Tow rope thimbles

[Bob Hills]

What is the consensus of opinion of using steel thimbles on aero tow ropes?

Currently we use some clear tubing in the loop to protect the rope from wear of the Tost tow ring. Not only does this allow the rope to bend sharply causing a weak point but it also seems to fill with dirt. This then manages to find its way to the sharp bend in the rope where it grinds away at the rope (poly) and precipitates a breaking point.
A steel (or other hard material) thimble in the loop would seem to eliminate this problem and look a lot more professional too.

Anyone out there do anything different or profess not use thimbles?

Bob 7U

[AS]

On Wednesday, December 16, 2020 at 9:31:34 AM UTC-8, Bob Hills wrote:
> What is the consensus of opinion of using steel thimbles on aero tow ropes?
>
> Currently we use some clear tubing in the loop to protect the rope from wear of the Tost tow ring. Not only does this allow the rope to bend sharply causing a weak point but it also seems to fill with dirt. This then manages to find its way to the sharp bend in the rope where it grinds away at the rope (poly) and precipitates a breaking point.
> A steel (or other hard material) thimble in the loop would seem to eliminate this problem and look a lot more professional too.
>
> Anyone out there do anything different or profess not use thimbles?
>
> Bob 7U

I assume you are using a hollow-braided plastic line? In this case, a metal thimble should not be used. There are several suppliers of Nylon thimbles out there, like this one:
https://www.westmarine.com/buy/whitecap--1-2-rope-thimble--17290263?cm_mmc=PS-_-Google-_-GSC%3ENonB%3EProduct%2520Type-_-17290263&product_id=17290263&creative=108171392404&device=c&matchtype=&network=g&gclid=CjwKCAiA_eb-BRB2EiwAGBnXXq6reEGASMsWmIv3aJVZcX99JxzLvq8wW-NJUaHZS6Y2eyNtdtF8txoClrQQAvD_BwE
I have used plastic thimbles on much larger Spectra ropes (7/8" or 1") with good success.
To put a thimble into the end of a plastic line requires the proper splicing technique to make sure the loop does not slowly stretch out.
Hope that helps,

Uli
'AS'

[Mark Mocho]

> To put a thimble into the end of a plastic line requires the proper splicing technique to make sure the loop does not slowly stretch out.

I have found that a Brummel Splice is the best non-slip. locking loop for a hollow braid rope. The following video demonstrates it on Dyneema, but it works equally well on hollow braid polypropylene. You do need a decent set of splicing fids.

https://www.youtube.com/watch?v=4WW7Qvg3VjI

https://www.coas****ersports.com/selma-fid-ultimate-rope-splicing-tool-p-2544.html?currency=USD

[Kenn Sebesta]

On Wednesday, December 16, 2020 at 12:31:34 PM UTC-5, Bob Hills wrote:
> What is the consensus of opinion of using steel thimbles on aero tow ropes?
>
> Currently we use some clear tubing in the loop to protect the rope from wear of the Tost tow ring. Not only does this allow the rope to bend sharply causing a weak point but it also seems to fill with dirt. This then manages to find its way to the sharp bend in the rope where it grinds away at the rope (poly) and precipitates a breaking point.
> A steel (or other hard material) thimble in the loop would seem to eliminate this problem and look a lot more professional too.
>
> Anyone out there do anything different or profess not use thimbles?
>
> Bob 7U

If you wish to keep the rope at full strength, then appropriate thimble use is required (unless the object spliced to itself is suitable larg in radius.

Sailors care lots about long-term rope survival, so this is a subject of keen interest and experience. https://www.practical-sailor.com/sails-rigging-deckgear/re-evaluating-eye-splice-thimbles goes into great detail on the various thimbles and ropes. Drew Frye, the author, is one of the most rigorous testers out there, writing for Practical Sailor, one of the most rigorous general sailing publications.

Note that the lowest D:d ratio is 1:1, and that's only for certain types of rope. However, I know of no rope used which is as small as the Tost tow ring so a thimble is almost certainly a very good idea if you want the rope to perform as rated. Please get an appropriately rated thimble so that it doesn't flatten under the load, depriving the rope of the needed bend radius.

Also, when selecting thimbles note that there are sailmaker thimbles and open-ended thimbles. The sailmakers resist the crushing load far better, but they are closed and so can only to on something which is open-ended. So you'd need a shackle-- I'd recommend a soft shackle-- to go from a sailmakers thimble to a Tost ring.

[Stuart Venters]

On 12/16/20 8:11 PM, Kenn Sebesta wrote:
> On Wednesday, December 16, 2020 at 12:31:34 PM UTC-5, Bob Hills wrote:
>> What is the consensus of opinion of using steel thimbles on aero tow ropes?
> Sailors care lots about long-term rope survival, so this is a subject of keen interest and experience. https://www.practical-sailor.com/sails-rigging-deckgear/re-evaluating-eye-splice-thimbles goes into great detail on the various thimbles and ropes. Drew Frye, the author, is one of the most rigorous testers out there, writing for Practical Sailor, one of the most rigorous general sailing publications.
>
> Note that the lowest D:d ratio is 1:1, and that's only for certain types of rope. However, I know of no rope used which is as small as the Tost tow ring so a thimble is almost certainly a very good idea if you want the rope to perform as rated.


