More scope in shallows, less in deep water

[Muaddib1116]

I've read and heard from multiple sources, somewhat axiomatically, that in shallow water you tend to need more scope and in deep water you can get away with less than usual scope. I trust that it's true, but I've tried and failed to find a good explanation of why this is. Can anyone help explain it?

[Reefmagnet]

The distance between anchor roller and sea surface is a constant whilst depth is variable so in some ways, this increases scope in shallow water in relation to total scope. Also, with less overall rode out, catenary - which acts as a shock absorber of sorts - is reduced so there is an argument there for increasing scope.


On the other hand, the angle of pull on the anchor is less in shallow water so this would kind of even things out.


Personally, I've always thought more scope in deeper water and less in shallow because of the greater risk of the anchor being pulled upwards by swell and also too much swinging room in shallow water might not necessarily be a good thing.

[rslifkin]

Originally Posted by Reefmagnet

Personally, I've always thought more scope in deeper water and less in shallow because of the greater risk of the anchor being pulled upwards by swell and also too much swinging room in shallow water might not necessarily be a good thing.

In deep water the vertical movement from the swell is smaller relative to the depth, so the angle change on the rode is smaller. Waves also tend to be less steep in deeper water, which contributes to lower loads on the anchor.

In 50 feet of water, putting out 5:1 scope could easily have you swinging a 500 foot circle with a 40-ish foot boat. In 10 feet, a 10:1 scope would only give a 260 foot circle. So in deep water, swing circle can become an issue quickly. Especially in an area with tight but deep spots where depth drops off quickly. Try for 7:1 scope and you just might not fit without running into land.

[Reefmagnet]

Originally Posted by rslifkin

In deep water the vertical movement from the swell is smaller relative to the depth, so the angle change on the rode is smaller. Waves also tend to be less steep in deeper water, which contributes to lower loads on the anchor.

In 50 feet of water, putting out 5:1 scope could easily have you swinging a 500 foot circle with a 40-ish foot boat. In 10 feet, a 10:1 scope would only give a 260 foot circle. So in deep water, swing circle can become an issue quickly. Especially in an area with tight but deep spots where depth drops off quickly. Try for 7:1 scope and you just might not fit without running into land.

I was kind of discussing in an "all things being equal" context. Although my scope preference is mine for the reason if I use too much scope where I like to anchor, I risk running aground in the shallows. I've anchored in areas with up to 10 m of tidal variation, and regularly anchor in areas with 6 m of variation. The problem with any tidal range is that the peak spring tides are always at the highest at around midday and midnight (in areas without weird tides, of course) so if anchoring overnight in these conditions, one needs to base scope on the high tide in order to enjoy a stress free sleep. In my case, I prefer to anchor in 3 to 5 metres at lowest water. I have a Rocna and all 10 mm chain so in anything short of seriously bad weather I should throw out a max of about 4:1 scope which equates to roughly 45 m of chain (during springs) in what will be, say, 4 m of water depth at low tide. Now that is a lot of scope at that depth and if the weather is benign enough, I'll tend to go 3:1 based on the highest expected tide. On the other side of the coin, when I need to anchor in deeper water it's usually affected by strong current or otherwise more exposed to swell than the shallows, so in this case more scope is the go for me.


But I do understand different situations will have different ideas, so I guess the ultimate answer is "it depends".

[Joh.Ghurt]

Originally Posted by Muaddib1116

I've read and heard from multiple sources, somewhat axiomatically, that in shallow water you tend to need more scope and in deep water you can get away with less than usual scope. I trust that it's true, but I've tried and failed to find a good explanation of why this is. Can anyone help explain it?

You'll want to check out this thread: https://www.cruisersforum.com/forums...pe-235053.html
It links to this page: https://trimaran-san.de/die-kettenku...UCeN1GLuBsVN-U

The short version: Using x-times depth for a scope calculation is only a very basic approximation. When you want to calculate the correct lengths, you need to use the chain-curves and things get very mathematical quickly. The deeper the water, the less the rule of the thumb applies. Thus the addendum that you need less scope in deeper waters.

