Aluminium anchor as main

[noelex 77]

Dan you may like to look at this anchor test :
https://www.georgekniest.nl/pdf/manuals/kobra.pdf

This was a large scale independent test (not paid for by a manufacturer). I would caution that anchor tests give variable results and reading only one test can be misleading, but the results of this test fit in with my own conclusion about the difference between the Steel and the aluminium Spade.

The holding in hard sand dropped significantly for the same sized aluminium version. From 1905 kg to 1052 kg. The shank of the aluminum Spade bent during the test, but if they had tested in softer sand or mud the results would have been much closer.

[anacapaisland42]

I've always felt that the anchor should just be the "thing" at the end of the chain that stops it slipping too much and that the chain is the main thing that stops you from going to far.
So.....instead of "setting the anchor" you should think about "setting the chain"
Bill

Originally Posted by tkeithlu

Any anchor itself is a compromise, because you anchor with varied bottoms and you can't carry them all. Every one I've owned has been improved markedly by all chain rode. Heavy, but it absorbs shock better than you would think, and does a much better job of keeping the anchor on the bottom than my nylon rodes did.

[S/V Alchemy]

A 14,000 pound cat is a pretty big cat, I think. I would suggest two options: a SPADE steel anchor as the primary, and a Fortress aluminum anchor (the 21-pound FX-37 should fit) as the "lunch hook" or "fair weather, stay aboard" anchor. You could keep the SPADE assembled, but lashed aft of the bow for weight distribution, should the weather pipe up, and keep the Fortress as the initial anchor. As a comparison, I have a 30 kilo SPADE as a primary on all chain, an FX-37 as a secondary/kedge on a chain leader and 5/8" nylon rode, and an FX-21 as a stern anchor on similar chain/rope rode. My steel cutter is currently 30,000 lbs. light load, so my suggestions for you are quite conservative.

[singlespeed]

Used an aluminum Fortress anchorage the primary anchor in a mud bottom on our Lagoon 42. Hurricane Irene passed right over us as a Cat. 1. Anchor reset after the eye passed over us with without issue. Had a big plow anchor out as back up, but the rode was slack throughout. Fortress was pretty much useless in grass/weed bottoms.

Continue to use the anchor for muddy bottoms, seems to work better than our Rockna in mud.

[SteveSadler]

Yes the anchor is not a good place to save weight. I bent my steel Delta one night in a blow. Aluminum is good for a stern anchor to be set by tender.
Smaller chain can save weight as well but catenary effect will suffer, needing yet more chain. I have 12 mil chain for my Jeanneau 49 DS, allowing me to short scope often. But in a blow, it still just needs more scope.

[SteveSadler]

I have to comment on Annacap islands post. Where he stated the anchor is not as important as the chain in anchoring. Sorry mainstream anchoring practises
disagree. Ahunk of lead followed by a length of chain does not hold as well as a well set anchor of any design attached to a nylon rode with no chain.
The anchor itself is what holds you. The chain and rode give it the scope it needs to stay dug in.

[FranklinGray]

Originally Posted by Dan GB

Going in where angels fear to tread, another anchor question...

...I am tempted by the aluminium Spade.

Cheers,

Dan

Years ago I did an anchor test for setting ability in hard bottoms. The bottom was a mix of coral shell and sand. It was pretty hard. I tested the Aluminum Spade (owner said it cost $3000) and it did not set. Light anchors hold just as good as heavy anchors but they do not set as well. Holding is about design and surface area, not weight.

Note: I also tested the Manson Supreme, CQR, Bruce, Fortress, Bubble, Danforth, Bulwagga and Mantus. The only ones that set and held were the Bulwagga and Mantus. I cruised halfway around the world anchoring most of the time using the Bulwagga and it never dragged and always set in a matter of inches. With that said, I just bought a new boat and just finished hooking up a brand new Mantus 105 lb that I paid $1000 for. I believe it is the best anchor out there with maybe the exception of the Ultra but that cost 3 times more.

