Catenary?

[Roberto]

I've often thought that using the engine to reduce the pull on the anchor might make the situation worse as it would take a skilled helmsman to keep the boat pointing at the anchor. Loads applied randomly by a yacht motoring might well upset the anchor. It'd be interesting to hear from someone who has done it as to whether it was easily done and whether it was a real benefit.

In really catastrophic conditions, I have seen a couple of cases (videos) during last summer mcs convective system that swept Corsica for an hour or so (reported winds 80/100kt, over 100 maydays, dozens of grounded boats, sinkings etc): a couple of boats tried and oppose the violent yawing by motoring (mind you, boats were heeling peak 40°+), it seemed to have some effect, but the amount of maneuvering the helmsman had to do would make it totally unfeasible for any extended period of time.

Just an idea of what happened, typical august calm day, then...

Here the skipper voice is congratulating the helmsman, he is "steering" under full engine power

 

[lustyd]

One case has high peak loads, the other doesn't. The average load is the same.

Not true. As I said, physics just doesn't work in that way. The loads would be identical.

Edited to add, this is a similar misconception to the old "two cars at 40mph crash head on" scenario. Physics says each experiences a 40mph crash while most people assume it's an 80mph crash.

 

[[2574]]

In really catastrophic conditions, I have seen a couple of cases (videos) during last summer mcs convective system that swept Corsica for an hour or so (reported winds 80/100kt, over 100 maydays, dozens of grounded boats, sinkings etc): a couple of boats tried and oppose the violent yawing by motoring (mind you, boats were heeling peak 40°+), it seemed to have some effect, but the amount of maneuvering the helmsman had to do would make it totally unfeasible for any extended period of time.

Just an idea of what happened, typical august calm day, then...

Here the skipper voice is congratulating the helmsman, he is "steering" under full engine power

Interesting. Must be impossible to know if the motoring was pushing the yacht over the top of the anchor, I guess it wouldn’t but how would one know, no transit would visible and gps unlikely to be of much use.

 

[noelex]

These photo show a different view, looking down the chain as well as a side view. It is an Ultra anchor in 6m of water at 3.5 : 1. The wind was 25 knots gusting perhaps to 30 knots. In this wind strength the chain was still on the seabed during the lulls, but nearly all the catenary was lost in the stronger gusts, as the photo shows.

[/QUOTE]

 

[[2574]]

These photo show a different view, looking down the chain as well as a side view. It is an Ultra anchor in 6m of water at 3.5 : 1. The wind was 25 knots gusting perhaps to 30 knots. In this wind strength the chain was still on the seabed during the lulls, but nearly all the catenary was lost in the stronger gusts, as the photo shows.

[/QUOTE]
I’m surprised that the anchor isn’t completely buried if that’s 30kts.

 

[noelex]

I’m surprised that the anchor isn’t completely buried if that’s 30kts.

Yes, good point. 25 knots of wind gusting 30 is only about the same force as applied by most sailboats when setting the anchor. This Ultra is slightly less buried than expected although no by a huge amount if it was hard sand.

In this substrate there was a thin layer of sand over rock. The sheets of rock can be seen in the top of the second photo.

In this difficult substrate the Ultra was doing well.

Anchor pull tests are nearly always conducted in good, uniform seabeds. The real world is not like that. The danger is expecting the holding power derived from tests in good substrates to always translate to real world anchoring. A 15kg can hold 2500+ kg in perfect sand, but in this type of real world substrate the numbers would be much lower.

 

[vyv_cox]

I’m surprised that the anchor isn’t completely buried if that’s 30kts.
[/QUOTE]
You illustrate the total general misunderstanding about oversized anchors. The recommended size is nowhere near buried at 30 knots and not a lot further in at 50 knots. Going to a bigger size achieves nothing. More info on my website Oversize anchors – necessary?

 

[[2574]]

I’m surprised that the anchor isn’t completely buried if that’s 30kts.

You illustrate the total general misunderstanding about oversized anchors. The recommended size is nowhere near buried at 30 knots and not a lot further in at 50 knots. Going to a bigger size achieves nothing. More info on my website Oversize anchors – necessary?
[/QUOTE]
I

 

[[2574]]

You illustrate the total general misunderstanding about oversized anchors. The recommended size is nowhere near buried at 30 knots and not a lot further in at 50 knots. Going to a bigger size achieves nothing. More info on my website Oversize anchors – necessary?

