Radar Arch Fail - Cautionary Tale

[Cadence]

Originally Posted by GordMay

Actually, Acceptable = agreeable; tolerable; allowable; suitable; adequate; satisfactory.

Whereas, Political correctness a isterm used to criticize language, actions, or policies seen as being excessively calculated to not offend or disadvantage any particular group of people in society.

And there is too damn much of it especially in the US.

[wgoodbye]

I mentioned in a previous post that I have the same arch. The mounting screws on the leg mounting plates are 1/4-20 stainless steel. Not sure of the grade. Possibly 316 or 18/8. These are oval head machine screws and fit nicely into the recessed 1/4 " mounting holes on the mounting plates. I think I found a simple solution and would welcome any opinions. I'd probably replace the 1/4-20 stainless oval head machine screws with 1/4-20 Titanium grade 5 oval head screws. I found a source for these on: https://www.alliedtitanium.com/. Minimum yield strength is 128,000 psi (880 Mpa) which is more than twice the strength of stainless. You also don't have to worry about crevice corrosion with Titanium, which may or may not have played a part in the failure of the stainless hardware. Thought anyone?

[Cheechako]

Originally Posted by wgoodbye

I mentioned in a previous post that I have the same arch. The mounting screws on the leg mounting plates are 1/4-20 stainless steel. Not sure of the grade. Possibly 316 or 18/8. These are oval head machine screws and fit nicely into the recessed 1/4 " mounting holes on the mounting plates. I think I found a simple solution and would welcome any opinions. I'd probably replace the 1/4-20 stainless oval head machine screws with 1/4-20 Titanium grade 5 oval head screws. I found a source for these on: https://www.alliedtitanium.com/. Minimum yield strength is 128,000 psi (880 Mpa) which is more than twice the strength of stainless. You also don't have to worry about crevice corrosion with Titanium, which may or may not have played a part in the failure of the stainless hardware. Thought anyone?

One thing to keep in mind, ti may gall on stainless and make a nut permanently seized. So buy Ti nuts if available, or use a good graphite (?) or approved lube on threads.

[model 10]

Originally Posted by sk Barnett

Allow me to introduce some fundamental principals of failure analysis used by engineers as I recall from my younger life as a mechanical engineer. The bolts in question as mounted on the hull are subjected to multiple stresses. First, the bolts are stressed by the tension introduced by tightening the nuts. Second, the bolts and mounting flanges/backing plates are subjected to the forces necessary to resist the torque of the load on the end of the arch -- this bending moment expressed in units of torque (ft. lbs.) is transferred to the several mounts and met with a combined counter torque by the bolts in shear once the friction between flanges/backing plates is overcome (if it is overcome). Third, the dynamic force of the load (dinghy) acting through the cantilevered arch as measured by the Mass x acceleration as the arch load cycles in a seaway. The vectorial sum of these forces acting on the cross-section of the bolts is the principal stress vector, which will be oblique to the cross section of bolt as it combines lateral and axial stress components. Interestingly, at the time of installation or maintenance of the bolts, an additional torsional stress is introduced by the act of tightening the nuts, and is often the time that fasteners will fail if that torsional component of the principal stress exceeds ultimate tensile strength in combination with other loads then acting on the fastener.

The principal stress, once determined is then compared to the yield stress of the material, discounted to accommodate stress concentrators such as cut threads and other abrupt changes in geometry, and deformation of the bolt shank if any, should the mounting flanges/plates have shifted. Cut threads are a severe stress concentrator as compared to rolled threads. The cyclic stress (such as the dink suspended on a bobbing boat) introduces the doctrine of fatigue, which can be quantified using empirically derived curves showing number of cycles at given stress levels. As I recall, carbon steel has an infinite life when subjected to cyclic stress less than 30% of the yield strength -- unlike all others such as aluminum and stainless which have finite cycle life at any level of stress. Cyclic stress militates changing fasteners from time to time in order to avoid fatigue failure.