[sailorboy1]

I pretty much use the same scope regardless except the few times that I was in water so deep that I have it all out, which has only been the case a few times. I would say I have short scoped more often in shallow water because that was a protected spot and there was a crowd.

[Shrew]

Originally Posted by rslifkin

In 50 feet of water, putting out 5:1 scope could easily have you swinging a 500 foot circle with a 40-ish foot boat. In 10 feet, a 10:1 scope would only give a 260 foot circle. So in deep water, swing circle can become an issue quickly. Especially in an area with tight but deep spots where depth drops off quickly. Try for 7:1 scope and you just might not fit without running into land.

You might wan to check your math a bit.

5:1 @ 50 feet = 244.949 foot radius

10:1 @ 10 feet = 99.499 foot radius

[rslifkin]

Originally Posted by Shrew

You might wan to check your math a bit.

5:1 @ 50 feet = 244.949 foot radius

10:1 @ 10 feet = 99.499 foot radius

I was thinking full circle, not radius. And also including the length of the boat. I also did the quick / lazy math. So I was a little off, it would actually be approximately 570 and 280 feet.

[SaltyMetals]

There was a different thread on topic a few weeks ago (referred to by Joh Ghurt at #5 above) which considered all the variations and the maths behind it. Very interesting and popular thread that has so far run to 25 pages, the last being post #372.

The maths can be done for you by using the App "chainsim" but approximately you could use another shortcut which more or less arrives at the same thing which is 15 metres (46ft) PLUS 2 x depth. Personally i use 16 metres (48ft) + 2 x depth since i am not sure if the suggested 15 metres(46ft) includes the height of the bow roller above the water. There are so many variables in anchoring as regards seabed, anchor type, weather, exposure to any swell, swinging space available but I think this is a good guide to start with.
Andrew

[Shrew]

Originally Posted by rslifkin

I was thinking full circle, not radius. And also including the length of the boat. I also did the quick / lazy math. So I was a little off, it would actually be approximately 570 and 280 feet.

Please bone up on geometry if you're going to engage in a discussion of geometry. What you're describing is the Diameter of the circle.

5:1 @ 50 feetL
-> Radius: 244.949
-> Diameter: 489 feet

10:1 @ 10 feet:
-> Radius: 99.499
-> Diameter: 198.998

No this is not pedantic. The scope (no pun intended) is around the math of calculating scope.

[rslifkin]

Originally Posted by Shrew

Please bone up on geometry if you're going to engage in a discussion of geometry. What you're describing is the Diameter of the circle.

5:1 @ 50 feetL
-> Radius: 244.949
-> Diameter: 489 feet

10:1 @ 10 feet:
-> Radius: 99.499
-> Diameter: 198.998

No this is not pedantic. The scope (no pun intended) is around the math of calculating scope.

Yes, I'm aware that I'm talking about the diameter. The diameter of the swing circle (plus accounting for boat length) is how much space you need to be able to swing safely in a wind shift without hitting land or a shallow spot. My point was that in deeper water, if you try for long scope, the swing circle can easily become very large, meaning you may not fit in the area you're trying to anchor in.

[Hardhead]

Math...

[zstine]

the reason you need a higher ratio in shallow is because a few feet of bow pitching has a much higher % of depth and more likely to pull out the anchor than in deep water where a few feet pitching has negligible impact on the anchor

[sailorboy1]

Originally Posted by Muaddib1116

I've read and heard from multiple sources, somewhat axiomatically, that in shallow water you tend to need more scope and in deep water you can get away with less than usual scope. I trust that it's true, but I've tried and failed to find a good explanation of why this is. Can anyone help explain it?

10' water depth + 5 foot to bow, 5:1 = 75' chain out, chain length to water depth = 75/10 = 7.5

15 water + 5' to bow, 5:1 = 100' chain out, chain length to water depth = 100/15= 6.67

so maybe it seems more chain out at 10' water depth

but for me if there wasn't a reason not I put out more anyway

[Hardhead]

I may be wrong, but I believe the catenary effect increases - as the length of the rode increases. Essentially - the sag in the rode increases with the length of the rode, and causes a lower angle of pull on the anchor.