Anchoring too close to other boats?

[Cooltechmd]

Great post, awesome views.

[noelex 77]

An anchor’s holding ability with varing scope is an important question that could do with more research, but there are at least some ball park figures available.

Some anchor manufacturers publish data. Fortress is typical and they publish the following results:

Scope Holding
10:1 100%
7:1 85%
5:1 70%
3:1 40%
2:1 10%

Fluke anchors generally have slightly better short scope performance so these percentages may be considered a little optimistic for other anchors. Other manufacturers published results are typically a little more pessimistic (the holding decreases more rapidly at shorter scopes).

The late Professor Knox did some great practical testing on this subject and published more optimistic results. For example at 5:1 the Spade was measured at 85% and the Delta at 80%. At 3:1 the Spade was 70% and the Delta was 60%. (Note: this is expressed as a percentage of the particular models maximum holding ability. The Delta’s maximum holding was significantly lower than the Spade’s).

The very large multi-magazine test conducted in 2006 measured holding at both 5:1 and 3:1 and while they did not analyse the 3:1 results the raw data produced more pessimistic results than the Fortress table suggests.

So there is some variation and we have not begun to consider important variables such as different substrates, but I think as starting point the table above (from Fortress) gives at least some idea how the the holding power is likely to vary with scope.

It is also worth remembering that it is the chain’s angle to the seabed at the anchor that is the determining parameter rather than simply scope.
Scope is a major factor in determining this angle, but there are other important factors, especially the bottom slope. For a more advanced view on the subject try and think about the likely chain/bottom angle rather than simply scope.

[StuM]

Strange concept. If your scope is less than one, your anchor is not on the bottom. How can there be any force at all?

[Skip JayR]

I keep my tone little bit straight forward, as I think, that casual talking in such a risky situation you brought yourself, your crew member and the boat neighbour into, could have ended badly. - I won't refer to technical aspects of anchoring, as others have posted. (Rec.: Regularly you can count at least 7-10 times water depth as very safe length of your anchor chan for bad weather conditions. For smooth conditions I'd calculate factor ~5. As relevant beside water depth is the effective depth, calculated by water depth + bow height above the waterline + LOA.)

You can give the answer yourself you imagine, if there would happen a sea accident by staying passively on your position and front sea court you'd later explain: "Yes, I'd felt unsafe. I thought its bad seamanship. I was aware that I was very (and too) close to my neighbour." - The court judge simply would pledge you guilty. Fair enough.

Keep people on board and ship safe, under any circumstances. Or to say it more formal citiating the law: "safety and security of the ship, crew, passengers, cargo and property and for the protection of the marine environment have highest priority". You have to follow IMO/SOLAS regulations, also on a recreational boat.

Remember the basic idea of COLREG (International Regulations for Preventing Collisions at Sea):

"Every vessel shall at all times proceed at a safe speed so that she can take proper and effective action to avoid collision and be stopped within a distance appropriate to the prevailing circumstances and conditions."

Anchoring is not directly referring to, as it is not a ship under way, but Colregs contain relevant safety aspects which should guide you for any decision having other boats/ships around: appropriate distance + circumstances & conditions.

The decision you should have done is very simple after you recognized, that you are in danger: Leave the anchor spot to move to a more safe place.

To dig little bit deeper to find the real reason how it came to this problematic situation and to learn from: Re-read your own description how it came to this unsafe situation. Citiation: "... dropped anchor in a fairly crowded harbor".
At sea court a jury would pledge you guilty, as you already noticed, that the anchor bay is over-crowded, giving you and also other boat owners and boat crews a higher risk at the moment you come in and drop your own anchor.

Always have a plan B and C available, if your regular plan A is not working. Pack plus 200% on top and you will have a safe journey. - And also be aware, that in "being overtired" physical conditions, you have higher risks, to make wrong decisions. Try to avoid to be in bad physical shape.

Good, that you did this post. It helps to do self reflection and to make oneself more sensitive against our own weaknesses as human.

[Dockhead]

You're right of course. Obviously a mistake in the table.

But see Noelex's more substantial discussion of this subject above.

Originally Posted by StuM

Strange concept. If your scope is less than one, your anchor is not on the bottom. How can there be any force at all?

