Which anchor set have you tried,,, tandem or V formation

[Storyline]

When I bought my Rocna, quite a few years ago, Craig Smith told me that the size advised for my boat was intended to be sufficient for any conceivable conditions, so that was what I bought. I cannot say I have tested it to the ultimate but it has never given me any problems, provided the bottom was reasonable. Conversely, a couple of seasons ago we were with a boat several feet shorter than ours with a new Rocna a size bigger than ours. He experienced considerable difficulty in getting it to set, despite using max revs astern. It seemed to me that the anchor was too big for the available pull, at least in that bottom. Ours set first time.

I enjoy reading posts like this. It is human nature (mine, anyway) to always wonder if when you see a larger anchor than yours on another boat of a similar size if yours is big enough. In our case it is a neighbours 25kg Rocna vs our 20kg. It got me looking at the Rocna site when I saw it to check i had the right size ! Same thing happened with the chain , should I have gone for 10mm then read that weight isn't everything. Keep them coming !

 

[GHA]

Conversely, a couple of seasons ago we were with a boat several feet shorter than ours with a new Rocna a size bigger than ours. He experienced considerable difficulty in getting it to set, despite using max revs astern. It seemed to me that the anchor was too big for the available pull, at least in that bottom. Ours set first time.

This has popped up on other forums I think, so maybe something in it. But I can't get my head around it, if the anchor isn't setting it's getting pulled across the bottom, how would it know how much power the boat pulling the chain has? All it knows is how fast the chain is being pulled.

 

[Neeves]

This has popped up on other forums I think, so maybe something in it. But I can't get my head around it, if the anchor isn't setting it's getting pulled across the bottom, how would it know how much power the boat pulling the chain has? All it knows is how fast the chain is being pulled.

We tend to treat anchors like dumb bits of steel, anchors are much more clever than we think no wonder they get huffy.


If you have a little sharp chisel and hit it into a piece of wood, it will penetrate, if you hit a large sharp chisel and hit it with the same force and into the same piece of wood the large chisel will penetrate less. Quickest analogy I could think of.

The yacht is limited to it ability to set an anchor by its engine size, and the need to use it all (if necessary). Its not that the big anchor did not penetrate - it obviously did not penetrate sufficiently.

Jonathan

 

[GHA]

We tend to treat anchors like dumb bits of steel, anchors are much more clever than we think no wonder they get huffy.


If you have a little sharp chisel and hit it into a piece of wood, it will penetrate, if you hit a large sharp chisel and hit it with the same force and into the same piece of wood the large chisel will penetrate less. Quickest analogy I could think of.

The yacht is limited to it ability to set an anchor by its engine size, and the need to use it all (if necessary). Its not that the big anchor did not penetrate - it obviously did not penetrate sufficiently.

Jonathan

Still doesn't make sense. If the anchor in vyv's example penetrated then the boat would stop and you would think of the anchor as set. Even if not a huge amount. From his post I took it that the anchor wouldn't set at all, ie it hadn't penetrated at all.

If it's down to engine power then should boats with small engines have small anchors as well?

 

[Storyline]

I cannot see that one or two anchor sizes up ie 5 or 10kg would make much difference to the available power of the engine. If the engine could not cope with that it would have to be tiny. If it is because of the larger surface area of the anchor then it would be also hard to set one with a large surface area like a Fortress. If a larger anchor is harder to set there must be something else going on.

 

[vyv_cox]

I don't know the answer and could only hazard a guess as to the cause. I only know that we accompanied a 32 ft boat with a 20 kg Rocna as he tried to anchor. He made numerous attempts, with the correct scope as far as I could see, both at low speed and higher in astern. The anchor did not penetrate the bottom, as viewed with a glass-bottomed bucket. He got it to set eventually after half an hour or more of trying. The bottom was a very typical Mediterranean one of sandy mud with a small amount of weed.

 

[GHA]

I don't know the answer and could only hazard a guess as to the cause. I only know that we accompanied a 32 ft boat with a 20 kg Rocna as he tried to anchor. He made numerous attempts, with the correct scope as far as I could see, both at low speed and higher in astern. The anchor did not penetrate the bottom, as viewed with a glass-bottomed bucket. He got it to set eventually after half an hour or more of trying. The bottom was a very typical Mediterranean one of sandy mud with a small amount of weed.

