How good is the Rocna?

[Dockhead]

Originally Posted by conachair

Thats a very big "therefore"...., a force of buoyancy is a constant if the liquid stays the same, how can the acceleration not be constant if the force is constant?
(ignoring a tiny change for increase in density).
Drag is completely different and changes with velocity through the liquid.
A bit of buoyancy inside an object just means the object is a bit lighter.

Yes, thank you -- what I wrote could be misunderstood. I guess I wrote it sloppily.

I meant gravity as a constant as Lodesman was thinking of it at first (tungsten and steel balls dropped in the water and landing at the same time).

The change to acceleration from buoyancy is of course constant, aside from, as you correctly point out, change of density with depth (a nice point).

What is not constant is 36 feet per second per second -- which is reduced PROPORTIONATELY by any counteracting force. A very simple formula stated by Isaac Newton as the Second Law of Motion. So 12.5% less than that, in case of an object made of steel with density 8000kg/m3.

[conachair]

Originally Posted by Dockhead

What is not constant is 36 feet per second per second -- which is reduced PROPORTIONATELY by any counteracting force. A very simple formula stated by Isaac Newton as the Second Law of Motion. So 12.5% less than that, in case of an object made of steel with density 8000kg/m3.

The graph might look something like this. (y=velocity, x= time) . though a bit of a guess, Velocity = acceleration x Time, but accel goes down quickly as the velocity goes up and drag becomes a dominant force. Factor of 6 x velocity total guess - anchor might top out at about 1.5m/s - 3 Kts? Fun to think about and visualize on a graph though.

[Dockhead]

Originally Posted by conachair

The graph might look something like this. (y=velocity, x= time) . though a bit of a guess, Velocity = acceleration x Time, but accel goes down quickly as the velocity goes up and drag becomes a dominant force. Factor of 6 x velocity total guess - anchor might top out at about 1.5m/s - 3 Kts? Fun to think about and visualize on a graph though.

Hah, I had given up on this thread, and now you come in with this.

Actually the terminal velocity of a steel ball is quite a bit more than 3 knots, but carry on

What you're getting at, is correct. And the more the effect of drag, the bigger difference between objects of different densities . . .


Edit:

So using this formula:



There's a handy calculator here: Terminal velocity (under buoyancy force) - fx​Solver equation calculator)

We get a terminal velocity of 5.26 m/s (about 10 knots) for Lodesman's steel ball.

The tungsten one has 8.57 m/s (about 16 knots).

That's using 0.6 as the drag coefficient for both balls, which is a guess. If there is no turbulent flow (?), the drag coefficient for a smooth sphere of this size is about 0.1. That would give 12.9 m/s and 21 m/s, respectively.


But getting back to anchors -- there are three forces which can create torque and spin the anchor around while it's falling -- the drag force, the righting moment from buoyancy, and momentum from balance. Not necessarily in the same direction! In fact I would guess that all these anchors we're discussing will tend to fall upside down if drag were the only force influencing the orientation -- the fluke would act like a parachute. So I guess they really do need as much righting moment as they can get -- hence the lead ballast.

Interesting, do Rocna anchors, which don't have ballast, tend to fall upside down, then roll over on their roll bars? I thought Panope wrote that all the anchors he watched fell fluke-down. If that's so, how to explain it?

I can say from considerable experience with both Rocna and Spade, that the Rocna has far more failed sets than the Spade. I always guessed it was because the ballast in the Spade kept it oriented correctly to dig straight in, but I've never watched them falling, so that's only a guess.

[Dave_S]

I believe the anchors setting ability is controlled largely by how it reacts to being pulled by the chain after it has landed anyway up and we are just discussing how the air pocket affects the anchors fall through water for fun.

here is another thought.

The acceleration and terminal velocity is a product of mass verses resistance. The cross section of the anchor with the air pocket is oval, the other is rectangular. The anchor with the air pocket has less resistance and may accelerate faster than the one without. If this is true the air pocket may increase the chances of the anchor landing poorly.

[Dockhead]

Originally Posted by Dave_S

I believe the anchors setting ability is controlled largely by how it reacts to being pulled by the chain after it has landed anyway up and we are just discussing how the air pocket affects the anchors fall through water for fun.

here is another thought.

The acceleration and terminal velocity is a product of mass verses resistance. The cross section of the anchor with the air pocket is oval, the other is rectangular. The anchor with the air pocket has less resistance and may accelerate faster than the one without. If this is true the air pocket may increase the chances of the anchor landing poorly.

This is a good case for wings on the shank to provide drag. It's a seriously good idea.

But I doubt that the slightly different shape of the Ultra vs Spade offsets the effect of air in the shank (if there really is any -- we haven't established that yet).

And the chain will be affecting the angle of the shank to the bottom, not rotation of the anchor away from its point. It's just a guess (unlike the hard physics which we've managed to do in this thread, which are not guesses), but I think that ballasted fluke/light shank has a really big, really practical effect on anchor performance.

[mako]

Wow guys, I haven't seen so much discussion about fluid dynamics since I was studying aerospace engineer at U of Mich decades ago!

Go BLUE!

[TheThunderbird]

Originally Posted by makobuilders

Wow guys, I haven't seen so much discussion about fluid dynamics since I was studying aerospace engineer at U of Mich decades ago!

Go BLUE!

You zeem to ignore the fact that the future iz an anchor of variable geometry, in tungsten and an air bladder adapting to the degree of salinity and sea bed conditions

[Dockhead]

Originally Posted by TheThunderbird

You zeem to ignore the fact that the future iz an anchor of variable geometry, in tungsten and an air bladder adapting to the degree of salinity and sea bed conditions

Actually a sharp tungsten point on an anchor like the Spade would be absolutely superb. I suggested it a few years ago.

Open 60's have tungsten in their keels. And tungsten bars are used as ballast in Formula 1 cars. It's pretty spendy at 22 euros or so per kilo for the low grade type, or about 10x the price of lead. But it's more than half again as dense than lead and nearly 2.5x as dense as iron. A couple of kilos of tungsten in the tip should make a significant difference in balance, and also solve the galvanizing problem at least on that part of the anchor

[groundtackle]

EXCEL has a stainless steel plate welded to the toe. Can be dented / dulled then reshaped without compromising the galvanising. Nice intelligent feature.
ce

[Souzag818]

I don't know how you would re-assemble a Spade anchor in the wrong way. The hole for the bolt will only line up with the shank one way- the right way. If you're hurrying to anchor, you're too late...