Ideal Catenary Angle

[O C Diver]

In my perfect anchoring scenario, the first 6' of chain are never lifted off the seabed or buried in the mud with the shank. While it doesn't completely eliminate the shock effect, the jerk isn't trying to break the anchor out.

Ted

 

[Nomad Willy]

While the rode is on the seafloor the shank may be well below the seafloor. When one pulls on the rode the shank end tends (strongly) to dive down into the seabed. And some seabeds are soft and some are hard. So as you anchor one night after another the set anchor may have various fluke angles. And fluke angles have a significant effect on holding power.

If you had an anchor that had a low throat angle having the shank end high will tend to allow to allow the anchor to break out in the direction of the rode. That is the anchor would go fwd but up instead of down.
If you had an anchor that had a high throat angle (w the shank end low) the anchor will tend to break out vertically. Somewhat like a Danforth w too much TA dragged on a beach tending to jack up the back of the fluke. This is the action that causes a vertical breakout. If you need a visual on that I can provide.
At any rate the fluke angle likely could be too great to create max holding. Kinda like an aircraft wing stalling.
So the long and short of it is holding power can vary even at a very long scope depending on anchor design. And it can vary such that holding can be less at long scope that somewhat shorter scope w the rode up off the seafloor. And it has to do w the anchor design. So some anchors can have increased holding by shortening up the scope. However the bottom line is it can happen. That dosn’t mean the effect it has is significant but if I’m correct in my recollection it caused one anchor to demonstrate a lower holding at longer scope at and during an anchor test. That could be a significant amount but probably not enough to warrant buying this anchor or that. Mostly because it effects only long scope performance ...... or does it? I think anchors tend to perform well at one end. The Supreme couldn’t best the Rocna on long scope but had no trouble showing a much better short scope performance. But either one may show a lot better performance at both ends .. as in overall. But re new gen anchors most probably shine above others only at one end .. of scope.

If I was an anchor manufacturer I’d prefer that my product scored better where max holding was achieved and measured. This is be cause most all boaters choose based on max holding.
But if it were me I’d choose the anchor that performed better at short scope because most of the time max holding (or anything close to it) is not needed. But a shorter swing frequently IS important. And there are other performance factors like setting that likely should be more important than max holding.

Getting off the subject ....

 

[O C Diver]

Eric, I'm a believer in no moving or screwed together parts on an anchor to mess up. I use a Rocna and before that an original Bruce. If the shank is resting on the seabed or buried in the mud, the fluke angle is correct and isn't going to change.

Ted

 

[Nomad Willy]

It changes relative to seafloor and the pitch or height of the end of the shank.
Sure the TA (throat angle) of the anchor does not change.
The operative element is that the shank controls the angle of the fluke to the horizon, the seafloor and the anchor’s direction of travel.
Too many variables?

I flew UL aircraft, owned five and built them all except one.
One is dependent on many bolts, nuts, pins ect ect.
You just hafta make sure they are as they should be.

 

[syjos]

I posted this on another rode discussion. The diagram is from Earl Hinz's book "Complete Book of Anchoring and Mooring" which is the definitive bible on anchoring.

The critical part of anchor - rode relationship is the angle of the fluke to bottom and the angle of the rode (lead) and shank to the horizontal when the rode is completely tight. Less angle, more horizontal pull, more holding power. The lead angle is calculated from the point of attachment on the boat to point of attachment on the anchor when rode is taut.

Most boat anchors are designed for a lead angle of around 8°. Scope is calculated for various bottom compositions, rode materials and combinations to maintain a lead angle of less than 8° when the rode is taut. Newer anchors supposedly can stay buried with less lead and shank angle but the angle reduction is less than 1°.

I don't believe that holding power diminishes with too much scope.

Having more scope than required is counterproductive because it creates issues with boats around us with a larger swing circle. More scope, more chain out, require us to anchor further from shore to avoid drifting into shallows. Anchoring further from shore in deeper water necessitates more scope and a bigger circle. That's counterproductive.

