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Old 14-09-2021, 10:06   #16
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Re: Ground Tackle for Larger Yacht

1) Decide the maximum gust wind speed you want to design for.
2) Look up, calculate yourself or contact the manufacturer to get the frontal area.
3) Calculate the peak load for the design delta for the assume gust.
4) Calculate the estimated peak jerk based on the road type and estimated stretch at the rode operating point.
5) Double the peak gust load (assuming 45 degree swing), add the peak jerk.
I usually iterate this process to get the working load limit of the rode (i.e. diameter) to match the peak load.
After estimating all these calculations you will be in the ballpark.
Then just decide the type of bottom you are designing for and look up ACTUAL
test data for the type of anchor you want (personal choice).
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Old 14-09-2021, 10:29   #17
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Re: Ground Tackle for Larger Yacht

Quote:
Originally Posted by Dockhead View Post
I'm helping a friend upgrade his newly purchased Discovery 67, a lovely fast cruising boat, and get her ready for an Atlantic crossing in December.


The ground tackle is a puzzle. The boat was delivered with the usual undersized Delta -- in this case, 45kg, 12mm chain, and the same windlass my much smaller boat (22 vs 50 tonnes) -- a Lewmar Ocean 3.


The seller had circumnavigated with the boat and said the ground tackle was great; never once dragged.



But honestly -- the Ultra sizing guide advises 80kg; Spade recommends 75kg. Rocna recommends 70kg. These anchors need 14mm chain.


So what to do? My friend is not keen on changing out the windlass, but I had a 55kg anchor on mine and I think that was about the limit.


55kg seems a bit light to me. And we would have to do something with the chain in any case, maybe G70 12mm would work.


In my experience, the larger the boat, the less demand on the ground tackle per unit of weight. And bigger anchors seem to work disproportionately better (although I can't think of any engineering principle by which that would be true). But 55kg -- pushing it?


Using the ABYC anchor load table and extrapolating 42kt loads from 60’ length to 70’, 70’ load would be 7200lb.

Beam may govern, what’s the boat’s beam?

WLL for 7/16 G40 is 7200lb, for 1/2” it’s 9000lb. 12mm is between those 2 Imperial sizes. As long as beam doesn’t bump up load they should be fine for chain size.
ABYC is a pretty conservative for chain sizing.

I would size anchor per manufacturer. I wonder is they like over sized chain because they can undersize the anchor slightly. Actually, I would go up one size from maker’s recommendation.
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Old 14-09-2021, 10:47   #18
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Re: Ground Tackle for Larger Yacht

The biggest unknown in anchoring is the state of the sea floor. Clay sea floor soils act differently from silts, and both act differently from sand bottoms.
Silica sand bottoms act differently from calcareous (coral detritus) sands.
Add a recommendation to your friend to include CHIRP sonar to his sensor suite so at least qualitative judgements be made for the type of seafloor where one anchors.
The sonar does a pretty good job of identifying rock or rocky bottoms. With experience, a soft mud bottom should give a different return wave form from a dense sand.
Unfortunately, my experience has nearly always been with sand bottoms where I cruise (Sea of Cortez north of LaPaz) but the small coral heads/boulders we have here are readily apparent on sonar.
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Old 14-09-2021, 20:16   #19
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Re: Ground Tackle for Larger Yacht

We're probably similar size to a Discovery 67, 65,000#'s with a 93' rig. We use a 121# second generation Rocna, Maxwell Nilson 3500 windlass and 400' of high tensile chain. I can't remember the size of the chain but it matches the recommendation of the anchor company. We're happy with the performance, we anchor quickly and securely in almost all bottoms. Oh, we also use a double bridle snubber.
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Old 14-09-2021, 22:06   #20
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Re: Ground Tackle for Larger Yacht

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Originally Posted by BillKny View Post
What you do not describe is the use this boat will be put to, and that's important.

One of the things that is missing (or relegated to the fine print) from most anchor sizing tables is the conditions for which the anchor is being sized for! To me this makes such a table totally useless.

While Rocna does have this information in their standard tables, it is hidden in the notes where people do not notice it. They size anchors for winds of <50 knots. In my opinion, this is suited for little more than casual coastal cruising since winds of greater than this can be found near a lot of cold fronts and other common weather conditions. I think it is a totally inadequate basis for a "real" cruising boat, no matter how much Rocna claims their sizing is "conservative."

We have been anchored in winds of 72 knots, the loading in these winds is about twice as high as it would be in 50 knots.

For the OP's boat in question (67 feet, 60,000 lb) the table from Mantus anchors would recommend a 57 kg anchor for <50 knots, and a 68 kg anchor for >50 knots. In line with what Rocna recommends for the lighter conditions, but much more honestly presented--in my opinion.

