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Old 14-09-2021, 06:50   #1
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Ground Tackle for Larger Yacht

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?
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Old 14-09-2021, 07:10   #2
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Re: Ground Tackle for Larger Yacht

Size the chain to the loads the boat will produce in heavy winds, not to the anchor. My 33kg Vulcan recommends 3/8" G43 chain, but I'm using 5/16" as there's no way my boat will exceed the WLL of 5/16" G43 even in 80 kts of wind. The anchor isn't sized to hold 100+ kts in a good bottom (even though it might do that), but it's sized to provide sufficient margin for 50 - 60 kts on slightly short scope in a not so good bottom.

As you noted, bigger, heavier boats are easier to anchor as they're less reactive to gusts, etc. so the loading on the anchor is more stable.
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Old 14-09-2021, 07:14   #3
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Re: Ground Tackle for Larger Yacht

I am by no means an expert, my practical experience is very limited, but I've done a fair amount of research on the subject. I would think 12mm G70 would work, as long as you could get the right size shackle to work. According to Rocna's chain chart, you could even use 10mm G70 for their 70kg anchor, but I imagine there would be issues getting an adequately strong shackle to work.

55kg definitely seems small for a 67ft, by the standards of today's common wisdom (largest primary anchor the boat/you can handle), but I agree that it is probably at the top end of what the Ocean 3 could/should handle. I have an Ocean 3 that is handling my 10mm chain and 50kg anchor pretty well, but when it needs to be replaced, I'll be moving up to a Lewmar V5 or equivalent. If he moves to a 70-80kg anchor, I think he would have to upgrade his windlass. It seems like a minor thing to balk at, given he just bought a Discovery 67...

My thoughts on why larger anchors work disproportionately better is that it's not a surface area thing, or even directly a weight thing, it's that the weight means that the edge/point pressure of the fluke is much higher and penetrates the substrate better, and also probably helps the anchor dive better, because the weight actually scales faster than the surface area. And I buy into Steve Goodwin's theory that diving is where anchors get their holding power.
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Old 14-09-2021, 07:16   #4
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Re: Ground Tackle for Larger Yacht

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Originally Posted by rslifkin View Post
As you noted, bigger, heavier boats are easier to anchor as they're less reactive to gusts, etc. so the loading on the anchor is more stable.
But wouldn't they have higher loads because of increased windage? And also in a wind shift where the boat builds up speed and the chain is slack, the weight and momentum of the boat will require huge loads to stop it.
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Old 14-09-2021, 07:19   #5
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Re: Ground Tackle for Larger Yacht

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But wouldn't they have higher loads because of increased windage? And also in a wind shift where the boat builds up speed and the chain is slack, the weight and momentum of the boat will require huge loads to stop it.

Yes, the loads go up due to higher windage. But as a boat gets bigger, weight increases faster than windage. So the boat reacts more slowly to wind gusts and the heavier boat will typically be less reactive to wave action as well. So the load on the anchor changes more slowly and the spikes are lower relative to the average load.
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Old 14-09-2021, 07:41   #6
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Re: Ground Tackle for Larger Yacht

That makes sense, I hadn't considered that before. I would still think that even if the shock loads from waves and wind gusts are less, and that windage loads are proportionally less than the weight, you still need the chain to be strong enough to take the shock of stopping the boat after it's drifted right over the anchor for a few hundred feet during a sudden wind shift, potentially getting the boat up to a few knots of speed. Assuming a good, large anchor that will reset immediately or stay engaged in the bottom, that load on the chain could be much higher than you might calculate for normal wind gusts.
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Old 14-09-2021, 08:40   #7
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Re: Ground Tackle for Larger Yacht

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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.
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.
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Old 14-09-2021, 08:49   #8
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Re: Ground Tackle for Larger Yacht

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).
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Old 14-09-2021, 08:57   #9
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Re: Ground Tackle for Larger Yacht

For what it is worth, I have an Oyster 655, which I believe is a similar size boat. I have a Lewmar V6 vertical gypsy/drum type windlass, and I use a 48kg CQR anchor with 12 MM chain.
First, the chain: using a 14mm (which I used to have) only allowed me to have about 220ft of chain, because of the weight on the bow. A 12mm chain allows me to have 300ft (same weight), so I can anchor in deeper water in the Pacific.
In regard to the anchor, maybe I am lucky? But, once the anchor is set, I have never had a dragging problem. Experienced 70 knot winds in West Palm, and other boats dragged into shore, my boat stayed completely still.
I think it is most important to be good at anchoring; getting the anchor properly set in the right ground, and having enough chain out. His current setup should be fine.
Not familiar with that windlass, so won't speak to that.
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Old 14-09-2021, 08:58   #10
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Re: Ground Tackle for Larger Yacht