I could probably replace my 66 Genuine Bruce with a 20 to 30 pound anchor on scopes of 5 to 1 and more and it would probably hold from the minimum horizontal pull, the weight of the chain and the resistance of the chain dragging across the bottom.

Finding a compromise between scope, swing circle and secure holding is what makes anchoring a mystery or challenge to boaters.

 

[Nomad Willy]

I hear what you are saying Eric, and it may be so in a strictly geometrical sense, but you are overlooking the other huge factor in that statement. That is the shock protection effect of a decent length and weight of rode between the shank and the place where the pull lifts the rode off the bottom. It is generally recognised I think that it is when the pull is so strong that all the rode is lifted off the bottom, (effectively becoming a straight line, with no catenary), and the shocks/jerking from wind and wave action is being directly passed to the shank, that the anchor will most likely break loose, even if at that angle it gives the whole rode the perfect fluke to shank angle. Just sayin'...

Peter B,
What I’m talking about has nothing to do w catenary or/and chain.
It’s about the relationship (angle wise) of the fluke, shank and direction of anchor travel in the seafloor.
I’m talking about generally recognized things that may not be completely true.

You’re trying to help me (as a good friend should) by convincing me do ditch my thoughts and notions and “do as da book says”. Everything that’s in those books got there somehow and at one time not thought of by anybody. You’ve got to ditch conventional thinking to understand something new. I’m trying to open people’s minds up see a new thing or thought to show us that old long held thinking is not absolute. The ability to accept or just understand new things is something we learn all our lives and it enables us to reach out and formulate/learn new things. Some of which will need to be discarded later as we learn and come in contact w even more new things.

HaHa I did get an A in philosophy.

 

[O C Diver]

I don't believe that holding power diminishes with too much scope.

Having more scope than required is counterproductive because it creates issues with boats around us with a larger swing circle. More scope, more chain out, require us to anchor further from shore to avoid drifting into shallows. Anchoring further from shore in deeper water necessitates more scope and a bigger circle. That's counterproductive.

I think you're confusing a regional problem with optimal anchoring. Clearly more scope is better to maybe 7:1 and maybe 10:1 or more for storms. I can appreciate the desire to accommodate more boats but that doesn't make anchoring elsewhere at 7:1 counterproductive.

Ted

 

[O C Diver]

It changes relative to seafloor and the pitch or height of the end of the shank.
Sure the TA (throat angle) of the anchor does not change.
The operative element is that the shank controls the angle of the fluke to the horizon, the seafloor and the anchor’s direction of travel.
Too many variables?

I flew UL aircraft, owned five and built them all except one.
One is dependent on many bolts, nuts, pins ect ect.
You just hafta make sure they are as they should be.

Eric I flew an Ultralight trike for a number of years and qualified to do my own annuals when it was N numbered. One of the first rules of aviation design is to avoid adding unnecessary failure points. Reliability of critical components through simplicity of design.

Ted

 

[markpierce]

Gee, if five-to-one scope isn't sufficient, perhaps one needs a heavier, better anchor.

 

[Nomad Willy]

Mark,
An anchor that’s better than a genuine Bruce?
Nawwww it couldn’t be .....

Seriously 4-5 to one is probably close to optimum for Claws. But all Claws aren’t the same .. one can clearly see just by looking. And I still don’t know much about what Claws are better. The Bruce brand is clearly a good Claw but there may be one better. Nobody knows.
Carry on mate .. your doing just fine.

 

[syjos]

I think you're confusing a regional problem with optimal anchoring. Clearly more scope is better to maybe 7:1 and maybe 10:1 or more for storms. I can appreciate the desire to accommodate more boats but that doesn't make anchoring elsewhere at 7:1 counterproductive.

Ted

More scope than necessary to hold the boat for conditions is counterproductive in any crowded anchorage.

We only anchor during summer when wind around the PNW is 15 to 25 knots in protected anchorages with minimal fetch. A strong wind during summer may hit 35 knots. Gales on the straits are much higher but we don't anchor on the strait. We use 3 to 1 in most conditions and 4 to 1 if we expect strong wind or raft a boat alongside.

