Anchor sizing questions

[Mike OReilly]

Originally Posted by wingssail

...Finally we learned to put a non-stretchy section of line in the snubber system. This non stretchy line does not heat up and it does not chafe going over the roller when encased in a section of hose. Beyond the non-stretchy line our snubber has 15ft of nylon which can absorb the waves and heavy winds which might otherwise shock the chain.

Good point, and good tip. Snubbers are an essential part of any all-chain setup. This is one of the big advantages to a nylon rode — the snubbers are built in.

Chafe would definitely be a problem for snubbers. I run mine, in a bridle, through the bow hawsers (chocks). No friction issues, at least none I’ve ever noticed. They do need to be replaced occasionally though, as they get stretched and worn out.

[noelex 77]

Originally Posted by kmacdonald

Mike, most here are saying use all chain rode. If you do that you could get by with a smaller anchor since the catenary effectively increases scope resulting in higher holding power. Now if you use nylon rode and a bigger anchor but less effective scope the holding power may be the same but way less total weight.

The concern with your approach is chafe. It does not take much to cut nylon. There is a surprising number of obstructions on the seabed. Man made junk, old mooring blocks and natural formations such as rock and coral.

I agree some boats have no choice, the weight of all chain rode is simply impractical. If you fall into this category you can select anchor locations that reduce the risk, but this approach precludes some of the nicer tropical anchorages, and many others. Even with careful selection it is impossible to eliminate the risk entirely.

Having a reasonably long length of chain with a rope rode back up that is only needed in deeper anchorages, or stronger wind, can be good compromise, but the gold standard for long distance cruising boats that need to anchor in a variety of locations is all chain.

[kmacdonald]

Yes, historically chain has been the gold standard but is quickly changing. Lighter, faster cruisers are more the norm as are weight conscious cruisers. In the photo above 10' of chain is all that's needed to protect the nylon rode. Newer line with polyester sleeves and nylon core double braids like polydyne help address the chafe issue.

[seadago]

Originally Posted by Mike OReilly

I boil all this down to a simple recommendation: for your bower anchor, get the largest new-gen anchor you and your boat’s system can reasonably manage. Use as much chain as your boat can manage without compromising other factors.

...Which is as good a generic recommendation as you are going to get, all other factors excluded. But the excluded factors; your boat's displacement, inertia, windage, how it behaves to wind shifts and waves, etc, etc, things that only you know, are just as important as sheer mts of rode and anchor size IMO.

On medium and low density substrates, weight of the anchor counts because of gravity, and also because once the anchor is set and buried, a heavier anchor is more difficult to dislodge than a lighter one.

Every performance test I've seen and/or read about on this type of anchors shows demonstrably good holding power with rode to depth ratios smaller than conventional wisdom would have dictated as minimum.

I'm afraid the notion that an all-chain rode adds significantly to overall holding power, is not supported by evidence. At least with new generation self-righting anchors. The articles below are informative in that respect.

https://www.petersmith.net.nz/boat-anchors/catenary.php

Rode Static Behavior 2

The length of chain lying on the bottom ensures the forces acting on the anchor are vectored in the right direction (i.e. angle WRT the bottom) to make the anchor dig deeper or stay buried with increased pull. The rest of the chain hanging in midwater, perhaps three to four fifths of the total rode, have no effect on the anchor, but have a punishing effect on the bowsprit and deck fittings. Also, chain has no elasticity; under tension it will transmit every jerk from yawing or pitching down the rode without dissipating any of those forces.

In the compromise we all have to make, and once I am confident that I have a sufficient length of chain lying on the bottom at all times (I highlight that condition!), if I have to choose where to add to my ground tackle, for my money I'd add weight/size on the anchor, not on the chain, and cable to my rode. And if I feel I need to improve my catenary, I ad a weight at the start of the chain section of my rode. Only a few kilos will chink the curve sufficiently to achieve the same result as many mts of chain.

[wingssail]

Originally Posted by kmacdonald

Yes, historically chain has been the gold standard but is quickly changing. Lighter, faster cruisers are more the norm as are weight conscious cruisers. In the photo above 10' of chain is all that's needed to protect the nylon rode. Newer line with polyester sleeves and nylon core double braids like polydyne help address the chafe issue.