I disagree, I think you don’t understand that a properly sized anchor will be achieving secure seabed penetration by 30kts, see the evidence on Steve Goodwin’s testing videos. I’m not a believer in over-sizing, I have a 25kg Vulcan on my 14t monohull - the recommended sizing from Rocna.
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[noelex]

25 knots of wind should not be a difficult test for an anchor. The anchor should have the capacity to dive much deeper to increase to increase in holding capacity should the wind pick up to say 50+ knots.

 

[Neeves]

I conducted some rode tension tests - the results are published in Practical Sailor. I did not use a snubber, bridle, I wanted the extreme. I did use a bridle - it read dyneema.

I was using a 15kg Excel anchor which, for the sake of argument, would hold 2,000kg in a good holding sand seabed.

At 35 knots and short scope I recorded snatch loads of 650kg (2t load cell). The snatch loads were like driving into a break wall. The snatch loads were sufficiently severe (and nothing I had ever experienced, before or since, that I terminated the test.

In terms of holding capacity - as has been mentioned - its an ideal figure - BUT that hold is more than you need as you will be scared witless long before the loads (snatch lots) reach even 50% of the holding capacity. Much is made of the concept that a bigger anchor is better in 'difficult' seabeds - if a smaller anchorage cannot set - then neither will a big one (in fact a smaller anchor might set better, more deeply).

The snatch loads I was experiencing demanded a change - if you were at anchor you would want to move (though this might not be possible). If there was no-one aft of you, nor nasty rocks - you could deploy more chain, you could also use an elastic snubber.

To answer Tranona - at least in part. If you are using all chain then there is no doubt that 8mm is better then 6mm - if you carry enough and can deploy more. If there are restrictions then downsizing chain (would make matters worse) but you could use a snubber, or bridle - the elasticity will replace any losses that develop for their being no catenary. The advantage of elasticity is that it offers respite - until it fails. Catenary has a finite usefulness, as already mentioned.

The limits of downsizing are the ability to choose and engineer a sensible snubber system (so as you replace catenary with elasticity) and sourcing chain with a WLL that meets the loads developed by your windage. We have had no issues with 6mm chain on a cat with the windage of a Bav 45 (but we use a snubber/bridle - that we can extend if the chips are down).


I don't believe yachts drag because their anchor has an inadequate holding capacity (as defined by a straight line pull). I find yawing and horsing (the yacht moving up and down - and the tension on the anchor thus moving up and down) are the major causes - if the rode is moving from side to side and up and down the shear strength of the seabed immediately adjacent (or the sand in which the anchor is set) will be reduced. A deeper set anchor (think small) will be less impacted as the anchor will be more deeply buried. Test results on yawing have been covered by Practical Sailor. None of this is improved with catenary but is minimised with a snubber which reduces peak loads (my snubber idea are also defined in Practical Sailor).

Jonathan

 

[lustyd]

Not with rock underneath though. Anchors find it fiendishly difficult to go deeper in rock.

 

[vyv_cox]

Not with rock underneath though. Anchors find it fiendishly difficult to go deeper in rock.

Whether undersized or oversized.

 

[Neeves]

Whether undersized or oversized.

Surely this is wrong - I have frequently read here, on these august 'pages' from recognised experts that a bigger anchor is what is needed in 'difficult' seabeds - and if rock is not difficult - what is?

Another 'Boat Unit' of currency wasted.

Jonathan

PS - if you deploy your anchor and allow wind, maybe a bit engine, to straighten the all chain rode - you can feel the anchor grumbling over the rocks if you simply rest your fingers on the rode (it transmits the vibrations quite effectively). You quickly learn to detect different seabeds.

J

 

[geem]

A bit of first hand experience. It's been gusting hard all night. It wasn't and still isn't forecast. Peak gusts over 30kts in squalls. 30kg Spade set on a slight down slope ( not ideal). 4:1 scope with a 9 metre snubber in 7.5m of water so 30m of chain deployed. Holding is sticky sand and seagrass. We have our large suncover up over the boom so a ton of windage. 44 ft ketch 19 t. No snatching. No dragging. Fetch of 0.3 nm to the reef.
10mm chain G43. The only stuff I could buy here last time I replaced it. All good

 

[vyv_cox]

Surely this is wrong - I have frequently read here, on these august 'pages' from recognised experts that a bigger anchor is what is needed in 'difficult' seabeds - and if rock is not difficult - what is?

Another 'Boat Unit' of currency wasted.