Difference from low to high tide can be 10 to 15 feet here. Anchoring in an anchorage with a tidal range of 20 to 35 feet of water, we would put out 105 feet yielding a scope of 3 to 1. At 7 to 1 scope, 245 feet of rode would need to be deployed and at low tide we would be swinging in a pretty big circle, resulting in a lot of pi%#ed off people.

Accommodating other boats is only one reason to be efficient with scope. The other is distance to shore. Excessive scope means big circle so anchor further out. Further out means deeper water necessitating more scope. On and on.

If 3 to 1 or 4 to 1 was inadequate to hold Sandpiper in typical conditions, I'd go to a larger anchor, which we did a few years ago when we started rafting a 56' Sonship alongside us. We went to a 66 Genuine Bruce from a 44 GB. Or go to bigger chain.

We sat out a gale up in the Broughtons once and we were the only boat in an anchorage next to a strait. We put out 5 to 1 to be safe and in hindsight 4 to 1 would have sufficed.

 

[O C Diver]

More scope than necessary to hold the boat for conditions is counterproductive in any crowded anchorage.

We only anchor during summer when wind around the PNW is 15 to 25 knots in protected anchorages with minimal fetch. A strong wind during summer may hit 35 knots. Gales on the straits are much higher but we don't anchor on the strait. We use 3 to 1 in most conditions and 4 to 1 if we expect strong wind or raft a boat alongside.

Difference from low to high tide can be 10 to 15 feet here. Anchoring in an anchorage with a tidal range of 20 to 35 feet of water, we would put out 105 feet yielding a scope of 3 to 1. At 7 to 1 scope, 245 feet of rode would need to be deployed and at low tide we would be swinging in a pretty big circle, resulting in a lot of pi%#ed off people.

Accommodating other boats is only one reason to be efficient with scope. The other is distance to shore. Excessive scope means big circle so anchor further out. Further out means deeper water necessitating more scope. On and on.

If 3 to 1 or 4 to 1 was inadequate to hold Sandpiper in typical conditions, I'd go to a larger anchor, which we did a few years ago when we started rafting a 56' Sonship alongside us. We went to a 66 Genuine Bruce from a 44 GB. Or go to bigger chain.

We sat out a gale up in the Broughtons once and we were the only boat in an anchorage next to a strait. We put out 5 to 1 to be safe and in hindsight 4 to 1 would have sufficed.

Again you're dealing with a regional issue, depth of water and small anchorages. 3:1 in 10' of water would have you dragging through the anchorage in 15 to 25 knot winds on the east coast. To say someone can anchor 3:1 in 10' of water because you do it in 35' is short sighted. There's a big difference in shock loading (jerking) between 30 and 105' of rode in the water.

Ted

 

[syjos]

Again you're dealing with a regional issue, depth of water and small anchorages. 3:1 in 10' of water would have you dragging through the anchorage in 15 to 25 knot winds on the east coast. To say someone can anchor 3:1 in 10' of water because you do it in 35' is short sighted. There's a big difference in shock loading (jerking) between 30 and 105' of rode in the water.

Ted

I don't recall mentioning anything about anchoring in 10' of water. Or correlating 35 feet of anchoring depth to 10.

The minimum depth we anchor at mid tide is around 30 feet and least chain we've deployed is probably 90' which should along with the chain riding stopper mitigate surge induced shock loads.

Anchoring in 10' of water here in the PNW will guarantee a beaching.

 

[psneeld]

I think that's Ted's point. 10' is a common anchorage depth in many East Coat anchorages and the theory is higher scope ratios are needed in shallower water for several reasons.

 

[syjos]

I think that's Ted's point. 10' is a common anchorage depth in many East Coat anchorages and the theory is higher scope ratios are needed in shallower water for several reasons.

I completely agree.

 

[Slider70]

My experiences after 54 years of anchoring on the BC Coast from Washington to the Alaska boarder, with 29 or those years commercial fishing and anchoring almost every night. Over those years I have used a number of different types of anchors and anchor rodes.

It would be nice if all anchor bottoms were the same - mud or parked sand. Unfortunately this is not the case. Bottoms come in dozens of different make ups. Worst is flat rock, usually granite with everything in between. Then we have tight areas with limited swing and large areas with major swinging room.