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Originally Posted by Muaddib1116 View Post
That makes sense, I hadn't considered that before. I would still think that even if the shock loads from waves and wind gusts are less, and that windage loads are proportionally less than the weight, you still need the chain to be strong enough to take the shock of stopping the boat after it's drifted right over the anchor for a few hundred feet during a sudden wind shift, potentially getting the boat up to a few knots of speed. Assuming a good, large anchor that will reset immediately or stay engaged in the bottom, that load on the chain could be much higher than you might calculate for normal wind gusts.

Thank you for bringing up boat speed. I believe most loading tables assume one knot, if it was taken into consideration at all.
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Old 14-09-2021, 08:58   #11
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Re: Ground Tackle for Larger Yacht

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...or if I was feeling all high tech and rich, maybe 10mm (3/8") g7(70).
The guy just bought a Discovery 67. If money is an issue, then he just made a big mistake. Unless I'm somehow missing something, this guy just spent upwards of $2M on a boat, spending $15k to get the ground tackle up to snuff is a no brainer, regardless of what type of cruising he's doing. How could you possibly feel comfortable on a boat that size sitting on a 45kg Delta?
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Old 14-09-2021, 09:00   #12
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Re: Ground Tackle for Larger Yacht

As far as chain is concerned the 12mm (or maybe ½') is certainly strong enough. Add some scope and it will do. On my 25+ ton yacht I downgraded från a Lewmar Delta 40kg (with was fine but obstructed my bowsprit) to a 30kg S/S Bruce. I'm very happy and has ridden out storms with both without problems. I think a 55kg Bruce or a slightly heavier Delta would do very nicely. If the bow roller has sufficient diameter and turns without too much friction, the load on the windlass is not a problem.
In any case, when winds start blowing above 50 knots you would launch a second anchor, just for a good nights sleep.
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Old 14-09-2021, 09:06   #13
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Re: Ground Tackle for Larger Yacht

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In regard to the anchor, maybe I am lucky? But, once the anchor is set, I have never had a dragging problem.
This is a common caveat with people recommending smaller anchors. It's kind of a get out of jail free card. If the anchor drags, it wasn't set correctly, it wasn't the fault of the anchor.

In great holding conditions, yes a smaller anchor can hold a bigger boat. If you ALWAYS anchor in deep firm mud or sand, sure skip to that smaller anchor.

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.

I really don't understand the benefit for recommending a smaller anchor. The difference between. For a 60 foot boat the difference between a "really big" and a marginal anchor is 50 lbs. Unless you are sailing an ULDB I very much doubt you would notice a difference in sailing performance with 50 lbs of weight on the bow. If it REALLY bothers you, cut off 30 feet of chain. You'll be better off!

But that's enough of this...you either like my advice or not. I have nothing else to add.
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Old 14-09-2021, 09:24   #14
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Re: Ground Tackle for Larger Yacht

Dockhead,


I've posted this link many times. It is from Calder's Cruisers Handbook.


Anchor System Sizing Tables (Reply #6) & Swivels http://c34.org/bbs/index.php/topic,4....html#msg30400


It seems to me that with these tables and with all the anchor sizing recommendations from the individual anchor manufacturers there should be more than enough information for you to help your friend.



Without getting into anything else, like techniques, personal choices, or any of the rest of the "usual suspects" that always come up during the anchor wars BS.


Good luck. With this information available to you, you shouldn't need luck, only your usual determined analytical skills. Treat it like one of your CTS drills! You know, a rational engineering exercise.
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Old 14-09-2021, 09:52   #15
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Re: Ground Tackle for Larger Yacht

the tables are guidelines, simple put.
No anchor is best no matter what size vessel or anchor.

The question is how will it be used, can the windless retrieve the weight of anchor and chain, and is it maintained.
I know a number of 65 foot/50 ton vessels with 80lb anchors that have held with 70 knot winds-
The failures tend to be the loads like sailing on the anchor that can cause a good deal of surging problems. The other issue is method of setting the anchor. That forum is another problem, but it is interesting to see so many experts weigh in on a subject includes all vessels. ships drag, oil rigs drag, yachts drag. If the anchor is to big to always prevent drag no matter what then one must decide if the yacht itself can handle the weather. Generally not.
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