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Old 26-01-2014, 18:09   #16
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Re: Distance from Anchor = Rhode Length

A Manson Supreme and 3/8 chain helps. Just got brand new chain. Good ground tackle is the first step. And familiarity with your boat, the bottom and conditions. And I've spent many nights in the cockpit. Not an issue, dress warmly.
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Old 26-01-2014, 18:22   #17
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Re: Distance from Anchor = Rhode Length

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It may be in the cases you cited the hooks were not set properly and the boats crept backwards slowly. Also in some cases due to the nature of the bottom and the type of anchor a boat might creep backwards.
This type of slow drag does not seem to be related to how well the anchor was initially set, but rather related to other factors such s the type and size of anchor softness of the substrate scope etc.
It most common with small convex anchors in softer substrates. When diving the anchor can be seen remaking completely buried, but the substrate above the anchor gradually moves like a burrowing animal. The anchor can remain stable like this and slowly drag many boat lengths. Often the boat will slowly drag into deeper water and the reducing scope will cause the anchor to break out and drag rapidly.

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In these cases it's usually slower and nothing like a good anchor drag where a boat takes off.
Agreed it is not as dangerous as the more common rapid drag, but it is still important to recognise.
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Old 26-01-2014, 18:34   #18
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Re: Distance from Anchor = Rhode Length

This thread needs a name change, how about 'How to make simple things complex' or 'Lets confuse the newbies'. There I feel better, carry on.

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Old 26-01-2014, 18:38   #19
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Re: Distance from Anchor = Rhode Length

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An anchor watch is sometimes necessary, but when spending winter in the Mediterranean I would be a tired, frozen cruiser if I set an anchor watch every time the windspeed reached this stage.

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You might be tired and frozen but that's no excuse for staying in bed when conditions are such that your boat might drag. When conditions are such that I might drag I keep an anchor watch. It's just good seamanship. I can sleep later. The conditions do not solely rely on windspeed. Fetch, nature of bottom, restricted manoeuvering room are a few of the many factors.
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Old 26-01-2014, 19:46   #20
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Re: Distance from Anchor = Rhode Length

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You might be tired and frozen but that's no excuse for staying in bed when conditions are such that your boat might drag.
I guess my point is that if I haven't dived on my anchor (which I usually, but not always do ) there is some possibility of dragging as low as 25 knots (for boats with smaller anchors the possibility exists in even milder conditions). The risk is very low at these sort of windspeeds, but the possibility exists nonetheless. I think it is important to have procedures in place that would alert me in these situations.

(This needs some explanation. I always set the anchor with full reverse which provides a force equivalent to 30-35 knots , but I have occasionally encountered situations where the chain has been caught around a rock and the anchor is totally unset. There are other scenarios where the anchor can drag in low windspeeds despite a sound technique)

Even when I dive and inspect the set of the anchor it has only been tested to 30-35 knots and then only in the direction of set. In the Med in winter it is not unusual to have windspeeds greater than this for a week or so.

I do agree that an anchor watch is prudent on occasions, sometimes at relativly low windspeeds when I am concerned about other boats dragging. I would typically set a full time anchor watch about 5 days a year (out of 300), but it is important to recognise that there is a (low) risk of dragging for many of the other 295 days. Having systems in place that provide adequate monitoring for these other days is important and the GPS is one of the best ways for doing this.

I am not a believer in making sailing uncomfortable. Staying warm down below while monitoring the GPS position is perfectly adequate in even very strong conditions where you can drag a considerable distance without hitting anything (and when there are no boats to hit you). (Occasional outside inspections for chafe etc may be needed).

There are times when you need to spend the night out in the cockpit monitoring transits, other boats, or the rocks just behind your stern, but often this reflects choosing a poor anchoring location in the first place.
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Old 26-01-2014, 20:09   #21
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Re: Distance from Anchor = Rhode Length

A point: all this talk about sitting in the cockpit all night monitoring transits?? In my experience, at night it is dark (maybe different in your neck of the woods, who knows?). I have had little luck in seeing transits in the dark.

