Radar Arch Fail - Cautionary Tale

[Cadence]

Originally Posted by DeepFrz

It looks like a lot of weight way up high. Long lever arm.

And the footprint looked small.

[Boatguy30]

I'm certain she will send you a free replacement so she can continue to claim her arches are the best ever.

Metal Creations came thru for me, but the shipping to CA would be a problem. Probably best just go remove it entirely if selling the boat.

[Sailor647]

Originally Posted by Sandero

If the bolts sheared then the forces on them were too much. This means the engineering was wrong. In addition to the static load (dead weight) you have dynamic loads created from the boat's movement and these sorts of loads are much greater than static loads. That is to say, this rig sitting at the dock would likely not fail. But take it out to sea in all manner of waves and you introduce forces which exceed capacity of the bolts. There are also different grades of bolts and you might have had the lowest grade which didn't help.

The design looks poor as well. The rear post is in compression and the forward one is in tension. Once the starboard side failed and the structure detached and fell... the remaining apparently pinned port connection was no longer in compression and the reduced weight was carried by the floating dinghy. So it seems.

Excellent explanation!

[hamburking]

Originally Posted by Sandero

If the bolts sheared then the forces on them were too much.

Disagree. If the forces were too great, the arch would have failed soon after installation, as soon as it was loaded and taken out into the waves (for the added "dynamic" load you discuss).

The problem is fatigue. Over the course of time, and constant movement, the bolts fatigued...like bending a paperclip back and forth until it breaks.

The solution is the same though...bigger bolts, stronger bolts, or reduced load.

[tdoster]

Originally Posted by funjohnson

Is there a way to make davits/arch cantilevered over the stern without the rear post in compression and front in tension? We're thinking of having an arch made and cant I see how you could do this.

Matt

The arch is not on the boat right now, but here are the mounts to handle both loads on the old boat. You can't see the stern too well, but it is goes down about twelve inches and attached with 4 bolts and both are backed with stainless plates. The fiberglass will let go before they do.

[Kenomac]

Just say no to monkey bars.

I've never understood why some people add monkey bars to a beautiful yacht. I'm sorry this unfortunate incident happened, maybe the bars belong elsewhere and not on a boat. IMHO

There are other more practical ways to haul a dinghy around, mount solar panels and mount radar.

[hamburking]

Originally Posted by funjohnson

Is there a way to make davits/arch cantilevered over the stern without the rear post in compression and front in tension? We're thinking of having an arch made and cant I see how you could do this.

Matt

You can reduce the forces by increasing the distance between the fore and aft posts.

The aft posts should sit flat on the deck...so the deck takes the force and the bolts only hold the mount in place. The foreward posts should also sit flat on the deck...this puts the whole of the bolts in tension, not shear.

Bolts work like a spring...a nice long spring (long bolt) distributes the load and smooths the forces. Bolts in shear (mounted on the side of the boat) have a stress point where the mount contacts the deck. This can be resolved somewhat by having the bolts really tight...the friction between the mount and the deck takes the shear, instead of the bolts themselves. However, this preload on the bolts will also increase the total load on the bolts...so big bolts are still a good idea.

[witzgall]

I cannot imagine attaching something like this with 1/4 inch bolts, way too small. They should have been 3/8 or maybe 5/16? Also, using something like 4200 or 5200 on both the backing plate and the foot can have a positive effect.


Sent from my iPhone using Cruisers Sailing Forum

[gbgreen59]

Originally Posted by Cheechako

Backing plates were not the issue unless you tore a chunk of deck out!

It seems like the bolts being tightened when backing plate or washers are on an uneven surface can cause the bolts to bend in an effort to seek a flat surface (this is what I'm seeing on my installation). Sure seems like that could contribute to a shear failure.


I have also thought about making new backing plates. The ones supplied with the arch are pretty thin...maybe 1/16" to 1/8". They bend pretty easily. I may replace with G-10

[wrwakefield]

Hi Bill,

I'm sorry to hear of your misfortune and glad there were no injuries and that it wasn't catastrophic.

