I rather assume a cruiser will be hundreds of miles from home at times. I'm not much for depending on taking a mooring. Me, I'd carry 200' of G43 chain on a boat with a windlass.
Anyway, that was my reasoning. You could also be safe with rope.
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Anyway, that was my reasoning. You could also be safe with rope.
You'll read all kinds of things about what's optimum. All chain or a mixed rode, and in what proportion. IMHO, chain on the bottom is good, for all the reasons recited by others, but you don't want to weigh your boat down with extra chain you'll rarely if ever put out.
I try to anchor in 10-15 feet of water, my scope is typically 5:1 to 7:1 or 50’ to 105’ of mixed rode. I use 60’ of chain because my boat is weight-sensitive. If I had a monohull or cruising cat I’d carry at least 100’ of chain on the rode, or more, backed by more rope.
Who wrote this rule? It defies logic or justification. The logic I follow is to base the amount of chain on how much I expect to have on the bottom, then back it with rope for extra capability. Therefore the amount of chain is determined by water depth (scope) not boat length.
I try to anchor in 10-15 feet of water, my scope is typically 5:1 to 7:1 or 50’ to 105’ of mixed rode. I use 60’ of chain because my boat is weight-sensitive. If I had a monohull or cruising cat I’d carry at least 100’ of chain on the rode, or more, backed by more rope.
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No. Just heavy and expensive. You indicated you wanted to sail in the Keys and thought $1000 for 200 feet of chain was expensive. I was just pointing out that you don't need that kind of length most places you go. The Caribbean is different. The volcanic nature of those islands means deeper water near shore.
It would hinder the anchor from setting only because it wouldn't allow the boat to move.
The cantenary curve of a good chain will absorb the shock of a lifting wave and the sheer weight will hold the boat steady against a chop. I'm unfamiliar with the transfer of shock from the anchor. I assume the bottom stays steady and there are no giant wahoo or grouper trying to yank the anchor around.
The difference between anchoring with rope rode and chain is that the weight of the chain acts like a meta-kellet and pulls the boat forward against the wind/current and a curve develops in the line. That curve encourages a better angle on the anchor which means less scope is necessary. 3:1 up to 5:1 is typical. A mostly rope rode means around 7:1. The more lead chain, the more it will affect like all chain, so one can "get away" with scope as short as 5:1 if half of it is chain. Keep in mind, the shallower your anchor depth, the less chain weight there is to help create that cantenary curve. More scope in shallow water, for chain, is a good idea.
The cantenary curve also takes up the shock of waves lifting and dropping the boat. If you have 3:1 scope out in 20 feet of water, that's 69 feet of chain (20' + 3' of freeboard). In 20 feet, I think 3:1 its pretty skimpy for both, the reason stated above, and that if a 6 foot wave came along, you will be in 23 feet of water at the top of that wave. That means your 3:1 scope is temporarily only 2.65:1. So, let a little more out.
I toured the Galapagos aboard an 80'+ tour boat once. They had all chain rode out and while anchored in a cove. One of the deckhands and I were snorkeling when he challenged me to swim to the bottom. We couldn't see it because of the particulates. I took him up on it and pulled myself down the anchor chain. In the beginning, it was straight down and I was constantly clearing my ears. Soon, however, the chain was more like a 45 degree angle and then it was much more straight out than straight down. I was starting to worry that I wouldn't reach the bottom because the anchor chain wasn't going to lead me there fast enough. Finally, I saw the bottom and left the chain to swim directly down. The captain told me they were anchored in 55 feet of water. There was a lot more horizontal length to the anchor chain, in that 55 feet of depth, than vertical length. Usually, a good 10% or more of chain is just laying on the bottom. That is shock absorbing weight the boat has to pickup off the bottom and stretch out nearly straight before it feels the tension on the anchor as a hard jerk. Perhaps for smaller lighter boats with light weight chain, that would be the case.
-Will (Dragonfly)
I think that is the case, especially in shallow water with 5/16” or even 3/8” the kellet effect is reduced except in light winds. In a heavy storm force wind I wouldn’t rely on a kellet or chain catenary as a shock absorber if in shallow water.
Some, but not really much. A 15-20 knot breeze can pull the catenary out of the chain, that's why a snubber is used. 5/16" chain weighs around a pound/foot. With 50 to 60 feet of chain lying on the bottom, that is not much weight.
If the cantenary is that easy to pull out of a chain, 7:1 should be the scope to use, otherwise, a straight 3:1 - 5:1 scope will not allow your anchor to hold. I've not done a scientific study of this, but I grew up anchoring on 3:1 scope aboard our family 56' live-aboard. We moved from rope rode to all-chain and my father, who has sailed all around the world, never looked back. Storm conditions and depth dictate a modification to the standards. I agree with that.
