Over-specifying a Rocna anchor on a cat

[arsenelupiga]

Originally Posted by 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.

HA !

my anchor winch failed in very inconvenient place and time and pulling it up saved the boat hitting another.

40kg anchor & chain try to pull up in a hurry

Heavier is better but not by as much as some would like.

[KatKokomo]

I have a Rocna 40 on my 50ft. Cat. Never had any problems. 55 Kg is a bit overkilled I think.

[Stumble]

Originally Posted by 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

If you feel you need more cantenary just add more chain. The relavent formula is

Catenary Force in kg = Specific chain weight x (chain length)2/(2 x Depth)

So using the above chain options, when anchoring in 5m going from 8:1 scope to 10:1 provides pretty much the same cantenary force.

3/8 - .816kg*(40m)^2/(2x5m) = .816kg * 1600m^2/10m = 130.56kg
5/16- .453*(40m)^2/(2x5m) = .453 * 1600m^2/10m = 72.48kg (115kg at 10:1 scope)

But since the stronger chain is so much lighter you could almost double the amount of chain on board for the same weight. But again, run the formulas, in almost any case in winds above 30kn the cantenary is gone anyway, and all that is going to help is a larger anchor. So invest in high strength chain, a big anchor, and learn to use a snubber instead of relying on chain cantenary that's pretty much gone by the time you need it.

I hate appeals to authority, but this is strait out of Dashew's playbook. One giant anchor, high strength chain, and a snubber. It isn't rocket science.

[conachair]

Originally Posted by 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.

Remember that it takes very little distance to straighten out a catenary in the sort of scopes we use, even 10:1, so the energy is being transfered very rapidly, the force on the chain is still high.

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[Uplander]

Greg, thanks for the information regarding the catenary effect. Had to look up Wiki to find out what it was! Following on from that logic would you therefore see little advantage in the use of Anchor Buddies and the like?
John

[Jim Cate]

Originally Posted by Stumble

If you feel you need more cantenary just add more chain. The relavent formula is

Catenary Force in kg = Specific chain weight x (chain length)2/(2 x Depth)

So using the above chain options, when anchoring in 5m going from 8:1 scope to 10:1 provides pretty much the same cantenary force.

3/8 - .816kg*(40m)^2/(2x5m) = .816kg * 1600m^2/10m = 130.56kg
5/16- .453*(40m)^2/(2x5m) = .453 * 1600m^2/10m = 72.48kg (115kg at 10:1 scope)

But since the stronger chain is so much lighter you could almost double the amount of chain on board for the same weight. But again, run the formulas, in almost any case in winds above 30kn the cantenary is gone anyway, and all that is going to help is a larger anchor. So invest in high strength chain, a big anchor, and learn to use a snubber instead of relying on chain cantenary that's pretty much gone by the time you need it.

I hate appeals to authority, but this is strait out of Dashew's playbook. One giant anchor, high strength chain, and a snubber. It isn't rocket science.

Good to put some numbers to the discussion,Greg. And all along I have agreed that in ultimate holding situations, there is no substitute for a big anchor. The combination that you and Steve propose is the best combination for ultimate holding power with the additional advantage of better holding in poor substrates in ordinary conditions.

In our experience, there are all too many anchorages where either small size or large populations limits the amount of chain that can be deployed, and that can impact the "just add more chain" idea. But, where it is feasible, more chain will certainly offset the lighter weight in the catenary.

Finally, as we've discussed before on CF, there are some disadvantages to G-70 chain, including high cost, smaller margins for wear related strength reduction, questionable re-galvanizing practices, and in many areas, difficulty in procurement (from trustworthy vendors). None of these factors mean that one shouldn't use G-70, but should be folded into the decision process.

Getting back to the OP's query, I think we all agree that his windlass is likely undersized for the job. I suspect that a replacement windlass of adequate size will be a bigger increment in bow weight than the difference between the two anchors under discussion! And, just perhaps, folks are overestimating the sailing performance degradation encountered with small increments of bow weight. I've often suggested that people who are worried about this factor simply put a sandbag or other temporary weight on their bow and go for a sail to windward in a chop and see if they can tell any significant difference. No one seems to want to do this simple experiment, preferring to rely upon folk wisdom.

Jim

[Stumble]

Originally Posted by Uplander

Greg, thanks for the information regarding the catenary effect. Had to look up Wiki to find out what it was! Following on from that logic would you therefore see little advantage in the use of Anchor Buddies and the like?
John

Anchor buddies, kellets, and the like are as close to worthless as you get...

Mathematical models prove that the closer to the anchor these are placed the better they function, but even better than moving them closer to the anchor is using an anchor larger by the same amount than a kellet. At best all they can do is to add their weight to the hold power of the anchor (so a 1:1 relationship ). While modern SHHP anchors generally generate 1:10 weight:holding power or better.

Basically a 10lb kellet adds 10lbs (max) to the anchoring system, while buying a 10lb heavier anchor adds 100lbs (minimum) to the anchoring system. You might be able to identify a system where using a larger anchor simply isn't possible and thus a kellet adds something, but more rhode would sell be a better use of weight.

Thus the only time a kellet is beneficial is when you are 1) using the largest anchor possible, and 2) more rhode is impossible. This is such an unlikely occurrence on a cruising boat I don't think it's really a practical issue any longer.