[senormechanico]

Originally Posted by StuM

Strange concept. If your scope is less than one, your anchor is not on the bottom. How can there be any force at all?

MANY years ago, I saw a photograph in a catalog (before photoshop even existed) of a Sears and Roebuck fishing boat anchor winch. The instructions clearly stated,
"Pay out reel until anchor contacts the bottom, then pull up 2 feet."


I wish I could find that ad again !

[CaptTom]

Originally Posted by StuM

Strange concept. If your scope is less than one, your anchor is not on the bottom. How can there be any force at all?

Actually, I imagine there would be some force with a sea anchor or drogue.

Interesting how quickly the line starts to flatten after 3:1. Of course, conditions in the real world are dynamic. I wouldn't suggest always using 3:1 or even 5:1.

[Wolfgang.Schau]

180ft of chain in 18ft of water seems to be a little overkill in a crowded harbor.
Minimum is 3 times depth, 5 times is safe and would require 90ft.

[noelex 77]

Originally Posted by Wolfgang.Schau

180ft of chain in 18ft of water seems to be a little overkill in a crowded harbor.
Minimum is 3 times depth, 5 times is safe and would require 90ft.

Don’t forget the bow roller height in the scope calculation.

[CatNewBee]

Originally Posted by noelex 77

Don’t forget the bow roller height in the scope calculation.

My chain "counter" is zeroed when the anchor is under the water suface, there is a zero mark on the chain, and from there every 10m plastic marks in different colors and counts,

10 is one red, 20 - 2 green, 30 - 3 yellow 40 - green, red, red, green, 50 - yellow green green green yellow, 60m is end of chain. Plus 5m for the bridle.

[noelex 77]

Originally Posted by CatNewBee

My chain "counter" is zeroed when the anchor is under the water suface, there is a zero mark on the chain, and from there every 10m plastic marks in different colors and counts,

This will still not give the correct scope calculation.

If anchoring at, for example 5:1, your chain needs to lengthened by 5 times the distance from the bow to when the anchor is just under the water (in addition to 5 times the depth of water). Your system is only allowing for 1 times this distance so your scope calculation will be wrong, especially in shallow water.

[CatNewBee]

Originally Posted by noelex 77

This will still not give the correct scope calculation.

If anchoring at, for example 5:1, your chain needs to lengthened by 5 times the distance from the bow to when the anchor is just under the water (in addition to 5 times the depth of water). Your system is only allowing for 1 times this distance so your scope calculation will be wrong, especially in shallow water.

Thats ok, the bridle adds 5m of depth.

Do you calculate your chain length depth sounder plus depth sounder distance from waterline plus freebord x 5 or do you calibrate the depth sounder to water line and add the freeboard?

[noelex 77]

Originally Posted by CatNewBee

Thats ok, the bridle adds 5m of depth.

In that case (if I understand correctly) your scope calculation will be correct at something around 4:1. You will have a higher scope than calculated at less than 4:1 and a lower scope than calculated at more than 4:1.

The error will not be significant at medium scopes so it is probably close enough in practice.

Originally Posted by CatNewBee

Do you calculate your chain length depth sounder plus depth sounder distance from waterline plus freebord x 5 or do you calibrate the depth sounder to water line and add the freeboard?

Our depth sounder is calibrated to the waterline. I prefer depth to be displayed this way.

Our first yacht had a retractable keel and of course calibrating the display to read depth under the keel does not work with this type of vessel. I have never quite understood anyone preferring the third option of depth under the transducer, but the mental arithmetic is hardly difficult so as long the calibration is understood by the crew any method is fine.

[StuM]

Originally Posted by noelex 77

Our depth sounder is calibrated to the waterline. I prefer depth to be displayed this way.

Me too. It makes it easier when comparing water and chart depth for in-close pilotage. You should know your draft, so you just make sure that your display depth stays above that number

[Dockhead]

Originally Posted by noelex 77

An anchor’s holding ability with varing scope is an important question that could do with more research, but there are at least some ball park figures available.

Some anchor manufacturers publish data. Fortress is typical and they publish the following results:

Scope Holding
10:1 100%
7:1 85%
5:1 70%
3:1 40%
2:1 10%

Fluke anchors generally have slightly better short scope performance so these percentages may be considered a little optimistic for other anchors. Other manufacturers published results are typically a little more pessimistic (the holding decreases more rapidly at shorter scopes).

