10:1 Scope?

[TomY]

We're lucky sailing here in New England and I suppose that is partly our preference for anchorages that are well protected and shoal.

Our 200' main rode of 150' 5/8" nylon and 40+' of chain allow us about a maximum of 5' LW, for 10 to 1 scope at HW(20' total from deck to bottom X -10=200') - which is the normal tidal range here.

Our preference over the years (as well as equipment) is the result of being able to avoid open roadstead anchorages. Cruising further from home (when younger), I still remember those open roadsteads during some high wind and waves.

It still happens (but rarely because of our sailing grounds). I remember being able to sleep soundly in open roadsteads - back then. But I'm afraid you lose (at least in our case), that calm mental state from lack of use.

These days we sleep soundly at anchor up to about,....--kts. We could still survive the open roadstead up to a point, but we won't sleep soundly!

First priority in my mind before shoving off is the upcoming weather, and where we will spend the following night. This is a learned thing, in our case.

Scope - 'Limp rode'; that's my goal for an overnight anchorage.

 

[Rick486]

Is that Vinylhaven Tom? Don't recognize the lighthouse.

 

[TomY]

Is that Vinylhaven Tom? Don't recognize the lighthouse.

Very close, Rick!

That's the light - referred to locally as the 'spark plug' - at the Eastern end of the Fox Island Thoroughfare. Deer Isle in the background looking over Western Penobscot Bay.

 

[Kings Gambit]

OK So it appears that 10:1 scope has found some application. The most I’ve deployed is btw 6:1and 7:1 anchored on chain in a sheltered site in 15 ft on a mud bottom w/gusts to 28 kt. (But, I’ve anchored on less scope in stronger wind as well.) As far as sleeping is concerned, that’s hard for me to do when anchored in an open roadstead, which is not uncommon at the northern Channel Islands.

 

[Rich Stidger]

I use all chain mostly because it runs through my windlass gypsy nicely. I shoot for 7:1 regardless of where I am. I carry 175 feet of chain so if I must anchor in depth over roughly 25 feet I will be less than 7:1. I have anchored with as little as 5:1 in deep spots, but I can't remember ever being less than that. Your original question of 10:1 - yes, if I expect a hard blow and I am exposed to it, and there is room in the anchorage, I will put out 10:1. I doesn't happen very often.

 

[Will Gilmore]

I think it is most important to pay attention to your equipment and your environment. 10:1 scope seems excessive, especially for chain, but familiarity with your equipment and conditions may dictate more conservative standards.

-Will (Dragonfly)

 

[Hayden Watson]

Jamesg161, here is what a 16 know current looks like. This is the Skookumchuck Narrows and the tidal current is driven by tidal changes up to 20'. Skookumchuck Kayak - Bing video

 

[Mikem]

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....

And the fuel you didn’t take

 

[Kings Gambit]

I use all chain mostly because it runs through my windlass gypsy nicely. I shoot for 7:1 regardless of where I am. I carry 175 feet of chain so if I must anchor in depth over roughly 25 feet I will be less than 7:1. I have anchored with as little as 5:1 in deep spots, but I can't remember ever being less than that. Your original question of 10:1 - yes, if I expect a hard blow and I am exposed to it, and there is room in the anchorage, I will put out 10:1. I doesn't happen very often.

For me to achieve 10:1 on my chain, I’d have to anchor in 16 ft or less. In SW FL and other GOM locations, where 12-ft depths or less abound, 10:1 would seem a regular option. Here, 16-ft depths for anchoring are rare. Most of the time you would be sheltered, as in a cove, to find those shallow depths and might not even need so much scope. So, I’d have to get into the 3-strand nylon to anchor 10:1 in, say, 25 ft. That means a few thousand $$$ worth of SS chain plus SS Plowmaster riding on a $20 rope-chain splice , not to mention the yacht itself. So, that’s one reason I avoid it, even if a questionable one. I’ll take the 5:1 scope and stay hooked into the chain, etc., as long as possible

 

[smokey73]

I use all chain mostly because it runs through my windlass gypsy nicely. I shoot for 7:1 regardless of where I am. I carry 175 feet of chain so if I must anchor in depth over roughly 25 feet I will be less than 7:1. I have anchored with as little as 5:1 in deep spots, but I can't remember ever being less than that. Your original question of 10:1 - yes, if I expect a hard blow and I am exposed to it, and there is room in the anchorage, I will put out 10:1. I doesn't happen very often.

Correct me if I"m wrong, but I am pretty sure you need to add the freeboard height where your chain goes over the anchor roller to correctly calculate scope. So, assuming about 4 feet (depends on the boat) for the freeboard height, anchoring in 25 feet of water to keep to all chain, would require 25+4=29 feet and for 7:1 scope would require 203 feet of chain, not 175. If you want to keep to the 7:1 with 175 ft of chain rode, you can only anchor in 21 feet at high tide. A 10:1 scope in 25 feet of water depth at high tide would require 290 feet of rode or ony 10 feet less than a football field!

