How good is the Rocna?

[LakeSuperior]

Originally Posted by noelex 77

.

Would anyone like to hazard a guess at the reduction in shank weight from the buoyancy?

To hazard a guess not knowing the wall thickness but understanding the forces and the material needed to withstand the forces my guess is about a 5 percent reduction to yield an actualized shank weight in water.

The specific gravity of steel is about 7.8. So displacing the water given the same internal shank volume filled with steel by air gives this buoyancy factor estimate.


Ok I will redo this above estimate with a paper and a pencil. I'm digging a hole here.

[noelex 77]

This Ultra owner has drilled a hole in the shank.

The photo gives some idea of the Ultra shanks construction and interior volume. While the photo is not directly comparable with the photo Spade I posted earlier the wall thickness of the Ultra shank looks less than the Spade. The overall width of the Ultra shank is also greater, especially as it widens towards the midpoint, so the internal volume is more significant. If sealed this volume will not fill with seawater. The buoyancy reduces the effective weight of the shank in water and helps the anchor rotate upright.

Another question is does the buoyancy also have an effect on the the tip weight in water?. There is obviously some reducion in the weight of the anchor overall, and hence the tip weight. It is hard to estimate the centre of buoyancy of the shank. This is probably well away from chain attachment point so this may compensate and actually increase the effective tip weight.

[Panope]

I just measured the internal volume of my 45 pound galvanized Spade anchor's shank by filling/draining with water: - 1.5 cups or 3/4 pound of buoyancy.

This "trapped" air will compress as the anchor descends in water (the shank is not sealed), so the actual buoyancy will be less than the above.

I've never had my hands on an Ultra anchor but it appears the shank volume could be perhaps 2 or 3 times that of a similarly sized Spade.

Here is one of my videos, showing that the air is likely to remain trapped in the Spade shank as the opening in the bottom of the shank never rises above the rest of the shank

Steve

https://www.youtube.com/watch?v=i8P-gS1Z-oI

[noelex 77]

Originally Posted by Panope

I just measured the internal volume of my 45 pound galvanized Spade anchor's shank by filling/draining with water: - 1.5 cups or 3/4 pound of buoyancy.

Thanks Steve . Could you also measure the total shank weight. This will give some idea of the percentage reduction in weight from the buoyancy.

As you point, out for the unsealed shank of the Spade, even if no air escapes, the volume will be half filled with seawater at around 10m as the air compresses. So in practice it will only be a small, but I still think helpful reduction in the effective shank weight.

The reduction in the shank weight of the Ultra will be more significant, given that its volume is greater and that it is sealed.

[a64pilot]

The reduction of shank weight due to buoyancy will be I bet insignificant, however the reduction in weight due to it being hollow, will be very significant.



Sent from my iPad Pro using Cruisers Sailing Forum

[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.

[noelex 77]

Originally Posted by a64pilot

The reduction of shank weight due to buoyancy will be I bet insignificant, however the reduction in weight due to it being hollow, will be very significant.

The fabricated shank of the Spade and Ultra as you point out does have significant potential weight savings over over a solid steel shank even ignoring the slight buoyancy issues. However fabricated shanks are constructed from only mild steel (Spade) or 316 stainless (Ultra), so the hi tensile steels used in the better conventional anchor shanks make up some ground. The fabricated shanks are also thicker reducing penetration although the effect of this is mitigated by the tapered profile (triangular for the Spade and Diamond shaped for the Ultra).

Overall I think if all other things are equal fabricated shanks are better, but I suspect the gains in overall performance are not great.

The other factor is the testing societies only measure the vertical strength of anchor shanks. The horizontal strength is not measured at all. So modern anchor shanks have tended to be very strong with a vertical direction, but they are more marginal with a horizontal direction of pull. Fabricated shanks gain a lot of strength in the horizontal direction, but not much in the vertical direction compared with steel shank of conventional profile. Hence for an anchor with a fabricated shank to pass the same tests as a solid steel anchor the weight saving is only small.

I suspect a new standard (or manufacturers ignoring the standard even if this costs them sales) would result lighter but stronger shanks and importantly higher performing anchors.

Modern anchors seem to be moving towards lighter shanks overall. There is still a demand for near industructable anchors but I think most consumers are looking for balance of high performance with reasonable strength.

[a64pilot]

If you look at the shape of the shank it of course tells you where its strength lies, it is strong proportional to the thickness, as it is thickest vertically, there is the strength.
Surprisingly most of the strength is on the surface of a steel shape, reason why a thin tube of the same weight is many times stronger than a solid rod, many times. So if weight is an issue then built up is best, but anchors are sold by the lb usually, so there isn't much reason to save weight. Fortress excepted, their niche seems to be high performance at a light weight.
Also I assume any hollow structure may be subjected to internal corrosion, big issue on tube steel airframes for example.
Plus built up is likely much more expensive to manufacture.


Sent from my iPad Pro using Cruisers Sailing Forum

[Panope]

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.

No one in this discussion is saying that any shank will float. What is being said, is that an air-filled shank will sink with LESS force than a solid one (of equal mass), and therefore create LESS negative righting moment.

Steve

[uncle stinkybob]

I just circumvented all this bull and bought a Mantus!

[noelex 77]

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 be take a sealed hollow shank like the Ultra it might weigh 9kg in air as you suggest. It we take an identical shank and punch some holes in it so that is no longer sealed it will still weigh 9kg.

When we immerse the two shanks in water both will weigh less but the sealed shank will weigh significantly less than the shank with the holes. No one is suggesting the sealed shank has become positively buoyant, but buoyancy of the trapped air in the sealed shank reduces the weight compared to shank with the holes that will fill with water.

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.

[chris95040]

Originally Posted by Lodesman

I can't believe all of you.

...

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.

[Panope]

Originally Posted by noelex 77

Thanks Steve . Could you also measure the total shank weight. This will give some idea of the percentage reduction in weight from the buoyancy. ..........

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

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.

Steve

[Fortress]

All the above discussions are probably a non-issue if the sea bottom is a harder soil in which most anchors will orient correctly with the fluke in the downward position as the anchor is being slowly pulled.

Where it becomes a larger issue is if the bottom is a very soft soil in which the anchor lands on its side or upside down, and the question then becomes whether or not it has the capability of properly orienting itself.

It has been observed that a rollbar anchor needs a harder soil for it to roll over, so to speak, and in a softer soil, it will simply slide along or through very soft mud (as an example) in whatever position that it lands.


Sent from my iPhone using Cruisers Sailing Forum

[Jim Cate]

Originally Posted by Fortress

All the above discussions are probably a non-issue if the sea bottom is a harder soil in which most anchors will orient correctly with the fluke in the downward position as the anchor is being slowly pulled.

Where it becomes a larger issue is if the bottom is a very soft soil in which the anchor lands on its side or upside down, and the question then becomes whether or not it has the capability of properly orienting itself.

It has been observed that a rollbar anchor needs a harder soil for it to roll over, so to speak, and in a softer soil, it will simply slide along or through very soft mud (as an example) in whatever position that it lands.


Sent from my iPhone using Cruisers Sailing Forum

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