Scope for chain vs. rope

[Steve W]

Simple question to all.

Assuming normal conditions for anchoring (subject to change for extraordinary wx conditions, extraordinary depth <more than 10'-15 depth at mean high tide, etc.) what scope might be 'normal' for an all chain rode. Assume a 35# CQR or equivalent anchor.

With a standard nylon all rope rode, I would use a minimum of 7:1 scope. Should I be "safe" with figuring a 5:1 scope with an all chain rode?

 

[RichH]

Re: scope for chain-v-rope anchor line

4:1 in benign conditions or 5:1 in small chop is typical for an all chain rode. 7:1 or greater in 'heavy' conditions. In heavy conditions, if the chain is pulling bar tight, you dont have enough chain out.

However, the consideration should be made to use a 'bridle' on the rode so that the rode 'equivalently' is attached to the bow AT where it meets the waterline - the bridle holds the rode so that it 'acts' as if its attached to the bow AT the waterline .... for LESS amount of scope needed. Use a 'chain hook' to connect the rode to the bridle, the bridle attaches to both sides and aft of the bow.

Anchor Rode Bridle example: http://forums.sbo.sailboatowners.com/showthread.php?t=140455&highlight=bridle ... post #15 The use of a bridle will greatly lessen how a boat dances around or 'sails' when at anchor.

 

[Stu Jackson]

Re: scope for chain-v-rope anchor line

"Assuming normal conditions" may be an oxymoron. No offense intended. Why? Because there are none.

For instance, here where I sail, the conditions are pretty predictable because unless it's winter with storm fronts moving in, the conditions do not change very much.

However, in almost all the rest of the country, thunderstorms can roll in at any time. You may have seen or heard of the horrendous conditions just the other day back east in the Chesapeake.

In that case, normal conditions should be considered as storm conditions.

I use an anchor SYSTEM on my 34 foot boat that is good for a 40 foot boat in 42 knots of wind, so I should be good for maybe 50 knots, if I ever considered anchoring in those conditions and I can assure you I wouldn't be out there if that was predicted. Again, predicitions of weather where I sail are pretty accurate and forecast well in advance because of the climate and the prevailing weather patterns.

If I lived almost anywhere else, my anchor SYSTEM would HAVE TO BE a lot heftier.

Instead of my 22# Rocna and 1/2" rode with 35 feet of 1/4" chain, anchoring in 12-15 foot depths (yes, I frequent certain anchorages repeatedly), I'd go to a 35# or 44# anchor with longer heavier chain, 100 feet of 5/16" or 3/8" chain with 5/8" rode so I could deploy all the chain plus some rode to avoid having to do the bridle and chain hook dance (a bear to retrieve - maybe not a bear but IS extra work, which as a singlehander I avoid like the plague!!!). It's a technique a friend of mine used on his cruise from Vancouver BC to Mexico, he wrote: I also wouldn't add more chain - this works perfectly in 25 to 30 feet of water - you let all the chain out and you tie off nylon at the preferred scope and don't bother with snubbers and chain hooks and all that stuff...

The answer to your question depends on where you sail and anchor.

I would also caution you to consider the newer generation of anchors, vs. a CQR. I do not intend to start an anchor debate yet again, but if you do a search on this board, or any other, you'll find lots of discussions suggesting that old technology anchors could and have been vastly improved upon. What's your boat worth to you? For a few hundred dollars you could get a much more substantial anchor.

 

[Ross]

Re: scope for chain-v-rope anchor line

I anchor out regularly with about 30 feet of 3/8inch chain on a 35 pound CQR with 5/8 nylon rode. If I expect any kind of a blow I will set out 10:1 or even 20:1 scope. I haven't dragged YET.

 

[jviss]

4:1 in benign conditions or 5:1 in small chop is typical for an all chain rode. 7:1 or greater in 'heavy' conditions. In heavy conditions, if the chain is pulling bar tight, you dont have enough chain out.

However, the consideration should be made to use a 'bridle' on the rode so that the rode 'equivalently' is attached to the bow AT where it meets the waterline - the bridle holds the rode so that it 'acts' as if its attached to the bow AT the waterline .... for LESS amount of scope needed. Use a 'chain hook' to connect the rode to the bridle, the bridle attaches to both sides and aft of the bow.

Anchor Rode Bridle example: http://forums.sbo.sailboatowners.com/showthread.php?t=140455&highlight=bridle ... post #15 The use of a bridle will greatly lessen how a boat dances around or 'sails' when at anchor.

Such a thing is called a snubber. It can be configured as a bridle, or just a single line. It doesn't change the amount of scope needed. the primary purpose is to act as a shock absorber, and to quiet the chain.

