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Old 21-01-2010, 12:25   #1
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Rocna Anchor Sizing Aids

After months of absorbing information from this site as we prepare to outfit our new boat (that should arrive in a few months ), I think it is time for me to give back a little. One big question I have had is regarding anchor selection, and based on the number of posts (and opinions) in this Anchoring & Mooring forum, I think this is fairly common. Being a visual kind of person, I struggled a bit with the sizing tables that manufacturers make for their anchors as they seemed a bit too cut and dry and I felt like a few questions were not answered well like "how big a jump is it really if I went with the next size up?"

As I have pretty much narrowed down my quest for an anchor to a Rocna, I created a more visual representation of the sizing chart on the Rocna website that I hope may help others. To use it, simply draw a line up from your boat length and across from your boat weight and where they intersect is where you boat falls into the sizing scheme of things. You can then make your own call on whether or not upsizing might make sense (i.e., are you close to a line or a fair ways below it? Does your boat have a lot of freeboard or sail at anchor, tending to increase loads?). Also note that I have left off Rocna's three largest anchors since including them made it difficult to interpret things at the lower / mid range I was most interested in.



I also found a lot of information regarding anchor size or weight that, while extremely helpful, didn't really apply to someone with a 37' / 20,000 lb. boat. There were several testimonials from people using 100+ lb. anchors on a boat of size X that held them for a day of 50+ knot winds, but how would that scale to my situation where there is no way I am putting a 100+ lb anchor on the bow? Especially with respect to a Rocna that supposedly has more holding power per lb. than a more traditional anchor? (And note the "supposedly", I am not saying this is necessarily the case so please don't fixate on that one statement!) After researching this, I have come to a personal conclusion that holding power isn't a linear phenomenon of anchor size and someone's experience with an anchor that weighs 150+ lbs couldn't really map to a West Marine test that looked at 30 lb anchors (hence the debate over why a Bruce seems rock solid over 100+ lbs but seems to have problems setting and holding in the 25-75 lb range).

Luckily I found the following information in a somewhat recent post at Steve Dashew's blog:


"At the risk of repeating ourselves (this topic is covered in great detail in our books), and using one of the new generation highly efficient hooks like the Rocna, Spade, or Bruegel, we would carry the following:
  • Sundeer 56/60 – 110lb/50kg
  • Sundeer 64/68 – 176lb/80kg
  • Beowulf 80 – 240lb/110kg
  • For yachts in the 40 to 50 foot range – 80lb/38kg
  • For yachts in the 30 to 40 foot range – 60lb/28kg
This assumes a chain rode."

SO, weeks of my agonizing over tons of information only to find things neatly summarized by a respected expert in the cruising field, for the anchor I am interested in at least. It is nice, however, to get some independent confirmation of my final decision for an anchor for our particular boat (prior to reading the Dashew's opinion, I had decided on the 55 lb Rocna even though according to their sizing table the 44 lb should work).

One last comment, the spreadsheets and discussion about anchor rodes on Alain Fraysse's website are a fantastic resource. The guy has written a doctoral dissertation on rodes and in my opinion deserves some sort of award for sharing all that hard work for (as far as I can tell) no personal benefit.

I hope this helps someone else...

Steve
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Old 21-01-2010, 15:50   #2
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Pyxis156,

Thank you for doing that table, I think that it is a helpful way to look at it. Hopefully you will be happy with new anchor.

I have actually found Peter Smith's website to have some very good information on it if you look past the arguments for different brands. It is PeterSmith.net.nz : anchors & anchoring, “Kiwi Roa”, photo journals from Patagonia & Antarctica He explains some really nice simulation work that he had done for him regarding storm conditions for small boats (that is the key, small boats are very different from large vessels at anchor). It leads to conclusions which might be surprising for many at first regarding rode type, scope, and kellets but are good conclusions from an engineering standpoint. Steve Dashew has done a good job of confirming the conclusions found in his analysis.
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Old 21-01-2010, 16:16   #3
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Charts and graphs make me dizzy......
I always figured that if the boat moves alot with the rock I've tyed to the end of the rope, I just get a bigger rock.....
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Old 21-01-2010, 18:34   #4
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Originally Posted by Pyxis156 View Post
SO, weeks of my agonizing over tons of information only to find things neatly summarized by a respected expert in the cruising field, for the anchor I am interested in at least. It is nice, however, to get some independent confirmation of my final decision for an anchor for our particular boat (prior to reading the Dashew's opinion, I had decided on the 55 lb Rocna even though according to their sizing table the 44 lb should work).
It's so funny for me to read this. I went through the exact same thing this Summer. I had decided on the Rocna. The charts suggested that the 88 lb one was appropriate. I called the Rocna rep in Canada who distributes them to North America and he said the 88 lb one was the right one. I couldn't decide between that and the 121 lb model. I finally decided on the 121 lb and I'm really glad that I did.

