Bent anchor!
- Thread starter Ian_Edwards
- Start date
prv
Well-Known Member
I believe it's solid in the smaller sizes, fabricated as a hollow triangle as you say in the larger ones. Not sure at what size they change over.
Pete
D
Deleted member 36384
Guest
Interesting failure, this too was on my list of possible replacements, favourite in fact; still is. Triangular shapes are prone to twisting when loaded laterally with the stress concentrating towards the apex. It is not a good shape for resisting sideways loads at all. I rectangular profile would have been better. then again the anchor would not have been designed to be locked in place and a huge side force applied, if that is what happened.
Seajet
...
It seems a sad comment on our times and merchandising practice that hollow anchors, some with 'buoyancy' are marketed as a Good Thing !!!
I think it was PBO who ran an April 1st issue a while ago about a 'new water ballasted anchor'
A sensible well designed shaped anchor - as heavy as one can sensibly handle - on the end of as long and heavy a chain and / or nylon warp as possible is about all one needs to know, though something like a fisherman or folding grapnel is handy on weed covered rocks, ie in emergency, or to use as an Angel.
I think it was PBO who ran an April 1st issue a while ago about a 'new water ballasted anchor'
A sensible well designed shaped anchor - as heavy as one can sensibly handle - on the end of as long and heavy a chain and / or nylon warp as possible is about all one needs to know, though something like a fisherman or folding grapnel is handy on weed covered rocks, ie in emergency, or to use as an Angel.
noelex
Well-Known Member
For most anchor designs a shank as light as possible helps weight distribution and therefore holding power.
This is why Spade has constructed a complicated and expensive fabricated hollow shank. Others have used high tensile steels when it would be cheaper to use a thicker section of mild steel.
It's not so much about making the anchor lighter, but getting the centre of gravity in the right place.
This is why Spade has constructed a complicated and expensive fabricated hollow shank. Others have used high tensile steels when it would be cheaper to use a thicker section of mild steel.
It's not so much about making the anchor lighter, but getting the centre of gravity in the right place.
Last edited:
D
Deleted member 36384
Guest
I don't agree with the heavy bit. It has long been concluded that weight has nothing to do with holding ability. Weight is needed for the tip to penetrate but even that has been reduced now due to better geometry. Why struggle with a heavy anchor when deploying or retrieving?
For example my 27 kg CQR can be replaced with: -
6.8 kg Fortress
9 kg Spade (aluminium)
18 kg Knox
20 kg Manson Supreme
20 kg Spade (Steel)
25 kg Rocna
The CQR probably has the strongest shank being a drop (IIRC) forged 'I' profile.
Goldie
Well-Known Member
I wonder if this was a steel Spade anchor, or an aluminium one?
I'm an extremely satisfied owner of a steel Spade which has given literally hundreds of nights holding.
I'm an extremely satisfied owner of a steel Spade which has given literally hundreds of nights holding.
Neeves
Well-Known Member
I vaguely recall that Spade reccommendation for their alloy anchors is one size bigger than their gal version (in terms of surface area), primarily because the alloy shank is not strong enough in the smaller size. So if 20kg gal is right for your yacht then the equivalent alloy would be the next size up to the 6.8kg you quote. The Fortress has a shank of really beefy proportions.
I have never understood Spades reluctance to change their shank design (for their gal models), in fact I'd welcome someone pointing out the logic. Its very complex and must cost a fortune to make. Its hollow, but can fill with water, unlike the Ultra which is sealed. They could use the same amount of steel with a hi-tensile steel as they do with their model, have (at least) 3 times the strength, same weight and a thinner section (and it would be cheaper and easier to make).
If I were the unfortunate owner of the Spade illustrated I'd be having a 800mpA shank made, simply cut from plate, to replace the one that bent.
Last edited:
Conachair
Guest
Ultra I believe is stainless, you can't hot dip galv steel with sealed voids, it will so needs a hole to release the expanding air.
