Rocna Anchor Sizing Aids

[impi]

Originally Posted by noelex 77

If the wind picks up it will set deeper. If you are concerned set it a bit harder, we increase the revs in reverse and end with full reverse for at least 15 seconds.
Bigger will always hold better. It is also a big help in penetrating weed. The weed does not change size. A weed root that is a significant obstruction to a 5 lb anchor is less of a challenge to 100lb anchor.

Yeah, actually you are 100% correct in saying that ... Ana is not always thrilled when I use high revs in reverse so I have been going a bit 'soft' on that of late ... thanks for reminding me

[DumnMad]

Hi folks, We are very new to boating and greatly appreciate all the experience being expressed here and can see there's no simple answer to an anchoring question.
I have designed many anchors for wharves and bollards etc. For a buried anchor increasing the surface area will almost proportionally increase holding power but double the depth of embedment and you will almost certainly have more than 4 times the holding power. Therefore, once the anchor is set it is more important to have burying ability than area.

[bobconnie]

Finely!! somebody got it right !! DumnMad Thank you Ive been trying to say just what you said, but could not say it right !! Depth is much more important no matter what type of anchor is used !! I thought that was the reason for the spade style anchors !! so really getting to big an anchor would be not be the way to go right ???

[noelex 77]

Originally Posted by bobconnie

so really getting to big an anchor would be not be the way to go right ???

A big anchor will set as deeply as a small anchor of the same design given enough force. In fact, the advanges of scale means it will generally set deeper.

However anchors develop an equilibrium with the forces applied, they will burry deeper untill the holding power matches to force applied then stop.

For example if we set a big and small anchor of the same design then apply more force because the wind has increased, both anchors will burry deeper, but the smaller one will be deeper than the large one because it needs to do that to match the force that is applied. If the wind builds further , both will burry deeper again, but a limit is reached where the anchor can no longer burry further. If the force from the boat is greater than the resistance of the blade area, the anchor will start to pull through the substrate and drag.this will happen to the smaller anchor before the larger anchor. The larger anchor will not have reached its burring limit.

This is a simplified explanation, but every crusing sailor knows that a larger anchor will always hold better than a smaller anchor of the same design. It would be a shame if we confused such a simple law.

[DumnMad]

Agreed Noelex, bigger is better if comparing the same design, thats why manufacturers rate larger ones for larger boats. However, ground conditions being equal, one type of anchor with more weight and larger area will not necessarily hold as well as another which is better designed and digs deeper. Don't want anyone to feel secure just because they've got big. I guess that's what you mean Bobconnie

[GordMay]

Greetings and welcome aboard the CF, DumnMad.

Thanks for that interesting information, simply, yet clearly stated.

[noelex 77]

Originally Posted by DumnMad

Agreed Noelex, bigger is better if comparing the same design, thats why manufacturers rate larger ones for larger boats. However, ground conditions being equal, one type of anchor with more weight and larger area will not necessarily hold as well as another which is better designed and digs deeper. Don't want anyone to feel secure just because they've got big. I guess that's what you mean Bobconnie

Yes agreed
Anchor designers need to trade off surface area and burying depth.

Part of the problem is that when the word "big" is used it can mean more weight or more sur,face area.
More weight is always good for holding. More surface area may or may not be good. If the designer has got his sums right, a design with more surface will be a plus in very soft substrates.

[Kettlewell]

More surface area may or may not be good. If the designer has got his sums right, a design with more surface will be a plus in very soft substrates.

Playing a bit of the Devil's advocate here, that isn't strictly true. There is a type of mud I have found in the Chesapeake with the consistency of Jello and you need a compact dense anchor to penetrate down to where there is mud with decent holding. This Jello layer might be 10 feet or more deep and you won't really get decent holding until you get below it. Something with a lot of surface area, like a Danforth or a Fortress, will just float on the surface of this stuff or at least it doesn't penetrate down quickly enough to be of use. You need something small and dense to scythe its way down to better holding.

On the opposite end of things, in bottoms that are extremely hard, like are found down in the Caribbean, particularly along the Mexican coast, the sharpness of the point with a concentration of weight on that point is what you need. Surface area is almost irrelevant because you just won't get that much penetration. I have hung for days in some of these anchorages, with stiff winds blowing, with nothing but the first inch or two of the point of the anchor hooked in. Something like an old-fashioned heavy fisherman anchor would be ideal. My Bulwagga, with two sharp points, hung in there in some bottoms where almost everyone else was dragging.

My point being that there is more to an anchor's performance in certain bottoms than weight and surface area.

[noelex 77]

Originally Posted by Kettlewell

Playing a bit of the Devil's advocate here, that isn't strictly true. There is a type of mud I have found in the Chesapeake with the consistency of Jello and you need a compact dense anchor to penetrate down to where there is mud with decent holding. This Jello layer might be 10 feet or more deep and you won't really get decent holding until you get below it. Something with a lot of surface area, like a Danforth or a Fortress, will just float on the surface of this stuff or at least it doesn't penetrate down quickly enough to be of use. You need something small and dense to scythe its way down to better holding.

There are some bottoms like this, but in general if the substrate is soft the tip weight and design is less important, and surface area is most important.

In hard or weedy anchorages a good design really earns it keep. If you dive and observe anchors when the bottom is hard you will very rarely see CQR set well.

If you only cruse a local area you can select an anchor that works well only for the these conditions, but for a long distance crusing boat an anchor that works it all conditions is much better. Fortunately the modern designs are versatile enough that they will cope with different bottom conditions. If you go slightly oversize there is reserve to cope with strong winds in all substrates.

Changing anchors to suit the bottom conditions, while crusing, is fortunately a thing of the past except in very exceptional circumstances.

[NorthPacific]

So some great advice and reasoning and first hand experience. So I decided to buy a ROcna 20 for my DUfour 35 (1984) boat.

Searching for a supplier in BC find a advisory from West Marine that there is a problem with the stock of these anchors. (China Manufactured)?

So what do you think.

Steve

[sabray]

Buy a Manson

[Steve Pope]

manson supreme, as good, maybe better? made in New Zealand, known quantity. cheaper.

[NorthPacific]

GRRR.......

It was much simpler with my Wayfarer!

OK I'll look at Manson.

Cheers

[NorthPacific]

From a search on Manson got a lot of concerns regarding construction and strength? Even on their website several of the tests have them behind the Rocna? Guess nothing is easy.

[virginia boy]

I was happy with my Rocna but chose to trade it in for a Manson. I haven't used it yet, but the crappy welding concerns me.