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Old 11-01-2020, 13:42   #1
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Anchor rode calculator

I made an online anchor rode calculator that can make some different calculations regarding anchor rode forces, scopes, etc. It can also do some calculations regarding dynamic loads which can be useful when choosing a snubber. It can be found here: anchor rode calculator

Let me know what you think and if you have any corrections and suggestions.
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Old 11-01-2020, 14:22   #2
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Re: Anchor rode calculator

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Originally Posted by BjarneK View Post
I made an online anchor rode calculator that can make some different calculations regarding anchor rode forces, scopes, etc. It can also do some calculations regarding dynamic loads which can be useful when choosing a snubber. It can be found here: anchor rode calculator

Let me know what you think and if you have any corrections and suggestions.

Very nice. I think folks will get useful information from this.


I suggest cross-checked the results with a load cell. I mention this because the windload calculation seems high. If it includes typical chop, which makes sense and smith may have included, then it is in the range I have measured on several boats.


Dynamic effects, including yawing and single-leg loading make it complicated!
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Old 11-01-2020, 14:42   #3
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Re: Anchor rode calculator

What is effective scope?
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Old 11-01-2020, 15:08   #4
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Re: Anchor rode calculator

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Very nice. I think folks will get useful information from this.
Thank you.

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I suggest cross-checked the results with a load cell. I mention this because the windload calculation seems high. If it includes typical chop, which makes sense and smith may have included, then it is in the range I have measured on several boats.
I suspect you are right: it is probably a bit in the high end, but until I have had a chance to experiment with a load cell myself, I figured it best to be a bit conservative.

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Dynamic effects, including yawing and single-leg loading make it complicated!
Yes, probably not really realistic to calculate. The good thing about at least having a simple dynamic energy tool like the calculator does is that it allows one to make relative comparisons between various snubber designs.
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Old 11-01-2020, 15:13   #5
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Re: Anchor rode calculator

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What is effective scope?
It is another way to express the angle between the rode and the seabed at the anchor. Say this angle is 7.18 degrees. This doesn't really mean a lot to me, but it is the same angle that a completely straight rode would have if it was a 1:8 scope. So I call the effective scope 8.00 in this case, regardless of what the actual scope is. This seems more intuitive than the anchor angle, so both are given.
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Old 15-01-2020, 08:16   #6
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Re: Anchor rode calculator

Here is an example of how I have used the calculator. A while ago I read this post about running a snubber along the deck:
http://www.cruisersforum.com/forums/...ml#post2425883
I liked the idea and used the calculator:
Calculator results
It seems that I can cut snatch loads in half compared to the bridle I have been using so far (additional load in this example is cut from 513 kg to 255 kg).

So yesterday, I installed some low friction rings as fairleads and I look forward to trying it out.
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Old 15-01-2020, 08:40   #7
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Re: Anchor rode calculator

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It is another way to express the angle between the rode and the seabed at the anchor. Say this angle is 7.18 degrees. This doesn't really mean a lot to me, but it is the same angle that a completely straight rode would have if it was a 1:8 scope. So I call the effective scope 8.00 in this case, regardless of what the actual scope is. This seems more intuitive than the anchor angle, so both are given.
Angle should not really be coming into play as that is a result of catanery. Scope is simply the ratio of the length of rode (attachment point to seabed) and the total depth (from the attachment point to the seabed). Angle of the rode will vary with the same scope depending on the wind and current.

Depth: 15 ft
Bow pulpit height: 6 feet
Total: 21ft.

5:1 scope: 105 feet (21*5)

The angle of the rode is irrelevant. I could be sitting 10 feet from my anchor with the chain vertical. This has no bearing on the amount of scope.
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Old 15-01-2020, 10:18   #8
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Re: Anchor rode calculator

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Angle should not really be coming into play as that is a result of catanery. Scope is simply the ratio of the length of rode (attachment point to seabed) and the total depth (from the attachment point to the seabed). Angle of the rode will vary with the same scope depending on the wind and current.

Depth: 15 ft
Bow pulpit height: 6 feet
Total: 21ft.

5:1 scope: 105 feet (21*5)

The angle of the rode is irrelevant. I could be sitting 10 feet from my anchor with the chain vertical. This has no bearing on the amount of scope.