I use anchor alarms on the GPS routinely when the conditions warrant such. Why would one not do so? Costs nothing, works well if there is room for a small amount of error. If one is so close to disaster to leeward that this is not so, then an anchor watch is prudent. Anchoring in such places is not (but sometimes may be unavoidable).

We've been cruising and living at anchor for almost 28 years. In that time, I've sat anchor watch only a handful of times, but have used the GPS many, many times. Worked so far!

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Old 27-01-2014, 08:26   #22
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Re: Distance from Anchor = Rhode Length

when an 85 ft heavy cargo schooner drags anchor straightback it is NOT slow drag, nor creep.is a DRAGGING of anchor.mebbe yŠll should reassess your parameters on anchor dragging to include this so you dont awaken on rocks.
remember I AM OUT HERE..not at a desk jotting notes and stating alleged facts as i play office manager.
i KNOW what i see,and i see a lot. remember, i AM out here , still, and will be for loong time, as this is my lifestyle, not my avocation.

btw...many incidents of dragging have been during 20-30 kt winds .... might wanna reassess your data.


i cannot see the transits in dark, but i can judge the distances and use the lights of town and houses to tell where i am.
here there are those lights and such to use as a guide. there are also jetties and breakwalls usable for judging distances as well.
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Old 27-01-2014, 09:01   #23
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Re: Distance from Anchor = Rhode Length

Have the scope be long enough to keep the shank of the anchor parallel to the sea floor under ANY conditions that you will encounter and forget the fuzzy math.
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Old 27-01-2014, 09:56   #24
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Re: Distance from Anchor = Rhode Length

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Have the scope be long enough to keep the shank of the anchor parallel to the sea floor under ANY conditions that you will encounter and forget the fuzzy math.
The math is simple for the "bar-tight rode" case:
Scope, Shank Angle
3:1, 20 degrees
5:1, 12 degrees
10:1, 6 degrees
20:1, 3 degrees

angle = ArcCos(rode length / anchor roller height above bottom)

An angle of zero = parallel to the sea bed.

Add in bottom slope and things get better or worse. Factor in chain catenary, things get better.
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Old 27-01-2014, 11:00   #25
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Re: Distance from Anchor = Rhode Length

Me thinks that there's a lot of engineers who could better serve their employer's this Monday mornings than doing hypothetical musings about anchoring their boats.
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Old 27-01-2014, 11:25   #26
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Re: Distance from Anchor = Rhode Length

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Me thinks that there's a lot of engineers who could better serve their employer's this Monday mornings than doing hypothetical musings about anchoring their boats.
I'm retired. I enjoy this stuff. Apparently others do too. Got a problem with that?
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Old 27-01-2014, 12:10   #27
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Re: Distance from Anchor = Rhode Length

No, I don't. I'm retired also. I enjoy this forum too. I like to share my sailing experiences, and sometimes express my opinion. Sorry you disapprove.
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Old 27-01-2014, 12:21   #28
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Re: Distance from Anchor = Rhode Length

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No, I don't. I'm retired also. I enjoy this forum too. I like to share my sailing experiences, and sometimes express my opinion. Sorry you disapprove.
regards
Disapprove? Not at all. Sorry for any misunderstanding.
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Old 26-08-2014, 09:50   #29
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Distance from Anchor = Rhode Length

In the merchant fleet our rule of thumb is to let out enough chain to give you 5-7 times the depth of the water. It'll keep the shank of the anchor closer to pulling parallel to the sea floor. I do this on my own boat too and it works well. Leave the complicated math at home.


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Old 26-08-2014, 11:22   #30
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Re: Distance from Anchor = Rhode Length

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In the merchant fleet our rule of thumb is to let out enough chain to give you 5-7 times the depth of the water. It'll keep the shank of the anchor closer to pulling parallel to the sea floor. I do this on my own boat too and it works well. Leave the complicated math at home.
You do need to allow for the distance from the water to the bow roller as well. If you don't, it will result in a significant error in shallow depths.

eg You have anchored in 3 m of water. According to your formula you would let out 15 m of chain if you decided conditions warranted a 5:1 scope.

If your distance to the bow roller is 1.4 m (as ours) your scope is:
15/(3+1.4) :1
= 3.4 :1
That is way off the expected 5:1.
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