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Anchoring on Short Scope?

Jul 27, 2011
3,363
Bavaria 38E Alamitos Bay
There's been a little, somewhat critical, discussion in the "Rode Courtesy" thread about anchoring on short scope, which I interpret to mean a scope of less than 3:1--the ratio of the amount of rode veered out to the depth of the water, as it is sometimes defined. Some might argue short scope would be anything less than 5:1; etc., who knows? But first, what depth of water do we mean? The depth where the anchor was dropped or the depth of the water under the boat as it lays to the anchor? Considering that the depth of water under the boat is not a constant, at least not in the ocean even if moored by bow and stern anchors, due to tidal flows, scope is something that varies while anchored if the latter definition of "depth" is applied, which is the prevailing view, of course.

So, that brings me to the question of what is "short scope" when anchored on a steeply-sloping bottom, as it often is the situation when anchoring in the PNW. I cite the Harmony Islands as one example.There, an anchor might be dropped in 90 ft, whereupon the boat is backed toward shore for making a stern tie. Hardly 150 ft (maybe less) of rode MIGHT be veered before the vessel finds itself in maybe 25 ft at mid-tide on the spring-tide series. Now what? Reset in, say, 180 ft of depth to gain maybe a few more feet of rode veered out, but which in terms of scope little would have changed? It gets more ridiculous much beyond that. When the tide floods, the boat may rise another 8-10 feet. This could be in the middle of the night. So what might arguably have been effectively 6:1(?) (150'/25') scope referencing to the depth under the boat when first anchored and stern-tied, laying to its anchor, becomes maybe 4:1 (150/35) a few hours later. And there might be other boats nearby which serves to limit anchoring options. Would anybody consider the anchored vessel as described being on short scope?
 
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Jul 1, 2010
689
Seaward 25, Catalina 350 Erie, Pa
I always figured the depth to be is the depth where the anchor is dropped plus the distance from the water to the bow roller...at high tide. The irresponsible guy is always the guy in the powerboat with the puny stainless anchor that is out on less than 3:1 and wants to anchor right next to you :) I did successfully chase one of these guys off a couple of weeks ago. To prove to his wife that no one made him move his anchoring spot, he left the anchorage. Good riddance. He jammed himself between us and another powerboat. I'm sure the other powerboat was as glad this guy left as well as we were.

From what I've read, you guys in the PNW have some challenging anchoring situations. Certainly, not typical of our anchorages on the east coast and great lakes.


Interesting article on "short scope" anchoring here:

 
Jan 12, 2016
212
Hunter 410 Ladysmith, BC
I met a charter Captain this summer in Secret Cove who worked 6 months in British Columbia and six months in the Caribbean. He told me some great stories over beer, and one bit of advice regarding scope that I took to heart. He said he had dragged in the Bahamas over sand with 7:1 out whereas with similar tackle/boat and wind conditions he was glued to the bottom with 3:1 when anchored locally over a mud bottom. For him, the bottom type was the most important factor, scope was the secondary concern.

Generally in our local mud bottom anchorages I try to anchor with 5:1 mean low tide scope knowing that it will shorten up to around 3.5:1 at max high tide. If the winds are expected to howl, and I have the room, 7:1 low tide will be my aim, but I've never dragged with as little as 3:1 over mud bottom with 30-35kts of gusts. Having 200' of chain plus another 150' of muliplait rope, an up sized rocna anchor, and 30' per side bridle/snubber. This summer in Squirrel Cove, Desolation Sound, we saw boats dragging in 25-35 kts while we swung laterally but never dragged. The boats that were dragging were high freeboard flybridge power yachts with fortress type anchors that seemed to have trouble getting a good set. The combination of massive freeboard, smallish anchoring tackle for their boat weight, and some umm... interesting technique, all seemed to combine to achieve less than stellar results.

If needing to anchor in really deep water, I would try to limit these to fairly calm nights where a stern line could be run ashore. I'm not fond of anchoring in more than 50' of water if it can be avoided.
 