Neeves
Well-Known Member
Sorry, yes I knew that, they blow up! I was really questioning the 'theory' behind the box shank. Ultras offer some buoyancy, though how much impact it has I have no idea. But the hollow shank of the Spade does not seem to offer any advantage over a simple hi tensile steel shank made from plate and must cost an inordinate amount.
aluijten
Well-Known Member
I'd say because a box profile (even with a triangular cross section) is much stronger then a massive cross section at the same total weight point)
Neeves
Well-Known Member
I understand that a box section is stronger than the same weight of steel sheet. But we do not have that with the Spade shank. We have a box section shank, made from a medium tensile strength steel, so it will be stronger than the same weight/dimensions of (similar tensile strength) plate steel. However there is the option of using steels of higher tensile strength, but as a simple piece of plate. One is therefore comparing a box section made from a low(er) tensile steel vs a plate steel shank utilising hi tensile steels - both having the same approximate shape (one is fat because it is hollow - but otherwise similar).
The thickness, amount, of metal used in a Spade is similar to the amount of steel used in the equivalent sized Manson's Supreme (double the Spade's shank wall thickness and you have the thickness of the Supreme). Manson steel has, at least, twice the tensile strength of that used by Spade. So is the box section design twice as strong, because it must cost more to make, than Manson's simple plate shank.
Somewhere, someone must be able to make the calculations.
My thought is
Spade has great reviews but is inordinately expensive. Lots of people would buy it, were it cheaper. Part of that expense is in the complex shank. If the shank could be made more cheaply from plate hi tensile steel (and have the same or better performance) what is the reasoning behind the continuing use of the hollow shank?
The thickness, amount, of metal used in a Spade is similar to the amount of steel used in the equivalent sized Manson's Supreme (double the Spade's shank wall thickness and you have the thickness of the Supreme). Manson steel has, at least, twice the tensile strength of that used by Spade. So is the box section design twice as strong, because it must cost more to make, than Manson's simple plate shank.
Somewhere, someone must be able to make the calculations.
My thought is
Spade has great reviews but is inordinately expensive. Lots of people would buy it, were it cheaper. Part of that expense is in the complex shank. If the shank could be made more cheaply from plate hi tensile steel (and have the same or better performance) what is the reasoning behind the continuing use of the hollow shank?
noelex
Well-Known Member
A solid hi tensite shank would be heavier for the same strength at least in the horizontal direction. Extra weight in the shank alters the balance and diminishes the performance.
The designer and manufacturer obviiously thought the extra cost was justified for the performance increase.
Neeves
Well-Known Member
Fine
Can you quantify the answer? The metal in the box shank has about half the strength of the hi-tensile plate shank. What you are saying is the box sections design imparts twice the strength - can you quantify, show the calculations?
I'm not doubting the design, then. But have hi tensile steels become more easily available since then, become cheaper? I do not know
I only suggest - Spade comes out top or near top in every test, people do not buy it becuase it is expensive - is there just the iota of a chance technology and costs have moved on and maybe it could be improved - it is after all almost 20 years old.
noelex
Well-Known Member
It has been a long time since I have done moment of inertia calculations etc, but you don't need to be an engineer pick up any box, T or I section and it is much stiffer than the the equalent weight in flat bar. The difference is dependent on the thickness, but will typically a lot greater than 2:1
Weight critical structures like aircraft spars are constructed from hollow sections and I beams because of these advantages.
Hi tensile steels have been available for a long time, why do you think the designer went for a more expensive, less corosion resistant hollow section?
Last edited:
Neeves
Well-Known Member
why do you think the designer went for a more expensive, less corosion resistant hollow section?
I do not know, that's why I asked
But thanks for the answer!
I do not know, that's why I asked
But thanks for the answer!
fishermantwo
Well-Known Member
The difference the hollow shank makes to the balance of the anchor is enormous. My replica is closer to an Ultra than a Spade but the geometry is very similar. I built the shank from 20mm SS tube and SS sheet to make a box section. By the time I added the lead in the tip the anchor almost doubled in weight. Its easy to carry one handed because of the weight distribution and it lands on the bottom one way only. In the water the weight of the chain and the lead in the tip and the possibly buoyant shank ensure that. The shank where it is welded to the blade is about 100mm x 25mm so there is quite an air chamber. The anchor weighs 13 kg.