replacing U bolt chain plates

[BlueCanoe]

My standing rigging needs to be replaced being between 20 and 36 years old. I don't like the U bolt chain plates. With the west marine bankruptcy I scored a bunch of rigging parts at about 20% of the original price from my local west marine including enough 1/2" toggles and turnbuckles to use at every rigging position. I am considering making a custom chain plate for these in one of the designs shown below. These initial designs are to illustrate the concept and I would do some further refinements to make them easier to produce. Most likely I would use either 316 SS or titanium (I have a big block of titanium that I obtained from a surplus store). I would also replace the inner aluminum chainplate blocks probably with 303 SS.

Before I do any of these I would make sure that the component I would me making is at least as strong as the rigging using finite element analysys in cad.

The reason I am bringing this to the forum is I am wonder if maybe I am re inventing the wheel.

My cost to make them is maybe an hour each, I am still justifying the CNC mill that I bought off craigslist in 2009.

 

[Prairie Schooner]

My standing rigging needs to be replaced being between 20 and 36 years old. I don't like the U bolt chain plates. With the west marine bankruptcy I scored a bunch of rigging parts at about 20% of the original price from my local west marine including enough 1/2" toggles and turnbuckles to use at every rigging position. I am considering making a custom chain plate for these in one of the designs shown below. These initial designs are to illustrate the concept and I would do some further refinements to make them easier to produce. Most likely I would use either 316 SS or titanium (I have a big block of titanium that I obtained from a surplus store). I would also replace the inner aluminum chainplate blocks probably with 303 SS.

Before I do any of these I would make sure that the component I would me making is at least as strong as the rigging using finite element analysys in cad.

The reason I am bringing this to the forum is I am wonder if maybe I am re inventing the wheel.

My cost to make them is maybe an hour each, I am still justifying the CNC mill that I bought off craigslist in 2009.



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Well, yes, you're reinventing the wheel. But it might end up being a better wheel. As I understand the chainplate system, the force is lead to the structure below the deck. These might lock the deck in place more, perhaps adding more stress to the deck than intended. When you do the FEA, will you try to recreate the entire assembly in CAD, including decking?

Is there galvanic compatibility between SS and titanium?

But the CNC mill, heck yeah. That was a good move.

 

[sdwnav]

I’d like to see results of fea, should be an interesting comparison if you model both original and the new design. The backing plates below deck secured by the rod are loaded in a similar way to your new design. I suspect that getting a big enough fillet radius at the intersection between the tang and base plate is key. I would also check out of plane loading; rigging isn’t perfect. Deflections are also probably important. The fiberglass deck is going to be along for the ride and won’t absorb any meaningful load; if deflections are too high you will crack fiberglass or have leaking.

 

[e38 owner]

I am no expert, but having worked on the navtec system here are my thought. The U bolts are fine unless you cannot get the seal between the ubolt and the deck water tight. A wider base may work but I think you give up strength and possibility of corrosion. Your CNC is perfect for the lower aluminum plates which should be stainless. as any water from the top causes problems

 

[Loren Beach]

Before going too far down the TI rabbit hole, check on the various iterations of TI. The wikipedia entry is quite detailed. The loads are high and specific for a shroud base, and choosing the correct alloy of SS is pretty easy, in comparison. For TI information, these folks might be able to advise. https://www.alliedtitanium.com/

As for the OEM weldment where it meets the deck, I would first advise over-drill/epoxy fill/ re-drill procedure that we utilized when we pulled all our deck fittings off during our 2021 refit. Once the deck in that area is solid glass composite and the base fitting is properly bedded, you will not have any moisture getting inside, where the aluminum block is, for another 30 years at least.
After all, the inside aluminum machined block and the Navtec headed rod in it will not suffer any corrosion as long as they are kept dry, as they were designed to be.

 

[BlueCanoe]

Thanks for all the responses.

I agree with Steve's statement above that the "deck is along for the ride". The existing U bolts have a shoulder that puts the deck in compression just like this design does.

For the FEA I will probably stop at the fasteners that are going into the deck. Doing FEA on composite systems is in my experience very complicated, I will make a comparison against the original U bolts/ To Steve's point the fillet radius is key. What's shown in the pictures above is just the starting point to illustrate the design, I might actually narrow the tang some and widen the fillet after FEA.

I do the over drill and epoxy whenever I replace though bolt deck hardware.

I have some experience with TI. I work as an ocean engineer (Mostly electronics). The motivation to use TI is mostly that I have a perfect sized block of grade 5 titanium (TI-6AL-4V) that I hauled through at least 3 moves having bought it as a remnant 20 years ago.

TI plays ok with 316 (very small voltage difference) which is what I would use for fastners including the toggle pin. Galling is always a concern, I would use tefgel in all interfaces. TI does not play well with bronze (learned that the hard way).

 

[Drewm3i]

The biggest concern is what block system underneath the deck you will use to take the rigging load from the deck and pass it via the tie-rod into the TAFG. The "u-bolt" chainplates are not really chainplates, but are more like passthroughs allowing the rig loads to be transferred from the deck (which would be a horrible idea as the deck was never made/designed for those types of tension loads) to the anchors set in the structural grid which is then heavily tabbed to the hull--which is the genius of the Ericson design (that many other manufacturers have since copied).

Garhauer actually makes far beefier and better sealing 316 SS u-bolts that have the oblong washer welded into the bolt instead, preventing the crevice corrosion that eats the OEM Navtec u-bolts over time at the deck junction.

I personally would say either purchase or duplicate the Garhauer design as it is fool-proof and better than OEM. If you could machine the replacement blocks/sockets (for the tie rod) out of 316 SS or Titanium it would be even better!

 

[BlueCanoe]

The biggest concern is what block system underneath the deck you will use to take the rigging load from the deck and pass it via the tie-rod into the TAFG. The "u-bolt" chainplates are not really chainplates, but are more like passthroughs allowing the rig loads to be transferred from the deck (which would be a horrible idea as the deck was never made/designed for those types of tension loads) to the anchors set in the structural grid which is then heavily tabbed to the hull--which is the genius of the Ericson design (that many other manufacturers have since copied).

Garhauer actually makes far beefier and better sealing 316 SS u-bolts that have the oblong washer welded into the bolt instead, preventing the crevice corrosion that eats the OEM Navtec u-bolts over time at the deck junction.

I personally would say either purchase or duplicate the Garhauer design as it is fool-proof and better than OEM. If you could machine the replacement blocks/sockets (for the tie rod) out of 316 SS or Titanium it would be even better!

I will make the blocks out of 303SS, much easier to machine than 316 and if aluminum lasted 36 years 303 will last certainly out last the boat.

I have a perhaps unfair aversion to U bolts. I have seen them fail in professional contexts multiple times.