Radar Arch Fail - Cautionary Tale

[Cadence]

I'm out of here. I thought I might put an end to a pissing contest. All of you "structural engineers" have at a pissing contest.

[UNCIVILIZED]

it's not about bearing area stress, it's about the weakest way/option in which the bolt can fail. The math of which has been shown in several different ways already.
Take the bolt's cross sectional area & multipliy that times it's tensile strength.

As to differing accelerations, when you hit the ground, holding the 2x4 does it seem heavier than when you're just standing holding it? Nod yes. Well then, that's the same thing which happens to a dinghy hanging from an arch.

[Jesse]

In photo #2 there seems to be holes on the outside of the support legs next to the mounting brackets. If so what is their function? Do the mounting brackets rotate as well as swivel? What is the size of the single bolt attaching the leg to the bracket? In photo #3 are broken bolts visible in three holes? I"m trying to visualize the force and stress that different loads would apply to the mounts. Wouldn't the pivoting attachment to the hull eliminate a considerable amount of shear force to the mount?

[boogalloo]

Oh Montenido
I feel your pain
All you wanted was a quick bit of advice - now I see 93 posts
I haven't seen so much ego driven ill-informed opinion for a long time (White House excepted). Only couple got it right
Best of Luck!

[Cadence]

Originally Posted by boogalloo

Oh Montenido
I feel your pain
All you wanted was a quick bit of advice - now I see 93 posts
I haven't seen so much ego driven ill-informed opinion for a long time (White House excepted). Only couple got it right
Best of Luck!

Drift! I love the White House exception.

[1affiah]

Does it matter, as a monohull with an arch, he motors more or less than a multihull with an arch?

[montenido]

I'm back again. Kenomac, yes, I am going to do it again with improvements to the fasteners and to the mounting orientation of the pads. Does that make me stupid? Maybe, but this arch traveled over 3000 miles carrying the panels, radar, and dinghy most of the time. It survived Hurricane Odile (local winds of 125kts+)a year ago with the panels attached, and has been reliable up until the day it failed. I can do many little improvements that will lighten the load and stress from the set up.

To answer a question about the bolt on the leg that is above the foot, it is rigid, and pins the leg and foot together. They are also epoxied together creating a pretty strong assembly.

I get that arches are a bit ugly and that some people seem to abhor them, but they are pretty handy, supplying a place for panels, radar, davits, etc.. That is why I will do it again.

BTW, I have included a picture of an arch installation from the company's website very similar to my installation. All 4 pads are mounted vertically.

Cheers, Bill

[1affiah]

Good on you, Bill. I'm going on 5 years with my AT arch I had modified from the original "Tower in a box". It was one of the best upgrades for my boat, I have recommended it to many, and will continue to do so.

[Kenomac]

Originally Posted by montenido

I'm back again. Kenomac, yes, I am going to do it again with improvements to the fasteners and to the mounting orientation of the pads. Does that make me stupid? Maybe, but this arch traveled over 3000 miles carrying the panels, radar, and dinghy most of the time. It survived Hurricane Odile (local winds of 125kts+)a year ago with the panels attached, and has been reliable up until the day it failed. I can do many little improvements that will lighten the load and stress from the set up.

To answer a question about the bolt on the leg that is above the foot, it is rigid, and pins the leg and foot together. They are also epoxied together creating a pretty strong assembly.

I get that arches are a bit ugly and that some people seem to abhor them, but they are pretty handy, supplying a place for panels, radar, davits, etc.. That is why I will do it again.

BTW, I have included a picture of an arch installation from the company's website very similar to my installation. All 4 pads are mounted vertically.

Cheers, Bill

Just curious... Why would you expect a different outcome the second time around? Same arch, same hardware, same vertical mounts?

When equipment lets me down... I get rid of the junk and upgrade to better, more reliable stuff. Seems odd when others keep doing the same thing over and over again... expecting a different result.

Me thinks you need to go back and re-read the good advice offered up by engineers on your thread regarding accelerated loads... Has nothing to do with how your arch fared in high winds.

[hoppy]

Originally Posted by Kenomac

Just curious... Why would you expect a different outcome the second time around? Same arch, same hardware, same vertical mounts?

Originally Posted by montenido

Hi folks,
Today, after more than 3000 miles in all kinds of conditions, it failed and fell off the back of my boat. I was moving my boat to a broker's dock in order to sell it.

Originally Posted by Kenomac

When equipment lets me down... I get rid of the junk and upgrade to better, more reliable stuff. Seems odd when others keep doing the same thing over and over again... expecting a different result.

