Anchor Scope - From Waterline or Bow Roller

[Mike OReilly]

There's nothing magical about 7:1, or 5:1 or 3.5:1. The thing to understand is that all small-boat anchors work by digging into the substrate. To do this they require a horizontal force pulling on the anchor.

Basic trigonometry tells us a direct horizontal pull is not possible while anchoring. The anchor and the bow roller are at opposite ends of the hypotenuse of a right-angle triangle. The longer the hypotenuse, the lower the angle at the anchor. This is why you want as long a scope as possible.

But each addition to the scope length (or hypotenuse) provides a diminishing benefit to the angle, so in practice we settled on these default ratios. But there is no absolute right answer. More is always better, but diminishing returns means, in practice, much beyond 7:1 is not generally helpful.

I'm unaware of the tests or research around scope vs maximum holding capacity for new-gen anchors. I'd love to see this data Carl. My understanding is that angles still matter with these anchors, and longer scope always give you better angles and therefore more holding power.

Panope's tests are intended to be extreme situations. They do not represent normal anchoring situations or techniques. This is what makes them so useful. But the range of results also shows that there are so many variables in real-world anchoring, that all tests can only give limited information.

With regard to tidal ranges ... of course one must calculate to max. height.

[noelex 77]

Originally Posted by CarlF

The likely reason that "old salts" anchor with 7:1 scopes is that they learned that when they started cruising in the 1970s and 1980's. I guess I would now qualify as an "old salt" and I was taught to always use 7:1 - and I did. The reason you had to do this was that anchors were pretty lousy back then. The most common on sailboats was the CQR. It was incredibly difficult to set so you learned to put out a lot of scope to improve your chances.

But if you don't still anchor with a CQR but a more modern anchor like a Rocna, Spade or Excel there's no reason to use 7:1. They reach their full holding power and resettability at much lower scopes. The excellent YouTube videos by SV Panope tests at 3.5:1 and 2.5:1 (measured to the roller) with extremely good results.

An anchor’s maximum holding ability is not reached until a scope of at least 10:1 is deployed.

A large anchor in relation to vessel size also increases the anchor’s holding ability so a large anchor at a shorter scope can equal a smaller anchor at a larger scope, but if bad conditions are predicted the best formula is deploying the largest anchor that is feasible combined with the largest scope that is feasible.

[MicHughV]

as one that has dove on my anchor hundreds of times....here's my take....

for one...I use a chain/rode combo....75' chain...and the rest nylon rode. I have used this combo for over 35 years, I love it and it has served me well, and I will likely never change it, but to each his own.

even in very shallow water, ie, B'mas....I let all the chain out as I don't want the chain on the rubber roller as it wears out, plus a small length of rode to ride over the roller...typically, around 6'.
with this setup, even in windy conditions, I have yet to see more than 20' of chain off the bottom....the remaining 55' lies flat on the bottom...

in deeper water, I obviously let out more nylon scope...I use 5:1 as a general rule, as measured from roller, including tidal range. anchoring in Georgia, for instance, one most allow for a 7-10' tidal range.

even in very windy conditions, I have never seen more than about 20' of chain lift off the seabed...so it's fair to say, that under most anchoring conditions, the pull on the anchor is horizontal...the rode lies in a curved arc type shape...there is no triangle shape....but to discuss this more will take far too long..

on occasion, an anchorage is crowded or experiences current changes or wind direction changes......in this case.... I will lay out a second anchor.....don't forget the 2nd anchor option....

there are countless anchor threads on this form......countless...scope...anchor type...etc....

but what is never mentioned is technique.....there is an "art" to laying out an anchor, you can't simply drop an anchor and peel of " x" ft of rode, it doesn't work that way...

to explain more is difficult.....but, for instance, dropping a big load of chain over an anchor is to be avoided...the anchor must be correctly "set" and " aligned" and a slow even pull used to dig the anchor in...etc..etc..

sticking to a 5:1 or 7:1 ratio does not guarantee nor imply any type of safe anchoring approach...nor does anchor selection, boat type, etc, ad infinitum...it takes time to develop and understand how best to anchor your particular boat.

[Mike OReilly]

Originally Posted by MicHughV

...but what is never mentioned is technique.....there is an "art" to laying out an anchor, you can't simply drop an anchor and peel of " x" ft of rode, it doesn't work that way...

Agreed. I've often said in these anchor discussions that the single most important aspect of anchoring is good technique. And that includes laying out sufficient rode length.

Proper technique includes choice of location, careful deployment, laying out of rode, setting, and then digging the anchor in. All are part of good technique.

Bad technique can be masked by good equipment, but if one anchors a lot in varied locations and conditions, eventually it comes back to bite. I also believe good technique can make less-good equipment work just fine. So the best option is to combine top gear with good technique.

[Mickeyrouse]

It has been alluded to earlier in the comments about informing new neighbors to the anchorage just where ones anchor is. I’ve been known to attach an anchor buoy to the anchor on a pennant of prox. depth of high tide water. Doesn’t need to be fancy- 1/4” line on a Clorox bottle, or anything with a handle to tie to. That way if there’s a late arrival to the anchorage, the new guy may not necessarily snap to “ anchor” but ( if he can see it- reflective tape helps) he’s less likely to anchor on top of it. And if we’re in a really crowded anchorage ( Offatts Bayou, Galveston, Labor Day, for example) a couple more at suitable intervals will ( sometimes) ward off outboarders and jet skiers. Not ideal anchorage under the circumstances, and takes a few more minutes to anchor and retrieve, but it’s a coping mechanism.

[MicHughV]

I've seen charter boats come into an anchorage.....drop the anchor straight down until it hits bottom, snub off the chain and walk back to the cockpit to drink a beer.....

therein lies the problem....old salts typically know what to do, when to do it, and how to do it......but a guy chartering a boat is almost always a loose cannon...scope to him means 1:1.....throw in a high windage cat and the situation compounds...

then some boats lay to their anchor in a docile manner and others look like they are under full sail...veering side to side in a huge arc....

some nervous nellie's or other newbies might lay out 100's of feet of rode in a relatively shallow anchorage..and then get pissed off when you anchor anywhere near their half mile of rode...

off course, don't forget the " go fasts"...usually a multi-engined fishing boat zipping thru' the anchorage at 60 mph...with a youngster on board steering with his feet...

there is no simple answer here....wish there was, but there is none.....each anchoring situation must be evaluated on it's own merits...depth, wind direction, bottom material, other boats, etc, etc.

the full anchoring experience is not complete until a sudden 80 mph squall hits the anchorage area...sideways rain....thunder and lightning.....coming from the opposite direction.....at night......when many boat owners might be ashore at the local pub...of yeah.....a good learning experience....often painful and costly....