Effect of chain on anchoring

[kefroeschner]

Re: evansstarzinger's suggestion of an electrical snubber -- Great Idea, been there but haven't done it yet.

We have designed and done the analysis for several types, from simple replacements of the usual rubber band with a linear PM generator Which hooks onto your chain in the same way and has an electrical cable which hooks to your charging system -- estimated output up to 100 watts average. And really big systems with Megawatt capacity for off shore installations in places like the Western shores of the Caribbean Islands, Belize, wherever nations are currently relying on diesel gen-sets powered by imported oil.

We have not pursued this beyond the paper stage, but will be putting a prototype system on Orion.

Any interest out there?

[Jim Cate]

Originally Posted by kefroeschner

We have not pursued this beyond the paper stage, but will be putting a prototype system on Orion.

Any interest out there?

G'Day mate,

Just noticed that you were a Livermoron, so from another retired LLNL physicist, welcome to CF!

And I'm certainly interested in your efforts, so keep us all informed as to progress. But, knowing how things go, one can envision deliberately choosing not just windy anchorages for power generation, but ones with a lot of sea action!!

Cheers,

Jim

[JonJo]

Originally Posted by Maine Sail

This is one thing that really bugged me about that article. The friction over the chock reduces the peaks the load cell sees and gives quite different readings then when ahead of the chocks..

I have done this both ways on my own boat. I simply "water proof" as best I can my load cell and leave it ahead of the chocks for the most accurate readings. Never had a single corrosion issue.. I used No-Ox-Id electrical grease to coat the cell and wrap the wire with self amalgamating tape.

I have measured peaks, using "peak hold" on our 36 footer in just 17 - 20 knots, and some associated chop, at 218 pounds. When I place the load cell aft of the chocks I do not see the same data. There are many boats who's cleats are right on the bow with no chock to pass through and I want to see the loads that type of cleat would see thus I place my load cell ahead of the chocks..

I'm not sure what you mean by chocks but assume you mean bow roller and if yours is anything like ours it has lots of friction (which presumably why synthetic bow rollers wear away). The Dyneema bridle led through large ball bearing blocks and according to Harken they are almost friction free and designed for Dynamic/snatch loads considerably higher than we were experiencing. The bridle touched nothing except the blocks. It was establishing the routeing of the Dyneema that caused us a lot of anguish to ensure it ran fair. To have not used the Dyneema would have caused readings of little value, the cat would veer all over the place in strong winds and the result Dynamic/snatch loads would have been higher than reality. We pre-stretched the Dyneema and the knots prior to recording.

Your method is valid for a monohull that would take the load on one central point, not so a catamaran. Unsurprisingly no-one would lend us a large monohull, 45' would be the size needed for the tests, they took days of experimentation and then days of recording and the loads would have discouraged most owners! We were testing in the same place around high tide, so same depth, same seabed - so directly comparable scopes. We only tested in afternoon summer seabreezes so same waves, which were small and same wind variation.

As an aside, we want to do the same tests, same place etc, but as a front passes through which will give us wave effects (as the wind would be in the opposite direction and there is 3-4 miles of searoom). How do we measure wave height?

In the 2.7:1 scope tests at just over 30 knots we were subjected to dynamic/snatch loads of 650kg (or 1,500lb in America speak) - it was these loads that caused us to terminate the recording of loads at that scope. We do not know if the 650kg was a max. There was/is no suggestion that 2.7:1 was a scope that anyone should use - it was simply set up to establish points on a graph.

We normally use 13m of nylon double braid run down each side deck as a bridle. It runs through small, different to the ones on the tests, turning blocks on the bow but touches the deck all the way. We secure to the aft horn cleats. We tried measuring the loads at the aft end of the bridle and never achieved anything over 100kg, no matter what we did. We put the low loads down to friction, so could not define what benefit the bricle gave as opposed to elasticty plus friction.

[SVNeko]

Originally Posted by JonJo

To have not used the Dyneema would have caused readings of little value, the cat would veer all over the place in strong winds and the result Dynamic/snatch loads would have been higher than reality.

This is confusing, as most cat owners do NOT use dyneema bridles and frequently DO have their cats veering around in strong winds. This would seem to me to replicate reality more than a pre-stretched Dyneema bridle.

[Kettlewell]

In my experience, cats using bridles do not "veer" around a lot, rather they react to changes in the wind quickly, and therefore they may be moving a lot, but not presenting a side of the boat to the wind, like a monohull typically does. My 32-foot cat would stay weathercocked right into the wind perfectly using a bridle rig, indicated by both legs of the bridle staying perfectly taught. If the boat were veering, one leg or the other would go slack as the boat slewed sideways to the wind. I think that most cats I have observed exhibit a similar excellent weathercocking effect. In the past I anchored amongst monohull numerous times in high winds and observed that my cat did very well with regard to what I call veering, compared to the typical mono, which generally sails back and forth a bit on the end of the rode. However, the average load on a cat is higher due to the much greater windage, but my feeling would be that the peak loads, due to lack of veering, would be less.

