What is a "thrasher chain"?

[skyflyer]

Can someone explain what the difference is, and the function of, a thrasher chain and a riser on a swinging mooring. I found out that the thrasher chain on mmy mooring is 32mm chain - so presumably the main function is dead weight? There is also a 1.5 tonne block

 

[lw395]

Not heard it called that, I guess it's the bit of chain that thrashes around in the mud and silt, mostly staying on the bottom?
In some places, the mud/gravel/rubbish is very abrasive, so massive chain is also good for wear resistance.

are there many swinging moorings in Worcester?

 

[oldharry]

Not heard it called that, I guess it's the bit of chain that thrashes around in the mud and silt, mostly staying on the bottom?
In some places, the mud/gravel/rubbish is very abrasive, so massive chain is also good for wear resistance.

are there many swinging moorings in Worcester?

Plenty of swingers in Worcester maybe, but mooring swingers?

The bottom chain (I've not heard of it is a thrasher chain either) has several functions. 1: to add weight to the cantilever, so as to reduce snubbing, 2: to ensure that when loading comes on the sinker it pulls sideways, avoiding any vertical pull that could break the sinker out, 3: to add weight around the sinker. 4: lack of movement means the eye or whatever that is cast in to the sinker doesn't wear so quickly, so sinkers last a lot longer. Except in specific conditions, the bottom chains rarely wear quickly: My local mooring contractor reckons some of his are 30 yrs old, and still as good as the day they were laid, while the riser chains need replacing every 2 - 3 years (drying moorings)

 

[lw395]

We find the bottom of the riser, where it is often on the bottom, but often moving, can wear quite quickly.
With 5m tidal range, it's not practical to have very heavy chain for everything that can touch the bottom.

Empty moorings seem to wear worse, they have more chain on the bottom, stirring around.

 

[emsworthy]

As I understand it a swing mooring is generally made up of either a single sinker or anchors joined by a heavy ground chain in the mud, from which is attached a riser made up of two different sizes of chain. A lower section of very heavy gauge chain (mine is 1.5" stud chain) and an upper section of lighter chain attached the buoy (mine is 3/4" chain). The length of each is such that the lighter upper part is rarely in contact with the seabed and the lower heavy section provides the damping and shock resistance so the sinker is rarely loaded at all.

Never heard it called a thrasher chain though.

 

[skyflyer]

sorry i didn't make myself clear - i don't understand what ANY of these different chains are!
What is attached to what - can anyone link a diagram?

 

[Lon nan Gruagach]

sorry i didn't make myself clear - i don't understand what ANY of these different chains are!
What is attached to what - can anyone link a diagram?

See http://www.brantpointmarine.com/moorings/moorregs.html

Seems that their heavy chain is what you call thrasher and their light chain is the riser.
Oldharry has it right with the functions, but I would argue with his word cantilever, centenary would suit better in this situation.

 

[oldharry]

Centenary has to do with my age! Because of which I wrongly used cantilever for catenary. Whichever, its the weight of the chain as the load comes on which helps prevent snubbing.

 

[skyflyer]

Brilliant -thanks everyone. The term "thrasher" is one used by the professional mooring contractor by the way!