Sorry kmacdonald, many areas have rocks, some quite sharp, which can quickly chafe through any line, sleeved or not. Even worse, there may be coral heads present and even though we attempt to avoid them, they may be unseen and a slight shift in the wind direction could put our anchor rode over a sharp portion of coral and slice any rope rode in an instant. None of this even mentions other hazards such as old metal debris present on the seafloor.
If you always dive on your anchor you might discover these hazards, then you can move, re-anchor, and dive again. And again.

To us, a chafe proof, all chain, rode, is not only better for holding, it is excellent protection against sharp objects on the sea floor.

[noelex 77]

Originally Posted by kmacdonald

In the photo above 10' of chain is all that's needed to protect the nylon rode.

No, the chain on the left side of the photo is leading up to the boat.

The section of rode close to the boat is the part that is moving the most as the vessel yaws from side to side, or travels the greatest distance with a windshift. It is often this part that is subject to most chafe, or is most vulnerable to becoming caught under, or around an obstruction.

Keeping the rope rode off the bottom is a help, but this requires much more than ten feet of chain, and the rode length needs frequent adjustment for the wind strength and state of tide.

Originally Posted by kmacdonald

Yes, historically chain has been the gold standard but is quickly changing.

That has not been my observation for long distance cruising boats. These vessels almost invariably use all chain. If you have a look at my anchor photos I can only think of one or two boats in the series that did not use all chain rode for their primary anchor.

[wingssail]

Originally Posted by seadago

... The rest of the chain hanging in midwater, perhaps three to four fifths of the total rode, have no effect on the anchor, but have a punishing effect on the bowsprit and deck fittings. Also, chain has no elasticity; under tension it will transmit every jerk from yawing or pitching down the rode without dissipating any of those forces.

No. Generally, with a 5:1 scope for example, 4/5 is lying on the bottom and 1/5 is vertical from your bow to the bottom. When the wind begins to blow the amount hanging down to the bottom becomes greater, as the windage of your yacht pulls back on the chain it lifts some more off the bottom. Only in quite strong winds does a majority of your chain pull off the bottom. Even then the catenary is such that the last bit before the anchor is nearly horizontal.

There is rarely much shock on your tackle with all chain because most of the loads are absorbed by straightening out of the catenary. This then springs back as the weight of the chain sinks back towards the seafloor. The idea of a bar tight chain transmitting loads to the bow of your boat is not likely but yes, possible in rough conditions and short rodes. Then is when you need more scope or a good snubber.

[seadago]

Originally Posted by wingssail

Sorry kmacdonald, many areas have rocks, some quite sharp, which can quickly chafe through any line, sleeved or not. Even worse, there may be coral heads present and even though we attempt to avoid them, they may be unseen and a slight shift in the wind direction could put our anchor rode over a sharp portion of coral and slice any rope rode in an instant. None of this even mentions other hazards such as old metal debris present on the seafloor.
If you always dive on your anchor you might discover these hazards, then you can move, re-anchor, and dive again. And again.

To us, a chafe proof, all chain, rode, is not only better for holding, it is excellent protection against sharp objects on the sea floor.

Well, that comes down to choosing the right place to anchor. We should not be damaging coral. If there's coral around, then the viz is also good enough to see 15 to 20 mts down. A quick inspection tour from the dinghy will tell us if we are likely to get tangled or cause damage. No need to dive. And if we get tangled, we need to dive to untangle anyway!

[LargeMarge]

a == What is the bottom?
b == How deep until you get bite?
c == Surge? Tides?

In our experience in the Oregon-to-Anchorage area and around Puget Sound, the beautiful green mountains mimic the underwater slope. Thirty-feet / 10m off the beach could be over a hundred-feet / 35m deep... rock with gnarly surge. And gnarly tides.

If I was me, I would go with the gnarliest anchor(s) SYSTEM your boat can carry.

And I would say 'gnarly' a lot. Work it into every conversation. "Did you see that brown swimming against the surge! Gnarly!"

"With this 'daylight savings' time change, my circadians are all messed up." "Gnarly!"

"Honey, how would you like your cocktail?" GNARLY!!!"