[JonJo]

Originally Posted by SVNeko

This is confusing, as most cat owners do NOT use dyneema bridles and frequently DO have their cats veering around in strong winds. This would seem to me to replicate reality more than a pre-stretched Dyneema bridle.

In Oz I have never seen a cat without a bridle. Here cats are delivered, locally made or from France (and now from Vietnam or Korea), with bridle attachment points at the ends of the forward crossbeam, they are commissioned with bridle ropes. Whether owners use them is another matter. But in stronger conditions I have never, ever, seen a cat at anchor not using a bridle (and we have a lot of cats now in Oz). Cats might not bother with a bridle at lunch time, that was not the focus of the tests. Bareboat charter cats might not use bridles, but they would not normally be at anchor in 30 knots. The Dyneema was only used to remove the elasticity - we wanted to measure the load on the anchor/yacht without any 'help'. There was no suggestion anyone should ever use Dyneema as a bridle, too expensive, no elasticity - but any sensible owner would know this.

However the tests were conducted on the basis of sensible yacht owners and there was an attempt to compare the results on a cat with a similarly sized, windage, monohull. We accepted that a similarly sized monohull, windage, has a different underwater profile and weight - and this was mentioned.

Contrarily - whereas our straw poll suggest all cat owners use bridles in Oz this is at complete variance with mono yacht owners as here they very seldom use a snubber and I can count on the fingers of one hand the number of yacht snubbers I see that are actually long enough. Consequently the loads found are relevant to the majority of yacht owners (accepting the impact of keels and weight). If you look at the lead image in the Practical Sailor article, the owner was not using a bridle nor knew about their advantages (I spoke to him subsequently) and at the time the picture was taken had completed 75% of a circumnavigation of the UK. The yacht in the image is 43' long, steel and weighs 20t. That's what I call a 'Dynamic' load!, wish he had had a load cell.

To return to the comment on the use of a bridle and it impacting results. Stationary the load cell measures the chain weight hanging to the seabed at which time there is no friction. If this weight is wrong (we know the length, lead line, we know the weight per metre etc) - the 'bridle' systems needs to be changed. If the resultant graphs, for loads in the 10 - 30 knot range, cannot be extrapolated back to that chain weight (or something very close) then there is friction or elasticty in the system and we discarded the results.

J

[SVNeko]

It was a short post but I guess I wasn't clear. I wasn't saying cats veer more than monos. And I wasn't saying cat owners don't use a bridle. I was merely suggesting that not using a stretchy bridle in the PS test to avoid snatch loads might not have been the best approach to replicate reality since most cats do in fact (or should) use a stretchy nylon bridle. Constructing a test to determine loads felt by mono owners who don't anchor properly with a snubber doesn't make a lot of sense to me either.

[JonJo]

Originally Posted by SVNeko

It was a short post but I guess I wasn't clear. I wasn't saying cats veer more than monos. And I wasn't saying cat owners don't use a bridle. I was merely suggesting that not using a stretchy bridle in the PS test to avoid snatch loads might not have been the best approach to replicate reality since most cats do in fact (or should) use a stretchy nylon bridle. Constructing a test to determine loads felt by mono owners who don't anchor properly with a snubber doesn't make a lot of sense to me either.

Most mono owners that use a 'snubber' actually use it to take the load off the windlass, its 1-2m long. Most cat owners will use a pair of 3m heavy duty snubbers/bridle. We find the load is on one side of the bridle, or the other - so using (in the case I quote) 3m at a time and because they are beefy, almost inelastic.

I'm guessing most people here would agree - even 3m is too short for a snubber, better than nothing but not enough.

There were enough variables in the test with scope and wind - anything else was initially too complex. Bridles vary in length from 3m to 13m (maybe longer). Most would be around 3m - if I'd used 3m someone would have said it was too short. Snubber types might be double braid, hawser laid, anchor plait. Thickness, for a 6t cat, could be 8mm - 12mm.

If you are setting up a test to measure loads on anchors you need to start somewhere - there are follow ups using different types, different lengths, different thickness etc of snubber. Buy Practical Sailor and sometime soon you can read the test on 'Anchor Shockles'. Without a base load of data you have nothing with which to compare. There is a follow up on 90 degree loading (which might even make your hair curl) and it does explain why some anchors bent! Tests take time, lots of it as they depend on the wind and oddly when you are testing winds are never strong, go on a cruise and it blows your boots into the sea! - you will need to be patient.

J

[SVNeko]

Fair enough. Thanks

[JonJo]

Is it out of turn, within this thread, to wonder what people use in addition to chain, swivels? and description of their snubbers. Particularly the latter as few, none(?), of us use chain alone and consequently the snubber has as much influence on the behavious of the chain/anchor as many other variables.

