Author Topic: Radar Arch Fail - Cautionary Tale  (Read 3878 times)

RHoodJr

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Radar Arch Fail - Cautionary Tale
« on: August 12, 2015, 05:26:17 am »
To those of us that have installed one of these. Saw the above post on the Cruising and sailing forums posted by Bill of S/V Odyssey a CSY 44.

We all may learn from this.

Sorry I couldn't figure out how to provide a direct link to the thread but you will find it if you search for the subject above.


Soggy Geek

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Re: Radar Arch Fail - Cautionary Tale
« Reply #1 on: August 12, 2015, 06:28:53 pm »
Thanks, Richard.  Here's a clickable link:

Cruiser's Forum, Radar Arch Fail

We mounted our arch with the front supports on deck and only the back supports on the side.  And we almost never carry the dinghy on the davits underway.  But this is a good warning to go carefully check on things.  And if you install one, think through the engineering a bit.

Sherry
Soggy Paws the CSY has been sold but not forgotten!
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Matthew Balch

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Re: Radar Arch Fail - Cautionary Tale
« Reply #2 on: August 12, 2015, 07:01:49 pm »
I appreciated that information also.  It would seem that 1/4" bolts mounted in shear are not sufficient to handle the repetitive loads.  Although I don't have the "arch in a box", I do have the aft supports on my arch loaded in shear with larger bolts, and this cautionary tale will have me checking out those bolts very soon!  Thanks again Richard and Bill.

Bill Bischoff

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Re: Radar Arch Fail - Cautionary Tale
« Reply #3 on: August 13, 2015, 09:15:23 pm »
Hi Folks,

As you might have guessed, it was my post on Cruiser's Forum.  I'm still at a bit of a loss to explain why the bolts sheared, though the arch and solar panels did ride out Hurricane Odile in La Paz (125+ winds locally).  However, I agree that the 1/4-20 bolts are under-sized for the vertical mounting.  Shelley at Atlantic Towers told me that they think the vertical mount is stronger.  I ordered another arch, but this time in schedule 80 tubing, which is much stiffer than the schedule 40.  I will up-size the bolts for the pads , as well as the pins that attached the arch to the pads.  I will only carry the dinghy on the davits on short trips, and I am going to lower my solar panels by about 3-4".  Hopefully, all of these big and little changes will keep things above the water where they belong.  The hull of my boat remained intact during the entire event, which just reinforces my confidence in the CSY build quality.

Cheers, Bill
Odyssey, 1979 44 WO
Lying La Paz, BCS

Soggy Paws

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Re: Radar Arch Fail - Cautionary Tale
« Reply #4 on: August 14, 2015, 05:22:09 am »
Hi Guys,

As Sherry mentioned our arch fwd bow legs are on deck and the bolts are therefore in tension (somewhat greater failure strength) not shear.  Even so, eight 1/4" bolts in shear have a lot of strength.  If there was say 500 lbs on the arch I think that even shock loading that would not be enough to have them fail.  I would think the aluminum pad welds would have failed first.

Have a look at this link for some useful info on bolt terms and strengths:

http://www.rockcrawler.com/2003/04/fasteners-making-the-grade-a-technical-discussion/

Although SS bolts aren't specifically mentioned I know from having compared 316 SS and mild steel (grade 3) shackles that the strengths are roughly equal.  The above link says that grade 5 bolts have an ultimate shear strength of 3700 lbs and tension strength of 4000 lbs.  That means that those 8 bolts in shear might have a shear strength of almost 30,000 lbs.   I say 'might' because the comparison between SS and grade 5 bolts might be a bit off and because threads and loose torque affect the shear strength negatively.  But they did fail, the question is why.

Bill, any opinion as to why the failure?  Two thoughts that come to my mind is crevice corrosion, in that the SS bolts are in an anoxic environment as they pass through the hull and weak bolt head to shaft joint.  Did they crack at the head joint or further down and is there any indication of crevice corrosion on the bolts?  I would not be surprised if weak/cheap bolts were the culprit.

Dave
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Bill Bischoff

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Re: Radar Arch Fail - Cautionary Tale
« Reply #5 on: August 19, 2015, 08:45:18 pm »
Hi Dave,

I did not observe any corrosion issues looking under a magnifying glass with a flashlight.  The bolts appeared to shear at the hull/pad joint.  The outboard heads were lost on the one pad, but the inboard bolt ends with washers and nuts were still there.  Only one pad out of the four sheared, but apparently that was enough to get things out of balance and to upset the integrity of the setup.  I'm honestly not clear on why this happened.  I think that riding through Hurricane Odile might have been a large contributing factor.

The replacement arch that I ordered is much stiffer, being schedule 80, not schedule 40.  I have also increased the pad mounting bolts to 5/16, which are surprisingly larger than the 1/4" bolts, and hence, stronger.  If I end up keeping Odyssey I might put in titanium fasteners on the pads. 

Cheers, Bill
Odyssey, 1979 44 WO
Lying La Paz, BCS

Chip

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Re: Radar Arch Fail - Cautionary Tale
« Reply #6 on: August 23, 2015, 07:58:02 am »
Having the bolts shear under relatively low load is probably the result of fatigue. Several cycles at an extreme but not catastrophic load will greatly reduce the number of cycles to failure. Only a handful of materials like carbon steel have an infinite fatigue life and even these loose that ability after an extreme load cycle. B8M bolts may be a better option for heavily loaded applications. This is a high grade 316 stainless which is used on pressurized tanks and piping. Industrial suppliers like McMaster-Carr stock them.