I made one from 5/8" nylon 3-strand, and spliced a galvanized chain hook to one end.

 

[jviss]

"Assuming normal conditions" may be an oxymoron. No offense intended. Why? Because there are none.

For instance, here where I sail, the conditions are pretty predictable because unless it's winter with storm fronts moving in, the conditions do not change very much.

However, in almost all the rest of the country, thunderstorms can roll in at any time. You may have seen or heard of the horrendous conditions just the other day back east in the Chesapeake.

In that case, normal conditions should be considered as storm conditions.

I use an anchor SYSTEM on my 34 foot boat that is good for a 40 foot boat in 42 knots of wind, so I should be good for maybe 50 knots, if I ever considered anchoring in those conditions and I can assure you I wouldn't be out there if that was predicted. Again, predicitions of weather where I sail are pretty accurate and forecast well in advance because of the climate and the prevailing weather patterns.

If I lived almost anywhere else, my anchor SYSTEM would HAVE TO BE a lot heftier.

Instead of my 22# Rocna and 1/2" rode with 35 feet of 1/4" chain, anchoring in 12-15 foot depths (yes, I frequent certain anchorages repeatedly), I'd go to a 35# or 44# anchor with longer heavier chain, 100 feet of 5/16" or 3/8" chain with 5/8" rode so I could deploy all the chain plus some rode to avoid having to do the bridle and chain hook dance (a bear to retrieve - maybe not a bear but IS extra work, which as a singlehander I avoid like the plague!!!). It's a technique a friend of mine used on his cruise from Vancouver BC to Mexico, he wrote: I also wouldn't add more chain - this works perfectly in 25 to 30 feet of water - you let all the chain out and you tie off nylon at the preferred scope and don't bother with snubbers and chain hooks and all that stuff...

The answer to your question depends on where you sail and anchor.

I would also caution you to consider the newer generation of anchors, vs. a CQR. I do not intend to start an anchor debate yet again, but if you do a search on this board, or any other, you'll find lots of discussions suggesting that old technology anchors could and have been vastly improved upon. What's your boat worth to you? For a few hundred dollars you could get a much more substantial anchor.

If you are going to be anchoring in varying depths and at varying degrees of crowdedness, and using all chain, you need the snubber so you can adjust the scope. For example, on a crowded weekend in Newport where the depth is 22', you need less scope than you would normally use so you don't swing into other boats. I carry 125' of 5/16" BBB galvanized chain, with a 35 lb. Bruce clone. I also carry a 22 lb. Danforth with 35' of chain and 150' of rope as a backup/alternate. I guess I could tie that rope to my chain rode if I ever needed more than 125'.

I've seen squalls that were not predicted by anyone that had winds to 60 kt., such as one a couple of years ago while anchored in Block Island (New Harbor); carry and use ground tackle for the worst conditions imaginable, and enjoy the show of the harbor master and Sea Tow wrangling the dragging and loose boats.

 

[Ross]

Re: scope for chain-v-rope anchor line

With 125 feet of chain you are limited to less than twenty feet of water when you anchor. I like many options for anchoring.

 

[jviss]

With 125 feet of chain you are limited to less than twenty feet of water when you anchor. I like many options for anchoring.

I disagree. At 5:1 I can anchor in 25'. At 4:1 up to 30. If it's deeper, or I really think I need more scope, I can tie the rope end of my other rode to this, but I haven't found that necessary yet.

The only place I usually anchor that's deep, like 22', is Newport, and you just can't let out much scope there without swinging into other boats.

Most of the time I'm in 14' or less. Block is the shallowest, at like my draft plus 6" at low tide.

 

[RichH]

Such a thing is called a snubber. It can be configured as a bridle, or just a single line. It doesn't change the amount of scope needed. the primary purpose is to act as a shock absorber, and to quiet the chain.

I made one from 5/8" nylon 3-strand, and spliced a galvanized chain hook to one end.

Noooooo ..... A snubber is to reduce shock load. A bridle also or additionally 'lowers' the attachment point making the total amount of scope needed less .... simple trigonometry and/or pythagorean theorem.

Example - If your bow is 5 ft higher than the water the total amount of scope is 'reduced' (by the SIN of the scope angle).
A 4:1 scope results in a 14° rode angle (at the anchor to rode connection).
Anchoring in 10 ft. of water from a 5ft. high bow (total of 15ft 'depth') at 4:1 scope (60 ft run per 15 ft. total depth) requires 62 ft. of chain
Anchoring in 10 ft of water with a Bridle set AT the bow waterline at 4:1 scope requires only 41.3 ft. of chain.
62 ft of chain is a lot longer then 41 ft.

