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-   -   Cast Thimble Modification for Dyneema (https://www.cruisersforum.com/forums/f116/cast-thimble-modification-for-dyneema-264850.html)

Manateeman 25-05-2022 12:51

Cast Thimble Modification for Dyneema
 
Weíve thought about fabricating a lashing ďdead eyeĒ using a cast thimble as a base. We would weld a bar across the wide part of the thimble so the lines would not climb over each other. This bar could also support a short section of pipe welded at 90 degrees to it for a pin.
Just an alternative to the aluminum terminal fittings.
We also thought about how to use plastic ...which was suggested before on the forum ...but we lost the post and are not sure which plastic would be ideal.
Looking forward to new ideas / suggestions.
Thank you.
Mark and Crew

Spot 26-05-2022 16:15

Re: Cast Thimble Modification for Dyneema
 
Hi Mark,
I am having a time trying to visualize what you are wanting to make.
Any chance the manatee crew could draw a sketch or make a reasonable facsimile out of discarded pizza crust?

Manateeman 27-05-2022 10:12

Re: Cast Thimble Modification for Dyneema
 
OK. We all know what the usual and rather expensive dyneema terminal fittings look like...sort of old wooden dead eyes but flat and made in aluminum. Nothing wrong with them, just the cost for some people.
Plain cast thimbles in stainless have some advantages but the wide curve tends to squeeze the lashings together and on top of each other.
If you cross the wider end of the thimble with a piece of heavy stainless bar stock at 90 degrees to the shroud or stay, the lashings would not cross on top of each other. With two thimbles, the lashings would be parallel. The line around the thimble is squeezing the thimble in compression and the crossbar... to which the lashing is attached ...is trying to push the thimble apart.
I think the crossbar should be about the diameter of the outside of the thimble but it could be larger or it could be fitted with a roller. The diameter is not limited by the thickness of the plate used in the existing dead eyes. The advantage is much less friction and a larger radius turn for the lashings.
As to cost. A cast thimble. A crossbar with or without a roller. Two simple TIG welds. Cast thimble to mast thru bolts would just be a simpler design where a plate fills the interior space of the thimble. It could be drilled and a short section of pipe could be welded to the plate at any angle desired.
We began the thread as an appeal to the forum members for new designs.
We will post our drawing as soon as we can but we would love to see what others can design. To us, cast stainless thimbles are a reasonably priced starting point to which a lot of interesting components can be easily welded.
Or maybe plastics are the way to go?
Mark and the manatee crew.

Manateeman 27-05-2022 11:19

Re: Cast Thimble Modification for Dyneema
 
1 Attachment(s)
Drawings.Attachment 258484

dfelsent 27-05-2022 11:59

Cast Thimble Modification for Dyneema
 
I wonder if the thimble would rotate slightly in the eye of the shroud. This would cause the lashings to slide to one side. This would cause more rotation and so on.
In other words this setup might not be stable.
You can test that.

Manateeman 27-05-2022 14:25

Re: Cast Thimble Modification for Dyneema
 
Rotation. Just donít see this as a problem. If you use a cow hitch for the lashings in the middle of the cross bar and work your way out toward the thimble, you can get a nice equal tension because you have far less friction vs a dead eye with smaller holes. Why would the thimble shift in the dyneema eye to begin with...just donít see why. Well you could wrap the dyneema in hockey tape.
The roller need not be straight across. You could cover the center with a series of little rollers...think a line of low friction rings. If the roller is plastic, itís easy on a lathe. Maybe no roller, just scallop out the crossbar. It can be larger in diameter than the thimble is wide.
The current aluminum dead eyes donít impress us. If you just use plain cast stainless thimbles, the lashings touch each other on the sides and are squeezed together by the curve of the thimble. Lot of friction. With a straight bar across the thimble you reduce this when you tighten things up. Iím more concerned with the best way to attach the dyneema lashing lines.
Looking for new thinking...new designs.
Testing will reveal the problems and the improvements.
The manatee pizza is done so weíll sigh off.
Mark

Muaddib1116 27-05-2022 14:50

Re: Cast Thimble Modification for Dyneema
 
If you can do the welding yourself, then this seems pretty reasonable. I think the crossbar should be as "low" on the thimble as you can while still having room for all your lashings side by side, this way it would inhibit rotational forces, if any. I don't know enough about synthetic lashings to know if a roller is necessary. The downsides of a plastic roller are that it's either a full cylinder and you'd have to cut and reweld to replace (assuming you could weld it in the first place without melting the plastic). If it's a split cylinder that you stretch around the crossbar, I'd be worried about the forces of the lashings shifting it out of place, or over time deforming it and loosening the lashings. For your mast fitting, again, if you have the welding equipment and the ability, that seems like a reasonable solution.

