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Old 01-07-2010, 11:38   #1
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Dragged Our Rocna Today

First time it lost it's grip in a variety of conditions over the past 3 years. The bottom was thick heavy weeds, but we didn't know that when we set it due to not being able to see the bottom (murky water).

Boat is a 40' catamaran with a 25kg Rocna on all 5/16" G4 chain.

Set it as usual: 5:1 scope and back on it with both engines from idle up to 2,000rpm for a minute. Bit and immediately set strong as always and we felt good. Stayed in place for 2 days. Today, a squall came through with 30-40kt winds, but we still held fast until a big blast hit us and we took off like a rocket.

It wouldn't reset, and when I pulled it up a big ball scoop of weeds were in it. No mud or other bottom - just 100lbs of weeds.

Only one other boat in the small cove not on a mooring and they dragged also. They had a Delta.

I would love to go back and reset on our Spade anchor , but there are too many moored boats and hard bits of land around to risk it.

Maybe this should be in the "confessional" section? Maybe titled "I think I need a CQR"?

Mark
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Old 01-07-2010, 11:49   #2
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I'm chuckling at this post. Could it be? Can this be so? A Rocna dragged???? Say it isn't so! Goodness gracious, must have been a 'knock-off', cheapo imitation.

Seriously, heavy weed is the bottom condition in my experience that defeats (dare I say it?) ALL anchors at one time or another. I had trouble once with a Rocna, (same size, boat, same size anchor, much less wind) in similar circumstances. Of course, I've also had trouble in similar bottoms, size boats, anchors with, (wait for it) a CQR as well!!

Sorry to hear it, glad you survived.
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Old 01-07-2010, 12:05   #3
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Not the awsome Rocna?

I guess once the leading edge gets blunted with weed, nothing will reset it.

Im still going to make my own later in the winter when I have time

How big might I need for a 20 ton 50 footer in a river?
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Old 01-07-2010, 12:55   #4
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OK...I need to know more facts - as best as you can remember. Here's what I'd like to know:

1. The exact number of feet of chain out.

2. The depth of the water at the time of dragging.

3. How far off the water in feet is the attachment point of the chain to the boat - not a snubber - but the actual chain connection point on the bow.


I've never had my Rocna out with less than 6:1 in 40 kt winds under all types of sea bottoms. I'm wondering how accurate the 5:1 really was because just a little change there (especially in shallow water) would make a great difference. At a true 5:1, I've been able to drag my Rocna under extreme reverse thrust (2 engines). At 6:1, I've never been able to move it as long as it was initially set (like yours was).
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Old 01-07-2010, 13:17   #5
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Originally Posted by colemj View Post
Boat is a 40' catamaran with a 25kg Rocna on all 5/16" G4 chain.
Just checked the Manta 40 specs and the Rocna chart which advises 1 size bigger for cats. Rocna 25 is definately 1 size bigger, maybe 2. Sorry about those good night's sleep for all Rocna owners.

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Old 01-07-2010, 13:25   #6
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3. How far off the water in feet is the attachment point of the chain to the boat - not a snubber - but the actual chain connection point on the bow.
Assuming the snubber does not break, the height of the attachment point of the snubber will determine the effective angle of pull at the anchor (together with the rode length).
On most yachts the height of the snubber attachment and chain attachment will be identical, but if a distinction is made why emphasize the chain attachment?
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Old 01-07-2010, 13:38   #7
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Originally Posted by noelex 77 View Post
Assuming the snubber does not break the height of the attachment point of the snubber will determine the effective angle of pull at the anchor (together with the rode length).
On most yachts the height of the snubber attachment and chain attachment will be identical, but if a distinction is made why emphasize the chain attachment?
Right. It's the height of the point where the snubber meets the boat, which determines scope. Everything else is irrelevant.
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Old 01-07-2010, 13:50   #8
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Set it as usual: 5:1 scope and back on it with both engines from idle up to 2,000rpm for a minute. Bit and immediately set strong as always and we felt good. Stayed in place for 2 days. Today, a squall came through with 30-40kt winds, but we still held fast until a big blast hit us and we took off like a rocket.
I often dive on my (and other anchors) and a factor never mentioned is occasionally the rode will take full reverse without putting any force on the anchor itself . The anchor is therefore completely unset.. The cause is the chain wrapping around a rock. If the boat swings the chain unhooks from the rock and the boat is lying to an unset anchor.
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Old 01-07-2010, 14:10   #9
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interesting. Ours also dragged for the irist time ever in Ajaccio harbour in a sudden blast of about 35 knots. We had our cockpit cover up with its sides on (not normally done) and a very sudden katabatic blast caught us from previous less than 10kts breeze. We had just set the anchor v thoroughly and in ways that have held in all sorts of conditions.

It reset almost immediately and was holding steady in the succeeding 25kts of wind no problem. That is a notoriously foul-bottomoed anchorage, and indeed when we hauled it up a week later the chain was wrapped around with building wire, so it may be that we had fouled but would never have known it without that particular set of circumstances.

We hauled it up because we didn't quite like the new position and found it had weed on it, but had penetrated and was holding fine. Some other friends with a rocna, similarly set , in the same anchorage at the same time but without the unusual windage, held absolutely solid.