J

[downunder]

Originally Posted by JonJo

Most mono owners that use a 'snubber' actually use it to take the load off the windlass, its 1-2m long. Most cat owners will use a pair of 3m heavy duty snubbers/bridle. We find the load is on one side of the bridle, or the other - so using (in the case I quote) 3m at a time and because they are beefy, almost inelastic.

I'm guessing most people here would agree - even 3m is too short for a snubber, better than nothing but not enough.

There were enough variables in the test with scope and wind - anything else was initially too complex. Bridles vary in length from 3m to 13m (maybe longer). Most would be around 3m - if I'd used 3m someone would have said it was too short. Snubber types might be double braid, hawser laid, anchor plait. Thickness, for a 6t cat, could be 8mm - 12mm.

If you are setting up a test to measure loads on anchors you need to start somewhere - there are follow ups using different types, different lengths, different thickness etc of snubber. Buy Practical Sailor and sometime soon you can read the test on 'Anchor Shockles'. Without a base load of data you have nothing with which to compare. There is a follow up on 90 degree loading (which might even make your hair curl) and it does explain why some anchors bent! Tests take time, lots of it as they depend on the wind and oddly when you are testing winds are never strong, go on a cruise and it blows your boots into the sea! - you will need to be patient.

J

Jon Jo

So do you suggest for a cat a adittional longer bridle for bad conditions. I think I recall Evans Starzinger carries several bridles for differing conditions with his mono. in the high latitudes.

[Seaworthy Lass]

Originally Posted by JonJo

Is it out of turn, within this thread, to wonder what people use in addition to chain, swivels? and description of their snubbers. Particularly the latter as few, none(?), of us use chain alone and consequently the snubber has as much influence on the behavious of the chain/anchor as many other variables.
J

Our system:

Snubber: 14m of nylon line with a splice loop at the chain end. A short prusik loop is attached to the chain with a Klemheist knot. A shackle connects the prusik loop and nylon line.

Swivel at anchor: none, as it is a possible weak link in the system (this had been hotly debated here before).

[JonJo]

Originally Posted by downunder

Jon Jo

So do you suggest for a cat a adittional longer bridle for bad conditions. I think I recall Evans Starzinger carries several bridles for differing conditions with his mono. in the high latitudes.

You could have different snubbers for a cat, but it would be incredibly complicated, or fiddly, changing them round. As you would need to hang over the bow and fit under the prodder and kicker wires (assuming you have them)

We put turning blocks on each bow. We secure our snubbers to the aft horn cleats and run each up the side decks. Our stanchions have 'slots' at the base through which we run the snubbers and furling lines etc. The snubbers go through the turning blocks on the bow to a common chain hook. We run the snubber through hose pipe where it runs over the crossbeam (or anywhere you think there might be high wear). Our snubbers stay in place 24/7/365 0r 366. After 3 years we have just end for ended them as there was wear from the chain itself when in use. We effectively have 13m each side, or thereabouts. We use 12mm double braid nylon. By choice, unlimited budget, I'd use anchor plait, only because I think it better, not that I know. You need nylon for the elasticity. We find that as the yacht continuosly swings the load is largely on one side or the other, so each side need take the full load. When not in use the chain hook is attached at the centre of the cross beam and its 'tight', so nothing dangling but loose enough to detach from the cross beam and attach to the chain. There is enough room on our horn cleats to still use them for mooring lines as well.

We only use this bridle set up now.

In use we have little out at the bow, maybe 3m each side. I know people who use very long snubbers forward of the bow, ie attached to the normal place on the cross beam. You end up with your snubbers dangling, and can be on the bottom in shallow water, or a long way ahead in any breeze.

I have also seen people using 4x4 snatch tapes, those retrieval tapes you get in 4x4 shops. I though them a good idea, but expensive and more importantly maybe not long enough.

Hope it helps, maybe someone else can comment.

J

[downunder]

Originally Posted by JonJo

We put turning blocks on each bow. We secure our snubbers to the aft horn cleats and run each up the side decks. Our stanchions have 'slots' at the base through which we run the snubbers and furling lines etc. The snubbers go through the turning blocks on the bow to a common chain hook. We run the snubber through hose pipe where it runs over the crossbeam (or anywhere you think there might be high wear). Our snubbers stay in place 24/7/365 0r 366. After 3 years we have just end for ended them as there was wear from the chain itself when in use. We effectively have 13m each side, or thereabouts. We use 12mm double braid nylon.

J

Could you manage a couple of pictures.

thanks

[JonJo]

Originally Posted by downunder

Could you manage a couple of pictures.

thanks

No problem,

But at 13m long its difficult to take'an' image. Tell me which part and I'll focus on that/them.

I might need to take images, so you'd then need wait till tomorrow night.

J