-Will (Dragonfly)
In my experience, if you are pulling the catenary out of your chain in a 15-20 knot breeze in calm water, then your chain is not heavy enough for your boat. I believe too many people do not size their chain to their anchor and boat (it's too heavy, and/or it's too expensive), completely forgetting that not only the anchor, but the chain make up a proper rode for the boat. I know of one couple who use a 125# Mantus with 3/8ths inch chain! One day that chain is going to break leaving their 125# anchor in place, but certainly not their boat.
One's anchor tackle is their number 1 safety and security and a lot more valuable than an insurance policy, so why not get it right?
Depending on the depth and scope, I''ve measured rode tension at 1.5 to 5 times wind force with all chain. The higher figure was caused by the chain getting straight, and I'm not guessing, I was watching a load cell. I repeated this on multiple days, in different locations, with winds from 10-40 knots. ABYC recommendation reflect this. The lower numbers were in deeper water or longer scope. Whether it will happen depends on all of the variables, but the main factor is pounds of chain out. Not scope. Not size of chain. Pounds. Makes sense, if you think about it.
Also, you may not feel the snubb. The speed and the jolt is not that high, even when the force is emense. I once exploded a 2000 pound load cell while I was standing next to it (it probably took 2-4 times that force), and I didn't really feel anything. No one on the boat noticed. But an express cruiser wake from many miles out caught the cycle just right.
Finally, in some cases what gives is the anchor. It moves a few inches. If it does that enough times, it may or may not trip.
One problem with chain is that it is very, very hard to gauge how much strain it is under. I will offer that very few sailors can estimate the load on their ground tackle within 200%. They only guess, based on... nothing, really.
Also, you may not feel the snubb. The speed and the jolt is not that high, even when the force is emense. I once exploded a 2000 pound load cell while I was standing next to it (it probably took 2-4 times that force), and I didn't really feel anything. No one on the boat noticed. But an express cruiser wake from many miles out caught the cycle just right.
Finally, in some cases what gives is the anchor. It moves a few inches. If it does that enough times, it may or may not trip.
One problem with chain is that it is very, very hard to gauge how much strain it is under. I will offer that very few sailors can estimate the load on their ground tackle within 200%. They only guess, based on... nothing, really.
I would love to read more about this experiment. Where was the load force measured, at the boat or at the anchor?
Wind pressure based on wind speed {How to Calculate Wind Loads From Wind Speeds} "(a 40-mile-per-hour (mph) wind speed creates a pressure of (0.00256 x (40)^2) = 4.096 pounds per square foot (psf)"
A rough approximation: For a boat that is 10' wide by 5 feet tall, without consideration of the drag coefficient ( treating it like a flat wall), that's in the neighborhood of 200lbs of force. Maybe double that adding in the rigging = 400lbs of force. Wave action and current will only add to it. You can see that a 20 knot wind would be factors less in force. It would be fun to do all the math. I wish I had time.
I believe you thinwater, but I would like to know more about the details of your experiment. I think it takes a lot of force to pull the cantenary curve out of 100 lbs of chain.
-Will (Dragonfly)
By measuring the rode tension anchored behind a break water with practically no fetch (no waves), using a long nylon rope, and averaging both windspeed and tension. But in fact, under such conditions, there is very little fluctuation.
I think it's all kinds of fun to play with graphs and tables, but in the real world boats don't sit bow dead into the wind and there's probably no accurate way to measure the pressure wind puts on an anchor through the rig. I'm sure that when someone's Hunter comes to the end of her sail and the anchor tackle violently jerks the bow at some 140 degrees, the forces are incredible, at least for a few seconds.
I think anchoring comes down to one very simple formula; if one can sleep well at anchor and the last time you drug anchor is almost beyond memory, then you have it down. But if you can't honestly say you sleep the night through and have perfect confidence in your anchor tackle, then you aren't doing it right and changes are needed. Whether it be a next gen anchor, more or heavier chain (generally, I don't consider a line/chain rode suitable for most full time cruising applications), or just re-evaluating what you are doing with what you have and making it work (adding kellet, snub, etc.).
It has cost me literally thousands of dollars to realize that more scope does not necessarily make for better anchoring under most conditions. Dragging lots of chain around the bottom just wears the galvanizing off much faster and since I've reduced my average scope to 3:1, my chain lasts longer, I have less trouble in anchorages with other boats coming into my swing radius and I believe the system works better as a whole. I see nothing wrong with allowing the boat to put some pressure on the anchor now and then, which is perhaps why, though we never back down on our anchor when we drop, (except when Med mooring or when there is no wind at all), we have yet to actually drag anchor.
