thimble testing

[estarzinger]

Somewhat related to the thread about anchor swivels . . .

I just tested a range of thimble designs. The loads quoted are all when a wall of the thimble started to distort sideways. They have already collapsed and elongated to some degree by this point.

There were two surprises. First is how poor the conventional open galvanized thimble (as used on many anchor rodes) is. Both the 5/16" and 1/2" sizes distorted at only 57% of the tensile of the appropriate size nylon 3 strand line. Second is how relatively good the closed nylon thimbles were (compared to the galvanized). They distorted at slightly higher load, and bounced back to near their original shape when the load was taken off. The open stainless thimble distorted at essentially 100% of the line tensile, so is "good enough". The clear winner is the closed stainless thimble (aka a sailmakers thimble). It was the only one which exceeded the nylon tensile strength - giving a safety margin for nylon and allowing for use with higher strength lines.

The picture below is of the thimbles after the test.

View attachment 40664

 

[oldbilbo]

Somewhat related to the thread.....

I'll shortly - insh'allah - be making up some twinned mooring strops to be fitted onto the BIG steel rings on my club's Hippos. I reckon some matching BIG shackles are warranted. S'pose I use s/s thimbles in the eye-splices at the 'far ends' of my lines, fastened onto the BIG shackles, can I expect accelerated wear due to a combination of mechanical friction and galvanic corrosion? Or not enuff to bother with...?

 

[William_H]

I would go stainless steel thimbles even on to GI shackles. Not such a good idea under water but OK above I think. The GI shackle will wear rather than the SS thimble.
Interesting the tests on thimbles but I wonder how much it matters if the thimble distorts. Surely it will still do its job after distorting from overload?
good luck olewill
PS what is a club "Hippo"?

 

[estarzinger]

Interesting the tests on thimbles but I wonder how much it matters if the thimble distorts. Surely it will still do its job after distorting from overload?

The distortion would only "matter" in two cases . . . One is if the side walls distorted enough to allow the loop to easily slip off the thimble, as might happen if you were sailing back and forth at anchor. That's why I tested to the wall distortion loading. The second is if they distorted (or broke) in some way that might have cut the rope. I did not see that behavior in my samples.

 

[rob2]

Hamble moorings prohibit the use of any stainless in a mooring tackle. Assuming you're using the maximum line size your bow roller and cleats will take, then a nylon thimble used with the appropriate sized shackle will not approach failure mode with the boat hanging in 30 knots. After serious bad weather the thimble will bounce back anyway and you'll be checking condition after such a blow...

Rob.

P.S. and the nylon won't squeak or rattle annoyingly when yu're trying to sleep.

 

[estarzinger]

Hamble moorings prohibit the use of any stainless in a mooring tackle. Assuming you're using the maximum line size your bow roller and cleats will take, then a nylon thimble used with the appropriate sized shackle will not approach failure mode with the boat hanging in 30 knots. After serious bad weather the thimble will bounce back anyway and you'll be checking condition after such a blow...

Rob.

P.S. and the nylon won't squeak or rattle annoyingly when yu're trying to sleep.

Avoiding stainless in a permenantly underwater mooring makes sense . . . But stainless is no problem for anchor gear.

The nylon thimbles are surprisingly good. However, my experience is that they fateigue. After about two years of cycling loads (elongate and contract cycles) they will crack.

 

[TradewindSailor]

The problem with stainless in anchor gear is that it gets covered with the expensive zinc that I prefer to stay on my chain.

 

[NormanS]

Avoiding stainless in a permenantly underwater mooring makes sense . . . But stainless is no problem for anchor gear.

.

I would question that. I left my boat anchored for four weeks, with a "Chum" attached to the galvanised chain, using a stainless shackle. On retrieval, the galvanising on the three or four adjacent links, had been destroyed. I won't do that again.

 

[oldbilbo]

PS what is a club "Hippo"?

This sort of thing....

....belonging to my club/moorings. There are many variants.

 

[William_H]

Be it right or wrong I have had an all stainless steel swing mooring for what must be 10 years. The chain is a heavy round link type which I believe was used in a lime kiln the shackles are just ordinary (Chinese) stainless steel. Chain is about 12mm shackles 10mm the swivel is built into the buoy with 12mm rod. (on a 1 tonne boat) One thing for sure is there is no sign of any wear on chain or shackles compared to rapid wear on GI that I had previously. Catastrophic failure, maybe I hope not of course but seems OK so far. olewill

 

[estarzinger]

I would question that. I left my boat anchored for four weeks, with a "Chum" attached to the galvanised chain, using a stainless shackle. On retrieval, the galvanising on the three or four adjacent links, had been destroyed. I won't do that again.

interesting . . . . . I am not sure what would explain the different experiences people have had with this. It would be interesting to know more about this.

I have used stainless anchor shackles for 20 years. They have never 'eaten' the galvanized chain links.

I use wichard shackles and G4 chain. I don't know if the composition or surface finish of either is different than what you were using, but they seem quite 'compatible'.

 

[NormanS]

interesting . . . . . I am not sure what would explain the different experiences people have had with this. It would be interesting to know more about this.

I have used stainless anchor shackles for 20 years. They have never 'eaten' the galvanized chain links.

I use wichard shackles and G4 chain. I don't know if the composition or surface finish of either is different than what you were using, but they seem quite 'compatible'.

Someone with more metalurgical knowledge might be able to shed some light. My chain is bog standard 10mm galvanised mild steel. However, the "Chum" is a 20kg lead weight with 1/2metre length of stainless chain attached. This in turn was shackled to the anchor chain with a stainless shackle of unknown type. At LW it would have been in contact with the mud. If using it in the future, I will use a galvanised shackle, as I would rather it got damaged, than the anchor chain, particularly as I have just had it regalvanised.

 

[William_H]

Further on this subject I did have a look at my mooring yesterday. The stainless steel chain has a joiner shackle that some professional fitted. No not my idea. It is about 18mm GI shackle which after 8 months shows signes of wear where the stainless steel shackle sits on both the pin and the bow. Despite its size it will almost ceertainly be the first part to wear through. So my experience GI does not like to sit on SS. olewill

 

[estarzinger]

^^

It would be interesting to know if there is any source for 'better' galvanized thimbles. All the commonly stocked ones I have tested have been quite weak. But it would certainly be possible to make a better stronger one.

 

[vyv_cox]

Someone with more metalurgical knowledge might be able to shed some light. My chain is bog standard 10mm galvanised mild steel. However, the "Chum" is a 20kg lead weight with 1/2metre length of stainless chain attached. This in turn was shackled to the anchor chain with a stainless shackle of unknown type. At LW it would have been in contact with the mud. If using it in the future, I will use a galvanised shackle, as I would rather it got damaged, than the anchor chain, particularly as I have just had it regalvanised.

It's an interesting one. Lead occupies almost exactly the same position in the galvanic series in seawater as 300 series stainless steel. My experience with stainless steel shackles in contact with galvanised chain is that the zinc is corroded extremely slowly, four or five years and some remnants remain. Adding the lead chum increases the surface area considerably, magnifying the driving force for the galvanic action. The example we use on training courses for this effect is putting an aluminium rivet in a steel plate versus a steel rivet in an aluminium plate. In the first case the aluminium corrodes rapidly whereas in the second both the steel rivet and aluminium sheet have a long life, as the aluminium protecting the steel is so much larger. So you might find that changing the shackle material will have no effect, assuming electrical continuity is maintained.