Oct 19, 2017
5,055
O'Day 19 Littleton, NH
KG, I thought your response to the short score criticism on the "Rode Courtesy" thread was excellent (Rode Courtesy).
Circumstances are often unique for something like anchoring. The commonly accepted standards, 7:1 rope rode/3:1 chain rode, are good and conservative guidelines. They assume a certain size anchor to boat tonnage/windage/draft ratio and/or a certain weight chain.
Frankly, I've never considered the problems inherent with a steeply sloped bottom, the East Coast, esp. the Gulf Coast of Florida doesn't really give one much experience with that.
In terms of scope, depth of anchor is the only consideration for scope, but individual situations such as a sloping bottom, the type of bottom, the sea conditions (open to the sea or protected lagoon), the relative size and configuration of your ground tackle, tide, current, wind direction and your neighbors, the experience of the sailor with the vessel and equipment and conditions are all to be factors.

-Will (Dragonfly)
 
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Nov 26, 2008
1,879
Endeavour 42 Cruisin
I used to follow Beth and Evens who were 'high latitude' sailors. They spent a lot of time in the Beagle channel down near Tiera Del Fuego where anchoring in 100' is common with williwaw's unexpectedly crashing down on them at 70 knots. They found that in deep water, 3:1 worked fine.
 
Jul 27, 2011
3,363
Bavaria 38E Alamitos Bay
No need to define scope. That’s well established. If you want science (not opinion or anecdotes) then look here Tuning an Anchor Rode
Very interesting, Captain Larry. I do believe I’ve come across that analysis before now. It’s easy to be impressed with it. But I note that the “science” to which you refer is a set of exact solutions to approximate anchoring models containing simplifications as well as caveats. The “opinions or anecdotes” represent the converse—approximate solutions to exact models; that is, approximate solutions to actual situations at hand, but ones which work and which are reported to other sailors, etc. Not to be ignored; it should not be an either/or call, IMHO.

Nowhere in the publication did I see MY basic questioned addressed. What is the proper scope NOT when anchoring on a level bottom, but on a steeply sloping one? I’m sure that can be modeled in the same fashion as the level bottom perhaps with few changes, but with the same shortcomings. The way I see it, as the boat backs away from the anchor toward shore and going shallow, the rode is paralleling the slope. So, it would not be pulling up at the anchor, but along the bottom keeping that angulation parameter of low value.;) So you would be achieving, effectively, a “horizontal” loading of the anchor. What would be the “proper” scope?

KG
 
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Jun 11, 2004
812
Oday 31 Redondo Beach
The way I see it, as the boat backs away from the anchor toward shore and going shallow, the rode is paralleling the slope. So, it would not be pulling up at the anchor, but along the bottom keeping that angulation parameter of low value.;) So you would be achieving, effectively, a “horizontal” loading of the anchor. What would be the “proper” scope?
I agree with your "horizontal loading" view. If you knew the anchor depth, depth under your boat and your rode length you could calculate your effective "level bottom equivalent" scope. Of course that would contemplate a constantly sloping bottom but I think it would be a useful consideration in determining what you think your "proper" scope should be under the circumstances. I don't "do the math" but definitely take that into consideration when anchoring on relatively steeply sloping bottoms (like Goat Harbor on the front side of Catalina)..
 
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Sep 11, 2017
164
Beneteau 373 Cape Cod
The depth of the water above the ANCHOR (plus the height of your deck above the water) is the depth that matters... Draw a picture of it, and you'll see that makes sense. Draw an extreme picture where your anchor is in 10 feet of water and your boat is floating above 1000 feet of water, and you'll see that the amount under your hull has NO impact on the angle your rode is pulling on the anchor... whereas the boat in 10 feet and the anchor in 1000 is impossible to anchor in. I'm an east-coast boater, so while the concept is the same here, in practice, what constitutes "crowded" and "deep" are very different. I've never anchored in anything deeper than 16-18 feet... mostly around 10 feet... add 4 feet of freeboard, and I almost always anchor 7:1 without getting the 100 foot marker on my nylon rode wet at all. With mostly nylon and 30 feet of chain at the end I generally do 6:1 or 7:1, and wouldn't consider going any less than 5:1. No snubber needed.
 
Jan 22, 2008
7,078
Beneteau 323 Annapolis MD
...So you would be achieving, effectively, a “horizontal” loading of the anchor. What would be the “proper” scope?
Seems to me you're making much ado about nothing. I tell crew to let out 75 feet, I make it bite in, then another 25 with a snubber.
 
