Double mooring strops twisting

[Jaguar 25]

I do accept that if you have two mooring strops that they will twist as the boat rotates. However, you would think that in a non-tidal lake, the rotation would be as much anti-clockwise as clockwise and therefore, twisting would be minimal. Seems not to be the case. Is there a logical reason for this?

 

[johnalison]

Coriolis forces, obviously. My remedy used to be to have one strop with a lot of slack, when little harm resulted. Alternatively, you may be near where the centres of low pressure pass, thus subject to an anticlockwise bias.

 

[C08]

Strange, I never had this over 20 years on 4 different swinging moorings - have you got a swivel top and bottom of the riser?

 

[Kelpie]

I've experienced twin strops twisting around each other- I now use a single fat strop, the biggest rope that will fit in the bow roller (allowing for chafe protection).
However there might be some merit in having one slack and one taut strop, some boats next to mine have this and don't seem to have problems.

 

[Jaguar 25]

Strange, I never had this over 20 years on 4 different swinging moorings - have you got a swivel top and bottom of the riser?

No, I took over the mooring this year and there is no swivel. However, some people put a swivel from the top ring on the mooring buoy and attach both strops to the swivel. This doesn't seem sensible to me as swivels (I am told!) are one of the main faiure mode.
I do have plenty of slack in the secondary strop.
However, my query was why does the boat mostly rotate in one direction? Coriolis may play a part but I would have thought that this would insignificant in a lake.

 

[Graham376]

swivels (I am told!) are one of the main faiure mode.

Swivels fail because some people think they're fit and forget and/or use undersized ones. Not having one either top or bottom of riser is asking for trouble, particularly with 3 strand risers. I've used twin strops for 25 years and have no problems with them twisting badly.

 

[Saltsail]

Wind rotates your boat, not tide, nothing to do with non tidal lake. Hope it helps

 

[William_H]

I think it must be related to Coriolis effect. Here in southern hemisphere certainly in winter witha front approaching wind always starts in NE goes to N then NW W then SW. So always winding the boat anti clockwise. So you should read clockwise for northern hemisphere. In summer wind however does tend to blow NE in morning stops then blows SW. So no clear rotation. Yes my twin tethers twist even witha swivel in the top. olewill

 

[Neeves]

We use a bridle, which obviously has a wide separation between the 2 arms - and both arms are quite tight. We have no issues with the bridle - but the bridle connects to 10m, or there about, of nylon which is roughly depth of water and is then connected to the sweep chains with a very oversize swivel.

We do not have a problem with the bridle nor the nylon pennant - and we enjoy roughly random rotation of the catamaran dictated by movement of fronts, development of a seabreeze and if there is no wind - tide. Our mooring is serviced annually and we need a new swivel every 3-5 years, it sits on a silica sand seabed. The pennant and bridle last much longer.

If you use a rated oversized lifting swivel, top and bottom, I find it difficult to imagine you will be any worse off. But you do need to check the swivels, especially the bottom one - as that is where our moorings fail as it is that 'point' that has the most movement over the seabed, getting progressively less as you near the sinker itself. You are unlikely to be able 'widen' the bridle, or not by much you could try shortening the arms, to tighten it up (which will also widen the arms a bit). But without a swivel you will get twists, as you have found - it might be better for you in the bridle, that you can see than the pennant, which is underwater.

For a mono - having one arm longer than the other sounds a good idea - as you can simply unwind the more slack of the 2 arms.

Jonathan

 

[Kelpie]

Why would you have a swivel at the top and the bottom of the riser? I've always managed just fine with one, and would place it high enough up the riser to avoid contact with the seabed. It's probably the single weakest point of a mooring and the only part that cannot be bypassed by a backup.

 

[Neeves]

Swivels are grossly ineffective, with 2 you might have a chance one will work. It might depend on how your mooring is constructed but we have no steel work above the seabed - its all chain, swivels or shackles - all monster, unless you think 2" chain is small and it is all on the seabed. We also have a 1" sweeper chain with the 1" swivel between chain and pennant/riser There are 2 issues, stopping the bridle twisting and stopping the pennant twisting. You assume the one at the top will work and will stop both the bridle and, in our case 10m, pennant will remain untwisted. Seems supreme confidence in swivels (and I've tested them and I don't share your confidence nor do I accept your, possible, false economy)

But if you are happy, go for it.

If you are servicing once per year and they last 3-5 years and cost peanuts I'm really to to sure what it is that you objet to, the cost of 2 swivels, Stg20 each?

Jonathan

 

[NormanS]

When I did all my own mooring work, we had a huge swivel joining the two ground chains (from the two anchors, latterly three) with the riser chain attached to the swivel. With the swivel at the bottom of the riser, it doesn't have the weight of the riser chain hanging on it, and so can rotate as and when required.

Our mooring work is now done by professional divers, and they insist that the swivel is fitted at the top of the riser, immediately under the buoy. I'm not convinced that the swivel is as keen to rotate, with 14m of vertical chain hanging on it, but I didn't argue about it. To be honest, our boat spends so little time on her mooring, that I don't think it matters.

The pennant to the boat is a single, huge octoplait, so we have no problems with twisting.

