Anchor sizing questions

[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:

Just noticing that the scale of this diagram is significantly exaggerated.
If we adjust the diagram to show true scale it looks like this (and this is with 7/16" chain? I am sorry, I am a bit skeptical.)

[kmacdonald]

The vast majority of cruisers day sail from one marina or mooring to the next and rarely get the hook wet. There are some that will anchor in well known anchor fields on occasion. Then there are a few that like to travel off the beaten path but their numbers are very small. Chain could be reasonably considered by that overwhelming minority. Final answer-----it depends.

[CatNewBee]

An anchor is a safety device, just like a PFD or life raft. The vast majority of cruisers day sail from one marina will not see them either and the numbers of sailors really using them is even smaller, but I would still recommend to have those on board - like an anchor and a proper size chain. The safety of the vessel may depend on it.

[seadago]

Originally Posted by wingssail

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.

I very well may be naive Wingssail, when it comes to cruising. Afer all, I am a relative newbie at it. And I am greatful I can learn from the experience and the mistakes of others, in a place like this forum. However, I do know an awful lot about coral, and about diving.

We should not be damaging coral. Or seagrass beds. Or any other sensitive marine habitat. I would like to believe this statement commands consensus among participants in this forum, and cruisers in general. That we may do it by accident qualifies as an accident. I'm not talking about accidents, but about habits and customary practices. I have seen first hand, and quantified, the damage on coral formations caused by heavy anchor chains, in places like Nosy Be and Inhaca Island.

Yes. Up to 15 meters you can see. Deeper too, if the light and viz are good. Not from the deck of your boat. That you can't. You need to stick your head in the water with a diving mask, or use an underwater viewer. If there are coral pinnacles on the bottom, you'll see them as sharp dark brownish patches against the white of the coral sand in between. Now, I don't want to cause damage to the environment, nor to my gear. When in an unfamiliar anchorage, anywhere, I believe if would be prudent to know, grosso modo, what type of bottom I'm dropping the hook into. And if am in the tropics and I discover brownish patches everywhere, with a bit of white stuff in between, I would take that as a good indicator to look for a better spot. But then again, that's just me.

In the same vain, yes, depth and water temperature permitting, once I dropped anchor, I had the habit of going down to inspect that the anchor was set correctly. Particularly if I was expecting a change in weather. I did not find this very difficult to do in 10 or 12 mts of water, with the aid of a weight belt. But that was quite a few yrs ago. Haven't tried recently.

Don't do that in the muddy shallow river anchorages here in the UK, though!!

Re the debate on the relative merits of an all-chain rode, you may have noticed I not once mentioned the issue of cable damage by chafe, or sharp metal debris lying on the bottom. Discussion centered on holding power, catenary curves, and stress on deck fittings.

Never said that I don't carry chain. If at all relevant to this discussion, I do. 50 mts of it. And then, another 60 mts of cable. And a kellet! Enough rode for my needs so far.

A catenary curve is what it is, a parabolic type of curve. Unless we redefine trigonometry, it will increase exponentially on one axis, per linear unit increase on the other axis. Irrespective of the scale it is displayed in a graph. Fact is, on any catenary curve under tension, the largest proportion of an all-chain rode will be off the bottom. Even if only by an inch, it will be off the bottom.

[CatNewBee]

Originally Posted by seadago

I very well may be naive Wingssail, when it comes to cruising. Afer all, I am a relative newbie at it. And I am greatful I can learn from the experience and the mistakes of others, in a place like this forum. However, I do know an awful lot about coral, and about diving.

We should not be damaging coral. Or seagrass beds. Or any other sensitive marine habitat. I would like to believe this statement commands consensus among participants in this forum, and cruisers in general. That we may do it by accident qualifies as an accident. I'm not talking about accidents, but about habits and customary practices. I have seen first hand, and quantified, the damage on coral formations caused by heavy anchor chains, in places like Nosy Be and Inhaca Island.

Yes. Up to 15 meters you can see. Deeper too, if the light and viz are good. Not from the deck of your boat. That you can't. You need to stick your head in the water with a diving mask, or use an underwater viewer. If there are coral pinnacles on the bottom, you'll see them as sharp dark brownish patches against the white of the coral sand in between. Now, I don't want to cause damage to the environment, nor to my gear. When in an unfamiliar anchorage, anywhere, I believe if would be prudent to know, grosso modo, what type of bottom I'm dropping the hook into. And if am in the tropics and I discover brownish patches everywhere, with a bit of white stuff in between, I would take that as a good indicator to look for a better spot. But then again, that's just me.

In the same vain, yes, depth and water temperature permitting, once I dropped anchor, I had the habit of going down to inspect that the anchor was set correctly. Particularly if I was expecting a change in weather. I did not find this very difficult to do in 10 or 12 mts of water, with the aid of a weight belt. But that was quite a few yrs ago. Haven't tried recently.

Don't do that in the muddy shallow river anchorages here in the UK, though!!