;-)

 

[jviss]

Noooooo ..... A snubber is to reduce shock load. A bridle also or additionally 'lowers' the attachment point making the total amount of scope needed less .... simple trigonometry and/or pythagorean theorem.

Example - If your bow is 5 ft higher than the water the total amount of scope is 'reduced' (by the SIN of the scope angle).
A 4:1 scope results in a 14° rode angle (at the anchor to rode connection).
Anchoring in 10 ft. of water from a 5ft. high bow (total of 15ft 'depth') at 4:1 scope (60 ft run per 15 ft. total depth) requires 62 ft. of chain
Anchoring in 10 ft of water with a Bridle set AT the bow waterline at 4:1 scope requires only 41.3 ft. of chain.
62 ft of chain is a lot longer then 41 ft.

;-)

I beg to differ. The simple fact is that the addition of the snubber, or bridle, doesn't change the attachment point of the rode to your boat. Maybe, if there was a significant angle where the snubber met the chain, but this is not the case. Whether in a calm, when the snubber and chain fall straight down from the bow, or in a bow, then they are stretched out straight, the attachment point remains the same. Perhaps you can explain by what logic of mechanical principle the use of a bridle lowers the effective attachment point of the rode to the boat, I might get your point.

 

[RichH]

I beg to differ. The simple fact is that the addition of the snubber, or bridle, doesn't change the attachment point of the rode to your boat. Maybe, if there was a significant angle where the snubber met the chain, but this is not the case. Whether in a calm, when the snubber and chain fall straight down from the bow, or in a bow, then they are stretched out straight, the attachment point remains the same. Perhaps you can explain by what logic of mechanical principle the use of a bridle lowers the effective attachment point of the rode to the boat, I might get your point.

Sorry, but you simply cannot 'negotiate' trigonometry. No maybes, no whethers, no perhapses.

The total scope is affected by the total depth from the attachment point of the boat to the bottom, not simply water depth. A bridle lowers the effective attachment point to near the boat's waterline at close to the bow .... you then can subtract the height from the water to the top of the bow ... and trigonometry takes over from there to require LESS total amount of chain or rope or whatever. Without a bridle the total depth for 'scope' purposes is the height of the bow from the water 'plus' the depth of the water; with a bridle holding at close to the waterline at the bow the 'depth' is the depth of water.
;-)
;-)

 

[jviss]

Sorry, but you simply cannot 'negotiate' trigonometry. No maybes, no whethers, no perhapses.

The total scope is affected by the total depth from the attachment point of the boat to the bottom, not simply water depth. A bridle lowers the effective attachment point to near the boat's waterline at close to the bow .... you then can subtract the height from the water to the top of the bow ... and trigonometry takes over from there to require LESS total amount of chain or rope or whatever. Without a bridle the total depth for 'scope' purposes is the height of the bow from the water 'plus' the depth of the water; with a bridle holding at close to the waterline at the bow the 'depth' is the depth of water.
;-)
;-)

Your insulting, pedantic manner aside, you still have not offered an explanation as to why the use of a bridle lowers the effective attachment point. It does not. You are wrong.

Don't give me this "you simply cannot 'negotiate' trigonometry" crap, just explain why the effective attachment point is lower with a bridle. I have studied mechanics, and if you can show me why the pull on the chain at the point where the bridle is attached suddenly becomes horizontal, I will pay close attention, and give you your due.

Magic aside, the pull on a rope or chain is along the axis of the rope or chain. Unless there's a distinct angle at the point where the bridle meets the rope, that remains when the pull is significant, then it doesn't work as you assert.

I have ridden out blows that straightened out the snubber to chain connection, at least what could be seen about the waterline. What would you say about the geometry in that case?

 

[Steve W]

Question answered! Thanks, all.

I thought the scope on chain would be less than nylon (4:1 or 5:1 as opposed to 7:1 on rope) and everyone's confirmed.

In terms of a snubber, I use it all the time. I've got a 10' line with a chain hook on one end and I tie it to a hefty cleat (for that purpose) in the anchor locker. When I've got the requisite length of chain out and the anchor's set, I tie off the snubber to the cleat, and hook the other end to a "reachable" link forward of the bow roller and then let the windless pay out the chain 'till the snubber is taut.

Don't ask the diameter of the line 'cause I'm not sure except I made it from cutting the existing rope rode when I switched out from line to chain.

Right now, I've got about 110' of 5/6" chain spliced to another 50' of rode secured to a fitting in the bottom of the locker.

Thanks again, all.