Jonathan

PS - if you deploy your anchor and allow wind, maybe a bit engine, to straighten the all chain rode - you can feel the anchor grumbling over the rocks if you simply rest your fingers on the rode (it transmits the vibrations quite effectively). You quickly learn to detect different seabeds.

J

The only time I anchored on rock, rather than rocks or pebbles, was in Mallorca. We stopped for lunch intending to stay overnight but the chain and the anchored sensation just felt wrong right away. I dived and found we were on a rock shelf with the anchor tip hooked over a small wrinkle in the almost flat seabed. It withstood our normal reversed pull but would have come off in a wind shift.

We moved on.

 

[noelex]

The substrate where the Ultra was dropped in post #64 was a sand layer over smooth rock. This is a not uncommon "difficult" seabed. No anchor can penetrate the rock so the holding is usually dependent on anchor gaining enough grip in the sand layer.

A larger anchor, with its larger fluke area will gain more grip on the sand layer than a smaller anchor of the same design.

In most types of “difficult” substrates such as thick weed, very soft mud etc etc a larger anchor is of considerable benefit. The only exception is rock.

 

[geem]

The substrate where the Ultra was dropped in post #64 was a sand layer over smooth rock. This is a not uncommon "difficult" seabed. No anchor can penetrate the rock so the holding is usually dependent on anchor gaining enough grip in the sand layer.

A larger anchor, with its larger fluke area will gain more grip on the sand layer than a smaller anchor of the same design.

In most types of “difficult” substrates such as thick weed, very soft mud etc etc a larger anchor is of considerable benefit. The only exception is rock.

The difference in hold in that situation would be very marginal. Neither anchor sets when it can't penetrate the seabed. It's really a poor argument for a larger anchor. You may argue that the smaller anchor may set in the shallower sand layer above the rock as the anchor is actually physically smaller.

 

[Neeves]

The substrate where the Ultra was dropped in post #64 was a sand layer over smooth rock. This is a not uncommon "difficult" seabed. No anchor can penetrate the rock so the holding is usually dependent on anchor gaining enough grip in the sand layer.

A larger anchor, with its larger fluke area will gain more grip on the sand layer than a smaller anchor of the same design.

In most types of “difficult” substrates such as thick weed, very soft mud etc etc a larger anchor is of considerable benefit. The only exception is rock.

Very soft mud, soupy mud, mud that is like old engine oil is hardly difficult - use a Fortress or the Lewmar Wanabe. Just read the Fortress Chesapeake tests for confirmation. The only difficulty is washing the mud off

I'm also not so sure of your suggestion that a large anchor is of considerable benefit in thick weed. The most common suggestion I see repeated for thick weed is a Fisherman or Admiralty Pattern - which is characterised by its small fluke.

Your suggestion and the common suggestion seem to be contradictory.

My suggestion for thick weed is - go some place else where you will not disturb the marine ecosystem that depends on the weed. Most weedy bays and shelter are well documented - there is little excuse for even trying to anchor. The exception would be weed with obvious sand patches, which are easy to see,

Maybe there are other 'difficult' seabeds where a large anchor might be better than one of the recommended size - but you are struggling with your arguments so far.

Jonathan

 

[geem]

Very soft mud, soupy mud, mud that is like old engine oil is hardly difficult - use a Fortress or the Lewmar Wanabe. Just read the Fortress Chesapeake tests for confirmation. The only difficulty is washing the mud off

I'm also not so sure of your suggestion that a large anchor is of considerable benefit in thick weed. The most common suggestion I see repeated for thick weed is a Fisherman or Admiralty Pattern - which is characterised by its small fluke.

Your suggestion and the common suggestion seem to be contradictory.

My suggestion for thick weed is - go some place else where you will not disturb the marine ecosystem that depends on the weed. Most weedy bays and shelter are well documented - there is little excuse for even trying to anchor. The exception would be weed with obvious sand patches, which are easy to see,

Maybe there are other 'difficult' seabeds where a large anchor might be better than one of the recommended size - but you are struggling with your arguments so far.

Jonathan

Anchoring in sandy patches may be possible where you cruise. Here everywhere is seagrass. It's impossible, sometimes to find a sandy patch.
The spot where we are currently anchored is a 1.5nm stretch behind a reef. Boats accumulate at one tiny spot at one end as the rest of the bay is scattered with coral heads. When you have miles of seagrass you don't worry about anchoring in one patch of it knowing the rest is perfectly well protected.
With a bottom covered in seagrass, an anchor with a sharp toe, capable of penetrating the tough seagrass is very desirable. I don't think an oversized anchor would give us any advantage