Over-specifying a Rocna anchor on a cat

[Uplander]

There are disadvantages having extra weight on the bow especially for a monohull. A yacht will seesaw more and not go to windward as well in a chop. In our anchorages if in doubt use more chain although I appreciate it may be a problem in crowded anchorages.
I somehow wonder at the wisdom of leaving a yacht on an anchor for extended periods. Recipe for a disaster somewhere down the line.

[Bulawayo]

Sure boats have been lost due to a variety of reasons but remember mankind has been anchoring boats for thousands of years - generally safely. We have left our own boat several times when we have gone land travelling - up to five months. If you have decent anchoring arrangments and locations then there is no special reason for concern.

Originally Posted by Uplander

There are disadvantages having extra weight on the bow especially for a monohull. A yacht will seesaw more and not go to windward as well in a chop. In our anchorages if in doubt use more chain although I appreciate it may be a problem in crowded anchorages.
I somehow wonder at the wisdom of leaving a yacht on an anchor for extended periods. Recipe for a disaster somewhere down the line.

[UNCIVILIZED]

I'll field the primary question in a minute. But first I have a (common sense) question or three. And they're not meant to be intrusive, but to find out how you plan to protect your investment in such an expensive vessel.
- What's your Plan; A, B, & C for caretakers/Skippers for the boat in your absence?
- What are their qualifications?
- Do you have the legal paperwork setup for them to do whatever is needed with/to the boat when you're not around?
- Will they (& a few crew) onboard her full time (or close to it) to monitor her closely?
- Also, if you're already planning to be away for the boat for as much as a year, why not wait until after that period of time is over & done with, & Then get the boat?
- Why not put the boat on the hard somewhere, so that you needn't worry about her much at all? Boat from an anchoring perspective. And from one of her having her expensive gear stolen. Or even her taken?


That said, when it comes to anchor size, I can't see any down side to going with the bigger anchor, other than the extra cost when you buy it. Especially as it sounds as if you need to upsize your windlass as it is.
And if an extra 15kg is enough to make you wonder whether or not your windlass is big enough, then it isn't even big enough right now!

The other missing piece here on anchoring though. Is that if anchoring conditions are either dicey enough to make you think about that bigger sized Rocna. Or you'll be away from the boat for quite a while.
Then you should be thinking/planning towards how best to set 2 anchors; with one another. And also in using 2 anchors to make a Med Moor. As well as a 3 anchor Med Moor.

Also, plan out how best to make multiple, massively strong connections from the apex point of a Med Moor, to your boat. And how best to have shock absorption systems on each them. Plus multiple layers of/systems of chafing gear on each as well.
Ditto if you're just using 2 anchors at a time, which aren't in any way connected to one another.

As, bottom line, you don't want to bet your boat on one anchor & rode, if there's no one around to adjust things, when conditions change. Or your anchor (however big) drags, etc.

For example, here in the US, you can buy 2 or 3 85lb Mantus Anchors, for the price of one of the big Rocna's which you're looking at. Or a mix of a couple of other substantially sized anchors, for the same price as the super big Rocna.
It's worth thinking about anyway. Especially as it's yet to be mentioned.


BTW, in addition to the above anchors & ground tackle, you should still have another anchor (or two) onboard, in reserve/just in case. Along with appropriate chain, rope rodes, shackles, thimbles, etc.
Especially if you're going to be gone for as long as you're stating.

[noelex 77]

Most of the weight is in the chain. The amount and size of chain is what is most critical in terms of windlass size.

In anchorages where the depth drops off quickly, it is possible to drag into deeper water. In this case the windlass must be capable of lifting the whole weight of chain and anchor.

Most windlass companies specify the lifting capacity should be 3x the total weight of chain and rode. The factor of 3x seems excessive, but the windlass specifications are for stall speed and by the time other factors are allow for such as the friction of the bow roller, and likely lower operating voltage, it seems more reasonable.

For example if you have 80m of 12mm chain the total weight is:

344 kg with a 40 kg Rocna, or 359 kg with a 55 kg Rocna.
The winch will need to be rated above 1032 kg (3x344) with a 40 kg anchor, or 1077 kg with a 55kg anchor.

With 80m of 10mm chain the total weight is:

208 kg with a 40kg Rocna or 223 kg with a 55kg Rocna.

The winch will need to be rated above 624 kg with a 40 kg anchor, or 669 kg with a 55kg anchor.

I am not sure of the rating of the Lewmar 1000. Is it 1000kg?

This is only a couple of examples, you chain may be longer or shorter. As you can see chain size and length is the biggest factor. The anchor size does not make much difference for the required rating.

Most of the time the required winch lift will only be lifting a short length of chain, or a short length of chain and the anchor. If the latter occurs, the anchor weight becomes more significant, but if the anchor winch is capable of lifting the total chain and anchor weight it will cope happily with these more common loads.

I think the best anchor size for a cruising boat is the largest you can comfortably handle. The winch size is often a limitation when looking to extend the chain, but not so much with the anchor. However the size of the bow roller etc can be a limiting factor.

The bow weight of a larger anchor is often mentioned. For a monohull once again nearly all the weight is in the chain and the anchor weight is not much of factor, but for a multihull this is not true. However, I find it hard to believe 15kg would have any practical effect on a 50 foot cat, or that you could not save 15kg with a bit of a clean out of the front lockers.

If you anchor overnight frequently, any extra weight in the anchor that you can reasonably manage will be some of the most valuable kilos (or pounds) you have on board.