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Jun 11, 2004
812
Oday 31 Redondo Beach
The depth of the water above the ANCHOR (plus the height of your deck above the water) is the depth that matters... Draw a picture of it, and you'll see that makes sense. Draw an extreme picture where your anchor is in 10 feet of water and your boat is floating above 1000 feet of water, and you'll see that the amount under your hull has NO impact on the angle your rode is pulling on the anchor...
Draw your picture the other way around and I think you will see that the outcome is completely different. Your rode is going to be pulling over the edge of a cliff, from deep to shallow, and it is going to be pulling the anchor parallel to the sloping bottom. Effectively infinite scope.
 
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Oct 19, 2017
5,055
O'Day 19 Littleton, NH
When anchoring with all chain, the deeper you anchor, the less scope you actually should need.
Since the point of "all chain" is to provide the weight to induce a natural curve that gives you a better biting angle on the bottom and a natural shock absorption as that curve is pulled out. The depth to length ratio needed to achieve that effect is less as more heavier and heavier chain is paid out.
Let's say your 7/16" chain weights about 200 lbs/100'. In ten feet of water, the relative weight of the chain will not be, in absolute terms not by ratios, as much more than the weight of a rope rode compared to the difference in 30' of depth. In 10' at 3:1, you will want about 40' of rode out. There will be somewhere in the neighborhood of 80 pounds of rode between the hosepipe and the anchor. To induce a bend in your rode, that puts a 10:1 angle on the anchor at the bottom on a day with 20 knots of headwind, you may need 100 lbs pulling down in the center of your anchor rode (I'm making this number up for the purposes of the illustration, I have no idea what would be an appropriate number). That pressure acts to keep your boat in place against the wind, combined with the holding power of your anchor at the 10:1 angle. In 10' of water, The induced bend will be minimal and a 4:1 - 5:1 scope might be more appropriate than a 3:1. The more chain you let out, the more weight there is to counter the pressure of wind and waves to move your boat back away from your anchor. Obviously, right?
In 30', at 3:1 scope, you have about 100' of chain out and that's a good 200# working to put that bend in the rode. Plenty of counter to straightening out your rode and maintaining 10:1 at the anchor. Go to a depth of 100' and 3:1 scope gives you over 600# of rode pulling down to hold that bend. Mostly, your rode drops straight to the bottom and you will have almost 200' of chain, just laying on the bottom to your anchor. That's 400# of chain the wind and waves have to pickup and straighten out to get your anchor angle steeper than 10:1. Shorten up that scope to 2:1 and you should still have about 70' of chain just laying on the bottom to act as a bend inducer to keep your shallow angle pull against the anchor.

You can color code your rode. Paint or stitch a colored stripe in your rode at 20', then put that same stripe in the rode at 60' (140' for rope rode). Add another color at 30' with a matching color at 90' (210'). You get the idea. Then there's little question of having the desired scope out.

-Will (Dragonfly)
 
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Jun 14, 2010
925
Quorning Dragonfly 1200 home
When anchoring with all chain, the deeper you anchor, the less scope you actually should need.
+1. Agree in principle. However, in storm conditions and less than extreme depth there is a possibility of pulling that chain “bar tight” and that snatch load can be at shorter effective scope - and yank the anchor out (a long snubber adds elasticity and safety). I only have 60 feet of 5/16” chain (approximately 1.1 pound per foot) and the remaining rode is brait, so I don’t rely on catenary to improve effective scope. Catenary is nice to keep the boat from meandering when the wind dies, but for holding purposes (when really needed) I ignore it as a factor.
 
Oct 19, 2017
5,055
O'Day 19 Littleton, NH
With 60' of chain, you are effectively using all chain until you've paid out about 70' of rode. In the depths that would work in, your 7:1 rule of thumb would be completely appropriate.
As far as storm conditions in deep water, with, in the case of my above example, a 2:1 score on 100' of depth, a wave would need to be over 50' tall to pickup 25' of that chain off the bottom and the wind and surfing forces pulling back on the boat would still have to overcome the 400# of force pulling down on the chain to straighten it out.
I wouldn't say it can't be done, of course it could, but those are extreme conditions that one would take special actions for in any case. One should appreciate also, that dragging in 100' of water presents the danger of moving into shallow water where the ratio of scope only improves. This, of course, discounts neighboring vessel's or verticle cliffs and other structures.