Very odd. Round the Atlantic on a 33' steel boat (an a tired 20hp engine) with a 20kg Rocna it dug in straight away nearly every time, once in the azores and again in the scillies it took about 15' before biting otherwise never missed a beat.

Still can't see what difference engine size would have to getting it to initially dig in, But it's a strange universe, maybe there is something going on.

 

[PhilipH]

Very odd. Round the Atlantic on a 33' steel boat (an a tired 20hp engine) with a 20kg Rocna it dug in straight away nearly every time, once in the azores and again in the scillies it took about 15' before biting otherwise never missed a beat.

Still can't see what difference engine size would have to getting it to initially dig in, But it's a strange universe, maybe there is something going on.

+1 . . . er .... what about setting an anchor under sail ... engine size seem like a red herring

 

[GrahamM376]

The yacht is limited to it ability to set an anchor by its engine size, and the need to use it all (if necessary). Its not that the big anchor did not penetrate - it obviously did not penetrate sufficiently. Jonathan

I also don't understand what bearing the engine has. Presumably as wind speed/load increases, the anchor will dig in further. Also, when boats swing with tide, anchors will/should re-set without any engine assistance.

 

[Neeves]

You could set an anchor by throwing it over the side and retiring for that welcome and refreshing drink.

An alternative would be to lower the anchor deploy, say, a 3:1 scope, back the engine up, gently, until it 'bites' and then increase engine revs. The higher the revs the deeper the anchor will set. Or to put it another way the higher the load the deeper it will set, the load could be engine or wind. But producing the load with the engine does give some indication that the anchor has 'bitten' and it is well set. Once you know the anchor is well set the scope required for the future weather can be deployed, snubber attached and the refreshing drink consumed as the casserole cooks gently in the oven You do need to consider Vyv's experience with the ball of fishnetting caught in the fluke that did not become apparent for 12 hours

Conjecture:

Some people consider that anchor performance has a strong relationship to anchor surface area (and not ignoring design). The larger the surface area the greater the 'holding capacity'. But to enjoy this 'holding capacity' of the large surface area anchor you need to be able to impose the larger load, so a 10 Hp engine generating, say, 100kg of load is not going to make much impact on a large anchor with a large surface area but that same anchor might appear as a toy with a yacht with a 125 Hp engine.

The problem might arise in a hard seabed where the anchor bites but there is simply insufficient power in the absence of wind and with only a small engine to drive the anchor sufficiently deeply to produce any hold and the anchor simply scrapes across the top surface.

There is the other extreme: all anchors have a finite diving depth, determined by seabed type, size of chain - a number of factors. If an anchor is too small it will achieve the finite depth quickly and because it is too shallow (and the load too big) then the anchor simply drags through the seabed (usually until it collects some debris - it then surfaces and drags, unless it drops the debris).

There are many respected cruising yachtsmen who swear by the use of modern anchors much bigger (in terms of by weight 50% bigger - which with modern anchors is a proxy for surface area) than the sizes that would be normally recommended. Consequently there is considerable 'weight' behind the concept.

Contrarily the evidence that Vyv provides, of the smaller yacht with the proportionally larger Rocna than his, might suggest there are other factors that might need considered. But it is difficult, in the absence of more examples, to 'know'.

But its an interesting area - but one that might need treated with some caution?



Jonathan

 

[GHA]

You could set an anchor by throwing it over the side and retiring for that welcome and refreshing drink.

An alternative would be to lower the anchor deploy, say, a 3:1 scope, back the engine up, gently, until it 'bites' and then increase engine revs. The higher the revs the deeper the anchor will set. Or to put it another way the higher the load the deeper it will set, the load could be engine or wind. But producing the load with the engine does give some indication that the anchor has 'bitten' and it is well set. Once you know the anchor is well set the scope required for the future weather can be deployed, snubber attached and the refreshing drink consumed as the casserole cooks gently in the oven:

So from that logic each anchor size should have an engine size recommended also and anyone anchoring under sail should be worried?
Come on, you're grasping at straws here to back up you're viewpoint.
I cruised for years with an ailing undersized engine, for the final set have a some of slack then a run up, when the slack runs out and the boat stops quickly with the chain bar tight you know the anchor's dug in. Then open a cold one

The problem might arise in a hard seabed where the anchor bites but there is simply insufficient power in the absence of wind and with only a small engine to drive the anchor sufficiently deeply to produce any hold and the anchor simply scrapes across the top surface.

