How good is the Rocna?

[Snowpetrel]

I could see a high strength aluminium shank, married to a high strength alloy or galvanised fluke with a stainless steel toe and a bolt on lead shoe being a neat design. Of course proper isolation would be needed to prevent corrosion..

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[noelex 77]

Originally Posted by Panope

Noelex, the shank weighs 12 pounds according to my digital bathroom scale.

Thanks Steve. 3/4 of a pound of buoyancy therefore reduces the shank weight by about 6% when just below the surface and 3% at 10m.

Originally Posted by Panope

My gut feeling is that the trapped air in a Spade anchor will provide an almost undetectable SETTING benefit while the captured air in the Ultra will provide a small, but noticeable setting benefit.

Yes I agree with that assessment.

I also like your emphasis on SETTING.

The effect of the buoyancy in the shank will be mostly on the ability of the anchor to set rather than necessarily the holding ability once (and if) the anchor has reached the correct orientation. However, I would also also stress the important resetting ability where the reduced shank weight will help the anchor maintain a more level orientation as it "shuffles" around to a new orientation.

It would be interesting to measure the volume of trapped air in a Mantus roll bar (and in a Rocna if you block the breather hole). There are also some convex plow anchors that can trap a small quantity of air in the ballast chamber. In the ballast chamber the buoyancy will have a small negative rather than positive effect.

With the exception of the Ultra, I think these are only minor points, but I think the effect of buoyancy has not been properly considered by many anchor manufacturers. Clever use of buoyancy has the potential to deliver better performing anchors, although I suspect from a marketing perspective it is tough to sell the general public on the benefits of buoyancy in an anchor design . This is probably why the Hydrobubble anchor failed and why Ultra makes no mention of the benefits of their sealed design.

[TheThunderbird]

Originally Posted by Lodesman

I can't believe all of you. If an object has the same density as the medium in which it is immersed (water, air, etc) then it is (as we divers say) neutrally buoyant and needs some other force to cause it to rise or sink. If it is less dense, then it will be positively buoyant and will rise. If it is more dense, then it is negatively buoyant and will sink.
If the shank is 9 Kg, which seems a reasonable estimate, then the equivalent weight of water would be 9 litres, (for fresh water - sea water would be 8.8 l) - or 8800 cubic centimetres. That's a column 10x10x88cm (4"X4" by almost 3'). The shank very clearly does not come anywhere close to that in volume, so it is negatively buoyant. That is, it sinks. It has no buoyancy, and therefore creates no righting moment.

Now iirc, Newton proved that everything falls at the same rate regardless of mass (in a vacuum). Aerodynamic and hydrodynamic drag would evidently slow the fall of items of large relative surface area compared to denser items of similar mass. The relatively large surface area of the Ultra's shank may provide greater drag against the water compare to the smaller shank of another anchor, but I would hazard to guess its streamlined shape and polished SS finish might cause it to fall faster than a lumpy galvanized, square-edged slab. What is more likely to keep the shank up and the point down when an anchor is dropping, is the anchor chain acting as a drogue.

An anchor with a weighted tip (using much denser lead) will tip point down, using the edge of the fluke (wing?) as a fulcrum on the sea bed - it does not necessarily fall faster than the relatively light shank through the water.

With all due respect, it was Galileo Galilei discovering the first principle of dynamics, of linear uniform speed motion, and the effects of gravity (with pendulum, and Pisa tower experiments)

[Dockhead]

Originally Posted by Lodesman

3'). The shank very clearly does not come anywhere close to that in volume, so it is negatively buoyant. That is, it sinks. It has no buoyancy, and therefore creates no righting moment.

This is false!

You do not need positive buoyancy, to change the center of gravity.


Easily proven with this thought-experiment:

Take one Spade anchor, 45kg. It is balanced so that it will land on its sharp tip, which Panope will attest to, having watched them being dropped.


Now take that same anchor, and tie a 20kg lead weight to the top of the shank. What will happen when you drop it? It will land shank-down, because you've screwed up the balance of it -- you've moved the CG up away from the tip.


So the opposite is also true -- trap some air in the shank, and you will reduce its weight in the water [also its mass -- anchor with water in the shank vs anchor with air in the shank], which will move the CG of the anchor down towards the tip. You don't need to reduce the weight in the water of the shank to zero or less, to get righting moment or a favorable shift downwards of the center of gravity. Any reduction in the weight-in-the-water of the shank, will move the CG.