If the fasteners were the only failure point then, as you have already mentioned, upsizing next time around makes sense.

Additional support for the davits (lever-arms) would be helpful as well. Are you able to clear the main sail and other rigging if you ran two lines [e.g., Dynema] from the masthead to the outer tops of the two davits? [like a split backstay only supporting the davits...] Or if that won't work, can you attach part way up the existing back stay to install "Davit topping lifts"?

My current boat is a ketch and the davit topping lifts originate from the mizen masthead. [Looks like a split backstay...] On two prior boats (sloops) I attached the davit topping lifts part way up the existing backstay to accomplish this.

On my current [last?] boat I regularly haul the dinghy with 15hp outboard on the davits- in protected waters and at anchor for security.... Offshore I will still remove the outboard and stow the dinghy on deck. (Caribe RIB)

The minor downside to this approach is when hauling the boat in a smaller boat lift, these "davit topping lifts" sometimes need to be slacked/removed to clear the frame on the boat lift...

In case this is helpful.

Cheers!

Bill

[zboss]

I saw an Island Packet that had wires running from the top outside edges of their arch to the top of their mast. Obviously a great way to support the arch but I wonder what happens when the mast pumps.

I had considered this arrangement as well.

[Cadence]

Supporting arch, davits, solar panels, radar, and dinghy from the mast head sounds like a accident looking for a place to happen. JMHO

[Bill Seal]

Originally Posted by funjohnson

Is there a way to make davits/arch cantilevered over the stern without the rear post in compression and front in tension? We're thinking of having an arch made and cant I see how you could do this.

Matt

Not without rewriting the laws of physics. Ask Archimedes.

1/4" screws are too wimpy for anything requiring strength, the minor diameter of a 1/4-20 thread is only 0.188", 5/16 -18 is 0.240". I'd go with 3/8" hardware and heavy back-up plates.

[boogalloo]

Sorry to hear of your misfortune

You have a big overhung load - rough weather plus sprung load and lateral buffeting shocks will easily add loads 3 or more times the static ones which will be high anyway because of the lever effect. The aft most bolted connection will be under the greatest load. Looking at your pic, this will be 2 to 3 times the total weight of dinghy and equipment

1/4" is way too small especially if 100% threaded- you should use 3/8 bolts at a guess with an unthreaded portion under shear.

Although they are classed as stainless, there are some real rubbish bolts around in some of the marine stores. Go to one of the engineering fastener suppliers on line and get A286 high tensile corrosion resistance bolts

Over-tightening of bolts is also a problem - download a torque table and use a torque wrench when fitting.

There is a little problem that poor quality stainless is especially subject to called stress corrosion cracking. Exactly what it says - where the bolt is under stress concentration such as the thread root or under the head, corrosion rates can be enhanced and tiny sites act as crack generators.
When you assemble make sure you get sealant under the head as well as everywhere else down the hole.

Lastly fatigue was mentioned - this and/or corrosion was certainly involved. The original bolts may have inadvertently been over tightened, greatly reducing fatigue life.

Load the bolt at only 50% of its maximum rated load - get this right with the torque wrench. Very very roughly the shear load you put on the bolt should only be half of the torqued load

When you look at another bridge examine the quality of the welds onto the mounting pads - if badly done this is another place where cracks will start.

Have great sailing!

[Cadence]

Originally Posted by Bill Seal

Not without rewriting the laws of physics. Ask Archimedes.

1/4" screws are too wimpy for anything requiring strength, the minor diameter of a 1/4-20 thread is only 0.188", 5/16 -18 is 0.240". I'd go with 3/8" hardware and heavy back-up plates.

And a good angle on the braces between anchor plates. If that makes sense? It looks like some are only maybe 15 degrees. I feel sure some one can do a vector analysis of that and the % of additional weight it would hold per degree?