We've used it now since August - and we anchor a lot - and it has been outstanding. It takes more to haul it in although my windlass can handle it. The only problem that I've had is that it takes between 20 and 40 minutes to retrieve it because after a night on the hook, it sticks in mud so hard at 1:1 scope (directly vertical) and takes time to bring in a couple of links, tie it off, let the boat work on it, and repeat. I would much, much rather spend that time in the morning and sleep well at night.

So my advice - if your windlass can handle it, go with the larger one you're thinking about. You'll sleep better.
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Old 21-01-2010, 21:54   #5
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20 to 44 min to retrive it??!!
Wow that would be a long time to wait for the darn thing to get aboard. My advice would be to get a bigger windlass. What if you have high winds, or current and you need get out of there fast ?

Now I believe as you do that being able to sleep well at night knowing the anchor is doing its job is nice. But in a real blow, I don't think I would /could sleep well being tied up to a jetty or mooring, let alone a anchor that can lose its hold. And any anchor can.
I have a lot of faith in Steve Dashew, but for the most part a day to day anchor I believe doesn't need to be that big for most of us. So for my 40 foot boat, I would like to have the 80 lbs anchor for high stress anchoring, but most days/nights a 50-60 lb would do fine.
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Old 21-01-2010, 23:10   #6
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I have a 37 foot 7 ton alloy boat and have a 20 kg Rocna it holds better than any other anchor I have ever used. One of the problems is that it grabs so quickly that if i back up to fast it strips the chain over the top of the gypsy. The beauty of the Rocna and the Manson Supreme is that they can be a lot lighter than the CQR type anchor that was the industry standard previously. the old weight method doesn't really apply any more, the surface area of the base plate is now the important factor. I have only dragged the rocna once and that was when the bottom was the consistancy of muddy soup. The only boats that didn't drag there were all on moorings. I would recommend either of the above anchors price being the only difference, they are equal in every other way.
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Old 22-01-2010, 04:08   #7
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Also, catenary calculating tool here...
Cable Sag Error (Catenary Curve Effect) Calculator
I just put in 250Kg load with stright line distance of 97m (results in a cable length of 100.2m, anchor being at the bottom of the catinary so length of chain would be 50m), 2.33Kg/m (10mm chain). Comes out with 10.93m sag. So according to the maths using 10mm chain, the chain will still be horizontal on the sea floor at the anchor up to 250Kg load down the chain when anchored in 10.9m water. Which is the 5:1 most of us seem to use.
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Old 22-01-2010, 04:21   #8
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20 to 44 min to retrive it??!!
Wow that would be a long time to wait for the darn thing to get aboard. My advice would be to get a bigger windlass. What if you have high winds, or current and you need get out of there fast ?
20 minutes is a long time - true. But I'm smiling the whole time.

The windless is plenty strong to pull the chain and anchor up. If my 40 ton boat can't unstick the anchor at the end, there's no windlass that could do it. The bow literally drops as you pull in the few links and wait for the boat's buoyancy to lift the anchor a little - takes a couple of minutes for the bow to rise. I think it digs deep in mud. I have found that backing up at 1:1 scope helps a lot if there's room. Going forward doesn't help at all.

If I'm ever in a super hurry, I've done something dumb. In most cases, I just stay put. In an emergency, I'd cut the chain.
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Old 22-01-2010, 08:15   #9
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Thanks for the comments klem. Peter Smith's site has some very interesting information, especially the page on Catenary & Scope that you mention. Buried in it at the begining of the simulations is the following statement:

"To illustrate the true effect of catenary, computer simulations courtesy of Alain Fraysse will be employed."