Here are the physics behind this:


"A pure force thorugh the center of gravity (with no net torque) will purely translate a rigid body (any point on the body).

A pure torque any point on the body (with no net force) will purely rotate a rigid body about its center of gravity"

http://physics.stackexchange.com/que...fect-free-fall


What this means is that any torque, and also any force applied other than exactly through the CG of an object, will rotate it around its CG.

That's why falling bodies land on their heavy ends unless a greater aerodynamic force dictates something different.

[Lodesman]

Originally Posted by Panope

No one in this discussion is saying that any shank will float.

Perhaps we have a different understanding of the word "buoyancy.":

Originally Posted by Kenomac

So yes, without a doubt the Ultra shank has buoyancy unlike the sheet metal shank on a Spade.

Originally Posted by noelex 77

As the anchor sets it has to rotate the shank upright. It also has to ideally keep the shank upright as it rotates to new wind direction. The less heavy the shank the easier it is to rotate or keep upright, because the centre of gravity is lower.

The shank does not have to positively buoyant to make the anchor perform better, just being less heavy is a help. The sealed shank is less heavy in water.

Originally Posted by chris95040

Even though the shank wouldn't 'float', obviously its density affects the center of mass of the anchor.

Consider 3 imaginary tweaks to the ultra and it's sealed shank:

If the ultra shank was the same dimensions, but solid, it would raise the center of mass and make the anchor less stable, period.

If the ultra shank was the same amount of material, but solid (so less shank volume) the center of mass would be largely unaffected, but the center of buoyancy would be lower, making the anchor less stable underwater.

If the ultra shank was the same dimensions, but open to the water (like the spade) the center of mass would be unaffected, but the center of buoyancy would be lower, making the anchor less stable underwater.

Originally Posted by Dockhead

Take one Spade anchor, 45kg. It is balanced so that it will land on its sharp tip, which Panope will attest to, having watched them being dropped.

Here are the physics behind this:


"A pure force thorugh the center of gravity (with no net torque) will purely translate a rigid body (any point on the body).

A pure torque any point on the body (with no net force) will purely rotate a rigid body about its center of gravity"

newtonian mechanics - Does mass distribution/center of mass affect free fall? - Physics Stack Exchange


What this means is that any torque, and also any force applied other than exactly through the CG of an object, will rotate it around its CG.

That's why falling bodies land on their heavy ends.

DH, the site you cite states that without external force applied to induce torgue, the item will not rotate - if external forces are applied (as in hydrodynamic drag) then the item will rotate around its CoG. That does not necessarily mean the item will land heavy side down, but how the drag orients the anchor would. I haven't seen Panope's video of the Spade being deployed, but the only true test would be to launch the anchor from various orientations, without an attached anchor cable, and observe how it falls.

A number of you continue to labour under the illusion that anchors have a centre of buoyancy - there is none, as there is no part of an anchor that is buoyant (save perhaps for the Hydrobubble - I'm unfamiliar with this design, but gather from the name it might have a buoyant chamber). No centre of buoyancy, then there is no righting moment - any moment around the CG is induce by form drag. Since most anchors are launched while dragging chain out of a chain locker, the chain provides the orientating drag to ensure a fluke down landing.

Back to physics - if I take two identically-sized perfectly smooth metal balls - one of steel weighing 10 kg, and one of tungsten that weighs 25 kg and drop them from the same height above the seabed, they will land simultaneously. Do we agree?
If I then connect the two balls with a short straight cylindrical shaft (like a barbell) the unit will have a CG that favours the tungsten ball, but if I orient it horizontally and drop it in the water, the two balls will behave as they did uncoupled, and will hit the bottom simultaneously.

I don't doubt that lightening the shank, and by encapsulating air that is the case, could change the CG of the anchor minimally. I don't see how that would have any real benefit, as you attach a whole load of heavy anchor chain to the end of it. Perhaps if you use a rope-only rode...

[Dockhead]

Originally Posted by Lodesman

. . .DH, the site you cite states that without external force applied to induce torgue, the item will not rotate - if external forces are applied (as in hydrodynamic drag) then the item will rotate around its CoG. That does not necessarily mean the item will land heavy side down, but how the drag orients the anchor would. I haven't seen Panope's video of the Spade being deployed, but the only true test would be to launch the anchor from various orientations, without an attached anchor cable, and observe how it falls.

