Anchor Scope - From Waterline or Bow Roller

[sv_pelagia]

Originally Posted by sv_pelagia

just not so good at understanding this difference between a snubber on a mono and a bridle on a cat.

In this case, I meant a "snubber" on a mono as long as a "bridle" on a cat. (Probably "snubber" not the best term.)

[StuM]

Originally Posted by Shrew

..
Actual Depth should include the offset for the depth sounder (sounder depth below waterline) to account for the difference between the depth sounder reading and actual depth.

So if:

Depth Sounder Reading: 10 feet
sounder offset = 1.5 feet (below surface) (+1.5)
.

You're one of the small minority who don't calibrate their sounder to either waterline or keel

[noelex 77]

But this only applies (in this example) for a scope of 5:1. If the scope deployed is not 5:1, especially when closer to a long or short scope this “rule of thumb” does not work.

Providing the scope is moderate or the water is not shallow, this rule of thumb is close enough, but the correct maths is not difficult. Just divide the length of rode deployed by the depth of water plus the height of the attachment point. The “rule of thumb” only saves adding together the depth of water and the attachment point height which hardly seems worth a worthwhile short cut.

Incidentally, if the bridle distance in front of the boat to height ratio is 5:1 as in Thinwater’s example, and the deployed scope is significantly longer than 5:1 (as it likely would be in strong wind such as 50 knots) simple trigonometry shows that the only way the bridal to chain connection point can be at or below the water surface is if there is significant catenary in the chain. An alternative explanation is that in rough conditions accurately estimating the position of the bridal to chain connection point relative to the water surface is difficult with wave action and the pitching of the vessel.

[jtsailjt]

Originally Posted by AiniA

I think you are overthinking things a bit. Anchoring is not that scientific and yes, experience matters

But I think it’s a good thing for the OP to be “overthinking” this as a starting point. As you suggest, it’s not an exact science but before you can develop the judgement necessary to become an artist, you’ve got to know the basics and only then start adding experience that gradually builds an encyclopedia of knowledge in your head that you can draw from without even realizing you’re doing it. Bottom type, bottom slope, type rode, weight and windage of your vessel, other nearby vessels, weather forecast, likely change in wind direction, shelter by nearby cliffs, sea state, proximity to a lee shore, type anchor, etc. are all things that experienced cruisers subconsciously consider every time they anchor so a casual observer wouldn’t even know they were thinking about all those things, but it all starts with understanding the basics and that’s what the OP’s question is an attempt to learn.

As others have mentioned, there are different norms in different areas. Some folks always use 7:1 and for others 4:1 is deemed sufficient, etc. While 7:1 is certainly conservative and we all want to be as safe as possible it’s probably pretty selfish in a crowded anchorage with good holding and calm winds but very appropriate if holding is marginal and high winds are forecast. Here in Maine where it’s easy to find a sheltered cove and during the summer the wind almost always dies to nothing at night and the bottom is mostly a very thick, clay muck that sticks to your anchor and chain. So, in those conditions it would probably be safe to set my anchor at 5:1 and then reduce to 2:1 or even less and I’d still be right there in the morning. But in the Bahamas with no high land for protection and a 15 knot steady breeze that could suddenly turn into a sudden squall in the middle of the night and with a sandy bottom providing only mediocre holding, anything less than about 7:1 would be asking for trouble. So when experienced cruisers from different areas have differing opinions about how much scope is enough, I try to understand why they think the way they do, and when I visit their area, I try to adopt their norms. Ive gradually learned that there’s probably a very good reason why they are doing things differently than I might do if I were back home in my normal cruising grounds. When in Rome...

[thinwater]

The challenge with and discussion of scope is that it is too easy to bring narrow experience to the question. What is the water depth (could be 3-100 feet)? What is the most wind you expect (could be 10-75 knots, excluding real storms)? Type of rode (chain or rope)? What size and type of anchor (some handle short scope better than others). And most importantly... what is the bottom, not in general, but exactly where the anchor landed (this can change the holding capacity by more than any other factor--10 times)?