[Jim Cate]

If you anchor overnight frequently, any extra weight in the anchor that you can reasonably manage will be some of the most valuable kilos (or pounds) you have on board.

Agreed, absolutely!

Jim

[Toys_with_time]

Originally Posted by DMCantor

My rule of thumb is that if the neighbors in the marina aren't giggling at how big your anchor is, you should go up a size. If the cost of the 55 vs. the 40 matters to you, then go with the 40. Otherwise, bigger is always better. 15 kg on the bow won't change anything about sailing performance or windlass sizing.

As an aside, we had a friend who proposed (I don't think ever executed) an April Fool's joke, cruising through the anchorage on a 45' boat with a dinghy-sized danforth on the bow, and a French flag. Sure to cause gasps among the cruisers. (I love French people, but I think their sailing education on anchoring leaves lots to be desired)

David
s/v Callisto

I have to beg to differ re French anchoring techniques - Glenans sailing school is one of the most famous in the world and the Glenans manual is one of the sailing bibles: "Every year up to 14,000 students and 800 instructors sail for 100,000 days". That's a lot of trained sailors.

IMHO it's not about nationalities, it's about experience and I can't think of any nationality that I have met that hasn't had a representative that has not given me pause for thought / worry. I regularly dive other boats anchors around me and if I see a potential problem I let them know. Some appreciate it and rectify the situation, others just shrug (or worse) and I then know to keep my eye on them or move.

By regularly checking my anchor, not just when first dropped, but also again and again, has taught me a lot about how my anchor acts in different winds / sea states / sea beds and I then feed this back into improving our anchoring techniques. 50 years on I consider I'm still learning.

TwT

[accomplice]

Originally Posted by UNCIVILIZED

[...]
As, bottom line, you don't want to bet your boat on one anchor & rode, if there's no one around to adjust things, when conditions change. Or your anchor (however big) drags, etc.
[...]

If you're leaving the boat for an extended period of time, a mooring might be a better choice than anchoring. Depending on what's on the bottom, and how deep it is, for relatively short money you can get a diver to put a few screws in a sandy bottom.

Sure, you cannot (easily) take the screws with you when you go, but you can take the chain. Of course, if you plan to return there you can use it again.

[sailnow2011]

Step 1, new windlass.
Step 2, always go bigger if the boat will hold it.
Step 3, back down on the anchor every time.

[arsenelupiga]

bigger anchor better is dangerous thinking.

Problem is with : this large anchor let me sleep well, ie overconfidence.

If you have anchor that is 50% heavier and useful area say 30% larger, however shape identical then what you get is extra 30% max pulling force before anchor breaks loose when comparing with lighter anchor. Wind & wave combination can produce extra 30% real quick.

Both anchors will set in same conditions because same shape.

Sorry, you only buy bit of extra power, but no security.

So you should not sleep well with either !!

[Stumble]

No boat has ever been lost because of too much holding power. Go with high strength chain to reduce weight and use that weight to buy a bigger anchor.

[Pauls]

I suggest you revisit the Rocna website and read the info there about anchor size selection. Last time I was there they had a recommendation that anchors over 100 lbs showed advantages. A couple examples of anchor size used by extremely experienced cruisers: Brent Swain, who has cruised for about 30 years full time and is always anchored out uses a 75 lb Delta on a 31 foot mono, and the Dashews, who are about as experienced a cruising couple as you can find, specify a 260 lb (from memory) Rocna on their 63' powerboat. This is a very low, sleek boat that probably has less windage than your cat.

So long as you can carry it bigger is always better.

[Uplander]

Wheee, not sure I can go along with:-

"Go with high strength chain to reduce weight and use that weight to buy a bigger anchor."

Certainly not our experience. Would be interested in any test results though.
The advantage of heavier/longer chain is that it acts a bit like a spring and reduces the forces on the anchor.

[a64pilot]

Lewmar 1000 max pull is I think 1000 lbs, not Kg


Sent from my iPad Pro using Cruisers Sailing Forum

[Stumble]

Originally Posted by Uplander

Wheee, not sure I can go along with:-

"Go with high strength chain to reduce weight and use that weight to buy a bigger anchor."

Certainly not our experience. Would be interested in any test results though.
The advantage of heavier/longer chain is that it acts a bit like a spring and reduces the forces on the anchor.

The cantenary (spring ) is gone typically by 30kn after which the heavier chain provides almost zero mathematical benefit let along practical benefit. Assuming the anchor system is weight limited, there is no question that using high strength chain of a smaller size with a larger anchor provides a higher ultimate holding power than the other way around.

As an example let's assume a weight budget of 525lbs.

-300' 3/8" BBB Chain weighs 480lbs with a MBL of 11,000lbs so we can handle a 45lbs anchor

While

-300' 5/16" G70 chain weighs 300lbs with a MBL of 14,100lbs allowing us a 225lbs anchor.


Trust me, the massive anchor in the second example, coupled with the stronger chain will provide massive amounts more holding power than a small anchor with heavy chain.

[Jim Cate]

Greg, while what you say is true, it ignores the significant advantage that heavy chain provides in absorbing more energy while the catenary is being "straightened" out. This absorbtion works in conjunction with the snubber in reducing peak loads. Once the catenary is gone, then the weight is immaterial of course... but meanwhile it has reduced the acceleration of the boat in puffs or wave strikes.

Jim