A number of you continue to labour under the illusion that anchors have a centre of buoyancy - there is none, as there is no part of an anchor that is buoyant (save perhaps for the Hydrobubble - I'm unfamiliar with this design, but gather from the name it might have a buoyant chamber). No centre of buoyancy, then there is no righting moment - any moment around the CG is induce by form drag. Since most anchors are launched while dragging chain out of a chain locker, the chain provides the orientating drag to ensure a fluke down landing.

Back to physics - if I take two identically-sized perfectly smooth metal balls - one of steel weighing 10 kg, and one of tungsten that weighs 25 kg and drop them from the same height above the seabed, they will land simultaneously. Do we agree?
If I then connect the two balls with a short straight cylindrical shaft (like a barbell) the unit will have a CG that favours the tungsten ball, but if I orient it horizontally and drop it in the water, the two balls will behave as they did uncoupled, and will hit the bottom simultaneously.
.

Lodesman, you know I have the greatest respect for you and your knowledge, and I've learned a lot from your on different subject, but where Physics is concerned -- back to the woodshed!

What you are saying amounts to saying that there is no such thing as balance!

Take a hammer and drop it, and watch what it does. Then come back to us.

If you are still puzzled about what you observe, then Google "physics of balance" and read up on it.

[LakeSuperior]

Originally Posted by Dockhead

Lodesman, you know I have the greatest respect for you and your knowledge, and I've learned a lot from your on different subject, but where Physics is concerned -- back to the woodshed!

What you are saying amounts to saying that there is no such thing as balance!

Take a hammer and drop it, and watch what it does. Then come back to us.

If you are still puzzled about what you observe, then Google "physics of balance" and read up on it.

Lodesman is correct if working in a vacuum. The problem at hand also requires the fluid. Think the barbell where the one ball weights 0.001 lbs in the water.

From an engineering POV balance is tied up in dynamic control systems like a human. Not relevant for setting anchors.

An anchor is more under the influence of hydrodynamic forces as it falls through the water column. Here the geometry is the overriding factor with the weight distribution being secondary. This problem is then confounded by whatever the anchor is attached to and the force it exerts on the shank.

[a64pilot]

Originally Posted by Jim Cate

I don't have Panope's ability to view my Manson Supreme on its way down, but to me it appears that if I am moving very slowly backwards (as I usually am whilst anchoring) the anchor is properly oriented by the hydrodynamic forces as it moves through the water, and should never land upside down or backwards.

Dunno if this is true of other designs, but suspect that it is.

Jim

Yes, and a way to demonstrate this is to let it free fall, My Rocna will always "fly" up right and blunt end forward, so if it's oriented the way it sits on the bow and I drop it, it will descend upright and will "fly" forward, away from the boat and land upright, correctly oriented.
I feel sure that having to drag chain out of the locker has a lot to do with it being upright and that if you dropped the anchor by itself, it would quickly invert.

Any boat speed of a kt or more will have the anchor orient itself with the point into the water flow, until I discovered this I wanted an Ultra flip swivel as my anchor always came up, upside down, but that was because I had forward boat speed.
Since then I now bring the anchor up and leave it just in the water and walk back to the cockpit where I slowly reverse the boat, this cleans any sand and mud off and correctly orients the anchor 100% of the time, I can then use the hard wired windlass switch in the cockpit to stow the anchor, I don't have to be on the bow to ensure the anchor is oriented correctly, as long as I'm making way in reverse it will be, and I don't need any kind of orienting / flip device.

I feel a little technique as in just a little speed in reverse while setting and retrieving the anchor will ensure it lands and stows correctly and you don't need to spend $ on something you don't need.

At least for a Rocna, and apparently a Mantus too


Put this in the frugal sailor thread

[Dockhead]

Originally Posted by LakeSuperior

Lodesman is correct if working in a vacuum. The problem at hand also requires the fluid. Think the barbell where the one ball weights 0.001 lbs in the water.

From an engineering POV balance is tied up in dynamic control systems like a human. Not relevant for setting anchors.

An anchor is more under the influence of hydrodynamic forces as it falls through the water column. Here the geometry is the overriding factor with the weight distribution being secondary. This problem is then confounded by whatever the anchor is attached to and the force it exerts on the shank.

Lodesman would be correct in a vacuum and IF any force exerted on the anchor, were exerted exactly through the CG. A hammer might also not fall on its head, in a vacuum, but ONLY if its dropped just so -- like balancing a penny on its edge.

Otherwise, even the slightest off center force will rotate the anchor around its center of gravity and even the slightest force will hold the light part back. That's how balance works, and it matters a lot. That's why the Spade has lead ballast.