You could write a book. Oh yeah... a number of people have. And then it takes practice and mental arithmetic including a lot more factors than scope. Or you could just use a rule of thumb, seak good shelter and known good bottoms, and there is a very good chance you will never have a problem, because the systems are over-engineered to allow for worst case situations. Actually, that means they are properly engineered, because eventually, that stuff happens.

[noelex 77]

Originally Posted by thinwater

You could write a book. Oh yeah... a number of people have.

Absolutely
https://www.amazon.com/Rigging-Moder...language=en_US

[MicHughV]

you could do what I've seen some charter boats do....

pull into an anchorage....lower your anchor...and when the anchor hits bottom, you can stop letting out rode......you've anchored....time to break out the brewski's.....

[thinwater]

Originally Posted by MicHughV

you could do what I've seen some charter boats do....

pull into an anchorage....lower your anchor...and when the anchor hits bottom, you can stop letting out rode......you've anchored....time to break out the brewski's.....

Or the ever popular "drop the anchor and back down fast, plowing furrows."


And lets not for get the "pick a spot near everyone else... and not worry that the boat will be be farther back and swing."

[Group9]

Originally Posted by MicHughV

you could do what I've seen some charter boats do....

pull into an anchorage....lower your anchor...and when the anchor hits bottom, you can stop letting out rode......you've anchored....time to break out the brewski's.....

Or just back down until you snag another well set anchor and chain!

[Shrew]

Originally Posted by StuM

You're one of the small minority who don't calibrate their sounder to either waterline or keel

Technically, I calibrate to waterline but I thought it was worth mentioning in proper calculations. If you know you've calibrated to waterline, then you don't need to add it. But you are only serving to reinforce my point. If the sounder is calibrated for keel depth then draft needs to be added to the scope calculation.

[gcaws]

Bow roller. If I have neighbours then I ask how much they have out and whether they’re using chain or rode then try to match (unless it’s sketchy of course). Plus it’s a good way to introduce yourself and show you care and that you’re thinking of everyone’s safety. Alone I always anchor conservatively so 7:1 or more with an over sized Rochna. I’ve got the chain and it’s not doing me any good sitting in the anchor locker. Plus I hate dragging. I really hate it. Yes, I know different scopes for different anchors but the more chain I have out , the better I feel. And over 25 years I’ve done a ton of anchoring. So I’ve lived through a lot of wind and bumpy anchorages. Did I say I hate dragging?

[Bill_Giles]

I’ve always measured the scope from the bow roller. My view is that it is not the angle between the anchor and the attachment point, if that were to be important your anchor is already on its way out. It is the amount of chain that you have on the bottom to force the pull to be along the sea bed. The 5x chain produces a suitable catenary (curve) in the chain for most mild weather conditions. Too little chain and there is not enough to remain on the bottom to produce the horizontal pull required. So take a pinch of salt with what the ‘Old salt’ told you, this old salt is telling you different. And if you want to add bridles, snubbers etc do just that ‘add’ them on after.

[kas_1611]

Originally Posted by gcaws

If I have neighbours then I ask how much they have out and whether they’re using chain or rode then try to match (unless it’s sketchy of course). Plus it’s a good way to introduce yourself and show you care and that you’re thinking of everyone’s safety.

This is one of the signs of a "good" skipper. Asking others for their information and using that shows intelligence, style and courtesy.

We have had a number of instances when on anchor other boats have come passed slowly and asked how much and where our anchor is (roughly) then have dropped theirs a safe distance from ours, let out a similar amount so we are all suitably spaced out. And we have asked boats already in the same. It's just good old fashioned common decency. We've even had one skipper ask, drop and set the anchor then row over with a couple of bottle of beer for a wee chat.

Cheers and cold beers

[dreuge]

SCOPE is well defined. What is an appropriate scope is subjective. SCOPE is by definition is a triangle relation of rise to run. If one imagines a taught anchor rode, then clearly the rise must include depth of water and attachment height above water.

[Amapola]

You are correct in your calculations.