 

[lw395]

Our experience is that our local mud is gritty enough that any swivel that ever touches the seabed won't rotate for very long.
Best results are a swivel under the buoy and a swivel on top of the buoy.
Capsize the buoy at low tide to check the swivel for wear.
Motorcycle chain wax is useful for lubricating swivels, it comes in a spray can.

 

[Neeves]

I have noticed that moorings in the UK are completely different to ours.

Our moorings are a concrete block, a few metres of very heavy chain (2") recycled from the mining industry, a few metres of sweep chain 1" and then a swivel - all of this sits on the seabed. None ft is galvanised Enough cordage to reach sealevel, spliced to the swivel with hose protection. I don't know how much they allow extra, over depth of water and tide. Ours has a bridle, as we are a cat. Our buoy is only for marking, it offers minimal flotation.

As far as I know - these are Sydney standard moorings.

Jonathan

 

[Gunfleet]

I've experienced twin strops twisting around each other- I now use a single fat strop, the biggest rope that will fit in the bow roller (allowing for chafe protection).
However there might be some merit in having one slack and one taut strop, some boats next to mine have this and don't seem to have problems.

I have this arrangement and it doesn't seem to cause any problems. The slack one of course is as a back-up.

 

[Kelpie]

Swivels are grossly ineffective, with 2 you might have a chance one will work. It might depend on how your mooring is constructed but we have no steel work above the seabed - its all chain, swivels or shackles - all monster, unless you think 2" chain is small and it is all on the seabed. We also have a 1" sweeper chain with the 1" swivel between chain and pennant/riser There are 2 issues, stopping the bridle twisting and stopping the pennant twisting. You assume the one at the top will work and will stop both the bridle and, in our case 10m, pennant will remain untwisted. Seems supreme confidence in swivels (and I've tested them and I don't share your confidence nor do I accept your, possible, false economy)

But if you are happy, go for it.

If you are servicing once per year and they last 3-5 years and cost peanuts I'm really to to sure what it is that you objet to, the cost of 2 swivels, Stg20 each?

Jonathan

2" chain is indeed monster stuff. I have 46mm stud link chain on the ground, and a 19mm riser. Half way up the riser I have a swivel, the biggest one that would fit the chain/shackles- this cost me £50. Cost isn't the issue though, it's all about eliminating failure points. A second swivel would be another weak point, and of course two extra shackles.
I've been on this mooring for four years, all year round, and have had no problems with the swivel seizing up. I check it every couple of months at low tide by hauling the riser aboard (need to use a rope onto the windlass drum due to the weight). It's never shown any sign of stiffness.

At a place I used to work we had swivels on top of the buoys, it was a weekly task to bash them with a lump hammer until they freed up. I think the oxygenated salt spray environment is about the worst possible place for a swivel- but the design of that mooring wasn't up to me!

 

[Neeves]

Swivels obviously will not turn if they are seized but that is only part of a problem. Swivels will not turn under tension unless the torque forcing the turn is considerable. If the swivel lies on the seabed it will take a lot of turns, of a yacht, to force it to turn - the turns will simply accumulate in the hanging portion (or bridle). Mooring swivels, which are simply devices taken from the lifting industry and our 'own' swivels that are used in the rode simply will not turn unless they are free to turn anyway (so hanging) and there is already about a twist per metre in the assembly (this assumes chain), and thats a lot of twists. Both types of swivels are very very crude - but lifting swivels are generally made to some sort of standard, at least for strength, and because lifting swivels are of an 'open' design you can see what is going on (not so our prettier versions where all the working parts are effectively hidden).

Good lifting swivels are no less safe than the chain or shackle, if sized correctly (and from reputable sources) - so the idea that having 2 swivels is unsafe is something I would not consider. Obviously if you useundesized, unrated swivels they will be a weak portion - but one has to assume a degree of professionalism. I watch our mooring being serviced, I know our mooring contractor well, and corrosion is not the issue - wear from the abrasive silica sand is. But our steel work is all at 10m depth and a mooring in shallow water on mud and or with big tidal ranges might need a totally different design to ours.

Some people here do not service their mooring and when they fail they alway fail at the chain/swivel or shackle interface at the rope/chain joint - this is logical as it enjoys the most movement and thus most abrasion (but it is the chain that fails at the first link because it is the smallest part of the assembly, everything else is oversized) Two oversized shackles will still leave the smaller sized chain as the weakest point -assuming that professionalism.

There is never one right answer.

 

[afterpegassus]

There is never one right answer.

Correct.
Mooring make up in my part of the world is dictated largely by location and depth of pocket being respectively, in my case, very sheltered and not very.
Under water I have only 1 shackle connecting a concrete block /heavy ground chain to multiplait riser.
My stainless swivel is on top of my large diameter suspension buoy ( large diameter to prevent the swivel contacting the hull) tied directly into the riser with a BFK which also stops the riser dropping back through the buoy. Not pretty, but very effective.
A single heavy pennant connects the boat to swivel.
Though sheltered, the mooring is subject to a lot of tidal movement, eddies etc. so plenty potential for twisting.
This has worked very well for me for many years with only one shackle underwater, and a swivel that I can inspect on every visit to the boat.