The back end of a tow rope is a strange use case. There is a lot of
dynamic loading, a maximum acceptable rope strength, and flailing seems
more of a wear contributer than loading. Also a flailing rope end can
hurt things. I only have experience on grass, but a hard runway should
bring another set of issues?


The thing that looks safest to my eye is what is usually done. Hollow
braid Polyprop chinese finger grip eye splice, chafe protection with a
piece of clear tubing, and a bit of electrical tape to keep it in place.
A possible improvement is to taper the clear plastic tubing in the
inside of the Vee where it usually wears first, but checking the splice
and freshening the nip when it gets frizzy seems more important.

The practical sailor says hollow braid lays flat in the load bearing
point and that would make the diameter of the big ring look really big
compared to the rope already. A metal thimble has a lot of relatively
sharp edges and these can cut the rope unless you can keep the splice
tight enough to keep the thimble in just the right spot. The article
suggests this is difficult and using a webbing chafe guard which might
be better in a situation where there is no inspection?

With occasional inspection has anybody seen a splice loop fail? It
doesn't happen often, but I've seen the rope fail just ahead of the
splice where the single rope goes into the finger grip. Things usually
look fresh at the tow plane end which leads me to think that flailing is
the major issue.

[Kenn Sebesta]

On Friday, December 18, 2020 at 9:45:04 AM UTC-5, Stuart Venters wrote:
> ... a maximum acceptable rope strength

This is a bit of thread drift, but I think this is the elephant in the room and warrants a bit of discussion. The weak link has a mandated maximum strength, which means that non-destructive testing can't tell us much about it.. If we test to destruction, we learned about the rope, but we also broke it in the process. Whoops.

With non-destructive testing, the most we can say about the weak link is that it will fail at most at 110% of the rope's rated breaking strength. This isn't very comforting, especially for life-saving gear which sees years of field service.

So without effective testing, we have no way of knowing if the safety system will perform as advertised, unless we can 1) precisely bound the upper breaking limit and then 2) somehow test the minimum breaking strength.

With this in mind, one possible solution is to use stitches to precisely calibrate the breaking load. Climbing gear companies do this in order to reduce the shock from falling: https://youtu.be/ABMLFed7TcE. The number of threads is linearly related to the breaking strength, so you can fine tune it to a great degree.

So a weak link could be made by simply stitching webbing together and then protecting the stitches from abrasion. However, the precision required means this is not something which can be done outside of controlled conditions. While I could do a great job on my home sewing machine, I could also do a crappy job. It's like repacking a parachute, anyone can do it but who do you trust to do it correctly?

In contrast, splicing onto thimbles has the nice feature of being something we can do at home and get very similar results to the professionals.

[Dan Daly[_2_]]

On Wednesday, December 16, 2020 at 12:31:34 PM UTC-5, Bob Hills wrote:
> What is the consensus of opinion of using steel thimbles on aero tow ropes?
>
> Currently we use some clear tubing in the loop to protect the rope from wear of the Tost tow ring. Not only does this allow the rope to bend sharply causing a weak point but it also seems to fill with dirt. This then manages to find its way to the sharp bend in the rope where it grinds away at the rope (poly) and precipitates a breaking point.
> A steel (or other hard material) thimble in the loop would seem to eliminate this problem and look a lot more professional too.
>
> Anyone out there do anything different or profess not use thimbles?
>
> Bob 7U
We protect the rope in the area and thimble with cloth hockey stick tape. It wears, nothing else does. We tape when new, and have a roll at the flight shack to retape as necessary. As a bonus, you can also tape your hockey stick. Four 15m rolls go for $CAD5.61 at Canadian Tire; this will last a season. Amazon dot com has it, but a lot more expensive.

[john firth]

On Friday, December 18, 2020 at 12:45:56 PM UTC-5, Dan Daly wrote:
> On Wednesday, December 16, 2020 at 12:31:34 PM UTC-5, Bob Hills wrote:
> > What is the consensus of opinion of using steel thimbles on aero tow ropes?
> >
> > Currently we use some clear tubing in the loop to protect the rope from wear of the Tost tow ring. Not only does this allow the rope to bend sharply causing a weak point but it also seems to fill with dirt. This then manages to find its way to the sharp bend in the rope where it grinds away at the rope (poly) and precipitates a breaking point.
> > A steel (or other hard material) thimble in the loop would seem to eliminate this problem and look a lot more professional too.
> >
> > Anyone out there do anything different or profess not use thimbles?
> >
> > Bob 7U
> We protect the rope in the area and thimble with cloth hockey stick tape. It wears, nothing else does. We tape when new, and have a roll at the flight shack to retape as necessary. As a bonus, you can also tape your hockey stick. Four 15m rolls go for $CAD5.61 at Canadian Tire; this will last a season. Amazon dot com has it, but a lot more expensive.

Rope flailing is indeed a destroyer of the rope end.
Many years ago, I , as CTP, introduced the experiment of a plastic funnel to keep the rope taut.
It stopped the flailing and made the rope end more visible when retrieving. However, club members found
replacing the worn funnel too much trouble and the idea died; we also tried cubes of foam
with a slot to take the rope, and bound with tape. This also works but requires maintenance.
Maybe support personnel are more motivated now, to repair things. Certainly RVSS seems to have
a hard working group.
John F