The late Professor Knox did some great practical testing on this subject and published more optimistic results. For example at 5:1 the Spade was measured at 85% and the Delta at 80%. At 3:1 the Spade was 70% and the Delta was 60%. (Note: this is expressed as a percentage of the particular models maximum holding ability. The Delta’s maximum holding was significantly lower than the Spade’s).

The very large multi-magazine test conducted in 2006 measured holding at both 5:1 and 3:1 and while they did not analyse the 3:1 results the raw data produced more pessimistic results than the Fortress table suggests.

So there is some variation and we have not begun to consider important variables such as different substrates, but I think as starting point the table above (from Fortress) gives at least some idea how the the holding power is likely to vary with scope.

It is also worth remembering that it is the chain’s angle to the seabed at the anchor that is the determining parameter rather than simply scope.
Scope is a major factor in determining this angle, but there are other important factors, especially the bottom slope. For a more advanced view on the subject try and think about the likely chain/bottom angle rather than simply scope.

As penance for my sin of having posted something oversimplified on the subject.


Scope gives you two things -- more catenary, which dampens dynamic loads and reduces the angle on the anchor provided the wind isn't so strong to pull the catenary out, and reduction of the angle on the anchor.


But catenary doesn't seem to influence holding power in really strong conditions, because eventually the catenary is all pulled out. Peter Smith wrote about this, and Dashew uses light chain and advocates putting all the weight possible into the anchor, because of the diminishing effect of catenary in strong conditions.


I was always somewhat skeptical about this, for years, but we had a big discussion on here about a year ago, and we ran all the numbers and did all the calculations and lo and behold it is correct even in deep water and even with quite heavy chain.

So that leaves us with the basic geometry of scope vs. angle on the anchor:



The definitive work on this was done by Alain Fraysse:

Anchor

Rode - Static Behavior

Different anchors have different angles of vanishing holding power, and respond differently to angulation, but good anchors keep around 60% of their holding power out to about 20 degrees of angulation, which is less than 3:1.




Anchor


That lines up well with Professor Knox's empirical work which you cited above.

If your anchor is sized to give you say 10x the holding power you need in a moderate bottom, as mine is, you see that 3:1 scope is going to be perfectly fine in almost all cases.

My theory for the "fetish for scope" is that excessive scope is used by many cruisers to make up for poor anchoring technique, particularly failure to set the anchor properly. It is true that a really large scope like 8:1 will often facilitate the anchor's setting itself, if the sailor fails to do so, because the mass of chain on the bottom will keep the shank down as the anchor drifts. If the anchor is not set properly in the first place, then 3:1 or 4:1 will not be enough.

But if you do, it is. And proper setting technique will also let you know exactly what kind of bottom you are dealing with. If you haven't bothered to back down hard and long and work the anchor into the bottom, you won't know. Therefore, you will be tempted to use a bunch of excessive scope "just to be sure".

You can increase the maximum holding power of the anchor by about half again, by increasing the scope from 3:1 to 10:1 (or maybe only 30% with a really good anchor like a Spade or a Fortress). Do you really need that extra holding power? If your anchor is adequately sized, and it is properly set, in a bottom which you know, because you have spent time setting the anchor, is at least medium quality, you should have such a safety margin that this extra bit doesn't make any difference, even in storm conditions.

Now if there is plenty of ROOM, you can put out more chain just for more comfort -- catenary dampens the dynamic loads. In deeper water and with heavier chain, you might not even need a snubber -- I don't use a snubber unless I'm expecting wind over 30 knots. A whole bunch of heavy chain in really deep water -- my 100 meters, weighing 330kg, in say 20 meters of water -- makes for a wonderfully smooth ride even in less sheltered anchorages.


But if you are in a crowded anchorage without the extra room, for God's sakes be considerate to others and don't lay out excess scope. Set your anchor properly -- and it can't be done in less than 10 or 15 minutes -- and keep it to 4:1 or so, or 5:1 if that is what others are using.


And if you are using excessive scope because you have a crap anchor, then do something about that. There's a wealth of choices for good anchors available these days, and the world has moved on very far since the 1990's.