusted my life with for 16-years and it has performed flawlessly. And - tens of thousands of other Atlantic Tower owners will report the same thing.

[Kenomac]

TacomaSailor,

When I look at the OP photos of the failed monkey bars... I see crap and corroded, undersized bolts.

When you look at your arch, even in light of the OP's failed monkey bar photos... bars that were made by the same manufacturer... You see a high quality, reliable radar arch.

We have a difference of opinion. Nothing wrong with that, I respect your opinion.

But You should also consider the possibility that the manufacturer might have changed the composition of the product since you purchased yours. Fifteen years apart. Maybe today... They aren't including high quality bolts or have recently chosen to spec a lower quality stainless steel from China?

[TacomaSailor]

Originally Posted by Kenomac

TacomaSailor,
We have a difference of opinion. Nothing wrong with that, I respect your opinion.

I am not stating an opinion - I am citing 16-years of facts. It is not an opinion that my tower has survived 16-years of abuse and about the most severe testing such a tower could endure - it is a FACT it has survived.

[Cadence]

Originally Posted by TacomaSailor

I am not stating an opinion - I am citing 16-years of facts. It is not an opinion that my tower has survived 16-years of abuse and about the most severe testing such a tower could endure - it is a FACT it has survived.

Ken was somewhat abrupt. I went back and looked at the OP pics. Do you have the identical set up? or just the same mfg.. I for one didn't like the set up but maybe that is just me? Yours maybe entirely different?
I doubt anyone doubts your 16 yrs. of good service.

[TacomaSailor]

My setup looks identical - except my feet mount on the deck and I have 2" of solid wood and then a stainless backing plate on the inside of the deck.

I have had had several stainless bolts in other applications fail due to crevice corrosion. The mounted bolt gets wet then the leak reseals before the interior of the bolt mount drys. No oxygen can get to the damp stainless and it corrodes due to oxygen deprivation.

Crevice corrosion is a nasty problem because it is almost impossible to detect before the failure.

No doubt the bolts in the OP bolt failed - I am just concerned why.

[Cheechako]

Originally Posted by TacomaSailor

It is hard to understand how folks here argue with engineering formulas and concepts.

The arch is static, as is every thing (radar, solar panels) attached to it. That weight is attached to the boat and does not generate any dynamic load just as a winch bolted to the deck does not generate a dynamic load.

The dynamic load is generated by the dinghy swinging. Using scaling and trig functions it is easy to determine the lever arm load of the dinghy hanging in a static position. The swing load is side to side - NOT up and down. A swinging side load on the arch would be a compression load on bolts as mounted on the boat in question.

I know the minor axis and can calculate the bearing surface of the bolt using the formula for the surface area of a cylinder. That is what I did. The minor axis of a 1/4x20 bolt is 0.1758". With 1" length of bearing surface (no spacer pad) - you get a bearing surface of 0.60 square inches - not 0.05" as mentioned above.

The tensile strength of 316 stainless is 81,400 psi. Plug that into the conversion of tensile to shear strength and you get
.60*81400*.3=14,652 psi for one bolt.

There are four (4) bolts on each foot giving a single foot strength of over 57,000 pounds.

How did an arch firmly mounted to a boat generate enough force to shear four bolts at 57,000 pounds? The arch does not move when the boat bounces, rolls, or yaws - at least mine hasn't in the last 10,000 miles.

Or, worse case how did that arch generate over 14,000 pounds to shear a single bolt? What could generate that dynamic load?

And, if the recommended 2" spacer was included in the formula the single bolt shear strength would increase to about 42,000 pounds with 316 stainless.

I have taken 35' free falls (generating about 4,000 pounds of force with a rope that stretches 10%) onto 1/8" bolts anchored in a cliff and not sheared the bolt - and we rock climbers really do calculate these things.

Engineering is a funny business - there are easily measured and calculated numbers.

I am not disputing the failure of the bolt - I am just pointing out that a 1/4" x 20 threaded, common/cheap, stainless bolt has way more shear strength than needed for this application.

As I said, I am not an engineer and maybe someone who is can critique my reasoning.

Well...... I am an engineer, and you seem to misunderstand a lot.

First, you are confusing bearing area of the bolt shank with cross sectional area. The cross sectional area is used to determine breaking.

Second you are wrong about dynamic forces. Think of it this way; First hold an 8ft 2 x 4 in your hands outstretched horizontal while standing on the tailgate of your truck. That is static.
Now jump off the truck. What happens? When your feet hit the ground the end of the 2 x 4 continues and strikes the ground. in simple terms, that is dynamic force.
The cantilevered arch is the same when the boat goes over a wave, as the boat stern comes down and then stops, the arch wants to keep going, putting stress on the fasteners.

Third, and most important for engineers, you are ignoring reality; The reality is that, even with all the high numbers you predict for a 1/4 bolt, they broke.
Put a 1/4-20 bolt in a vice and break it in shear... it is surprisingly easy.
Just sayin......

