10:1 Scope?

dlochner · Sep 30, 2019

If a little rode is good, a lot must be better.

We typically anchor in mud or sandy gravel, unless we are plowing up weeds. I shoot for about 5:1 with an all chain rode. At least that's what I think I use, since I can't remember if I marked the chain at 20 or 25 foot intervals.

Davidasailor26 · Sep 30, 2019

It is outside the norm, but maybe not so crazy in some situations. In Barnegat Bay NJ for example, one might anchor in 5’ of water, plus maybe 4’ of freeboard. At that depth 10:1 would only be 90’ of rode. Given that the prevailing wind switches overnight and often picks up, a person might put down the 90’ and sleep better than just 50’ or 70’. It’s a pretty large anchorage and not very crowded overnight, especially on a week night, so the extra few feet of swing isn’t really a problem. I wouldn’t necessarily feel like I hat to let out that scope, but I might finish dinner, look around, and think “why not”.

Nodak7 · Sep 30, 2019

In SW Florida the anchorages are pretty shallow. 6-12' with a sandy bottom. We put out 10:1 if there are no other boats around. If the wind shifts there is not much room to drag before you are on the bottom (especially with tide).

JamesG161 · Sep 30, 2019

I have used 10:1 in a 15 foot sand bottom for 2 reasons..

1) Dropped near the Gulfport shipping channel during a 35 knot wind squall. I didn't want to drag into a big Vessel.
2) Dropped in an area that had a 25 knot current at tidal changes.

IMHO what the 10:1 gives you is less strain on your boat and anchor, in high seas and winds.

Do the trigonometry for the less strain.

Jim...

markwbird · Sep 30, 2019

The rule of thumb I know is between 4:1 and 7:1.

thinwater · Sep 30, 2019

I've done a lot of testing and publishing regarding anchoring. This is one of those topics that takes at least a few chapters to answer. Declarative statements are made based on personal experiences that do not cover all situations.

a. Shallow water. At 7:1 scope I've watched the chain (all chain) start lifting clear at 15 knots. Had the water been 20 feet deep, this would have taken 35 knots. But some people never anchor shallow.
b. Strong weather. Some people think 35 knots is windy. Some 60 knots. Some 80 knots. Hint: 35 knots is just a breeze and saying the anchor held just fine in 35 knots is not really saying anything.
c. Bottom. Good sand that can take a little pumping? Mud that will liquefy with any motion?
d. Yawing. Does the boat hold still? Is the wind shifty? If either is true, you want to keep some chain on the bottom to keep the anchor from feeling it.
e. Anchor sizing and style. 'Nuf said.
f. 10:1 is NOT the magic upper limit. For example, tandem anchors in shallow water require more. But that would require a whole separate thread to explain it.

So, if I anchor in 15-25 feet with all chain, over sticky mud and don't think I'll see more than 30 knots, 5:1 is more than enough and I'd sleep well with 4:1 if the anchor is modern and large. If I anchor in 4 feet over pudding mud, expect 60 knots from a microburst, and my boat is a dancer at anchor, 10:1 with all chain is not going to be enough. I'll need to move or set a second anchor.

If there is a "formula," it includes relative windage, depth, yawing, bottom, rode type, exposure to waves, and wind. I've used everything from 3:1 to 15:1, and I can make the case for both.

smokey73 · Sep 30, 2019

JamesG161 said:
I have used 10:1 in a 15 foot sand bottom for 2 reasons..
2) Dropped in an area that had a 25 knot current at tidal changes.

Jim...

Jim, 25 knot current at tidal change - where is that. Why would you ever want to anchor where there is a 25 knot current?

JamesG161 · Sep 30, 2019

smokey73 said:
Jim, 25 knot current at tidal change - where is that.

We have Barrier Islands from Texas to Tampa Bay Florida. We like to anchor on the Leeward side of an Island for comfort. There is an actual ≈25 foot deep trench about 100 feet off shore, that is swept clean by these Tidal changes on both sides of Ship Island.

We did not anchor in that trench, but further offshore, beyond that trench. First time we slept, with Anchor Alarm on and 5:1 scope, we were moved by the combo of winds and current.

The winds were bow on and the current was abeam and the anchor was slowly dragging.

Never move with a 10:1!
Thus, we let out 100' of chain and sleep soundly.

Jim...

dlochner · Sep 30, 2019

smokey73 said:
Jim, 25 knot current at tidal change - where is that. Why would you ever want to anchor where there is a 25 knot current?

I think there is a missing decimal point, 2.5 knot current.

Tides Near Me

tidesnear.me

The tidal range is fairly small.

Station Home Page - NOAA Tides & Currents

JamesG161 · Sep 30, 2019

dlochner said:
I think there is a missing decimal point, 2.5 knot current.

That first link was Ship Island was in Alaska!!!

Your second link is correct.
Tide Predictions - NOAA Tides & Currents

The current is caused by mid tides as water rushes between the gaps of Cat Land and Ship Island.
Trust me...
~~25 knots~~ is correct.
[After being blasted about this I am correcting it to ≈10 knots.]

Did not realize how fast it was and swam to shore at High Tide [no current], and tried to swim back at Mid Tide.

The Admiral, on the boat, thought I was being swept out to sea when I tried to return to the boat.

Obviously I made it. The story would make laugh until tears.

Jim...

PS: There is no Current measurements on that Ship Island link.

dlochner · Sep 30, 2019

JamesG161 said:
That first link was Ship Island was in Alaska!!!

