Rocna Anchor Sizing Aids

[cfarrar]

We use a 25 Kg (55lb) Rocna on our Sabre 426 (42 ft & 25,000 lbs).

Summers in Maine mud, gravel and eel grass. Winter in the Bahamas, including some 45kt Christmas winds. We've never dragged, and we sleep well at night.

[ActiveCaptain]

Originally Posted by Pyxis156

It sounds like much of your anchoring is in great holding, but being in Maine I wonder have you had a chance to try your Rocna in poorer holding areas that have rock? I am curious about how it will work in those conditions.

I've used it from Maine through to where we are now - Vero Beach, FL (although we're on a mooring here - great holding!). We'll be continuing on to Key West and I suspect the results will be similar. I noticed that the Rocna came up easier (20 minutes) when anchored in Melbourne in mainly sand and crushed shell - at least that's what was all over the chain and anchor. The Chesapeake is known for soupy mud and difficult holding on the rivers. We anchored all around both sides for 2 months and never experienced anything other than an immediate set.

I haven't done any anchoring around hard bottoms and rock.

[ActiveCaptain]

Originally Posted by Jmolan

I have my chain set up, so that it ties the end of the chain to a frame in the locker, and the rope is long enough to come up to to the deck. I have it tied with a slip type knot so I do not even need a knife.

I have the same setup...but not the slip knot. That's a pretty good idea.

I've gone scuba diving for chain before. If you are moving while letting out the chain creating a long line of chain, it's quite simple to find it - a grappling hook would do it most of the time. Obviously a buoy would be nicer.

[Pyxis156]

Jmolan,

Given that according to the chart you are pretty close to the line for the 22 lb anchor and your trimaran has more windage than a monohull (the basis for these tables), you might want to consider moving up to the 33 lb anchor. The windage isn't hugely different (about 20% more than a monohull using info at Alain Fraysse's website here) but it may be enough to consider upsizing, especially since you list your cruising grounds as Mexico to Alaska. While adding 11 lbs to the anchor weight, it does bump you up fairly significantly in holding power (according to the chart at least). This is just my opinion, I am certainly no expert!

Steve

[noelex 77]

Originally Posted by Pyxis156

what am I missing?

I don't think your missing anything. If your boat and gear can handle it a larger anchor gives you great peace of mind, and some reserve for those horrible nights.
I have used a Rocna 55 (121Lb) on my 47 foot yacht for the last year and couldn’t be happier.

.

[Sharkman]

Another factor for going up a size from the manufacturers general advice is not just the weight, but the surface area once dug in - see how well the Fortress range perform for such light anchors.

We have a Rocna 33 as our main anchor with a 25 as a back up (we have other types, too), and the 33 is noticeably better in general, and copes with weed and very soft mud better. In the first case, the weight probably helps in terms of penetrating the weed, and in the latter it must be the larger surface are. Not very scientific, I know, but....

[Pyxis156]

Thanks for the replys noelex 77 and Sharkman, I appreciate the feedback. Both of your comments are exactly what I am basing my decision to upsize one size upon. My hope is that the larger surface area will help offset questionable anchorages and Sharkman's post certainly leads me to believe that will be the case.

Out of curiosity, does 13.4m and 10,300kg sound about right for your yacht Sharkman? And what is the approximate weight of your yacht noelex 77? I am doing an unscientific colation of responses from people using Rocna's trying to see where they fall in the overall chart.

[noelex 77]

Originally Posted by Pyxis156

And what is the approximate weight of your yacht noelex 77? I am doing an unscientific colation of responses from people using Rocna's trying to see where they fall in the overall chart.

14 t ( 31000 lb) design displacment. With fuel water and lots of crusing supplies a bit more maybe 18t (40000 lb)

[Sharkman]

Originally Posted by Pyxis156

Thanks for the replys noelex 77 and Sharkman, I appreciate the feedback. Both of your comments are exactly what I am basing my decision to upsize one size upon. My hope is that the larger surface area will help offset questionable anchorages and Sharkman's post certainly leads me to believe that will be the case.