Corrosion cracking, among other sources of introducing significant stress concentration, drastically affects the cyclic life of a fastener, and will lead to failure as the crack propagates toward the center of the fastener from its outside surface. Far less stress is required to propagate a crack to failure than would defeat a smooth surface. Torsional stress and bending moment related stress are transferred to the outside surface of the fastener, the center of the bolt being its neutral axis. Tension force stresses the entire diameter uniformly, but surface cracking or sharp deformations will diminish the useful loading of any fastener.

Fatigue failures generally show concentric rings across the diameter of the bolt working their from the point of highest stress inwards, and leaving a portion of the bolt that shows either a shear or tension failure when its ultimate strength across the diminished cross sectional area is reached.

I won't pretend to analyse the failure at hand here, but perhaps the foregoing principals will be useful as you consider how the deck arch mounting hardware is loaded statically and dynamically. Consideration of one or another of the stress components is inadequate as a basis of analysis. It is the vectorial sum of all stresses acting on the fasteners that must underlie any investigation of a failure -- and should govern design and installation means and methods.

With Best Regards,

All that and you did not mention that the cross section of the threaded portion is much smaller than 1/4" and fraught with sharp edges.

[natraps116]

Wow this is kinda scary, I have an arch from the same company. Mine was semi custom though because they did not make one wide enough for my application. I told them i would have up to 400lbs on the back just to be safe. But seeing this makes me re consider keeping the dinghy back there during passage. We have had our dinghy back there for about 4k miles so far with no issues. But now im gonna check all my bolts tomorrow before we leave to Colombia from Guanaja

[montenido]

Hi folks,

I just returned from the boat. I disassembled the arch and hauled everything home. I removed the broken bolts from the hull that were left when that pad sheared its fasteners. BTW, all the other 3 pads held securely, even with the stress of the tower being in the water. As far as I can tell, looking at the broken ends of the bolts, one might have been broken already, as the surface of the break looks a bit oxydized, but the others are shiny and clean. One of the bolts is bent, but not the others.
There is no sign of corrosion near the breaks, all are clean (they were encased in 5200). So, no real smoking gun as far as I can tell.

I spoke with Shelly yesterday and she told me that they prefer vertical mounting for their arches. Something about more surface area to add strength, I think. She told me that she has never had one of their arches fail like this, and asked that I send her some pictures. She asked what I was lifting with the arch and davits and I told her about the solar panels and the dinghy. She scoffed at the solar panels, explaining that the arches can carry hundreds of pounds on top with no problem. She was more interested in the weight of the dinghy.

So, I have ordered a new arch, and will using larger bolts with smooth surfaces on any bearing points, not threaded. That should increase the shear strength by a lot. As I mentioned earlier, the other 3 pads remained securely fastened, and the larger bolts that pin the arch to the pads were all intact. All the welds and bolted joints of the arch were solid and intact also. I will also trim a few inches off the height of my solar panel mounts. Every little bit should help.

Somebody mentioned changing out the smaller fasteners on an annual basis, or something like that. I think that if you are doing some heavy cruising or experience a hurricane (like me), it would be a wise move.

Well, that's it. Let the arrows fly. Thanks again for the thoughtful responses.

Cheers, Bill

[redsky49]

Originally Posted by Guy

All that and you did not mention that the cross section of the threaded portion is much smaller than 1/4" and fraught with sharp edges.

Actually, I thought that the post from SKBarnett was the most cogent non-hysterical remark to date!

[sk Barnett]

While this thread has primarily focused on the fasteners, it is important to consider the fact that holes (small radius holes) are drilled through the hull in a place close to the connection to deck, and where the surface of the hull is not flat. Small radius holes represent greater stress risers than those of a larger radius. The presumably flat mounting hardware (bulkhead fitting and backing plate) may crack the hull during installation if the bedding material is not allowed to cure prior to torquing the fasteners. And perhaps most importantly, the proximity of the drilled holes and mounting hardware to the flange joining hull to deck may induce some form of hull failure if by virtue of the arch installation this critical joint is compromised.

for what it is worth, I am always wary of OEM mounting recommendations made absent knowledge of the particular boat design and manufacture. I am confident the OEM has published all the usual disclaimers of responsibility and waivers of implied warranties such as fitness for purpose in their terms and literature accompanying the goods sold. Given the broad array of hull materials, thickness and construction, not to mention the multiple methods of joining hull to deck, I suggest that the OEM representative mentioned here has reached beyond her knowledge in expressing a preference for hull mounting.