[Cadence]

Originally Posted by TacomaSailor

My setup looks identical - except my feet mount on the deck and I have 2" of solid wood and then a stainless backing plate on the inside of the deck.

I have had had several stainless bolts in other applications fail due to crevice corrosion. The mounted bolt gets wet then the leak reseals before the interior of the bolt mount drys. No oxygen can get to the damp stainless and it corrodes due to oxygen deprivation.

Crevice corrosion is a nasty problem because it is almost impossible to detect before the failure.

No doubt the bolts in the OP bolt failed - I am just concerned why.

If yours are deck mounted it is an entirely different animal even if the superstructure looks the same. Your bolts will depend on tensile strength not shear forward mounts, next to nothing on the aft ones since the mounts are basically under compression. JMHO I hope you get another successful 16 yrs. from yours.

[hpeer]

Not to get into the middle of a congenial pissing contest but..!!being an engineer (electrical) Iooked it up in a table.

1/4-20 steel....safe working load
Sheer at the root 200 pounds
Sheer at full thread 370 pound

Breaking strength
Grade 2 2350
Grade 5 3800
Grade 8 4750

I think this gives an indication of the amount of reduction one need to consider due to fatigue. IMHO. SWL is very safe, however it is not to be discounted.

SAE Bolt Strength Specs

Less for stainless

I'm off to check my bolts

Cheers

[TacomaSailor]

Cheecako - thanks for replying as an engineer. Maybe you can help me understand what I mis-understand. You say:

"Second you are wrong about dynamic forces. Think of it this way; First hold an 8ft 2 x 4 in your hands outstretched horizontal while standing on the tailgate of your truck. That is static.
Now jump off the truck. What happens? When your feet hit the ground the end of the 2 x 4 continues and strikes the ground. in simple terms, that is dynamic force.
The cantilevered arch is the same when the boat goes over a wave, as the boat stern comes down and then stops, the arch wants to keep going, putting stress on the fasteners."

My apparent mis-understanding is based on my understanding of F=ma

the shear load is the Force (F)
the mass does not change based on boat movement
a is acceleration which I understand to be change in velocity per unit time
when at rest the a = gravity
when moving the a is due to change in velocity but the hull and the arch experience the same change in velocity - if they did not you would see differential movement in the arch/hull joint - Yes?

How can the acceleration of the arch be different than the acceleration of the boat hull? They are bolted together so in my thinking - the acceleration of the arch would be equal to that of the hull therefore no increase in F because of the equal acceleration of hull and arch.

How can two objects bolted together experience a differential acceleration?

As I said - I am no engineer so I guess there is something fundamental I missed.

[Kenomac]

Originally Posted by TacomaSailor

I am not stating an opinion - I am citing 16-years of facts. It is not an opinion that my tower has survived 16-years of abuse and about the most severe testing such a tower could endure - it is a FACT it has survived.

Then I guest we can agree on two facts.

1) Your arch has survived for 16 years in an upright manner.

2) The Op's identical arch is a twisted up piece of crap hanging off the back of his boat.

[TacomaSailor]

I am confused about the formulas. The engineering calculator website shows:

Shear Stress ave = F/(pi * r * r) or F / area

F=applied force (lbs)
R=bolt radius (inches)

Bearing Area Stress Area = thickness * diameter
Which results in square inches of bearing surface

Assume a 1,500 pound load on the bolt and NO safety factor – we are looking for breaking loads – not the SWL. Assume 500 pounds mounted on the arch and a 3g dynamic load.

Their calculator for a ¼ x 20 bolt with a minor axis of 0.1758” and 3.5” length and ultimate yield (min) of 85,000 psi (Stainless 316) reports the following.

Section area of bolt = 0.025 square inches
Bearing area stress = 2,778 psi
Shear Stress ave = 58,947 psi
Allowable Stress = 85,000 psi (NO safety factor)

Those calculations seem to show there is no problem using ¼ x 20 bolts even if 100% of the load is imposed on a single bolt even when there are 16 bolts in the mounting system.

If we assume four times the load, 6,000 pounds, and that load is imposed on only four of the 16-bolts then each bolt would feel only 1,500 pounds .

Again, ¼ x 20 stainless bolts seem plenty strong.

Link to engineering calculator web site:
https://www.engineersedge.com/materi...hear_calcs.htm

What am I doing wrong in my calculations? I just plug the numbers into the calculator:
- Force
- minor axis
- 316 tensile strength

[goat]

The bolts sheared. If I recall correctly the old 'rule of thumb' was; shear strength is 60 to 80% of tensile strength. Since the main difference between the OPs structure and Tacoma's was the mounting (and maybe a little more decorations hanging off the tower) I think I'd be inclined to rebuild the tower with deck mounts.
I'm watching this one with interest as I need to fix a few solar panels and trying not to ruin the lines of the ol' girl or end up with pieces flying.
Good luck on the repairs Montenido.

goat