Well, Alaska is just around the corner from Mississippi, isn't it?

How's this link:

Ship Island, 0.5 nm NW of, LB 26 Currents

Or this:

Tides Near Me

tidesnear.me

JamesG161 · Sep 30, 2019

That is the Channel current for the Shipping channel.
Look closely at this old picture. The Gap shown is now filled in by the Army Corps.

You can see the shallow area below surface just before the trench, which is darker blue.
Next you see the further offshore sand appearing again. That dark area between is the TRENCH.

I can swim in a 2.5 knot current.

Jim...

dlochner · Sep 30, 2019

JamesG161 said:
I can swim in a 2.5 knot current.

So can I, as long as I'm going in the same direction as the current.

thinwater · Sep 30, 2019

Kings Gambit said:
Thanks for your input here. But FYI, according to Beaufort, 35 kt is a gale. Fresh and Strong "breezes" are in the 20's. BTW. I've seen many boats start dragging long before 35 kt.

"Breeze" in that context was a figure of speech. At sea, it's a gale, in harbor, not really. Just weather where you are glad to be in harbor.

Squalls are not really a thing in CA, but they are on the east coast. In certain summer weather, if you are not prepared for 50-60 knots, you aren't prepared.

rgranger · Sep 30, 2019

I think the right answer is "maybe". What you need to lay out to stay in place depends on the bottom, the sea state, the anchor, where you cruise, etc.

For example, I generally cruise in either a lake or in the North Carolina sounds (Outer Banks = OBX). In the OBX the sounds are very shallow but the fetches can be 50 to 100 miles. Chop builds quick if the wind is from the wrong directions. A 4' swell is not a big deal unless they are only 4' apart. Then it is a big deal and that has happened to me in the Abamarle sound. When I anchor in the OBX, I look for a protected cove AND I lay out at least two anchors in a V- pattern in case the wind shifts (I've even laid three anchors once in a Y-pattern because of a predicted wind shift). I lay out at least 10:1 scope. If I have the cove to myself, I lay out all the rode I have. BUT.... I would not do that under the conditions that most of you all cruise in.

AND when I find a nice protected cove on my MOUNTAIN LAKE, with a sticky mud bottom I lay out 5:1 or less. Sometimes a LOT less, because I don't want to swing to close to land and get swarmed by mosquitoes. I'm not worried about chop on the lake, the bottom is good and the cove protects me from the wind. I probably could stay put with a simple mushroom anchor on but that goes against my sensitivities.

A big difference for me relative to what @Kings Gambit is dealing with is that I anchor in shallow water with either mud or sand. I have good holding, I'm not dealing with coral so I only need 10' of chain for a little additional weight (I use a cheap/light Danforth). I have room to carry a lot of nylon rode. I also suspect that if I was carrying an all chain rode and a heavy CQR, I'd make different choices. Likewise, if I cruised in different conditions, I'd have to outfit my boat differently and again... make different choices.

What is appropriate varies from boat to boat and from anchorage to anchorage.

Rick486 · Sep 30, 2019

Kings Gambit said:
I’ve read repeatedly here and elsewhere expectations for 10:1 scope in anchoring. I’m not opposed in practice or in “philosophy” to carrying hundreds of feet of anchor rode in case it might be needed. Certain special needs can & do arise. But really, does anyone here deploy 10:1 scope, especially of chain? I can say that I never have. I carry about 360 ft of anchor rode on the principal bower, 160 ft of it chain. But not so I can achieve a 10:1 scope anywhere I typically go. It’s based on a formula of 7:1 scope in 50 ft. Anchoring in 50 ft here in Southern California, e.g., Long Beach Harbor and “neighborhood”, as well as at the islands, is where depths of 50 ft are common. In places where the average anchor depth is less than 20 ft, a 7:1 formula would equate to 140 ft @ 20 ft; 105 ft at 15 ft depth. With all chain, you don’t need even 7:1. At 5:1 it’s between 100 and 75 ft of rode. Whence do those comments re: 10:1 scope arise?

Put out all that chain rode, remember you have to get it back up, or leave it. Frankly, my windlass has failed me more times than my anchor has ever dragged with the scope I use.

The most useless things in aviation are runway behind you and altitude above you....same goes for rode in the locker. I often do 10:1 or higher when the anchorage permits it. Why not? We sleep more soundly knowing dragging is just not going to happen. In a crowded anchorage, we rent a mooring....

Gene Neill · Sep 30, 2019

My philosophy is largely based on MaineSail's excellent article.

I see precious little benefit over about 7:1, and all sorts of potential problems when you get up to 10:1. Running aground or into other boats, needless destruction of the sea floor, entanglement, abrasion (if rope is involved), and so on.

If I ever felt like 5:1 was not sufficient for 98% of my needs, I'd be looking for a bigger/better anchor.

gettinthere · Sep 30, 2019

Beth and Evans Starzinger cruised the Beagle Channel at the tip of S America about 15 years ago. She now works at Boat US. They were anchoring in 70 or more feet. They found that 3:1 worked fine in deep anchoring situations. They had regular willawahs bursting down the mountain at 60 to 70 knots and didnt drag.
I've anchored in 20' with 4:1 with 3 knot reversing currents and fluctuating winds in the 20's and didnt drag a bit, if I had I would have been into a bridge. 10:1 seems very excessive unless facing a hurricane. 5:1 is all I have done for 6 years of full time cruising in a wide variety of conditions and have had zero problems except when accidentally anchoring in thick weeds. Throttling up full in reverse revealed that problem and I moved.