anchor scope

[noelex 77]

The minimum anchor scope needs the input from a lot of variables.
Ideally I look at:
1. The set of the anchor. If you can dive and see the anchor. This gains a great insight into the holding power.
2. The bottom composition
3. The expected force on the anchor wind, waves and current.

I also know that if my anchor holds in full reverse for 30 seconds (my normal set) it will hold up to 30-35k of wind.

The above factors are weighed and I try to estimated a minimum scope.

If I can exceed this scope safely, without causing problems for others I always do so.
If I cannot meet the minimums it is necessary to find another option.

The minimum scope may be from 2:1 to 8:1, or more.

I do this not only on my my own anchor, but others in the anchorage, especially those boats that may drag into me. I can often predict depending on how the anchor is set, the type of anchor, the distance to set and the scope the approximate wind speed the anchor will drag. It helps if you have seen the technique used when anchoring.

It's not a perfect science occasionally a poorly set anchor will set correctly as the windspeed picks up but generally I get reasonably close to predicting the wind speed when boats will drag.

[Kettlewell]

It's not a perfect science occasionally a poorly set anchor will set correctly as the windspeed picks up but generally I get reasonably close to predicting the wind speed when boats will drag.

Maybe you can come up with a DragApp that calculates that and then warns everyone around them what windspeed they will drag at.

I've gotten pretty good at figuring this out just from looking at the boat, the operators, the equipment, and how they handle it. If it's looking bad I send my wife up to the bow to give them the evil eye.

[noelex 77]

Originally Posted by Kettlewell

Maybe you can come up with a DragApp that calculates that and then warns everyone around them what windspeed they will drag at.

I've gotten pretty good at figuring this out just from looking at the boat, the operators, the equipment, and how they handle it. If it's looking bad I send my wife up to the bow to give them the evil eye.

I agree if you watch them anchor, the scope, type of anchor, and how it sets gives you a reasonable idea without diving.
It is amazing how many boat apply reverse power, drag backwards, without noticing

Evil eye and hands on hips. Yes most crusers try this on occasions. Generally it much easier to move. I have only lost my cool at one boat in last 1500 nights at anchor.
They deserved it

[barnakiel]

I guess this has been posted:

Pour francophones seulement

Regards,
b.

[Kettlewell]

Evil eye and hands on hips. Yes most crusers try this on occasions. Generally it much easier to move.

You haven't seen my wife's evil eye! It's rather hilarious to watch someone scuttle right out of the way when she gives them her stare. They usually anchor over on the other side of the harbor after that.

[Hydra]

Originally Posted by barnakiel

I guess this has been posted:

Pour francophones seulement

Regards,
b.

The English version is here and in the next pages:
http://alain.fraysse.free.fr/sail/rode/rode_b.htm

People interested in the static and dynamic behavior of mooring lines should look at this:
Principia - Deeplines : Global analysis of risers, moorings and flowlines

Alain

[barnakiel]

Hey Alain!

Thanks for the xls. Most helpful when choosing our gear and warp/chain ratio!

Regards,
barnakiel

[Hydra]

barnakiel, you are welcome.
All these computations are very interesting. I discovered them maybe 10 years ago in Alain Poiraud's book. However, it seems that the hydrodynamic drag on the boat isn't taken into account. In fact, it will provide some damping to the oscillations.

Alain

[Hudson Force]

When I arrive at an anchorage that is already occupied by a few boats I pass by my potential neighbors and ask them how much rode they have out. This allows me to choose to match their swing radius and also allows for a neighborly opportunity to exchange any concerns and reassure them of my intentions.

[capttman]

50 years of anchoring and trying different scopes and anchors i have to say 5:1 is the way to go. It gives comfort knowing if a swell comes up at 2am I'm still hooked. Just saying
One key is diff anchors for different bottoms

[Kettlewell]

This allows me to choose to match their swing radius and also allows for a neighborly opportunity to exchange any concerns and reassure them of my intentions.

Then you run across the guy who has out 200 feet of rode in 10 feet of water and expects you to stay clear of his swinging circle.

[Jimbo485]

Originally Posted by Kettlewell

Then you run across the guy who has out 200 feet of rode in 10 feet of water and expects you to stay clear of his swinging circle.

She is a gal, not a guy, and is on the west coast of Mexico somewhere!

[Hudson Force]

Originally Posted by Kettlewell

Then you run across the guy who has out 200 feet of rode in 10 feet of water and expects you to stay clear of his swinging circle.

Surprisingly, I have not had that happen. More often they'll have 50 to 80 feet out and I'll fit in fine. The second likelihood is that they say that they have an undetermined amount out,..."enough"...."a good amount", etc. and I'll leave them a little more space. I have far more frequent difficultly with people having too little rode than too much.

[barnakiel]

Originally Posted by Hydra

(...)

All these computations are very interesting. I discovered them maybe 10 years ago in Alain Poiraud's book. However, it seems that the hydrodynamic drag on the boat isn't taken into account. In fact, it will provide some damping to the oscillations.

Alain

Alain: Was it you who posted load cell anchoring measurements the other day in one of sailing magazines?

I remember someone measured the loads and the boat was swinging too, the measured loads came way BELOW the theoretical loads as calculated from plain wind force x exposed area.

Apparently, when the boat is side to the wind there is max exposure to the wind force coupled with max dampening from the lateral area. Then again, a boat sailing wildly may work the anchor thru the bottom. Etc., etc..

In fact, many, many factors that contribute.

b.

[sailorboy1]

So for those that are using numbers (especially 10' depth) was stage of tide are you basing this on?

In the NE area I spend my time in there is almost a 10' tide. I normally always base my scope on high tide. If I am in 10' at high tide I started out on shore.