?? So if someone backs up with a quarter revs their anchor will just drag across the surface but on full revs it will dig in? Try it and post a youtube if you can make this happen. Sounds completely implausible.

 

[Neeves]

GHA, I'm terribly sorry to have upset you, please accept my abject apologies.

I obviously did not explain too clearly, if the anchor is 'the right size', like that small chisel I tried to use as an illustration - it has a chance to dig into a hard seabed. The bigger anchor, with its thicker sections, and like that bigger chisel needs more oomph to get it to penetrate and set.

But I obviously have no idea and I am doubly sure you are correct.

Jonathan

 

[GHA]

GHA, I'm terribly sorry to have upset you, please accept my abject apologies.

I obviously did not explain too clearly, if the anchor is 'the right size', like that small chisel I tried to use as an illustration - it has a chance to dig into a hard seabed. The bigger anchor, with its thicker sections, and like that bigger chisel needs more oomph to get it to penetrate and set.

But I obviously have no idea and I am doubly sure you are correct.

Jonathan

Not upset at all, as the smilies in the post indicated. I'm asking questions. The chisel analogy doesn't really stand up either I'm afraid it's still holding.
I just can't see and real world examples of your "if you have a bigger anchor you need a bigger engine" tack.
There might be something in a smaller engine not getting an anchor to bite in the first place but it seems doubtful, if you have any evidence to back this claim up it would be very interesting to all on here I'm sure.

And you really don't need to be sarcastic, it just ends up in a shouting match.

 

[Storyline]

I wonder if the boat that could not set it's oversized anchor also had oversized chain. If so, maybe the engine could not lift that. Shooting that reasoning down, it may be that even the smallest engine could lift large chain, I can never remember the amount of force required to lift a given weight of chain.

 

[vyv_cox]

I wonder if the boat that could not set it's oversized anchor also had oversized chain. If so, maybe the engine could not lift that. Shooting that reasoning down, it may be that even the smallest engine could lift large chain, I can never remember the amount of force required to lift a given weight of chain.

I don't remember but I suspect it was 8 mm, which is probably about right. Extending the small engine/large anchor scenario I would have thought that heavier chain might assist the anchor to set, as the pull might be more horizontal.

 

[NormanS]

I don't remember but I suspect it was 8 mm, which is probably about right. Extending the small engine/large anchor scenario I would have thought that heavier chain might assist the anchor to set, as the pull might be more horizontal.

Oh no! Neeves has a theory against that

 

[Neeves]

Oh no! Neeves has a theory against that

I wish I knew what it was

Jonathan

 

[Neeves]

Vyv,

I am sure its posted but I have forgotten, what sizes are your yacht and anchor and the size of the yacht and anchor (of the anchor that was difficult to set).

Jonathan

 

[vyv_cox]

Vyv,

I am sure its posted but I have forgotten, what sizes are your yacht and anchor and the size of the yacht and anchor (of the anchor that was difficult to set).

Jonathan

We are a Sadler 34, actually almost 35 ft in length. 8 mm chain, 15 kg Rocna (or is it 16?)
The other boat was 32 ft, 8 mm chain I think, 20 kg Rocna.

 

[Neeves]

My memory is obviously at fault, I had thought we were dealing with bigger yachts and bigger anchors.

To me a 15kg Rocna and a 20kg Rocna are not significantly different, I had imagined we were talking of the 40kg Rocna and next size up a 55kg model.

Why a 15kg Rocna set easily and the 20kg model almost impossible to set can only be ascribed to the superior skill of the owner of the 15kg Rocna and/or extreme bad luck in a specific area of seabed by the owner of the 20kg model.

I am guessing if the toe of the 20kg model was bent (previous retrieval from rock), that would impact performance, but you would have mentioned same. After multiple tries a jammed shackle, allowing a sideways pull, would have been noticed and corrected. Chain twist, same.

Another of life's great mysteries.

Thanks Vyv,

Jonathan