I will leave this dead horse lie now, only wishing that with this post you may not be misled by the unknowledgable as you seek the optimal solution to fitting a replacement arch.

Best,

[TacomaSailor]

sk Barnett said:
"I will leave this dead horse lie now, only wishing that with this post you may not be misled by the unknowledgable as you seek the optimal solution to fitting a replacement arch."

Thank you very much for the very informative and well written explanation of the engineering considerations. I wish you had been able to offer all those details earlier in the discussion so we "wannabe engineers" would not have wasted our time arguing about things we little understood.

Again, Thansk

[montenido]

Well, I'm back once more. In addition to larger fasterners that are not threaded on the bearing portions, the new arch that I ordered is made of schedule 80 pipe, not schedule 40 as the previous arch was made of. As I mentioned earlier, in confused seas there was a re fair amount of side to side flex in the old arch, placing repetitive stress on the mounting bolts. The new arch should be incredibly stiff and much less susceptible to this flexing.

As a side note, my boat construction (CSY) makes the hull to deck joint and stern corners about the strongest areas on the boat for mounting things. Throughout the ordeal of the arch failing and dragging and flogging in the seas, there was no damage to any of the mounting sites, and they were subjected to stress from every direction possible, with loads well beyond those experienced while mounted on deck.

Cheers, Bill

[sk Barnett]

You are very kind to say so, Tacoma. Over time I have had most of my corners knocked off or ground away by things I didn't know or failed to do. the trouble is that the list just keeps growing as time goes on!

best,

[Cadence]

Originally Posted by montenido

Well, I'm back once more. In addition to larger fasterners that are not threaded on the bearing portions, the new arch that I ordered is made of schedule 80 pipe, not schedule 40 as the previous arch was made of. As I mentioned earlier, in confused seas there was a re fair amount of side to side flex in the old arch, placing repetitive stress on the mounting bolts. The new arch should be incredibly stiff and much less susceptible to this flexing.

As a side note, my boat construction (CSY) makes the hull to deck joint and stern corners about the strongest areas on the boat for mounting things. Throughout the ordeal of the arch failing and dragging and flogging in the seas, there was no damage to any of the mounting sites, and they were subjected to stress from every direction possible, with loads well beyond those experienced while mounted on deck.

Cheers, Bill

I don't think you need to worry about your hull. I was expecting to see a chunk pulled out on the side that didn't part company

Best of luck with the replacement.

[montenido]

Hi folks,

This is my final update. I did receive the new arch after quite a long time. It seems that the schedule 80 tubes need special presses to bend them. Anyway, I installed it in the same location with the help of a buddy. I used slightly larger fasteners, 5/16 instead of 1/4, and used bolts that had no threads on the bearing surfaces.

Since I was selling Odyssey anyway, I left off the solar panels, and only installed the radar and davits. It is really solid and looks nice. I was very meticulous in every aspect of the install (as I was on the original) and could do pull ups from this thing.

I have a buyer under contract, and have disclosed the entire event from failure to replacement. Well, that should wrap things up. Thanks for the very intelligent and lively conversations.

Cheers, Bill

[Cadence]

Originally Posted by montenido

Hi folks,

This is my final update. I did receive the new arch after quite a long time. It seems that the schedule 80 tubes need special presses to bend them. Anyway, I installed it in the same location with the help of a buddy. I used slightly larger fasteners, 5/16 instead of 1/4, and used bolts that had no threads on the bearing surfaces.

Since I was selling Odyssey anyway, I left off the solar panels, and only installed the radar and davits. It is really solid and looks nice. I was very meticulous in every aspect of the install (as I was on the original) and could do pull ups from this thing.

I have a buyer under contract, and have disclosed the entire event from failure to replacement. Well, that should wrap things up. Thanks for the very intelligent and lively conversations.

Cheers, Bill

Grats on the contract and for making a disclosure of the ordeal.

[zboss]

You used the same arch manufacturer? Any changes besides the bolts?