Mooring Swivel

[noelex]

There are plenty of G3 and G4 rated shackles available if you feel the rating is important in a mooring shackle that will be generously oversized to give acceptable corrosion resistance. The Van Beest catalogue is available here:

http://www.vanbeest.com/getattachment/aa492c75-5752-4fe6-84fc-b506b6fe93ab/Chapter01-Shackles.aspx

Van Beest have rated (WLL and UTS) G3 mild steel shackles from 13mm to 36mm and rated G4 shackles from 5mm to 67mm. Other companies will have similar products.

 

[Kelpie]

What is the effect of steel grade (HT) upon corrosion resistance? This is generally a more important factor than abrasion, in my experience.

By the way the assertion that 32mm gear is not suitable for a yacht mooring is simply not true. My own mooring which I do not consider particularly oversized consists of two 150kg anchors, a length of 46mm studlink ground chain, and 19mm riser chain which has 25mm shackles in it at the swivel. I would have used bigger shackles if they had fit the riser. I can't remember what size the shackles between ground chain and anchor are, but they are likely at least 32mm.
Cost wasn't really an issue. I get everything from the same people who supply the aquaculture and fishing industries, and it is far better value for money than yachty stuff.

 

[vyv_cox]

What is the effect of steel grade (HT) upon corrosion resistance? This is generally a more important factor than abrasion, in my experience.

High strength and toughness in steels are achieved by increasing carbon content and adding alloying elements. I can think of very few examples in which this would not improve corrosion resistance.

I suspect that the wear mechanism underwater is mostly corrosive wear. Relative movement between two components, chain links and/or shackles, removes any protective film, allowing the nascent surfaces to corrode. Further wear removes the corrosion product, re-starting the process. But there is no doubt that adding an abrasive increases the wear rate even further, as this beautiful example shows. It is the lower end of a riser that dragged across the sandy bottom as the boat veered with the wind.

 

[ffiill]

Problem is they take a rated galvanised
lifting eye designed to be bolted down onto something and combine it with a lifting eye with a threaded end or just two designed to be bolted down.
Then add a nut or nut and bolt-weld the bolt to one eye fit it through the other eye add a nut and weld.
Problem then is none of it is any better than the bolt and all has been subject to a lot of heat!

 

[Neeves]

Interesting image Vyv. It shows to me that wear between the crowns is no worse than wear along the length. This suggests wear on the seabed from sweeping the sand is 'as bad' as wear between the crowns caused by sand (as an abrasive medium) with tension. This might indicate, also, the mooring was not used very much.

I see a similar wear pattern on anchor chain - wear between the link seem no worse than wear caused by chain sweeping the seabed.


If the only criteria for a mooring was resistance to abrasion there would be an easy answer - simply buy huge chain and fittings cheaply from China. They would be cheap, have good abrasion resistance because they are huge and there would be no doubts on quality/strength - as again they are huge.

However you would end up with snatch loads and there is, in reality, a balance between monster fittings, helpfully linked to by Noelex, and fittings that allow the sweeper chain to be lifted and offer some snatch load dampening.

Jonathan

 

[Kelpie]

I doubt my mooring offers any significant dampening/catenary. I have 19mm chain all the way to a large rigid buoy, then a long rope strop made from chain plus rope; it is the latter which absorbs the snatch loads.

 

[NormanS]

I doubt my mooring offers any significant dampening/catenary. I have 19mm chain all the way to a large rigid buoy, then a long rope strop made from chain plus rope; it is the latter which absorbs the snatch loads.

The normal attitude of your riser is vertical under the buoy. As soon as the boat puts any load on the system, the chain is pulled sideways from the vertical. This gives dampening/catenary.

Second thoughts. That's how it works with my mooring in 14m at LW. However, I think you dry out at LW? In which case you won't get much effect.

 

[Neeves]

I'd hope no effect at low water

Jonathan

 

[lw395]

High strength and toughness in steels are achieved by increasing carbon content and adding alloying elements. I can think of very few examples in which this would not improve corrosion resistance.

I suspect that the wear mechanism underwater is mostly corrosive wear. Relative movement between two components, chain links and/or shackles, removes any protective film, allowing the nascent surfaces to corrode. Further wear removes the corrosion product, re-starting the process. But there is no doubt that adding an abrasive increases the wear rate even further, as this beautiful example shows. It is the lower end of a riser that dragged across the sandy bottom as the boat veered with the wind.

I've seen a few like that.
What is worth bearing in mind is that the UTS of even the mildest steel is what 300MPa? That's 300N or 30kg for every square millimetre?
When they come up broken, it's often surprising what force the crane can get up to before it lets go.
Now all we need is an idea of the actual forces involved....

You do get the odd one come up easily looking in good nick, but no longer shackled to anything.

 

[NormanS]

High strength and toughness in steels are achieved by increasing carbon content and adding alloying elements. I can think of very few examples in which this would not improve corrosion resistance.

I suspect that the wear mechanism underwater is mostly corrosive wear. Relative movement between two components, chain links and/or shackles, removes any protective film, allowing the nascent surfaces to corrode. Further wear removes the corrosion product, re-starting the process. But there is no doubt that adding an abrasive increases the wear rate even further, as this beautiful example shows. It is the lower end of a riser that dragged across the sandy bottom as the boat veered with the wind.

That's why I always drop my riser over the winter. It stops the length of chain equal to the tidal rise and fall, from wearing away due to the constant movement.

 

[vyv_cox]

That's why I always drop my riser over the winter. It stops the length of chain equal to the tidal rise and fall, from wearing away due to the constant movement.

Yes, we did the same. Unfortunately in the Menai Strait the marker buoy was vulnerable to various persons who wished it not to be there, so it had limited success.