We are not talking about scope, but the term "effective scope" that I have used as described in post #5 above.
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Old 15-01-2020, 11:15   #9
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Re: Anchor rode calculator

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We are not talking about scope, but the term "effective scope" that I have used as described in post #5 above.
My response was specifically to Post #5, where you repeatedly discuss "angle", which really has absolutely nothing to do with scope.
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Old 15-01-2020, 11:18   #10
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Re: Anchor rode calculator

Anchoring scope is always an N:1 ratio. Calling it 'Effective Scope' by dropping the ':1' and referring to it by the 'n' value doesn't make it 'Effective Scope' it is simply an abbreviated notation for the same value.
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Old 15-01-2020, 11:45   #11
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Re: Anchor rode calculator

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The angle of the rode is irrelevant. I could be sitting 10 feet from my anchor with the chain vertical. This has no bearing on the amount of scope.
The angle of the rode at the anchor relative to the seabed is the factor that influences the anchors holding ability.

Scope is a more familiar term, but this number only provides an approximate idea of this angle. It ignores important parameters such as the angle of the seabed.

The term “effective scope” translates the important seabed to rode angle into a number that is likely to be more familiar, so this can be useful concept.
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Old 15-01-2020, 12:16   #12
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Re: Anchor rode calculator

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The angle of the rode at the anchor relative to the seabed is the factor that influences the anchors holding ability.

Scope is a more familiar term, but this number only provides an approximate idea of this angle. It ignores important parameters such as the angle of the seabed.

The term “effective scope” translates the important seabed to rode angle into a number that is likely to be more familiar, so this can be useful concept.
Yup, seems clear to me. Scope is just a proxy for rode angle.

I've been playing with the calculator BjarneK. It seems very interesting. Comparing my limited personal experience to the calculated outcomes, it seems to reflect my reality. But I would love to see comparisons to real-world data, as Thin mentions.
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Old 15-01-2020, 22:53   #13
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Re: Anchor rode calculator

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Originally Posted by noelex 77 View Post
The angle of the rode at the anchor relative to the seabed is the factor that influences the anchors holding ability.

Scope is a more familiar term, but this number only provides an approximate idea of this angle. It ignores important parameters such as the angle of the seabed.

The term “effective scope” translates the important seabed to rode angle into a number that is likely to be more familiar, so this can be useful concept.
I'm not seeing the advantage in using effective scope the way it is presented in the calculator. Scope as we normally use it is an indicator of the rode angle to a horizontal bottom at a pull that makes the rode effectively straight.
The effective scope calculated by the calculator indicates the rode angle on a horizontal seabed at the given wind speed. Not sure what value this is to me. If the wind picks up, the effective scope goes down to eventually reach the standard definition of scope. So?
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Old 16-01-2020, 02:44   #14
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Re: Anchor rode calculator

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The effective scope calculated by the calculator indicates the rode angle on a horizontal seabed at the given wind speed. Not sure what value this is to me. If the wind picks up, the effective scope goes down to eventually reach the standard definition of scope. So?

This is not true if the seabed where the anchor has been dropped is sloping.
While some anchorages are effectively flat, it is not unusual to be anchoring on a significant up or downslope.

This thread explains the concept well:

http://www.cruisersforum.com/forums/...ng-137516.html

The important point is that while scope is a basic, simple concept, it does not take into account some important variables that influence the critical parameter, which is the angle of pull on the anchor.

Unfortunately, the calculator does not take into account seabed slope, only the wind force, which is a pity.

For a flat seabed the calculater does give an idea what the chain angle may be with different wind strengths, and I think this is useful, especially as the manual calculation of the catenary curve is quite tedious. You are correct that at high wind strengths the catenary is generally effectively lost completely so the angle becomes almost the same as the result that the simple scope ratio provides.
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Old 16-01-2020, 09:41   #15
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Re: Anchor rode calculator

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Originally Posted by noelex 77 View Post
This is not true if the seabed where the anchor has been dropped is sloping.
While some anchorages are effectively flat, it is not unusual to be anchoring on a significant up or downslope.

This thread explains the concept well:

http://www.cruisersforum.com/forums/...ng-137516.html

The important point is that while scope is a basic, simple concept, it does not take into account some important variables that influence the critical parameter, which is the angle of pull on the anchor.

Unfortunately, the calculator does not take into account seabed slope, only the wind force, which is a pity.

For a flat seabed the calculater does give an idea what the chain angle may be with different wind strengths, and I think this is useful, especially as the manual calculation of the catenary curve is quite tedious. You are correct that at high wind strengths the catenary is generally effectively lost completely so the angle becomes almost the same as the result that the simple scope ratio provides.
That's why I stated a horizontal seabed. In anchoring you do need to consider the seabed angle when it is not a gentle slope. Sometimes the slope is in your favor. Like when the wind turns to put you on a lee shore. Sometimes you are anchored in a hole and all directions are up,
I usually only consider the the seabed bottom when I know it to be very steep. In which case I might adjust scope so it is still useful if we drag back 150ft.
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