I have found that the old, dropped forged Northhill the best all round anchor with heaviest chain rode one can handle. Nest the plow type, then the navy type (poor in most bottoms) and last the Danforth (great on some bottoms but terrible on rocky ones. Rocky ones being prevalent in many BC Coast anchorages.

I often go with 3 to 1 scope but if swing room is OK kick it up to five to one. Rarely going more unless exceptional strong winds are present or forecast. If extreme I have added a heavy lead weight a few fathoms from the anchor to create more grab and reduce the shock when the rode gets snapped tight.

To answer the original question, I find increase scope produces better hold but often swing requires adjustment of ideal.

 

[RickyD]

There is much talk about catenary and scope. Often, more often than it would seem necessary or appropriate.

But in many anchoring discussions I think it’s assumed that the anchor is in an ideal position somewhere north of 6-7 to 1 scope. And 10-1 talk sounds like “you can’t get better than that”. This post is saying you can.

Every anchor probably has an ideal angle of rode to fluke. At 10-1 scope w the rode at the anchor shank and laying flat on the seafloor one tends to think it’s an ideal arrangement. I think it may .... but it may not. Relative to a specific seafloor every anchor has an ideal angle to it’s fwd progress in the substrate relative to the angle of pull. The angle of pull is not the throat angle of the anchor.
If you discard the shank and put an eyebolt at various fore and aft positions along the fluke center the anchor fluke will assume a position based on a balance and when that balance positions the fluke at an ideal angle maximum resistance will give the anchor it’s maximum holding power.

But when all these positions, angles and balance come together it may not be at a 10-1 or even 7-1 scope. It may even happen at 5-1. No anchor test has ever addressed this issue or concept.
I read somewhere that one anchor in a test did better at a moderate scope than at a long scope ... like 10-1.
But I don’t see anchor tests turning into scope/holding power tests. First it would be too much work. But more importantly it would take down (IMO) the assumption that more scope is always better. I think that it’s possible 4-1 or 5-1 scope may be the best scope for performance.

Anyway I welcome any ideas that may shed light on the above. When I think about the ideal anchor I sometimes think of a joint in the shank that would allow the fluke to find it’s ideal angle. There are some big boat or ship anchors that articulate around a point in between the fluke tip and the crown. The Danforths articulate around and at the crown. There’s a lot to be gained by this (I think) and it’s related to ideal scope.

In this picture the articulation point is not at the crown like a Danforth. The articulation point is further fwd toward the fluke tip. This way there is not big pressure differences around the flukes but a more even fluke loading. With more even loading higher loading can be had overall.

Ok, I'm going to build an anchor with a motorized shank to fluke adjustment system that is controlled by an app on your smart phone. So, trouble holding in mud?.....increase fluke angle. Skipping on the surface of hard bottom?......decrease fluke angle. Don't seem to be holding in a blow?.....Try different fluke angles. Worm gear drive provides strength rivaling bolt together systems (Mantus). Optional underwater antennae for signal detection over 1000 meters in salt water. Everyone send me money for the app and I'll try to have the anchor ready in a couple years, no guarantee.

 

[Andiamo2018]

So many different issues raised here. Many misleading. This isn't a 'truth is somewhere in the middle thing' or Fords vs Chevys etc. At best it is a theoretical discussion with little or no relation to anything that can actually happen in the real world even under the perfect conditions.

Max holding power comes from the anchor being pulled horizontally. Period.

You need enough scope to achieve that horizontal pull. Of course there are times there are too many boats too close together to let out so much scope that you can be nearly certain the pull is horizontal.

BUT, if Captain Marvel and Thanos had a fight who would win?

 

[Andiamo2018]

Perfect solution to anchoring issues!!

Perhaps the advanced 'gold' model would have its own onboard computer to work automatically? Tie it into your topside equipment it could even scan the bottom when your autopilot gets you to your destination and it would deploy itself in the best place possible for whatever goal you wish (close to shore, far from others etc).

You are going to be rich patent it quick!!