[seadago]

Originally Posted by wingssail

No. Generally, with a 5:1 scope for example, 4/5 is lying on the bottom and 1/5 is vertical from your bow to the bottom. When the wind begins to blow the amount hanging down to the bottom becomes greater, as the windage of your yacht pulls back on the chain it lifts some more off the bottom. Only in quite strong winds does a majority of your chain pull off the bottom. Even then the catenary is such that the last bit before the anchor is nearly horizontal.

There is rarely much shock on your tackle with all chain because most of the loads are absorbed by straightening out of the catenary. This then springs back as the weight of the chain sinks back towards the seafloor. The idea of a bar tight chain transmitting loads to the bow of your boat is not likely but yes, possible in rough conditions and short rodes. Then is when you need more scope or a good snubber.

This is interesting. I don't dispute your experience, but every catenary curve I've ever seen, modelled, or by actually diving under a boat, points to the contrary.
Example below:

[wingssail]

Originally Posted by seadago

Well, that comes down to choosing the right place to anchor. We should not be damaging coral. If there's coral around, then the viz is also good enough to see 15 to 20 mts down. A quick inspection tour from the dinghy will tell us if we are likely to get tangled or cause damage. No need to dive. And if we get tangled, we need to dive to untangle anyway!

I am sorry if I sound dismissive of your comment, but I think your answer is hopelessly naïve, that is all I can think of to say.

However,

"If there is coral around the viz is good enough to see 15-20 mts..." Cough cough, really? You can see the bottom in 15-20 mts enough to see a sharp rock or a coral head? I don't believe it.

"A quick inspection from our dingy will tell us..." OK, so you stop your vessel launch the dingy, and inspect the potential anchorage area before anchoring? In decades of cruising I have NEVER seen or heard of anyone doing anything like this, and looking over the side of your dingy is not EVER going to reveal much about the bottom in the surrounding area.

"if we get tangled, we need to dive to untangle anyway" Well, if your rode is cut by a rock or a coral head you didn't see, then you have nothing to untangle. And if it is not cut, when you pull it up and find the rope shredded almost through, then what do you do? Get out your spare rode?
Do you always have dive tanks? We don't, many cruisers don't. We got around a rock (with our chain) in 26 feet, and free diving it was difficult to get down to it and stay down long enough to untangle it. A rope rode could have definitely been damaged before we got it free.

If I may say so, I think your answers are more of justifications for why you don't want to buy or carry a chain rode. I don't like the weight or cost either, but in my opinion it is essential for offshore cruising.

[Jim Cate]

If I may say so, I think your answers are more of justifications for why you don't want to buy or carry a chain rode. I don't like the weight or cost either, but in my opinion it is essential for offshore cruising.

Agreed, in spades. (No, not the anchor)!

We've personally seen enough parted rope rodes to understand that the use of such when cruising in unfamiliar waters is not acceptable for general use.

Jim

[wingssail]

Originally Posted by seadago

This is interesting. I don't dispute your experience, but every catenary curve I've ever seen, modelled, or by actually diving under a boat, points to the contrary.
Example below:

Yes, I agree, in 20 knots of wind much of your chain will be off the bottom as shown in the diagram. I guess I am just accustomed to anchoring in calm places where I look over the bow and see the chain hanging straight down and then wandering along the bottom in the direction of the anchor.

For the OP, I guess this catenary would look dramatically different with a chain/rope rode.

[wingssail]

Originally Posted by Mike OReilly

Chafe would definitely be a problem for snubbers. I run mine, in a bridle, through the bow hawsers (chocks). No friction issues, at least none I’ve ever noticed. They do need to be replaced occasionally though, as they get stretched and worn out.

Mike, I am sure you have seen this. The chafe comes into play when the portion of the snubber between a cleat or bollard and the chock stretches. Then the place where it runs through the chock moves and it will chafe. If your snubber is made fast to a cleat just near the chock then the amount of stretch and chafe may be minimal, however, in strong wind and wave conditions even that set up may reveal chafe going through the chock. If your vessel swings around so that the place where the snubber touches the chock rubs back and forth, under enough load, it may also chafe.

I never noticed any chafe on my snubbers until I anchored in a bay with winds over 45 knots and two foot chop. Then I had to solve the problem!

[boom23]