[Fidsplice]

Dan, you mention “once set” more than once and it is the key. The problems with anchoring (presuming an adequate anchor and enough rode) are mostly with resetting. Tide turns, anchor pulls out, tumbles, needs to reset. So is the anchor strong enough to survive the severe forces of twisting it out of a well set bed, and once out is it clean enough to reset? Many new designs have hoops to help the initial set, but often hold a bog of mud that inhibits the next set! Damn! Isn’t there a great single answer? Nope. Of course not, that’s why the argument rages.

But to your question. Aluminium is great if it’s strong enough. But once bent, doesn’t accomodate being straightened. I like aluminium, but once bent it’s a throw away.

[Training Wheels]

Once bent, you call Fortress and get a new part.

[thinwater]

Originally Posted by noelex 77

Dan you may like to look at this anchor test :
https://www.georgekniest.nl/pdf/manuals/kobra.pdf

This was a large scale independent test (not paid for by a manufacturer). I would caution that anchor tests give variable results and reading only one test can be misleading, but the results of this test fit in with my own conclusion about the difference between the Steel and the aluminium Spade.

The holding in hard sand dropped significantly for the same sized aluminium version. From 1905 kg to 1052 kg. The shank of the aluminum Spade bent during the test, but if they had tested in softer sand or mud the results would have been much closer.

Look at the static forces in the "fact box" on page 27. I compared these to ABYC table one, my testing, and the testing of others. They are simply wrong, about 4 times higher than reality. In the text it makes it clear that this is the static load, not that including yawing and snatch loads. This mean wind only. For example, it says that the rode tension on a 6M boat in 15 knots would be 40KG. Please, in reality you could hold that pinched between two fingers. A 12M boat would be 130 KG, yet we know that wouldn't be difficult to pull in hand-over-hand.


I think this is an editing error. This is the worst-case load, with yawing, anchored with all chain in shallow water and no snubber. In that case it is still higher than ABYC table 1, but is in the range of reality. The actual wind load is 4-5 times lower than this table or no one would ever recover an anchor. It is a conservative value to base chain working load on. If the snatch loads were 4 times greater, as the table suggests, we'd be snapping chains all the time.

[FranklinGray]

For static wind, it is an easy formula:

for mono-hull
pull in lbs = .00256 * knts squared * ((length * height) * .3) * 1.4

for multi-hull
= .00256 * knts squared * (length * height) * 1.4

Not totally exact but very close. So I agree with thinwater.

[Jim Cate]

Originally Posted by FranklinGray

For static wind, it is an easy formula:

for mono-hull
pull in lbs = .00256 * knts squared * ((length * height) * .3) * 1.4

for multi-hull
= .00256 * knts squared * (length * height) * 1.4

Not totally exact but very close. So I agree with thinwater.

"Length" and "height" of what? And no factor for beam? Seems odd!

Jim

[FranklinGray]

Originally Posted by Jim Cate

"Length" and "height" of what? And no factor for beam? Seems odd!

Jim

My mistake...for multihull that is width by height. Mono-hull has more surface area from the side when it swings so width is not used.

[thinwater]

Originally Posted by FranklinGray

For static wind, it is an easy formula:

for mono-hull
pull in lbs = .00256 * knts squared * ((length * height) * .3) * 1.4

for multi-hull
= .00256 * knts squared * (length * height) * 1.4

Not totally exact but very close. So I agree with thinwater.

Are you sure? This suggests that a multi is 3.3 times as wide as a mono. More, yes, but I think perhaps there is a problem with the multihull multipliers. I've measure the load and get about 1/2 of that. Yes, they are wider, but the mast is shorter and they are much lighter. The conventional wisdom is ~ 140-160%.



The mono number is close.

[Jim Cate]

Originally Posted by FranklinGray

My mistake...for multihull that is width by height. Mono-hull has more surface area from the side when it swings so width is not used.

I ask again: Length and height of what?

Jim