Re the debate on the relative merits of an all-chain rode, you may have noticed I not once mentioned the issue of cable damage by chafe, or sharp metal debris lying on the bottom. Discussion centered on holding power, catenary curves, and stress on deck fittings.

Never said that I don't carry chain. If at all relevant to this discussion, I do. 50 mts of it. And then, another 60 mts of cable. And a kellet! Enough rode for my needs so far.

A catenary curve is what it is, a parabolic type of curve. Unless we redefine trigonometry, it will increase exponentially on one axis, per linear unit increase on the other axis. Irrespective of the scale it is displayed in a graph. Fact is, on any catenary curve under tension, the largest proportion of an all-chain rode will be off the bottom. Even if only by an inch, it will be off the bottom.

only if the tension is high enough.

[Souzag818]

Originally Posted by Kenomac

Buy a Spade or Ultra anchor according to the size chart.

+1

[seadago]

Originally Posted by CatNewBee

only if the tension is high enough.

evidently! Any tention higher than gravity. A catenary curve is that adopted by a chain hanging under gravity. Hence the name. That's what happens when the tide turns, or the wind shifts 180 degs. Boat goes right over the anchor with the rode hanging vertically from the bowsprit, and most of it lying on the bottom. A potential tangle hazzard in itself, but lasting for a short time only.

[zboss]

Originally Posted by boom23

Eyschulman,



Keeping every other variable the same, there is no way to convince me, or most everyone I think, that anchor size does not make a difference... Yes, the newer generation anchors are better than the older generation ones. They have to be.



MANY factors affect anchor performance, like boat weight, freeboard, windage, hull type, chain length and weight, bottom type, swell/wave height and direction, etc. Anchor manufacturers cannot account for all of that when they spec their anchors. They average. It is up to the captain to decide what he is comfortable with and for what conditions.



I prefer to have the odds on my side.

I’m definitely in the group that believes in a larger than necessary anchor weight wise but I have also come to believe that fluke area makes a major contribution.

I would ideally like to use one large size up on the anchor with g70 chain followed by an octoplait for those special times.

I didn’t know that 5/16” g70 would fit a 3/8” BBB gypsy.

[kmacdonald]

I don't think anyone is arguing that size doesn't make a difference. We all agree that a larger anchor of the same model will hold more that a smaller anchor of the same model. What we disagree on is how to size the anchor. A lot of people think they know more that the manufacturers of the anchors and suggest sizing it one or two sizes larger that the manufacturer suggests. I don't buy into it.

[arsenelupiga]

Originally Posted by kmacdonald

I don't think anyone is arguing that size doesn't make a difference. We all agree that a larger anchor of the same model will hold more that a smaller anchor of the same model. What we disagree on is how to size the anchor. A lot of people think they know more that the manufacturers of the anchors and suggest sizing it one or two sizes larger that the manufacturer suggests. I don't buy into it.

this is only true to one point. Increasing anchor after that dimishes the effect. That is why you have recommended sizes where anchor size and boat weight are optimized. In most terrains increased anchor will not provide more holding as new anchors are made to dig in with extra force.

Rest is just suburbian legend, probably from many centuries ago. But yeah, go for 200 kg spade and sleep tight!

[dfelsent]

A lot of people know far more about how and where THEY use an anchor than the manufacturers.
AND the manufacturer is not directly on the hook for the consequences of a failure.
Yes I would hope the seller knows their product. But it is ME who has to accept the consequences of using the product.
I use manufacturers recommendations, community input, my experience and my usage expectations to choose equipment.
Sometimes that means undersized. According to the brochure. Sometimes that means oversized.
It means right sized for me.

The manufacturer is never never perfect.

[Mike OReilly]

Originally Posted by kmacdonald

I don't think anyone is arguing that size doesn't make a difference. We all agree that a larger anchor of the same model will hold more that a smaller anchor of the same model. What we disagree on is how to size the anchor. A lot of people think they know more that the manufacturers of the anchors and suggest sizing it one or two sizes larger that the manufacturer suggests. I don't buy into it.

Manufactures recommend sizes based on a few simple sets of parameters. Windspeed, LOA and sometimes displacement. If that’s enough for you, then go with what they say. But most of us who anchor out a lot in a wide variety of situations know that the range of variables we encounter are far greater than anything that can be captured in a two or three dimensional mass-market recommendation.

[model 10]

The mfg has no idea what it will be like in the real world. You can't test for a 2 am squall. Bigger is better.

[dfelsent]

Originally Posted by Ecos

The mfg has no idea what it will be like in the real world. You can't test for a 2 am squall. Bigger is better.

^^^^^. This
Plus weeds over rock plus fetch.
Plus far from any assistance.

[kmacdonald]

Originally Posted by Mike OReilly

Manufactures recommend sizes based on a few simple sets of parameters. Windspeed, LOA and sometimes displacement. If that’s enough for you, then go with what they say. But most of us who anchor out a lot in a wide variety of situations know that the range of variables we encounter are far greater than anything that can be captured in a two or three dimensional mass-market recommendation.

Fair enough. What's your specific formula for anchor size so we can plug in our numbers and have it spit out the perfect size anchor?