Anchor chain scope, double the depth?

[SV THIRD DAY]

Just let it all go.....
https://www.facebook.com/beingmarinr...6253890076011/

[noelex 77]

Originally Posted by kas_1611

Easiest way to remember that is MAX Water Depth expected plus bow roller height.

I always think of scope in a tidal area as a constantly changing number, assuming the amount of rode is not adjusted.

For example, if we have a depth of water at low tide +bow roller height of 5m and we let out a rode of 40m our scope at low tide is:
40/5 =8:1.

It there is a 3m tidal rise at high water our scope will be:
40/8 = 5:1.

Many text books reinforce the view expressed by Kas and state the correct scope is measured at the maximum water depth. So the scope in this case would be expressed as 5:1, but in reality 5:1 is more correctly (in my view) thought of as the minimum scope.

If we get a squall at around low tide the true scope is 8:1 not 5:1. The anchors holding ability is related to the scope at the time. So 8:1 is the important number. As the tidal height varies the scope changes. If there is strong wind at the time of high tide the anchor will need to cope with this wind strength (briefly) at a scope of 5:1.

In most cases these minor variations are of little consequence, but occasionally in tight anchorages the differences can be important. At low tide swinging room is more often restricted and this is generally the time when letting out more rode can be impossible. If strong wind arrives at low tide it is silly to worry the "text book definition" says your scope is 5:1 when as far as your anchor is concerned the correct number is 8:1.

[SV THIRD DAY]

I have to tell you...this is where Chat Room Cruising is so different than Real Life Cruising (and actually why real life cruising is so much easier).

I don't recall doing any math while dropping anchor. We would get to a spot we liked in Mexico always in under 30ft of water...drop our 100lb Manson anchor on the ground with 150ft of 1/2 chain then pull back in full reverse with our 120Hp motor. If the Boat wasn't there when we came back from our trip ashore it's because someone stole it. Same thing when we were cruising on our 36ft boat, the 45lb Manson and 150ft of 5/16 would go overboard and we slept well. 15ft...25ft...30ft...it didn't matter. Maybe that is why we went 4yrs of daily anchoring without ever dragging anchor? Who knows, but it was damn easy.

In 4yrs of every day anchoring...we only anchored in deeper than 30ft of water ONCE and that was at Catalina Island in California because we were too cheap to pick up a mooring ball. Why would I waste my time varying my rode for the pluses and minuses of an anchorage under 30ft...and many of the cruisers we were with did the same thing. If the anchorage was too crowded for the "150ft drop it and forget it method", tell me again why I'm going there? We didn't sit around at cocktail hour reviewing our scope we were worried about who was bringing the ice.

[Vasco]

Originally Posted by SV THIRD DAY

Just let it all go.....
https://www.facebook.com/beingmarinr...6253890076011/

Saw that done once on a 10,000 ton passenger boat we were towing. Trying to anchor on a shelf and slipped off (depth sounder wasn't working). Should have seen the sparks fly. 15 shackles of chain (1350 ft) and a great big anchor at the bottom of the Banda Sea.

[conachair]

Originally Posted by SV THIRD DAY

I have to tell you...this is where Chat Room Cruising is so different than Real Life Cruising (and actually why real life cruising is so much easier).

I don't recall doing any math while dropping anchor..

There's also no rules against doing some sums in your head to get a bit of a better handle on what's actually happening to your anchor chain. Even if "I've always done it like this and it's always worked" (and more than likely will continue to do so).

Finding out how things work can be fun for some

[Lodesman]

Originally Posted by RichMac

How do you calculate the amount of chain to put out for various depths? The simple 'scope' multiplier requires judgement, because the number varies between shallow and deep water. Shallow water needs a high scope multiplier, deep water maybe down to 3:1.

The calculation I use is "Double the depth plus 50ft (15m)", which seems to work for all depths. This is a simple formula, good for average conditions, then vary the 50ft if necessary to suit special situations.

Like you, I'm not fond of the simple scope ratio - it's unsuited to anything other than fairly shallow anchorages. In the navy we used the square root of the depth multiplied by a factor to give the number of shackles (90 ft lengths) of anchor chain - this would hold a destroyer in a hurricane. It at least has the advantage of adjusting reasonably to greater depths. So I use a modified version: square root of depth in feet x 30 for feet of chain. I think if you have rope or mixed rode, then the basic ratio is probably a better method, to ensure an adequate angle to the anchor.