-Will (Dragonfly)
 
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Jun 14, 2010
925
Quorning Dragonfly 1200 home
With 60' of chain, you are effectively using all chain until you've paid out about 70' of rode. In the depths that would work in, your 7:1 rule of thumb would be completely appropriate.
As far as storm conditions in deep water, with, in the case of my above example, a 2:1 score on 100' of depth, a wave would need to be over 50' tall to pickup 25' of that chain off the bottom and the wind and surfing forces pulling back on the boat would still have to overcome the 400# of force pulling down on the chain to straighten it out.
I wouldn't say it can't be done, of course it could, but those are extreme conditions that one would take special actions for in any case. One should appreciate also, that dragging in 100' of water presents the danger of moving into shallow water where the ratio of scope only improves. This, of course, discounts neighboring vessel's or verticle cliffs and other structures.

-Will (Dragonfly)
In a protected anchorage I'd be less concerned with wave size than wind, in storm conditions. On the east coast US one would not be anchored in 100' depth.
 
Jul 27, 2011
3,363
Bavaria 38E Alamitos Bay
Draw your picture the other way around and I think you will see that the outcome is completely different. Your rode is going to be pulling over the edge of a cliff, from deep to shallow, and it is going to be pulling the anchor parallel to the sloping bottom. Effectively infinite scope.
Yeah. The “purpose” of adequate scope is to keep the angle of the pull on the anchor relative to the bottom (i.e., the angulation) low. Secondarily, to cushion hard jerks and tugs, i.e. shock loads, that might jerk the anchor out of the bottom. With the anchor in deep water of, say, 90 ft and the boat floating in 25 ft at the shallow end of a steeply-sloping bottom, what still matters is that angulation. The shoaling bottom is matching (paralleling) the angle of the of rode toward the surface to the boat, effectively keeping the angle of the pull relative to the bottom low. However, the calculated scope here with 150 ft veered would be only about 1.5:1, using the depth of the anchor as the divisor. But, the “effective scope”, evaluated in terms of that angulation, would be much greater, even with the rode fully stretched out. Effectively infinite, as Richard points out above.

If, conversely, the boat swung away from the sloping bottom into even deeper water, say, 600 ft under the boat, no amount of rode veered would improve (diminish) the angulation when fully stretched out. The boat would still be pulling the anchor at high angulation; i.e., AWAY from the bottom, etc. It would not change.

So my point, perhaps lost in some responses among competing versions of what constitutes adequate “scope”, is not really meant as one of how much rode is veered, but one of by how much, i.e. of how effectively, that angulation parameter is kept low in one’s own anchoring situation. On level bottoms, a lot of heavy chain might do the trick in normal, or calm, conditions. But IMHO one cannot conclude that ?:? scope (ft of rode veered) is inadequate, or not, from the “armchair” based on formulas.
 
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Mar 26, 2011
2,384
Corsair F-24 MK I Deale, MD
Very interesting, Captain Larry. I do believe I’ve come across that analysis before now. It’s easy to be impressed with it. But I note that the “science” to which you refer is a set of exact solutions to approximate anchoring models containing simplifications as well as caveats. The “opinions or anecdotes” represent the converse—approximate solutions to exact models; that is, approximate solutions to actual situations at hand, but ones which work and which are reported to other sailors, etc. Not to be ignored; it should not be an either/or call, IMHO.

Nowhere in the publication did I see MY basic questioned addressed. What is the proper scope NOT when anchoring on a level bottom, but on a steeply sloping one? I’m sure that can be modeled in the same fashion as the level bottom perhaps with few changes, but with the same shortcomings. The way I see it, as the boat backs away from the anchor toward shore and going shallow, the rode is paralleling the slope. So, it would not be pulling up at the anchor, but along the bottom keeping that angulation parameter of low value.;) So you would be achieving, effectively, a “horizontal” loading of the anchor. What would be the “proper” scope?

KG
Simple. Add the bottom angle to the angulation figure in the spread sheet. You have a good point, but not a complicated one. Scope is affected by bottom slope. The other thing to consider is that steep slopes are often hard, or they wouldn't stay there.

The math is pretty good and I don't wee how it conflicts with anything said here. Deeper water means more chain is out means less scope is needed, within the limitations of the bottom and all that.
 
Feb 26, 2004
20,736
Catalina 34 224 Maple Bay, BC, Canada
KG, this is from Maine Sail's old website --- I'm glad it's still there 'cuz he hasn't migrated everything to the new site and if some of this priceless stuff is lost it would be a real loss.


One could make the point that by essentially tilting the bottom based on the slope in your example, the trig could be easy to know the relative similarities of angles, thus translatable to "scope" in flat bottoms.
 
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