I guess if the chain is secured to a rope bridle, the height to add to the depth at high tide could be less if your boat design has a bow roller that is canted up some but it probably won't impact the calc number much for most boats. Where we live with an 8 foot tide range, if I anchor in 12 foot of water at low tide then I'll be a in 20 feet by the time the tide comes in so I have to set the scope based on that expected 24 feet calc (168 feet of rode out) If I drop the anchor in 6 foot of water at low tide (only 1 foot below my keel ), I still have to set for 18 feet at high tide (128 feetof rode)

 

[Rich Stidger]

Correct me if I"m wrong, but I am pretty sure you need to add the freeboard height where your chain goes over the anchor roller to correctly calculate scope. So, assuming about 4 feet (depends on the boat) for the freeboard height, anchoring in 25 feet of water to keep to all chain, would require 25+4=29 feet and for 7:1 scope would require 203 feet of chain, not 175. If you want to keep to the 7:1 with 175 ft of chain rode, you can only anchor in 21 feet at high tide. A 10:1 scope in 25 feet of water depth at high tide would require 290 feet of rode or ony 10 feet less than a football field!

You are correct Dan, but I did say "roughly"! I agree with the calculation, but I choose to ignore the free-board height.So it is not exactly 7:1 but still very adequate. I always do figure the high tide depth when determining the length of chain to deploy.

 

[JamesG161]

25 knots is correct.
[After being blasted about this I am correcting it to ≈10 knots.]

When I was "rescued" by the Miss Marine Patrol, they told me 25 knots.
I must have had water in my ears.

I swept by the Admiral [on the boat] and I tried to grab a line she threw me. Less than 2 seconds.
I was about 2 feet from the line and didn't get it. I yelled..
"Why did you pull that line in?
Admiral said "I didn't !!"

Thanks to my Naval Survival Swimming training and I casually swam back to the island until my feet hit sand and i walked about a 1/4 mile back to the boat spot.

She used FM 16 to get help, since no way could i swim across that Tidal current. I even walked up current and tried to cross swim it as i drifted down. No luck.

Funny story today, if I told the rest of it.

Jim...

PS: 6 knot currents are easily found in most passes.

 

[Hayden Watson]

Sorry, not trying to be contrary or mean but from someone who regularly has to transit a 6 knot current with our 15' tidal exchange on huge bodies of water I know that your estimate of current is way off. 2' way in 2 seconds = 1foot/second which is 0.59 knots. in a 10-knot current you are traveling at 16.9 ft/sec. This is the speed that a white-water river travels.

 

[Will Gilmore]

An average swimmer of good ability can swim about 2 mph while Michael Phelps has been recorded at a speed of about 6 mph. I'm sure that 6+ to 10 knot currents all seem pretty much the same to us mortal aquaticans. Unnavigable.
However, I think any sailboat anchored in a 25 knot current is planing at anchor. So, that might actually offer less strain on the anchor than an 8-10 knot current.

-Will (Dragonfly)

 

[MikeHoncho]

Sorry, not trying to be contrary or mean but from someone who regularly has to transit a 6 knot current with our 15' tidal exchange on huge bodies of water I know that your estimate of current is way off. 2' way in 2 seconds = 1foot/second which is 0.59 knots. in a 10-knot current you are traveling at 16.9 ft/sec. This is the speed that a white-water river travels.

Wow. I never would have guessed that and have to admit I looked it up. Right on the money.

 

[Kings Gambit]

8

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. ....

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.

Thank you for this, thinwater. I suppose one of my “issues” is reading about recommendations, and even instructions, to newbie sailors about the general need or desirability of 10:1 scope for secure anchoring. It’s like one has to offer them something to memorize rather than a functional understanding. I swear I’ve watched in astonishment at boaters veering what looks to be 200 ft of chain to anchor overnight in 25-30 ft, under shelter from seas, on a night where the wind is not expected to, and never does, rise above 15 kt. If they happen to be in a flotilla of boats from a school, then they’re all doing it, although not all boats will have chain. I suppose if there is ample room around, then why not? But that’s not always the case. It almost seems at times the boaters seek deeper water (i.e., further from the shore) to have room to veer more rode? As it was said above, however, the rode is not doing you any good in the locker!

 

[rgranger]

It almost seems at times the boaters seek deeper water (i.e., further from the shore) to have room to veer more rode?

That would defeat the purpose... but for ghunkholling I like to anchor away from shore so the mosquitoes don't find me.

 

[Will Gilmore]

One issue with anchoring on short chain rode in an anchorage where everyone else is on 7:1 rope rode, is if you have a neighbor behind you on long scope (7+:1) while you're on short scope (3-5:1) and you're sitting just ahead of his anchor, when the current shifts and he's in front of you, he's not just over your anchor, he's over you. Sometimes, it's best to do what the Romans do, when in Rome.

-Will (Dragonfly)

 

[Hayden Watson]

Will, I agree. I will usually try to cruise past my intended anchor buddies and ask what they are doing before finalizing my plan.

 

[rgranger]

All this uncertainty about scope makes me think I need to upgrade my ground tackle

At 26,000 pounds.... should hold my H26 on a 1:1 scope.

This guy is selling three of these on the Mobile Craigslist. Anyone need a mooring anchor?