There used to be an argument that pulling up a 40lbs kellet and 40lb anchor was easier than pulling up a 80lb anchor, but with reliable electric windlasses this is no longer true.

[Stumble]

I was able to track down the expected holding power of the original Ronca by weight and included it below. Keep in mind that the holding power of the chain cantenary we discussed above was about 280lbs with 130' of 3/8" chain deployed. Or about the same holding power as a 10lb Ronca would generate.



Holding
Power(lbs) .......Weight (lbs)
229 .................... 9
331 .....................13
561 .....................22
841 .....................33
1122 ...................44
1402 ...................55
1861 ...................73
2244 ...................88
3085 ...................121
3927 ...................154
6196 ...................243

[UNCIVILIZED]

Originally Posted by Stumble

I was able to track down the expected holding power of the original Ronca by weight and included it below. Keep in mind that the holding power of the chain cantenary we discussed above was about 280lbs with 130' of 3/8" chain deployed. Or about the same holding power as a 10lb Ronca would generate.



Holding
Power(lbs) .......Weight (lbs)
229 .................... 9
331 .....................13
561 .....................22
841 .....................33
1122 ...................44
1402 ...................55
1861 ...................73
2244 ...................88
3085 ...................121
3927 ...................154
6196 ...................243

Thanks for posting these. As they definitly give one pause, regarding how big an anchor to choose. And I'm Very surprised at how low the numbers are.
Out of curiosity, where are they to be found?

[Jim Cate]

Holding power? At what scope? In what sort of bottom? Buried how deep?

Numbers like these give me heartburn... my feeling is that they don't mean very much. Numbers quoted in various anchor tests have been much higher than these IIRC.

And what do you mean when you say "Keep in mind that the holding power of the chain cantenary we discussed above was about 280lbs with 130' of 3/8" chain deployed."?

The catenary has no holding power at all...

This is getting confusing!

Jim

[UNCIVILIZED]

Originally Posted by Jim Cate

Holding power? At what scope? In what sort of bottom? Buried how deep?

Numbers like these give me heartburn... my feeling is that they don't mean very much. Numbers quoted in various anchor tests have been much higher than these IIRC.

And what do you mean when you say "Keep in mind that the holding power of the chain cantenary we discussed above was about 280lbs with 130' of 3/8" chain deployed."?

The catenary has no holding power at all...

This is getting confusing!

Jim

Jim, your questions are more than valid, when one considers the after action reports generated by some boats/sailors, who survived Hurricanes at anchor.
Wherein, many a shackle with several times the breaking strength of the chain it was attached to, was broken. And or multiple chain rodes on a single boat were parted. Or that truly massive anchors held, but were incredibly mangled.

Which is part of why I inquired where such figures came from. As when I saw them, some light started to flash in the back of my head, wondering much the same questions which you're asking. I just wasn't able to articulate them then.

BTW, where is it written that all of a chain's catenary effect is gone by the time the wind reaches 30kts? Especially as there are a Huge number of variables in that statement/idea.
Particularly as compared to the real world wind loadings on boats at anchor. And how much they can vary from one to the next.

[noelex 77]

Originally Posted by a64pilot

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

Thanks. Most anchor winches are specified in lbs. I sort of hoped Lewmar had gone metric with their designation.

It is hard to imagine a manufacturer specifying a 1000 lb winch for a 50 foot cat, but here is the proof.

[Jim Cate]

Originally Posted by noelex 77

Thanks. Most anchor winches are specified in lbs. I sort of hoped Lewmar had gone metric with their designation.

It is hard to imagine a manufacturer specifying a 1000 lb winch for a 50 foot cat, but here is the proof.

Oh, maybe they thought that 1000 lbs was all the load that the bow of their cat design could absorb... (that's a joke, alla youse guys).

Jim

[noelex 77]

The table suggests that an anchor's holding ability is directly proportional to the weight. This has been shown to be roughly true (for anchors of the same design and material). Any variation in the substrate can of course produce an enormous change in holding power, but the relationship between weight and holding power seems to hold true (although it would be nice to see more research in this area) for most substrates (ignoring rock).

As most of the weight is in the chain rather than the anchor this has the effect that if we increase the anchor's size we can substantially increase our holding ability with only a very modest increase in the overall weight of our ground tackle.

For the OP's example, increasing the anchor size from 40kg to 55 kg would increase the holding power by 38% (according to this table). The overall increase in ground tackle weight is only 7% (assuming 80m of 10mm chain) or 4% (assuming 80m of 12mm chain), further showing the value and efficiency gains of maximising the anchor size when this is reasonably feasible.

[conachair]

Originally Posted by UNCIVILIZED

BTW, where is it written that all of a chain's catenary effect is gone by the time the wind reaches 30kts?

You can crunch the numbers yourself, sites like this..

http://www.spaceagecontrol.com/calccabl.htm

bearing in mind that's horizontal and 2 x length of an anchor chain. But even so, it takes a lot less force to get a chain well off the seabed than many people assume. And importantly a very short distance to do so. So if you want to keep the force down then a good snubber is a much better option, taking the catinery out of the chain just doesn't lower the force much in extreme conditions with the boat sheering about.

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