Ok, I'm going to build an anchor with a motorized shank to fluke adjustment system that is controlled by an app on your smart phone. So, trouble holding in mud?.....increase fluke angle. Skipping on the surface of hard bottom?......decrease fluke angle. Don't seem to be holding in a blow?.....Try different fluke angles. Worm gear drive provides strength rivaling bolt together systems (Mantus). Optional underwater antennae for signal detection over 1000 meters in salt water. Everyone send me money for the app and I'll try to have the anchor ready in a couple years, no guarantee.

 

[markpierce]

I think that's Ted's point. 10' is a common anchorage depth in many East Coat anchorages and the theory is higher scope ratios are needed in shallower water for several reasons.

Perhaps East Coast boaters are not factoring the vertical distance from waterline to bow roller. or is it the short rode has limited catenary.

 

[psneeld]

Yes we know about roller height.....

Correct about short scopes and catenary.

Shallower depths require greater rode ratios because as Ted mentioned, the rode length becomes ridiculously short in very shallow depths.

Take the Indian River lagoon in Florida.... something like 150 miles long, miles wide and many of the anchoring spots I use are 6 to 8 feet deep. Most of the lagoon out of the channels is only 6 to 8 feet deep. Much of it has only inches, maybe a foot depth change over 24 hours.

So with a roller 4 feet off the water and a 3 or 4:1 ratio... the scope would be 30 to 40 feet .....much less wind can straighten out 30 to 40 feet than the usual 75 or so I use. Normally there is no one nearby and not much chop but a few decent wakes. So my ratio is more like 7:1.

 

[esheufelt]

Change the question from Ideal to appropriate

I think we're tangled up because the question wasn't specific enough.


Two questions one could ask are;


One, at what scope is there no increase in holding power.

Two, what's the right scope for a given boat and anchorage. (and weather and other boats and duration and whether the boat will have someone aboard ....)


The second question is the one we have to answer every time we anchor and the truth is there isn't a fixed answer without answering all the other dimensions.


My own practice was to never put out less than about 70' even in shallow water and to shoot for 3-5 to 1 in settled conditions. Ive never needed more than 7 to 1 but i had realistically sized gear. 3/8 HT chain and a 66# Bruce on a 48' sailboat.


Some times the right scope is 0:1 in that its a bad idea to anchor. Too crowded, foul bottom, poor holding, exposure to wind shifts without safe swing area. Some places anything works and others nothing works.



I used to try to talk with folks that anchored unsafely near me but any more i just move. Its less stress in the end. My wife an i have a bunch of of funny one liners we use from the days of trying to talk with folks.



I love anchoring, it's a real challenge to try to foresee all the issues and try to pick the right spot. I drive my wife crazy because i usually circle around pick a spot, anchor then sit for a while and decide we need to move just a bit that way.

 

[rslifkin]

Catenary is not the only reason why more scope is needed in shallow water. And based on a recent discussion on another forum, catenary is pretty worthless in shallow water regardless of how much scope you use.

One big factor is waves. Even a 1 foot wake from a passing boat is a pretty significant change when you're in 8 feet of water. But in 50 feet, it's nearly irrelevant in terms of impact on angle of pull on the anchor. That smaller change also means that if the rode is already tight, the boat doesn't have to move forward as far to allow the angle change. That means the additional pull applied to the rode from wave action is less in deeper water (being that the boat will resist being pulled forward).

 

[O C Diver]

Perhaps East Coast boaters are not factoring the vertical distance from waterline to bow roller. or is it the short rode has limited catenary.

It's like this:
If you anchor in 10' of water with 3:1 scope, that's 30' of rode in the water. With wind or waves, every time the boat draws the rode taught (even with a snubber), there is a significant to shocking jerk on the chain because the slack goes away almost instantly. Now consider that same situation with 70' (7:1) of rode. A much reduced jerk on the anchor and the draw bar is much closer to the seabed (horizontal pull).

Ted

 

[gilbertpark]

Interesting discussion, my thought is that chain in the locker isn't doing anything, unless there is a reason not to let a good amount out (I use 5:1 normally more if the wind and waves are picking up) I can't believe there is any downside. The chain also act as a dragging weight enhancing the effect of the anchor.