I think anchoring comes down to one very simple formula; if one can sleep well at anchor and the last time you drug anchor is almost beyond memory, then you have it down. But if you can't honestly say you sleep the night through and have perfect confidence in your anchor tackle, then you aren't doing it right and changes are needed. Whether it be a next gen anchor, more or heavier chain (generally, I don't consider a line/chain rode suitable for most full time cruising applications), or just re-evaluating what you are doing with what you have and making it work (adding kellet, snub, etc.).
It has cost me literally thousands of dollars to realize that more scope does not necessarily make for better anchoring under most conditions. Dragging lots of chain around the bottom just wears the galvanizing off much faster and since I've reduced my average scope to 3:1, my chain lasts longer, I have less trouble in anchorages with other boats coming into my swing radius and I believe the system works better as a whole. I see nothing wrong with allowing the boat to put some pressure on the anchor now and then, which is perhaps why, though we never back down on our anchor when we drop, (except when Med mooring or when there is no wind at all), we have yet to actually drag anchor.
Rode tension, measured at the boat. Unless the anchor is burried quite deeply and the wind has changed direction, the anchor will feel effectively the same force (chain may have sideways resistance in soil, but not end-to-end--measured these too).
The square relationship is quite reliable.
Also consider that if your boat yaws (most swing 60 degrees--check your compass) the area is a lot more than the frontal area, plus there are momentum effects. I've found that by altering the yaw characterisics of my boat, I can change the anchor load by over 200% and reduce the anchor holding safety factor by 600% (anchors loosen). This latter is another place chain helps.
The DIY way to estimate catenery curve is to Google a catenary calculator. If you want to know the tension where the chain is lifting, for example, use only 1/2 of the span length. If you want to calculate the angle at the bottom after the chain lifts, you need to get into the math.
Curiously, chain stretch is not zero and becomes important at high load.
This was written up in Practical Sailor and in my book.
Not always so. If the angle of pull changes due to the cantenary curve, the sum of forces change because gravity has a different vector effect. Also, if chain is laying on the bottom, there is resistance to the tension forces not felt at the boat. However, if it is effectively "bar straight" then yes, tension at one point is the same as tension at any point along that chain.
-Will (Dragonfly)
Nope. Check your engineering. Other than friction, a line or chain can only transmit for in tension and the the value is fixed over its length. That it is laying on the bottom is no different than running across a series of pulleys. Make sense? If you want to know what the friction of chain is on sand or mud, drag some. But this part has as nothing to do with vectors.
That is not to say that the tension on the rode will not increase with (short) scope. Simple trig we all learned in high school, that one. At long scope the correction is tiny compared to other variables (waves, gusts, and yawing).
I wish there was more time. I would love to get into this discussion much "deeper"
. This doesn't seem correct to me. A chain, for example, hanging straight down with an anchor dangling freely at the end has a tension on the lower end of the anchor weight only. The top of the chain has tension of both the anchor weight and the chain weight. This is even true when straight. Gravity is a graduated effect along the length of a cord with mass. When applying additional tension to the chain in a fixed position, it is addative. Deflect the lower end of the chain and there is a perpendicular vector force of deflection added or subtracted based on support of the anchor or not.If I'm wrong, I will be thrilled to learn the correct physics, but it will take some convincing. I would be happy to discuss this in a PM or a thread dedicated to this problem. I don't want to continue hijacking a thread only tangentially related.
-Will (Dragonfly)
Shock loads will negate all of your load calculations. To visualize, place a nail upright on a 2 x 4, place a hammer on the nail, press down on the hammer with all of your weight. Next strike the nail with the hammer. Compare results.
Good point on the weight of the chain. I did not understand your point. But it's in the math when you use a cantary calculation.I wish there was more time. I would love to get into this discussion much "deeper"
If I'm wrong, I will be thrilled to learn the correct physics, but it will take some convincing. I would be happy to discuss this in a PM or a thread dedicated to this problem. I don't want to continue hijacking a thread only tangentially related.
-Will (Dragonfly)
This really nails it. I can do all of the calulations, but by the time you consider...
- catenary
- yawing
- momentum
- chain and rope stretch
- change in windage with yawing
- effect of keel moving through water
- that these are vary with time in a complex way
And that is what I did with a load cell. All chain (several sizes). All rope. Differing depths and scope. Differeny wind strength. Snubbers. Variable yawing. Unsurprisingly, there is a lot of scatter in the data, but trends are clear. The two really bad things are yawing >60 degrees and all chain in shallow water. As for the rest, you won't see me being hard line, because there are too many variables.There are many acceptable ways to anchor, and a few really bad ones.
One DIY option is to use a snubber, look up the stress/strain curve (which changes as it ages), and measure the change in length. The other is to buy a cheap load cell.