Out of curiosity, does 13.4m and 10,300kg sound about right for your yacht Sharkman? And what is the approximate weight of your yacht noelex 77? I am doing an unscientific colation of responses from people using Rocna's trying to see where they fall in the overall chart.

Hi Pyxis

Our boat is around 13m - the extra bit is really the stem head fitting which is long. 10,300kg dry but probably around 12,000 in cruising trim (if we throw out this winters book collection!).

And another non-scientific thought - nobody ever congratulated themselves for picking the next size down when the wind started to get up.

Best wishes

[Jmolan]

Originally Posted by Pyxis156

Jmolan,

Given that according to the chart you are pretty close to the line for the 22 lb anchor and your trimaran has more windage than a monohull (the basis for these tables), you might want to consider moving up to the 33 lb anchor. The windage isn't hugely different (about 20% more than a monohull using info at Alain Fraysse's website here) but it may be enough to consider upsizing, especially since you list your cruising grounds as Mexico to Alaska. While adding 11 lbs to the anchor weight, it does bump you up fairly significantly in holding power (according to the chart at least). This is just my opinion, I am certainly no expert!

Steve

I have a friend with a 40' version of my boat. 12,000 Displacement. He has the 33 lb. with 300' of 1/4" chain. I was considering the same set up. The thing with our boats and anchor and chain they are narrow bows. We have to get the weight back. It can be done, just has to be buillt in. Thanks......oh yea....
Steve, I do not mean to mislead anyone on my cruising grounds. I fish in Alaska, and my time is split between there, Oregon and Sea of Cortez. My boat is in Mexico, and I have no intention of going north with it.
My anchor in Alaska is 2,000 lb and 7 shots of 1 1/4" stud link chain powered by a 60 gallon a min. hyd. pump.....

[barnakiel]

That's interesting most of the discussion goes on around weight and so little is said about the area. I believe once the hook digs in, the weight is less important than the area (and shape) of the flukes. Sure, weight counts, but in the chain, to get the catenary to pull the stock flat.

How important the area is do you think?

b.

[klem]

Originally Posted by barnakiel

That's interesting most of the discussion goes on around weight and so little is said about the area. I believe once the hook digs in, the weight is less important than the area (and shape) of the flukes. Sure, weight counts, but in the chain, to get the catenary to pull the stock flat.

How important the area is do you think?

b.

I think that you are exactly correct that area is king in holding provided that it is at the correct angle. The only time this isn't true is when anchoring on a bottom where no penetration is possible (ledge) when weight is king. The reason that we talk about weight is that anchors are advertised by weight rather than area but the heavier anchors do have more area for a given brand. Weight does play a role in helping the anchor to set but it is certainly not the only factor. The other thing that can matter in some bottoms is how deep into the bottom the area is since some bottoms have much better holding further down. Hands down the highest holding power for weight is a fortress so if holding power were the only factor we would all use one of them. The reason that this anchor holds so well is related to its very large area.

Catenary could be the subject of several threads but putting a lot of weight in your chain will not necessarily increase holding power for a small boat. As is pointed out in the links, when your anchors holding power is truly being tested, there is no catenary so the chain provides no real shock absorption. This is one of the reasons for using a snubber. The issue with involving line in your rode is that chafe becomes a big problem but it can make a good mooring line. On bigger vessels with very heavy chain, catenary does matter. In 9 years of working on a 92' schooner with 3/4" chain, I never saw that chain fully stretch out even in very strong winds.

One note on chain is that breaking strength needs to be looked at. Grade 30 chain has a shockingly low working load limit. Taking weight out of the chain is a great way to save weight but higher grade chain must be used.