Manateeman 28-05-2022 07:59

Re: Cast Thimble Modification for Dyneema
 
For now weíll take plastics off the table and return to a stainless crossbar. Welding it to the cast thimble is pretty straightforward and even if you bring the thimble to the shop, itís not a lot of bench time.
We think the cast thimble with a single hole for a pin or a toggle would take much more labor but the cost might be worth the advantages of stainless over aluminum. It would be interesting to see prices for solid center cast stainless thimbles. You could drill out the pin size you need and weld in a short reinforcing tube or washer at any angle required. By solid center, we donít see the center plate thickness equal to the thimble width...just a plate. Not sure there is a good rule of thumb but some added strength would improve the current cast thimbles which are point loaded if you need to connect them to a pin in a toggle.
For example.
We have nice thick aluminum chainplates welded in the hull. From the mast, we now have a nice dyneema line hanging down with a manatee designs cast stainless thimble with welded in crossbar.
We would like to attach lashings between this thimble and the chainplates. So now we need one more cast thimble with crossbar and the smaller end now points down to the chainplate. We could add a double jaw toggle IF there were a reinforced hole in the thimble...or maybe a second crossbar.
If you had a second crossbar, you have a lot more options .
Thatís why we posted the thread. What are the options besides flat black aluminum terminal fittings.
Hand tossed manatee pizza recipe for the best NEW suggestions or drawings.
Happy trails.
Mark and the crew.

Manateeman 28-05-2022 08:28

Re: Cast Thimble Modification for Dyneema
 
1 Attachment(s)
Drawings of alternative modifications to cast stainless steel thimbles.
Attachment 258508

Muaddib1116 28-05-2022 08:42

Re: Cast Thimble Modification for Dyneema
 
What are the two middle right sketches of? What is the purpose of the add-ons at the narrow part of the thimble?

Manateeman 28-05-2022 09:07

Re: Cast Thimble Modification for Dyneema
 
The question marks indicate the two areas where we are not sure what to do.
Question mark 1 is the area inside the thimble. The two thimbles on the right are just suggested solutions. The middle one is a short second crossbar showing where a pin would be located on top of and at 90 degrees to the second lower crossbar. I just drew it as a cut piece, end view, just to show the position of the pin sitting on top of the second crossbar. The furthest right thimble has a plate with a hole for a pin which would be at 90 degrees to the top crossbar. One crossbar, one plate filling up the small end of the thimble.
Not shown would be the toggle or turnbuckle or plates connecting the chainplate in the deck to the lower thimble.
Question #1 is...what goes in this space.
Question #2 is...what connects the lower thimble to the deck chainplate.
Iíll try to make and photograph some models as soon as I can.
Thank you!
Mark and crew.

Muaddib1116 28-05-2022 09:52

Re: Cast Thimble Modification for Dyneema
 
Oh, I see, you are trying to figure out the bottom fitting to the chainplate. I'm don't think the thimbles will be strong enough, without the dyneema spliced around it, even with some welded bars. I think maybe the best solution would be to use the lower thimbles as thimbles with a dyneema strop around the thimble, lashings around the two lengths to keep it tight to the thimble, and then two spliced eyes over a pin through the chainplate. Or a lower leg of dyneema with a thimble on each end, and the lower thimble attached to the chainplate with a shackle or something.

smj 28-05-2022 10:19

Re: Cast Thimble Modification for Dyneema
 
1 Attachment(s)
A simple way to go from turnbuckle to lashing?
Attachment 258519

Manateeman 28-05-2022 15:32

Re: Cast Thimble Modification for Dyneema
 
SMJ ...Looking at the very bottom thimble. Here you have a flat toggle against a curved thimble. The flat surface of the toggle rides on the curved inner surface of the thimble. Itís a very small contact area. Itís a sharp 90 degree flat bar against the round inner surface of the thimble.
This is exactly the kind of point loading Iím trying to avoid.
It would be ok if it were welded, but it flexes.
Iíve got to draw out the above post before I comment but I do very much appreciate the response.
Lastly, Wichard has a photo of a rather beefy thimble but no details. Itís the thimble above their line clamps . The thimble has crossbars cast into it and a hole for a pin. Anyone recognize this thimble?
Operator manatee signing off for dinner.

Muaddib1116 28-05-2022 15:43

Re: Cast Thimble Modification for Dyneema
 
I'm not sure that's their product. The photo I found was for the clamp they sell. That beings said, I still think you need to have the load be born by the dyneema line that goes around the thimble, the thimble itself is not designed to be structural. So for the lower part of the lashings, I think you'll still need the thimble to be wrapped in dyneema and then attached to your chainplates somehow. In the photo that SMJ posted, if you remove the bottom thimble, and have two eye splices is the dyneema, each one on one side of a pin going through the chainplate, then you won't have a point load like with that toggle. Alternatively, you could maybe use a D shackle with the pin through the chainplate and the D through the thimble. You'll still get a point load, but on a rounded contact surface, which is probably better. That being said, I'm not sure that the point loading you see with that flat section toggle is a real problem. The thimble will deform a bit at the corner points, but the thimble is not the true load bearing structure, it is transferring the load to the dyneema line. Also, as you see in that photo, the lashings are all next to eachother in the curve of the thimble, and it doesn't look too crazy with overlapping and such. Are you sure you aren't going down an unnecessary rabbit hole because of theoretically un-ideal minor things.


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