This is the individual I mention at the bottom of my post and his spreadsheets that will allow you to make your own simulations are available to all from his website (link in my original post). Alain's spreadsheets and Peter's interpretations of the results are what guided my decision to go with an mixed rode of 80' of 3/8" chain and then 400' of 5/8" 6-strand nylon braid (from Samson). This will save me considerable weight over having an all-chain rode here in the Pacific Northwest where anchoring in deep water happens fairly frequently.
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Old 22-01-2010, 08:21   #10
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ActiveCaptain, it is reassuring to hear that you are happy with your decisions! I did upgrade the windlass on our boat from the "stock" one that came with it and based on your comments suspect I will be glad to have done so. While there is a pretty huge difference between the 55 lb. anchor I have selected and your behemoth, one thing I have learned while cruising on our friend's Catalina 42 is that not having a working windlass (or insufficient windlass for the job) is a big PITA when it comes time to weigh anchor!

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.
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Old 22-01-2010, 08:38   #11
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I have the 25 kilo rocna on a 46' 30,000 lb displacement sloop and mostly anchor in mud and silt.

Holds great. It's never yet dragged or failed to set. It will be plenty of anchor for your boat.
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Old 22-01-2010, 09:00   #12
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Originally Posted by Pyxis156 View Post

For yachts in the 30 to 40 foot range – 60lb/28kg
A 60 lb Rocna seems overkill for a 30 foot boat. It may provide much needed peace of mind for a long range cruiser anchored 300 days a year in all weather conditions, but it would be a major inconvenience for a costal cruiser on a two-week sailing vacation who is anchoring daily in a good weather in familiar area. On a boat without a windlass, may I add.
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Old 22-01-2010, 09:32   #13
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Lost Horizons, I had a few people suggest that a 55 lb anchor would be overkill but I am curious as to what the major inconveniences you mention in your post would be? The 55 lb anchor is only 11 lbs heavier than the 44 lb anchor Rocna recommends for my boat so the weight difference is not really significant. However, having a larger anchor with more holding power allows me to more confidently use a mixed chain / nylon rode and that does save me significant weight over an all chain rode. This is especially true here in the Pacific Northwest where anchoring in deep (> 50') water isn't that uncommon and I plan on carrying 480' of rode. It also gives me more confidence if I have to short-scope in a tight or busy anchorage. So from a day-to-day standpoint I see quite a few pros but few cons.

Plus, given my own situation here, I do not want to carry a storm anchor as well as a day-to-day anchor (and that isn't to say I won't have a backup anchor, just not a monster sitting in the lazarette that may never get used). Why not just size your primary anchor accordingly to a realistic worst case scenario and use it every day? My coastal cruising will involve Alaska so having confidence in my anchor for the occasion(s) I may run into something nasty seems prudent. Given that I am talking about a 55-60 lb anchor, I don't see what the inconveniences are for a coastal cruiser... what am I missing?
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Old 22-01-2010, 10:08   #14
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I am curious as to what the major inconveniences you mention in your post would be?
From my perspective, it would be very inconvenient to manually lift a 60 pound anchor (plus some 50-60 pounds of chain).

Regardless, my comment was meant to express a surprise regarding a quoted advice for a 60 lb anchor on a 30 foot boat, which is a factor of two increase in weight (and even more so in price) over what I consider a proper weight for a good anchor. If your boat is such that a 44 lb anchor is properly sized for it, but you go for a 55 lb, I think it is quite reasonable.
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Old 22-01-2010, 10:11   #15
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If I'm ever in a super hurry, I've done something dumb. In most cases, I just stay put. In an emergency, I'd cut the chain.

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. Hopefully I have the presence of mind to attach a buoy (with a tag line) before I let er' go. And make sure the buoy can clear the bow roller and bow pulpit. Good things to think about way ahead of time.

Thanks for the Rocna sizing graph. My 34' Tri with 7k displacement comes in pretty light. 10 kg? 22 lb. I will have to think about that one.
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