[thinwater]

Originally Posted by RichMac

How do you calculate the amount of chain to put out for various depths? The simple 'scope' multiplier requires judgement, because the number varies between shallow and deep water. Shallow water needs a high scope multiplier, deep water maybe down to 3:1.

The calculation I use is "Double the depth plus 50ft (15m)", which seems to work for all depths. This is a simple formula, good for average conditions, then vary the 50ft if necessary to suit special situations.

The chain hangs in a curve, and we want at least the last link to be on the bottom. The technically minded can study the graph below, which shows a reasonable match between the double depth formula and the catenary. That is the theory, but I'm interested in comparing the formula to what other sailors do.

What chain lengths would you use for various depths, eg 10ft, 20ft, 50ft.? How does the double-the-depth formula compare?

I like it. But does it need a windspeed factor? At some point, even in deeper water, you might need a little more scope. Or maybe 3x depth + 40?

I like it.

[wesevans]

When computing your scope DO NOT forget to add the height of the bow( or point where anchor is attached to boat) to the water depth.

[captain58sailin]

Vasco, I think the term you are looking for is "shots" @ 90' per shot. The rule I have always used is 5:1 in nice weather, 7:1 in bad weather or holding ground. There are times when even that isn't enough and you have to let out more.

[noelex 77]

Scope is for wimps. This how real men anchor.

[Juho]

The proposed formula makes a lot of sense to me. It is useful to know at what distance the chain (and/or rope) starts pulling the anchor upwards.

I'd like to have another curve too, for higher loads. Then I could pick my curve depending on what kind of wind and waves I expect to get.

If I want to have some extra chain lying at the bottom of the sea to strengthen the grip, then I can simply add the required number of meters/feet to the end result.

What I like in this formula is that it gives me an exact mathematical formula (based on the catenary curve) that tells me how strong my ground tackle is. In the figure of the first post you can see that between depths (plus bow roller height) of 10' and 60' the formula gives you max 10' too much chain length (highest at both ends of the scale).

The best part of this approach is that instead of dropping few tens of feet extra rode just to be sure, or instead worrying at night if the rode is long enough, I now know how strong the ground tackle is (the rode part, excluding the holding power of the anchor).

The ideal way of using this formula would be to have two boat specific formulas, one for light and one for strong winds/waves. Then it would be easy to pick the appropriate (minimum) length of rode (somewhere between or outside of those two numbers).

It would make sense to always add some chain length (to lie at the bottom of the sea). This chain can also compensate a poorly set anchor. It would be nice to know how many feet of chain (at the bottom of the sea) would be sufficient to take all the load (240 lb in the original example) even if the anchor would have no grip at all. This way you could take into account in the equation also the expected minimum holding power of the anchor. The formula would still be simple enough to make all the necessary calculations in your head while dropping the anchor (first count the catenary part, then add the bottom part with chain and anchor).

[El Pinguino]

Originally Posted by sailorboy1

What do people do in a crowded anchorage with little room do when a front is coming in and there isn't room for using more scope?

If what I saw in Papeete ( B. Carrefore )a few years ago is anything to go by .... they drag.

True story..... 15 to 20 knots and yachts dragging right through the anchorage.

[knockabout]

2xdepth pull in/ let out 2xtide range check pulled in/let out 2x distance from bow roller to water if still pulled in your at min anchor rode length/if in strong wind with chop x2 that rode length/ after a while it becomes a habit to anchor at your experienced safe depth with some string on the rode at that length/30 feet 100 foot mark with this boat/ all boat anchoring behaviours vary (try not to bluff mother nature she is a born admiral)

[StuM]

Originally Posted by captain58sailin

Vasco, I think the term you are looking for is "shots" @ 90' per shot.

YART.

It depends on when and where you learnt your trade.

Shackle is the common term in many circles.

A shackle used to be 12.5 fathoms as defined by The Admiralty. They changed it post WW2 to 15 fathoms.

[kas_1611]

Originally Posted by knockabout

2xdepth pull in/ let out 2xtide range check pulled in/let out 2x distance from bow roller to water if still pulled in your at min anchor rode length/if in strong wind with chop x2 that rode length/ after a while it becomes a habit to anchor at your experienced safe depth with some string on the rode at that length/30 feet 100 foot mark with this boat/ all boat anchoring behaviours vary (try not to bluff mother nature she is a born admiral)

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