[DumnMad]

Originally Posted by montenido

I'm back again. Kenomac, yes, I am going to do it again with improvements to the fasteners and to the mounting orientation of the pads. Does that make me stupid? Maybe, but this arch traveled over 3000 miles carrying the panels, radar, and dinghy most of the time. It survived Hurricane Odile (local winds of 125kts+)a year ago with the panels attached, and has been reliable up until the day it failed. I can do many little improvements that will lighten the load and stress from the set up.
Cheers, Bill

Bigger bolts is the answer. No wear in holes so its likely the shear load was transferred by shear friction with the bolts in tension. The tubes are loading the pad eccentically so bolts are in shear plus tension. Loading is cyclic which points to fatigue.
1/4" bolts can easily be overstressed when tightened up and my guess is that the bolts were fairly highly stressed to start with leading to a shorter fatigue life.
Bigger bolts means you will put less stress into the bolts for the same tightening torque. Either bigger bolts or get your wife to tighten them up this time.

[poiu]

Originally Posted by montenido

I'm back again. Kenomac, yes, I am going to do it again with improvements to the fasteners and to the mounting orientation of the pads. Does that make me stupid? Maybe, but this arch traveled over 3000 miles carrying the panels, radar, and dinghy most of the time. It survived Hurricane Odile (local winds of 125kts+)a year ago with the panels attached, and has been reliable up until the day it failed. I can do many little improvements that will lighten the load and stress from the set up.

To answer a question about the bolt on the leg that is above the foot, it is rigid, and pins the leg and foot together. They are also epoxied together creating a pretty strong assembly.

I get that arches are a bit ugly and that some people seem to abhor them, but they are pretty handy, supplying a place for panels, radar, davits, etc.. That is why I will do it again.

BTW, I have included a picture of an arch installation from the company's website very similar to my installation. All 4 pads are mounted vertically.

Cheers, Bill

You need to rule out stress corrosion cracking, crevice corrosion and fatigue. Get the screws out and look at them carefully. The signs will hopefully be clear. Crevice corrosion will show loss of material around the outside and indicates sea water got in. SCC will show micro cracks everywhere and a very rough surface and may indicate the presence of sea water also. Fatigue will show maybe beach marks. If you don't rule this out and deal with it if it is a problem then it will probably happen again. Maybe next time, much more quickly and before you get the chance to sell it .

I'd re-do it with a stiffer structure and use shoulder screws rather than plain screws. The 1/4" screws will be strong enough, but are much more prone to fatigue than thicker screws at say 3/8" or 1/2" Make them a bit bigger and ideally not of 304 or 316 stainless, though in 1/2" with lots of tight supporting material they should be bullet proof. It's my guess at this stage that 316 material failure is the root of it. Water cannot be allowed in.

[hamburking]

The OP mentions the arch survived hurricane ODILE. That one instance might have overstressed the bolts, or even broken one or two...and then it was just a matter of time before the catastrophic failure which followed.

Imagine a bolt as an elastic band, which can be stretched many times without damage...until it is overstretched and SNAP, its broken. Without that one overstretch, it might have outlasted the boat.

If thats the case, then the entire installation, including bolt sizes, is likely quite adequate for normal use. And as the OP said, it survived the hurricane...but perhaps not without unseen damage. In this case, I applaud the OP for the thread, as his disaster, and others, could be avoided simply by checking for broken bolts as part of the regular maintenance routine.

Should everything on a boat be built to withstand a hurricane? What catagory? If the bolts were stronger, perhaps the mounting plates would have been ripped from the hull.

[Kenomac]

Originally Posted by hamburking

The OP mentions the arch survived hurricane ODILE. That one instance might have overstressed the bolts, or even broken one or two...and then it was just a matter of time before the catastrophic failure which followed..........

Should everything on a boat be built to withstand a hurricane? What catagory? If the bolts were stronger, perhaps the mounting plates would have been ripped from the hull.

You and the OP are not understanding the recommendations make by the engineers in prior posts. Read them again. Most likely the cause of the sheared bolts was due to accelerated loads bearing down and essentially cutting through the undersized bolts. It's unlikely the wind was the cause of the failure.

[sailorboy1]

Originally Posted by montenido

Thanks for the replies. I will try to add more information. The arch came with its own mounting hardware, hence the 1/4" bolts. It does have backing plates, and having to do it again I would use more robust fasteners. I think the arch failed due to the fasteners failing (shearing), not due to structural weakness.

Yes they sheared. But that means the system got overloaded either by the total weight or from cyclic stress from the dinghy bouncing. I wouldn't just accept that replacing the bolts with something bigger as the answer as I bet that arc was designed for those bolts and larger bolts may just result in the arch itself breaking.