[Pyxis156]

barnakiel, I agree and have seen several places that indicate that the ultimate holding power of an anchor isn't in its weight (though it can be estimated as a function of weight) but in the weight of the ground it displaces. This would suggest that when upsizing, the larger area of the anchor is doing more than the weight increase. While the weight of the anchor may help determine how fast the anchor initially digs in, it seems to me that the anchor's surface area will dictate how much force the anchor will ultimately resist.

With respect to your catenary comment, it is very interesting to read the information on Peter Smith's website regarding catenary and scope. It makes it pretty clear that on smaller boats chain weight isn't that big a factor and it doesn't take too much wind to straighten the chain. Ultimately it is the anchor's holding power (and scope) that will dictate success or failure in a blow.

[cfarrar]

Pyxis156,

This is in response to your question about anchoring in Maine with our Rocna. As context, we've used a Bruce and a CQR in the past, and we now use a 25 Kg Rocna on a 42 ft boat with 25K+ displacement.

We have anchored in rock many times in Down East Maine. The anchor tends to catch hard and fast - too fast! With chain road and a significant wind or a current you've got to be ready to pay out the rode as the boat sets back, and you need to get your snubber quickly so you don't hurt the windlass. In these cases, the anchor's chiseled point probably catches under rock or ledge. However, Practical Sailor did a piece last year on anchoring in fields of large glacier rock down in Chile, and they concluded that a large Bruce anchor was best in those conditions. I suppose the reality is that, depending on the size and shape of the rocks, and a variety of anchors would work in on a rocky bottom.

In Maine we also run into thick eel grass at times. At one spot in particular we used to have real trouble with our Bruce anchor. It just couldn't penetrate the grass, so we gave up on that spot. Since switching to the Rocna, however, we've had no problems there. To be fair to the Bruce, our Rocna is also a heavier size, so it's not a direct comparison. Probably any anchor with sufficient weight and a sharp point would do well on this bottom, since after you penetrate the weed there's good mud underneath.

You also asked about soft mud. We've done the ICW both ways and had no problem with Chesapeake Bay mud, including one night when everyone around us dragged. I suspect the fairly large surface area of the anchor is the key factor in soft mud. A Fortress might be optimum in these conditions, but I'm not going to choose my primary anchor based soft mud bottoms.

Finally, the Bahamas usually offer decent holding in sandy bottoms, but sometimes the sand and silt sits over a hard coral / limestone substrate. If the sand is scoured out, the holding can be tricky. There's no substitute in this case for choosing your anchorage well, and a quick dive on your anchor provides peace of mind. However, I can say that during the course of a last winter in the Bahamas we never dragged, while boats around us did drag on several occasions. I suspect that in these conditions our Rocna worked because of the concave shape and surface area - basically the anchor needs to make the most of the whatever loose, thin sand is available.

Conclusion: We wanted a primary anchor that would perform in the range of conditions that we experience. For us, the newer generation anchor design (ie, Rocna, Spade, Manson Supreme) seemed promising. Why has our Rocna worked well? It's hard to draw conclusions - the weight doesn't hurt. The chiseled point probably helps, too. For the most part, however, I think that the concave shape and surface area is the key. I am reminded of this every time I bring up 20 pounds of mud and sand when weighing anchor. Perhaps that's the only real downside of this anchor. I've learned to always have my boat hook and washdown hose ready when hoisting, so I can deal with copious quantities of sea bottom that the anchor brings up.

[Pyxis156]

Cfarrar, thanks so much for the detailed feedback, it is very helpful. In our cruising so far, we have found all the conditions you describe except for the sand, and it is reassuring to know that the anchor performs well in all of them. On our friends Catalina 42 he uses a Bruce and we have found the same issue with eelgrass. There have been a few times where we ultimately moved to another anchorage because we couldn't get the anchor to bite. I am now looking forward to going to a few of those places and trying out the Rocna, especially since those anchorages tend to be less crowded because of the poorer holding.

I appreciate you taking the time to respond and sharing your experience in detail.