Loading is a very complex and difficult thing to get a hand on. The biggest variable is the amount of shock absorbing stretch in the rode. People who set anchors with a tiny 5 foot long snubber impose loads enormously higher exactly the same setup with a 30 or 50 foot snubber. That's just physics. For that boat I would use 12mm(1/2") g4(43) chain, or if I was feeling all high tech and rich, maybe 10mm (3/8") g7(70).

The stretchy snubber thing is baloney as boats get big and heavy. You won’t find one on the mega yachts or large commercial vessels. We are 58 feet, 40 tons. We destroy 1-1/2” 3-strand nylon snubbers in very short order. We still need to take load off the windlass and mechanicals so our very short snubber is 1” dynema from port and starboard. It is heading into year four of daily anchoring in the Caribbean. Our chain is 10 mm. Anchor is 54 kilo Rocna.

Also, big boats do swing and the moving mass on impact is impressive. In higher wind we let out 7:1 or more. The catenary is the ‘stretchy thing’ and you can radically change the period (frequency) of your swing by letting out chain. We can go from high impact to benign in very short order by making the stored chain useful.

I agree, the anchor seems small.
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Old 14-09-2021, 22:34   #21
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Re: Ground Tackle for Larger Yacht

Quote:
Originally Posted by Dockhead View Post
I'm helping a friend upgrade his newly purchased Discovery 67, a lovely fast cruising boat, and get her ready for an Atlantic crossing in December.


The ground tackle is a puzzle. The boat was delivered with the usual undersized Delta -- in this case, 45kg, 12mm chain, and the same windlass my much smaller boat (22 vs 50 tonnes) -- a Lewmar Ocean 3.


The seller had circumnavigated with the boat and said the ground tackle was great; never once dragged.



But honestly -- the Ultra sizing guide advises 80kg; Spade recommends 75kg. Rocna recommends 70kg. These anchors need 14mm chain.


So what to do? My friend is not keen on changing out the windlass, but I had a 55kg anchor on mine and I think that was about the limit.


55kg seems a bit light to me. And we would have to do something with the chain in any case, maybe G70 12mm would work.


In my experience, the larger the boat, the less demand on the ground tackle per unit of weight. And bigger anchors seem to work disproportionately better (although I can't think of any engineering principle by which that would be true). But 55kg -- pushing it?
Lovely boat!
Are you saying boats displacement is 50 tonnes?
That seems a bit high and I would guess closer 36 -40T , loaded an 32T light

My own steel pilothouse schooner @ 65ft is 42T loaded

Easy enough to test existing setup in good holding before deciding if an upgrade is warranted, as well as to consider his cruising area and style.

In my case, cruising in Typhoon waters, my Excel #13 at 63kg is sufficient, with 100m of 13mm chain.
Perhaps because of my large ship and Superyacht experience, I actually place more value on the size of chain and anchoring technique, than soley on the weight of the anchor
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Old 14-09-2021, 22:55   #22
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Re: Ground Tackle for Larger Yacht

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Originally Posted by BillKny View Post

Word of advice from an experienced cruiser: sometimes you will anchor by choice or necessity, in places with crappy holding where getting a good set on the anchor is hard.
That is true, but I think in those cases, a slightly bigger anchor does not exactly solve the problem!

Just accept you are in a crappy anchorage, allow for dragging room, keep a good anchor watch and always be prepared to drag.

Far too much faith in next Gen anchors rather than conservative management.

I always assume I've hard set my anchor just in front of a buried plastic bucket and that little extra nudge will have me merrily skating downwind.
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Old 14-09-2021, 23:22   #23
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Re: Ground Tackle for Larger Yacht

We are 64’ but only 16’ beam and 25 metric tons.

We have a Maxwell 3500 windlass, 400’ of 3/8” G70 chain, a Maxwell chain stopper (most crucial part of the system) and an 80kg (176lb) Bruce anchor.

This has held in a category 5 hurricane as well as several minor hurricanes.

If buying new, the only change I would make is the anchor. A steel Spade or that new Mantus M2 once they get that right. Same weight class.

We use a 5/8” 3-strand nylon snubber. A snubber isn’t a chain stopper. If you don’t have a chain stopper, your system is seriously flawed and you’ll have to rig a Dyneema workaround until you can get a real chain stopper, but you still need the nylon snubber for comfort at anchor, especially in shallow waters.
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Old 14-09-2021, 23:31   #24
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Re: Ground Tackle for Larger Yacht

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We are 64’ but only 16’ beam and 25 metric tons.