You can balance a penny on its edge if you do it very careful. But it only takes a very slight force to put it in its natural position with the CG as close as possible to the table. Same with anchors.

[conachair]

Originally Posted by Dockhead

Lodesman would be correct in a vacuum and IF any force exerted on the anchor, were exerted exactly through the CG. A hammer might also not fall on its head, in a vacuum, but ONLY if its dropped just so -- like balancing a penny on its edge.

Otherwise, even the slightest off center force will rotate the anchor around its center of gravity and even the slightest force will hold the light part back. That's how balance works, and it matters a lot. That's why the Spade has lead ballast.

You can balance a penny on its edge if you do it very careful. But it only takes a very slight force to put it in its natural position with the CG as close as possible to the table. Same with anchors.

fast forward to just before 3.00.
Drag either in air or water will far outgun any kind of movement on release as you seem to be talking about. And just to be clear, without drag or some external force there is NO turning force on the falling object. only any motion which was there before the object was dropped. Otherwise - where does the force come from?

https://www.youtube.com/watch?v=E43-CfukEgs

[noelex 77]

Originally Posted by Lodesman

A number of you continue to labour under the illusion that anchors have a centre of buoyancy - there is none, as there is no part of an anchor that is buoyant (save perhaps for the Hydrobubble

Buoyancy is a force that exerts on any object that is not in a vacuum. The object does not have to float. I think this is where you are misunderstanding the subject.

Buoyancy is the only force we have to explain why an object weighs less in water than it does in air.

All objects have some buoyancy in water. Hence all objects have a centre of buoyancy, even objects that sink.

Originally Posted by Lodesman

Back to physics - if I take two identically-sized perfectly smooth metal balls - one of steel weighing 10 kg, and one of tungsten that weighs 25 kg and drop them from the same height above the seabed, they will land simultaneously. Do we agree?

No the same sized ball made of the denser material (tungsten) will arrive at the seabed first. Consider the same sized ball made of a plastic, or perhaps hollow aluminium, where the material was only very slightly negatively buoyant (only slightly more dense than seawater). If the density was very close to seawater such a ball could drift down to seabed very slowly indeed. This shows that the density is a factor in the speed of fall.

My underwater camera (in its housing) is like this. If I let it go it very slowly sinks. If I added some lead weights inside the housing (the size and shape would be unchanged, only the mass is altered) the camera would sink more quickly. With enough extra mass it would sink rapidly.

If the enclosure had a leak seawater would enter the housing displacing the air and increasing the mass. Once again the size and shape would be unchanged but the camera would sink more rapidly.

If dropped in a vacuum all three balls (and the camera with and without the lead weights) would fall at the same rate, but anchors don't work well in a vacuum .

[LakeSuperior]

Originally Posted by Dockhead

Lodesman would be correct in a vacuum and IF any force exerted on the anchor, were exerted exactly through the CG. A hammer might also not fall on its head, in a vacuum, but ONLY if its dropped just so -- like balancing a penny on its edge.

Otherwise, even the slightest off center force will rotate the anchor around its center of gravity and even the slightest force will hold the light part back. That's how balance works, and it matters a lot. That's why the Spade has lead ballast.

You can balance a penny on its edge if you do it very careful. But it only takes a very slight force to put it in its natural position with the CG as close as possible to the table. Same with anchors.

Pretty tortured logic. Concepts of statics and dynamics are well known and have been studied for hundreds of years. The ideas are institutionalized. Suggest you read the textbook.

[groundtackle]

a64pilot, agree reversing cleans and orientates most anchors. There are situations when that is not possible. Other vessels, approaching shoreline, swell /wind, dark/scary and lack of time. Sometimes the bow bouncing in a slop will flip the anchor just as it approaches the roller. I like to think worse case.

[a64pilot]

No harm, until I saw yours, only option I knew of was the Ultra, and that was more money than I wanted to spend, for that kind of money I can use a boat hook.

[groundtackle]

I've been touring every anchorage in reverse my whole life untll now.
This shank topic is educational, wish I had been attentive in school.
Applying my worse case logic again... I usually operate the windlass with the admiral driving via hand signals, trying to get that perfect set and lay down the rode. We get messed up on occasion staying on station, it's always her fault.
You drive over it, wind/ current, boats that won't behave it reverse etc. The anchor design must be the overiding factor when it finally gets loaded and starts to set, don't you think?