I also use an angel - I know there is controversy about angels but I sleep better when its out, especially in crowded anchorages - like we have in the UK.

 

[AUXOPdwp]

Ideal Catenary = </= 8 degree lead angle

Jim Hancock (Masters Degree in Naval Architecture & Marine Engineering MIT) mathematically modeled his alternatives as he sailed the Pacific. His findings were published in an article "Scope For Improvement" in the July 2004 issue of Sail Magazine. His findings are entirely sensible, yet surprising in some respects; e.g. more scope is required in shallow depths than we normally suspect. I've tried to upload jpegs


"It depends on the bottom" is the truest statement in this thread IMO. While delivering a trawler from Ft Lauderdale to Annapolis I had to seek shelter in "Mile Hammock Bay" a man made small harbor on Marine Base Camp LeJeune, off the AICW. https://www.google.com/maps/place/M...1a0462b162f72a!8m2!3d34.5509661!4d-77.3245453 A 35# Delta Fast Set wouldn't hold in 9' over soupy mud in merely 7 knot winds, despite 4 tries! Upon seeing the soup after 4 retrievals, I SLOWLY paid out 140' of 3/8" BBB and relied on surface tension to avoid dragging.



A U-Bolt installed near the waterline can secure a snubber to the rode and reduce the swinging circle considerably by eliminating shortening the deployed rode. If water to anchor roller is 9 feet in a 7:1 circumstance, the 9 X 7 = 63' less rode deployed.



Hopefully my jpeg attachments are hereby available to you.

 

[O C Diver]

"It depends on the bottom" is the truest statement in this thread IMO. While delivering a trawler from Ft Lauderdale to Annapolis I had to seek shelter in "Mile Hammock Bay" a man made small harbor on Marine Base Camp LeJeune, off the AICW.

Makes me laugh.
Mile Hammock bottom varies quite a bit. Most would tell you it's sticky mud and a royal PIA to clean off your chain and anchor. Other parts have lighter mud. Have anchored there more than a dozen times, once with 23 other boats. Very popular spot with us Snowbirds. I'm never less than 7:1 there.

Ted

 

[AUXOPdwp]

Makes me laugh.
Mile Hammock bottom varies quite a bit. Most would tell you it's sticky mud and a royal PIA to clean off your chain and anchor. Other parts have lighter mud. Have anchored there more than a dozen times, once with 23 other boats. Very popular spot with us Snowbirds. I'm never less than 7:1 there.

Ted

Yep. In a half dozen previous visits, my 44# Rocna performed well enough on 7:1 scope with my 42' Sloop - although the soupiness of Mile Hammock was quite apparent. The subject incident made me appreciate the limitations of Delta Fast Sets. They're obviously not 'new gen geometry', which is typically important in soft bottoms. I've recently acquired a 47# Sarca Excel that incorporates both concave and convex surfaces to succeed. I'm eager to assess results.

 

[esheufelt]

Interesting discussion, my thought is that chain in the locker isn't doing anything, unless there is a reason not to let a good amount out (I use 5:1 normally more if the wind and waves are picking up) I can't believe there is any downside. The chain also act as a dragging weight enhancing the effect of the anchor.

I also use an angel - I know there is controversy about angels but I sleep better when its out, especially in crowded anchorages - like we have in the UK.

I've had one situation where chain in the locker was really useful. If the bottom is really foul and your chain keeps fouling so you're essentially at 1:1 scope having some left to get you through the night is good.


One trick i learned was to float the chain with fenders in to a controlled catenary. This protected to coral if the sand patch wasn't big enough to swing. Spent 2 weeks anchored in Fakarava in the Tuamotus and learned several painful lessons.


Broke a 1/2" nylon snubber twice in one night.

 

[gilbertpark]

Thanks Jim for a very interesting article you posted. In the UK (especially in the summer) there isn't usually enough room in popular anchorages to have all that chain out, especially in a motorboat that tends to swing around. I note the other names you have for an Angel and will try and use them in future, that said I do fine it useful to limit the swinging arc if placed right under the boat.