We have a Maxwell 3500 windlass, 400’ of 3/8” G70 chain, a Maxwell chain stopper (most crucial part of the system) and an 80kg (176lb) Bruce anchor.



This has held in a category 5 hurricane as well as several minor hurricanes.



If buying new, the only change I would make is the anchor. A steel Spade or that new Mantus M2 once they get that right. Same weight class.



We use a 5/8” 3-strand nylon snubber. A snubber isn’t a chain stopper. If you don’t have a chain stopper, your system is seriously flawed and you’ll have to rig a Dyneema workaround until you can get a real chain stopper, but you still need the nylon snubber for comfort at anchor, especially in shallow waters.
That's an intersting setup Nick and it obviously works!

Why 3/8 chain (9.55mm)?
I guess for the extra length, but I've always found smaller diameter chain more slippery along the bottom
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Old 15-09-2021, 00:26   #25
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Re: Ground Tackle for Larger Yacht

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That's an intersting setup Nick and it obviously works!

Why 3/8 chain (9.55mm)?
I guess for the extra length, but I've always found smaller diameter chain more slippery along the bottom
Main reason is to keep weight from the bow. The chain locker is about 8’ back from the bow to help with that too.
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Old 15-09-2021, 00:34   #26
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Re: Ground Tackle for Larger Yacht

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Originally Posted by Dockhead View Post
.



But honestly -- the Ultra sizing guide advises 80kg; Spade recommends 75kg. Rocna recommends 70kg. These anchors need 14mm chain.

Those ultras aren't very good eh?

We are high windage 65 tonner
Run a 68kg Manson supreme actually sized by Manson, not a chart and I did try to talk them up a size.
13mm chain on a Maxwell HWC 3500 24v
20mm nylon snubber around 5m long.

Have seen 80+ knots and multiple 40+ knots
Have deformed chain
But have not moved
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Old 15-09-2021, 00:41   #27
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Re: Ground Tackle for Larger Yacht

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The stretchy snubber thing is baloney as boats get big and heavy. You won’t find one on the mega yachts or large commercial vessels. We are 58 feet, 40 tons. We destroy 1-1/2” 3-strand nylon snubbers in very short order..
Strange
We run a 20mm nylon snubber on 65000kg (71 ton) and have never had one close to being destroyed or even showing signs of wear but replace every 18mths anyway.

How are yours destroyed?
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Old 15-09-2021, 00:52   #28
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Re: Ground Tackle for Larger Yacht

Chain size is determined primarly by the vessel, not the anchor. Therefore if upsizing the anchor, there is no need to upsize the chain.

Having said this, 12mm G3 or G4 chain is on the light side for a Discovery 67.

For a long distance cruising boat the best sizing guide is to fit the the largest anchor that can be comfortably managed. I have been on a Discovery 55 and that boat had a reasonably sized anchor winch and sturdy bow roller arrangement that would have managed ground tackle of a size that provides the versatility to anchor in areas of poorer holding, shorter scopes etc. I have not seen a Discovery 67.
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Old 15-09-2021, 02:45   #29
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Re: Ground Tackle for Larger Yacht

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Atlantic crossing - meaning canaries to Caribbean and flaff around and then return?

If so, he's probably just fine with what he has.

If he (or you) feel the need to 'upgrade' get a bigger/better anchor and leave the chain and windless alone.

Canaries to Caribbean then East Coast U.S. and back to Europe via the Northern route, Newfoundland, Iceland, Faroes.



I'm not sure the Ocean 3 can pull up an 80kg anchor in deep water. It seemed marginal with my previous 55kg Rocna.
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Old 15-09-2021, 02:52   #30
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Re: Ground Tackle for Larger Yacht

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Originally Posted by Nicholson58 View Post
The stretchy snubber thing is baloney as boats get big and heavy. You won’t find one on the mega yachts or large commercial vessels. We are 58 feet, 40 tons. We destroy 1-1/2” 3-strand nylon snubbers in very short order. We still need to take load off the windlass and mechanicals so our very short snubber is 1” dynema from port and starboard. It is heading into year four of daily anchoring in the Caribbean. Our chain is 10 mm. Anchor is 54 kilo Rocna.

Also, big boats do swing and the moving mass on impact is impressive. In higher wind we let out 7:1 or more. The catenary is the ‘stretchy thing’ and you can radically change the period (frequency) of your swing by letting out chain. We can go from high impact to benign in very short order by making the stored chain useful.

I agree, the anchor seems small.

I don't need a snubber with my 25 tonnes (loaded) boat and 12mm chain, except in extreme weather. 330kg of chain is a lot of catenary.


Your short dyneema is not a snubber -- it's a strop.. I use the same thing except made from chain.
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