We were actually comforted rather tahn the reverse by the rapid reset of the anchor as we think that would not have happened with our old CQR - certainly not in weed.
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Old 01-07-2010, 14:12   #10
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Originally Posted by noelex 77 View Post
I often dive on my (and other anchors) and a factor never mentioned is occasionally the rode will take full reverse without putting any force on the anchor itself . The anchor is therefore completely unset.. The cause is the chain wrapping around a rock. If the boat swings the chain unhooks from the rock and the boat is lying to an unset anchor.

I thought a good anchor is one that resets with a tide or wind shift. That happens a lot I have heard.
In that case what is the difference here?
Boat swings, chain moves off rock, anchor sets.
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Old 01-07-2010, 14:24   #11
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OK...I need to know more facts - as best as you can remember. Here's what I'd like to know:

1. The exact number of feet of chain out.

2. The depth of the water at the time of dragging.

3. How far off the water in feet is the attachment point of the chain to the boat - not a snubber - but the actual chain connection point on the bow.


I've never had my Rocna out with less than 6:1 in 40 kt winds under all types of sea bottoms. I'm wondering how accurate the 5:1 really was because just a little change there (especially in shallow water) would make a great difference. At a true 5:1, I've been able to drag my Rocna under extreme reverse thrust (2 engines). At 6:1, I've never been able to move it as long as it was initially set (like yours was).

OK, we can do a post-mortem here, but I want to emphasize HEAVY WEED. Lots of it. A giant 70-100lb wad of it came up with the anchor (I mean really big), but no indication of any good bottom (and the Rocna always brings up a good bottom sample). I believe the anchor was well buried in the weeds, but there was only weed to be buried into.

So, more details then.

The wind wasn't 40kts. It was 5-10kts until a squall cell moved through, when it suddenly increased to 30 going to 40. We held through all that, but then a HUGE single blast sent us for a ride.

1. Two 25' snubbers in a bridle (catamaran) were attached to the chain at the 55' point.

2. We were anchored on a bit of a sloping shore that was 7' deep when we swung toward shore and 10' deep at the furthest point away from it. So the anchor was in ~8-9 ft of water.

3. The bridle attachment points are hardened padeyes on the bows below the deck level. So the attachment is ~3' above the water. The scope is increased beyond 55' by the bridle, but a trig wiz will have to figure out by how much. Here are the parameters: a bridle triangle 25' on two sides and 19' on the base. Attachment 3' above the water.

I don't think scope was the issue. I mentioned 5:1 in the original post, but that was just to indicate that I wasn't French.

I think it was the weed. Not used to the weed they have down here !

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Old 01-07-2010, 14:31   #12
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Just checked the Manta 40 specs and the Rocna chart which advises 1 size bigger for cats. Rocna 25 is definately 1 size bigger, maybe 2. Sorry about those good night's sleep for all Rocna owners.

Greg
Yes, it is one size larger - we chose that specifically.

3 years without incident in all types of bottoms and all types of weather from Massachusetts to Florida, through the Bahamas and the entire Caribbean to Grenada. I have used Delta, CQR and Bruce extensively in the past and had them drag in much calmer conditions and much better bottoms.

But you weren't trying to start an anchor war, where you?

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Old 01-07-2010, 14:32   #13
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Roaring Girl, is your anchor a Rocna?

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Old 01-07-2010, 14:59   #14
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The Rocna guy is going to sue you for libel. :-)
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Old 01-07-2010, 15:12   #15
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Assuming the snubber does not break, the height of the attachment point of the snubber will determine the effective angle of pull at the anchor (together with the rode length)
You're right - I didn't explain it well. There are a lot of people who think that putting on a snubber reduces the connection point to the point where the snubber grabs the chain. It's really the point where the snubber attaches to a fixed point on the boat.

I'm not sure I have enough numeric details yet but the picture is clearer.

There was an anchor - then 55' of chain - then a 25' snubber but certainly the entire snubber wasn't out, was it? How long was the distance between the point where the snubber attached to the chain and the point where it attached to the boat? A very loose calculation is that scope was (55+X)/(10+3) where X is the length of the snubber between the chain and boat.

In order to have 5:1 scope, the snubber length had to be 10 feet. Any less and the scope was less. More would make the scope better than 5:1. In order to achieve 6:1, the snubber would have to be 23' which sounds unlikely given the snubber is 25' long and probably had a nice tail on it.

It's impossible to know now, but I have a feeling that letting out just 15' more chain would have removed the drag potential. That's easy to say on "Monday morning".

One last question - did you have an anchor alarm set? If so, which one? I'm in the process of writing a new type of anchor alarm. None of them are done correctly. They all have a 2X error where X is the distance from the bow to the GPS on the boat. It's simple to fix that with today's mobile phones (of all things). It's actually a substantial error especially for most sailboats who often keep the GPS on the stern. So a 40' boat would have 80' of error even when the boat was still in the safe zone. In the case above, 80' alone should have made the alarm go off. This error is the reason we all greatly lengthen our anchor alarm distances to stop them from going off. It's much better to have one that just works correctly instead!
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